Patent Publication Number: US-2023137285-A1

Title: An aerosol-generating article comprising a mouthpiece assembly

Description:
The present invention relates to an aerosol-generating article comprising a mouthpiece assembly. 
     Some aerosol-generating articles heat rather than combust an aerosol-generating substrate, such as a tobacco-containing substrate. An aerosol may be generated in such aerosol-generating articles through the transfer of heat from a heat source to a physically separate aerosol-generating substrate or material, which may be located in contact with, within, around, or downstream of the heat source. During use of the aerosol-generating article, heat is transferred from the heat source to the aerosol-generating substrate, which may release volatile compounds. These volatile compounds are entrained in air drawn through the aerosol-generating article by a user. As the released volatile compounds cool, they condense to form an aerosol. The aerosol may be inhaled by a user through a mouthpiece. 
     It would be desirable to provide an aerosol-generating article that can cause more of the released volatile compounds to condense, which may provide for increased flow of aerosol through the mouthpiece. This may provide a better user experience. 
     There is provided an aerosol-generating article for producing an inhalable aerosol upon heating. The aerosol-generating article may comprise a mouthpiece assembly. The mouthpiece assembly may comprise a first tube. The mouthpiece assembly may comprise a second tube. The mouthpiece assembly may comprise a third tube. The aerosol-generating article may comprise an aerosol-forming substrate. The first tube may abut a downstream end face of the second tube. The third tube may abut an upstream end face of the second tube. An internal diameter of the second tube may be smaller than an internal diameter of the first tube. The internal diameter of the second tube may be smaller than an internal diameter of the third tube. The internal diameter of the first tube may be at least 3 mm. 
     There is also provided an aerosol-generating article for producing an inhalable aerosol upon heating, the aerosol-generating article comprising: a mouthpiece assembly comprising: a first tube, a second tube and a third tube; and an aerosol-forming substrate; wherein the first tube abuts a downstream end face of the second tube, and the third tube abuts an upstream end face of the second tube; wherein an internal diameter of the second tube is smaller than an internal diameter of the first tube; wherein the internal diameter of the second tube is smaller than an internal diameter of the third tube; wherein the internal diameter of the first tube is at least 3 mm. 
     There is also provided an aerosol-generating system comprising an aerosol-generating device and an aerosol-generating article, the aerosol-generating article comprising: a mouthpiece assembly comprising: a first tube, a second tube and a third tube; and an aerosol-forming substrate; wherein the first tube abuts a downstream end face of the second tube, and the third tube abuts an upstream end face of the second tube; wherein an internal diameter of the second tube is smaller than an internal diameter of the first tube; wherein the internal diameter of the second tube is smaller than an internal diameter of the third tube; wherein the internal diameter of the first tube is at least 3 mm. 
     The aerosol-generating article comprising a mouthpiece assembly being formed from a plurality of tubes, with the second tube a narrower diameter than the first and third tubes, may result in an increased amount of aerosol that can be drawn out of the aerosol-generating article. This increased amount of aerosol can improve the user&#39;s experience. 
     Aerosol formation, in particular droplet size, depends upon multiple factors such as temperature, and air pressure. 
     During use of the aerosol-generating article, volatile compounds within the aerosol-forming substrate are vaporised e.g. by thermal vaporisation. The vapour cools and nucleates to form an aerosol. As a user inhales at the downstream end of the aerosol-generating article, air is sucked towards the downstream end and the moving air entrains the aerosol and the vaporised volatile compounds. 
     In a typical aerosol-generating article, the aerosol would flow directly out of the downstream end of the aerosol-generating article and it would then be inhaled by a user. However, in the aerosol-generating article according to the invention, the narrower diameter of the second tube constricts flow of the air as it passes through the mouthpiece assembly. In other words, the second tube provides a venturi effect. When the air with entrained aerosol is drawn out of the second tube and into the first tube, the wider diameter of the first tube allows the air to expand and cool down, which can cause more droplets to form in the aerosol. As with typical aerosol-generating articles, the aerosol is then inhaled by the user through the downstream end of the first tube. 
     Accordingly, the aerosol-generating article may provide an increased amount of aerosol droplets, which provides an improved user experience. 
     In addition, since the first tube has a larger diameter than the second tube, the resulting aerosol expansion can improve the user&#39;s filling perception. 
     Furthermore, forming the mouthpiece assembly from a series of tubes may provide a number of additional advantages. 
     Firstly, handling of the aerosol-generating article can be improved because the first tube, the second tube and the third tube may provide increased firmness compared to, for example, a paper shell. 
     Secondly, the aerosol-generating article may be easier to manufacture because it is relatively straightforward to machine a plurality of tubes and then assemble the tube together. 
     Thirdly, use of a plurality of tubes provides increased flexibility in terms of inner diameter and length. 
     The term “aerosol-generating article” is used herein to denote an article wherein an aerosol-forming substrate is heated to produce and deliver inhalable aerosol to a consumer. As used herein, the term “aerosol-forming substrate” denotes a substrate capable of releasing volatile compounds upon heating to generate an aerosol. 
     A conventional cigarette is lit when a user applies a flame to one end of the cigarette and draws air through the other end. The localised heat provided by the flame and the oxygen in the air drawn through the cigarette causes the end of the cigarette to ignite, and the resulting combustion generates an inhalable smoke. In contrast, in a heated aerosol-generating article, an aerosol is generated by heating a flavour generating substrate, such as tobacco. Known heated aerosol generating articles include, for example, electrically heated aerosol generating articles and aerosol generating articles in which an aerosol is generated by the transfer of heat from a combustible fuel element or heat source to a physically separate aerosol forming material. For example, aerosol-generating articles according to the invention find particular application in aerosol-generating systems comprising an electrically heated aerosol generating device having an internal heater blade which is adapted to be inserted into the rod of aerosol-forming substrate. Aerosol generating articles of this type are described in the prior art, for example, in EP 081212670. 
     As used herein, the term “aerosol-generating device” refers to a device comprising a heater element that interacts with the aerosol-forming substrate of the aerosol generating article to generate an aerosol. 
     As used herein, the term “longitudinal” refers to the direction corresponding to the main longitudinal axis of the aerosol-generating article, which extends between the upstream and downstream ends of the aerosol-generating article. As used herein, the terms “upstream” and “downstream” describe the relative positions of elements, or portions of elements, of the aerosol-generating article in relation to the direction in which the aerosol is transported through the aerosol-generating article during use. 
     As used herein, the term “abut” is used to describe a component, or portion of a component, being in direct contact with another component, or portion of a component. 
     The mouthpiece assembly may be located towards the downstream end of the aerosol-generating article. The mouthpiece assembly may be located at the downstream end of the aerosol-generating article. 
     One or more of the first tube, the second tube and the third tube may be a cellulose acetate tube. In other words, one or more of the first tube, the second tube and the third tube may be formed from cellulose acetate. For example, the first tube may be a cellulose acetate tube. The second tube may be a cellulose acetate tube. The third tube may be a cellulose acetate tube. 
     A cellulose acetate tube may alternatively be referred to as a “hollow acetate tube” or a HAT. 
     Advantageously, forming the first tube from cellulose acetate can further improve the rigidity and resilience of the mouthpiece assembly, which improves the user experience. In addition, since cellulose acetate is substantially impermeable to water, forming the first tube from cellulose acetate may result in a mouthpiece assembly that is less sensitive to the humidity of a user&#39;s mouth. 
     The aerosol-generating article may comprise a fourth tube. The fourth tube may be located in an opening defined by the third tube. The fourth tube may be located within the third tube. The third tube may surround the fourth tube. The fourth tube may line the internal surface of the third tube. 
     The fourth tube may be formed from a substantially non-porous material. For example, the fourth tube may be formed from cardboard. 
     Advantageously, the fourth tube may ensure that the largest part of the aerosol flows along the longitudinal axis and towards the second tube and the third tube, and not radially through the surrounding material of the third tube. 
     The aerosol-generating article may comprise a wrapper. The wrapper may be provided on the external surface area of the mouthpiece assembly. The wrapper may be provided on the external surface area of the first tube, the second tube and the third tube. The wrapper may be provided on an external surface area of the aerosol-generating article. The wrapper may be formed from a non-porous material. In one example, the wrapper is formed from cellulose acetate paper. 
     Advantageously, when the aerosol-generating article comprises a wrapper, radial air flow through the first, second and third tubes may be reduced. A reduction in radial air flow through the first, second and third tubes may increase the flow rate of aerosol out of the mouthpiece. 
     A ratio of the internal diameter of the first tube to the internal diameter of the second tube may be between 1.2 and 5. A ratio of the internal diameter of the first tube to the internal diameter of the second tube may be between 1.4 and 4. A ratio of the internal diameter of the first tube to the internal diameter of the second tube may be between 1.6 and 3. A ratio of the internal diameter of the first tube to the internal diameter of the second tube may be between 1.8 and 2.5. 
     A ratio of the internal diameter of the first tube to the internal diameter of the second tube may be 2. 
     In some examples, the ratio of the internal diameter of the first tube to the internal diameter of the second tube is a trade-off. Advantageously, maximising this ratio may improve the expansion effect of the aerosol, which may improve a user&#39;s experience. However, if this ratio is too high then the resistance to draw of the second tube may be too high, which can result in the aerosol-generating device being too difficult to use. 
     A ratio of the internal diameter of the first tube to the internal diameter of the third tube may be between 0.5 and 2. A ratio of the internal diameter of the first tube to the internal diameter of the third tube may be between 0.7 and 1.3. A ratio of the internal diameter of the first tube to the internal diameter of the third tube may be between 0.8 and 1.2. A ratio of the internal diameter of the first tube to the internal diameter of the third tube may be between 0.9 and 1.1. A ratio of the internal diameter of the first tube to the internal diameter of the third tube may be between 0.95 and 1.05. 
     A ratio of the internal diameter of the first tube to the internal diameter of the third tube may be 1. 
     A ratio of the internal diameter of the third tube to the internal diameter of the second tube may be between 1.2 and 5. A ratio of the internal diameter of the third tube to the internal diameter of the third tube may be between 1.4 and 4. A ratio of the internal diameter of the third tube to the internal diameter of the third tube may be between 1.6 and 3. A ratio of the internal diameter of the third tube to the internal diameter of the third tube may be between 1.8 and 2.5. 
     A ratio of the internal diameter of the third tube to the internal diameter of the second tube may be 2. 
     In some examples, the ratio of the internal diameter of the third tube to the internal diameter of the second tube is a trade-off. Advantageously, maximising this ratio may improve the Venturi effect of the mouthpiece assembly, which may improve nucleation. However, if this ratio is too high then the resistance to draw of the second tube may be too high, which can result in the aerosol-generating device being too difficult to use. Additionally, or alternatively, if this ratio is too high then the wall thickness of the third tube may be too low, thereby making the aerosol-generating article difficult to handle. 
     The first tube may be located downstream of the second tube. The first tube may be located downstream of the third tube. The first tube may be located at the downstream end of the mouthpiece assembly. 
     The longitudinal cross-sectional shape of the first tube may be circular. The longitudinal cross-sectional shape of the first tube may be annular. 
     The first tube may have a uniform internal diameter. In other words, the internal diameter of the first tube may be the same along its whole length. 
     In an example of a first tube having a uniform internal diameter, the internal diameter of the first tube is considered to be the fixed diameter of the first tube. 
     Alternatively, the first tube may have a changing internal diameter. In other words, the internal diameter of the first tube may vary along its length. For example, the internal diameter of the first tube may increase from one end to another. The internal diameter of the first tube may decrease from one end to another. 
     In a particular example, the internal diameter of the first tube may increase from the upstream end of the first tube to the downstream end of the first tube. In other words, the internal diameter of the first tube at its downstream end is larger than the internal diameter of the first tube at its upstream end. Advantageously, this “funnelling out” of the internal diameter of the first tube improves the taste of the aerosol. 
     In an example of a first tube having a changing internal diameter, the internal diameter of the first tube is considered to be the mean diameter of the first tube. 
     The internal diameter of the first tube may be larger than the internal diameter of the third tube. 
     Advantageously, the internal diameter of the first tube being larger than the internal diameter of the third tube may further improve the user&#39;s filling perception. 
     The first tube may have an internal diameter of at least 3 mm. The first tube may have an internal diameter of at least 3.25 mm. The first tube may have an internal diameter of at least 3.5 mm. The first tube may have an internal diameter of at least 3.75 mm. The first tube may have an internal diameter of at least 4 mm. The first tube may have an internal diameter of at least 4.25 mm. The first tube may have an internal diameter of at least 4.5 mm. The first tube may have an internal diameter of at least 4.75 mm. The first tube may have an internal diameter of at least 5 mm. 
     The first tube may have an internal diameter of less than or equal to 8 mm. The first tube may have an internal diameter of less than or equal to 7.75 mm. The first tube may have an internal diameter of less than or equal to 7.5 mm. The first tube may have an internal diameter of less than or equal to 7.25 mm. The first tube may have an internal diameter of less than or equal to 7 mm. The first tube may have an internal diameter of less than or equal to 6.75 mm. The first tube may have an internal diameter of less than or equal to 6.5 mm. The first tube may have an internal diameter of less than or equal to 6.25 mm. The first tube may have an internal diameter of less than or equal to 6 mm. The first tube may have an internal diameter of less than or equal to 5.75 mm. The first tube may have an internal diameter of less than or equal to 5.5 mm. The first tube may have an internal diameter of less than or equal to 5.25 mm. The first tube may have an internal diameter of less than or equal to 5 mm. The first tube may have an internal diameter of less than or equal to 4.75 mm. The first tube may have an internal diameter of less than or equal to 4.5 mm. The first tube may have an internal diameter of less than or equal to 4.25 mm. The first tube may have an internal diameter of less than or equal to 4 mm. 
     The first tube may have an internal diameter of between 3 mm and 8 mm. The first tube may have an internal diameter of between 3.25 mm and 8 mm. The first tube may have an internal diameter of between 3.5 mm and 8 mm. The first tube may have an internal diameter of between 3.75 mm and 8 mm. The first tube may have an internal diameter of between 4 mm and 8 mm. The first tube may have an internal diameter of between 4.25 mm and 5 mm. The first tube may have an internal diameter of between 4.5 mm and 8 mm. The first tube may have an internal diameter of between 4.75 mm and 8 mm. The first tube may have an internal diameter of between 5 mm and 8 mm. 
     The first tube may have an internal diameter of between 3 mm and 7.75 mm. The first tube may have an internal diameter of between 3 mm and 7.5 mm. The first tube may have an internal diameter of between 3 mm and 7.25 mm. The first tube may have an internal diameter of between 3 mm and 7 mm. The first tube may have an internal diameter of between 3 mm and 6.75 mm. The first tube may have an internal diameter of between 3 mm and 6.5 mm. The first tube may have an internal diameter of between 3 mm and 6.25 mm. The first tube may have an internal diameter of between 3 mm and 6 mm. The first tube may have an internal diameter of between 3 mm and 5.75 mm. The first tube may have an internal diameter of between 3 mm and 5.5 mm. The first tube may have an internal diameter of between 3 mm and 5.25 mm. The first tube may have an internal diameter of between 3 mm and 5 mm. The first tube may have an internal diameter of between 3 mm and 4.75 mm. The first tube may have an internal diameter of between 3 mm and 4.5 mm. The first tube may have an internal diameter of between 3 mm and 4.25 mm. The first tube may have an internal diameter of between 3 mm and 4 mm. 
     The first tube may have an internal diameter of between 3.3 mm and 6 mm. The first tube may have an internal diameter of between 3.4 mm and 5.5 mm. The first tube may have an internal diameter of between 3.5 mm and 5 mm. The first tube may have an internal diameter of between 3.6 mm and 4.75 mm. The first tube may have an internal diameter of between 3.7 mm and 4.5 mm. The first tube may have an internal diameter of between 3.9 mm and 4.25 mm. 
     In one example, the first tube has an internal diameter of 4 mm. 
     The first tube may have a length of at least 4 mm. The first tube may have a length of at least 4.25 mm. The first tube may have a length of at least 4.5 mm. The first tube may have a length of at least 4.75 mm. The first tube may have a length of at least 5 mm. The first tube may have a length of at least 5.25 mm. The first tube may have a length of at least 5.5 mm. The first tube may have a length of at least 5.75 mm. The first tube may have a length of at least 6 mm. 
     The first tube may have a length of less than or equal to 6 mm. The first tube may have a length of less than or equal to 5.75 mm. The first tube may have a length of less than or equal to 5.5 mm. The first tube may have a length of less than or equal to 5.25 mm. The first tube may have a length of less than or equal to 5 mm. The first tube may have a length of less than or equal to 4.75 mm. The first tube may have a length of less than or equal to 4.5 mm. The first tube may have a length of less than or equal to 4.25 mm. 
     The first tube may have a length of between 4 mm and 6 mm. The first tube may have a length of between 4.25 mm and 6 mm. The first tube may have a length of between 4.5 mm and 6 mm. The first tube may have a length of between 4.75 mm and 6 mm. The first tube may have a length of between 5 mm and 6 mm. The first tube may have a length of between 5.25 mm and 6 mm. The first tube may have a length of between 5.5 mm and 6 mm. The first tube may have a length of between 5.75 mm and 6 mm. 
     The first tube may have a length of between 4 mm and 5.75 mm. The first tube may have a length of between 4 mm and 5.5 mm. The first tube may have a length of between 4 mm and 5.25 mm. The first tube may have a length of between 4 mm and 5 mm. The first tube may have a length of between 4 mm and 4.75 mm. The first tube may have a length of between 4 mm and 4.5 mm. The first tube may have a length of between 4 mm and 4.25 mm. The first tube may have a length of between 4.25 mm and 5.75 mm. The first tube may have a length of between 4.25 mm and 5.5 mm. The first tube may have a length of between 4.5 mm and 5.75 mm. The first tube may have a length of between 4.5 mm and 5.5 mm. The first tube may have a length of between 4.75 mm and 5.5 mm. The first tube may have a length of between 4.5 mm and 5.25 mm. The first tube may have a length of between 4.75 mm and 5.25 mm. In one example, the first tube has a length of 5 mm. 
     The second tube may be located in between the first tube and the third tube. The second tube may be located at the middle of the mouthpiece assembly. 
     The longitudinal cross-sectional shape of the second tube may be circular. The longitudinal cross-sectional shape of the second tube may be annular. 
     The second tube may have a uniform internal diameter. In other words, the internal diameter of the second tube may be the same along its whole length. 
     In an example of a second tube having a uniform internal diameter, the internal diameter of the second tube is considered to be the fixed diameter of the second tube. 
     Alternatively, the second tube may have a changing internal diameter. In other words, the internal diameter of the second tube may vary along its length. For example, the internal diameter of the second tube may increase from one end to another. The internal diameter of the second tube may decrease from one end to another. 
     In a particular example, the internal diameter of the second tube may increase from the upstream end of the second tube to the downstream end of the second tube. In other words, the internal diameter of the second tube at its downstream end is larger than the internal diameter of the third tube at its upstream end. Advantageously, this “funnelling out” of the internal diameter of the second tube reduces filtration of aerosol into the second tube. 
     In an example of a second tube having a changing internal diameter, the internal diameter of the second tube is considered to be the mean diameter of the second tube. 
     The second tube may have an internal diameter of at least 1 mm. The second tube may have an internal diameter of at least 1.25 mm. The second tube may have an internal diameter of at least 1.5 mm. The second tube may have an internal diameter of at least 1.75 mm. The second tube may have an internal diameter of at least 2 mm. 
     The second tube may have an internal diameter of less than or equal to 3 mm. The second tube may have an internal diameter of less than or equal to 2.75 mm. The second tube may have an internal diameter of less than or equal to 2.5 mm. The second tube may have an internal diameter of less than or equal to 2.25 mm. The second tube may have an internal diameter of less than or equal to 2 mm. The second tube may have an internal diameter of less than or equal to 1.75 mm. The second tube may have an internal diameter of less than or equal to 1.5 mm. The second tube may have an internal diameter of less than or equal to 1.25 mm. 
     The second tube may have an internal diameter of between 1 mm and 3 mm. The second tube may have an internal diameter of between 1.25 mm and 3 mm. The second tube may have an internal diameter of between 1.5 mm and 3 mm. The second tube may have an internal diameter of between 1.75 mm and 3 mm. The second tube may have an internal diameter of between 2 mm and 3 mm. The second tube may have an internal diameter of between 2.25 mm and 3 mm. The second tube may have an internal diameter of between 2.5 mm and 3 mm. The second tube may have an internal diameter of between 2.75 mm and 3 mm. 
     The second tube may have an internal diameter of between 1 mm and 2.75 mm. The second tube may have an internal diameter of between 1 mm and 2.5 mm. The second tube may have an internal diameter of between 1 mm and 2.25 mm. The second tube may have an internal diameter of between 1 mm and 2 mm. The second tube may have an internal diameter of between 1 mm and 1.75 mm. The second tube may have an internal diameter of between 1 mm and 1.5 mm. The second tube may have an internal diameter of between 1 mm and 1.25 mm. 
     The second tube may have an internal diameter of between 1.3 mm and 2.7 mm. The second tube may have an internal diameter of between 1.4 mm and 2.6 mm. The second tube may have an internal diameter of between 1.5 mm and 2.5 mm. The second tube may have an internal diameter of between 1.6 mm and 2.4 mm. The second tube may have an internal diameter of between 1.7 mm and 2.3 mm. The second tube may have an internal diameter of between 1.8 mm and 2.2 mm. 
     In one example, the second tube has an internal diameter of 2 mm. 
     The second tube may have a length of at least 4 mm. The second tube may have a length of at least 4.25 mm. The second tube may have a length of at least 4.5 mm. The second tube may have a length of at least 4.75 mm. The second tube may have a length of at least 5 mm. The second tube may have a length of at least 5.25 mm. The second tube may have a length of at least 5.5 mm. The second tube may have a length of at least 5.75 mm. The second tube may have a length of at least 6 mm. 
     The second tube may have a length of less than or equal to 6 mm. The second tube may have a length of less than or equal to 5.75 mm. The second tube may have a length of less than or equal to 5.5 mm. The second tube may have a length of less than or equal to 5.25 mm. The second tube may have a length of less than or equal to 5 mm. The second tube may have a length of less than or equal to 4.75 mm. The second tube may have a length of less than or equal to 4.5 mm. The second tube may have a length of less than or equal to 4.25 mm. 
     The second tube may have a length of between 4 mm and 6 mm. The second tube may have a length of between 4.25 mm and 6 mm. The second tube may have a length of between 4.5 mm and 6 mm. The second tube may have a length of between 4.75 mm and 6 mm. The second tube may have a length of between 5 mm and 6 mm. The second tube may have a length of between 5.25 mm and 6 mm. The second tube may have a length of between 5.5 mm and 6 mm. The second tube may have a length of between 5.75 mm and 6 mm. 
     The second tube may have a length of between 4 mm and 5.75 mm. The second tube may have a length of between 4 mm and 5.5 mm. The second tube may have a length of between 4 mm and 5.25 mm. The second tube may have a length of between 4 mm and 5 mm. The second tube may have a length of between 4 mm and 4.75 mm. The second tube may have a length of between 4 mm and 4.5 mm. The second tube may have a length of between 4 mm and 4.25 mm. 
     The second tube may have a length of between 4.25 mm and 5.75 mm. The second tube may have a length of between 4.25 mm and 5.5 mm. The second tube may have a length of between 4.5 mm and 5.75 mm. The second tube may have a length of between 4.5 mm and 5.5 mm. The second tube may have a length of between 4.75 mm and 5.5 mm. The second tube may have a length of between 4.5 mm and 5.25 mm. The second tube may have a length of between 4.75 mm and 5.25 mm. In one example, the second tube has a length of 5 mm. 
     The third tube may be located upstream of the first tube. The third tube may be located upstream of the second tube. The third tube may be located at the upstream end of the mouthpiece assembly. 
     The longitudinal cross-sectional shape of the third tube may be circular. The longitudinal cross-sectional shape of the third tube may be annular. 
     The third tube may have a uniform internal diameter. In other words, the internal diameter of the third tube may be the same along its whole length. 
     In an example of a third tube having a uniform internal diameter, the internal diameter of the third tube is considered to be the fixed diameter of the third tube. 
     Alternatively, the third tube may have a changing internal diameter. In other words, the internal diameter of the third tube may vary along its length. For example, the internal diameter of the third tube may decrease from one end to another. The internal diameter of the third tube may increase from one end to another. 
     In a particular example, the internal diameter of the third tube may decrease from the upstream end of the third tube to the downstream end of the third tube. In other words, the internal diameter of the third tube at its upstream end is larger than the internal diameter of the third tube at its downstream end. Advantageously, this “funnelling in” of the internal diameter of the third tube improves nucleation of the aerosol. 
     In an example of a third tube having a changing internal diameter, the internal diameter of the third tube is considered to be the mean diameter of the third tube. 
     The third tube may have an internal diameter of at least 3 mm. The third tube may have an internal diameter of at least 3.25 mm. The third tube may have an internal diameter of at least 3.5 mm. The third tube may have an internal diameter of at least 3.75 mm. The third tube may have an internal diameter of at least 4 mm. The third tube may have an internal diameter of at least 4.25 mm. The third tube may have an internal diameter of at least 4.5 mm. The third tube may have an internal diameter of at least 4.75 mm. The third tube may have an internal diameter of at least 5 mm. 
     The third tube may have an internal diameter of less than or equal to 8 mm. The third tube may have an internal diameter of less than or equal to 7.75 mm. The third tube may have an internal diameter of less than or equal to 7.5 mm. The third tube may have an internal diameter of less than or equal to 7.25 mm. The third tube may have an internal diameter of less than or equal to 7 mm. The third tube may have an internal diameter of less than or equal to 6.75 mm. The third tube may have an internal diameter of less than or equal to 6.5 mm. The third tube may have an internal diameter of less than or equal to 6.25 mm. The third tube may have an internal diameter of less than or equal to 6 mm. The third tube may have an internal diameter of less than or equal to 5.75 mm. The third tube may have an internal diameter of less than or equal to 5.5 mm. The third tube may have an internal diameter of less than or equal to 5.25 mm. The third tube may have an internal diameter of less than or equal to 5 mm. The third tube may have an internal diameter of less than or equal to 4.75 mm. The third tube may have an internal diameter of less than or equal to 4.5 mm. The third tube may have an internal diameter of less than or equal to 4.25 mm. The third tube may have an internal diameter of less than or equal to 4 mm. 
     The third tube may have an internal diameter of between 3 mm and 8 mm. The third tube may have an internal diameter of between 3.25 mm and 8 mm. The third tube may have an internal diameter of between 3.5 mm and 8 mm. The third tube may have an internal diameter of between 3.75 mm and 8 mm. The third tube may have an internal diameter of between 4 mm and 8 mm. The third tube may have an internal diameter of between 4.25 mm and 5 mm. The third tube may have an internal diameter of between 4.5 mm and 8 mm. The third tube may have an internal diameter of between 4.75 mm and 8 mm. The third tube may have an internal diameter of between 5 mm and 8 mm. 
     The third tube may have an internal diameter of between 3 mm and 7.75 mm. The third tube may have an internal diameter of between 3 mm and 7.5 mm. The third tube may have an internal diameter of between 3 mm and 7.25 mm. The third tube may have an internal diameter of between 3 mm and 7 mm. The third tube may have an internal diameter of between 3 mm and 6.75 mm. The third tube may have an internal diameter of between 3 mm and 6.5 mm. The third tube may have an internal diameter of between 3 mm and 6.25 mm. The third tube may have an internal diameter of between 3 mm and 6 mm. The third tube may have an internal diameter of between 3 mm and 5.75 mm. The third tube may have an internal diameter of between 3 mm and 5.5 mm. The third tube may have an internal diameter of between 3 mm and 5.25 mm. The third tube may have an internal diameter of between 3 mm and 5 mm. The third tube may have an internal diameter of between 3 mm and 4.75 mm. The third tube may have an internal diameter of between 3 mm and 4.5 mm. The third tube may have an internal diameter of between 3 mm and 4.25 mm. The third tube may have an internal diameter of between 3 mm and 4 mm. 
     The third tube may have an internal diameter of between 3.3 mm and 6 mm. The third tube may have an internal diameter of between 3.4 mm and 5.5 mm. The third tube may have an internal diameter of between 3.5 mm and 5 mm. The third tube may have an internal diameter of between 3.6 mm and 4.75 mm. The third tube may have an internal diameter of between 3.7 mm and 4.5 mm. The third tube may have an internal diameter of between 3.9 mm and 4.25 mm. 
     In one example, the third tube has an internal diameter of 4 mm. 
     The third tube may have a length of at least 4 mm. The third tube may have a length of at least 4.25 mm. The third tube may have a length of at least 4.5 mm. The third tube may have a length of at least 4.75 mm. The third tube may have a length of at least 5 mm. The third tube may have a length of at least 5.25 mm. The third tube may have a length of at least 5.5 mm. The third tube may have a length of at least 5.75 mm. The third tube may have a length of at least 6 mm. 
     The third tube may have a length of less than or equal to 6 mm. The third tube may have a length of less than or equal to 5.75 mm. The third tube may have a length of less than or equal to 5.5 mm. The third tube may have a length of less than or equal to 5.25 mm. The third tube may have a length of less than or equal to 5 mm. The third tube may have a length of less than or equal to 4.75 mm. The third tube may have a length of less than or equal to 4.5 mm. The third tube may have a length of less than or equal to 4.25 mm. 
     The third tube may have a length of between 4 mm and 6 mm. The third tube may have a length of between 4.25 mm and 6 mm. The third tube may have a length of between 4.5 mm and 6 mm. The third tube may have a length of between 4.75 mm and 6 mm. The third tube may have a length of between 5 mm and 6 mm. The third tube may have a length of between 5.25 mm and 6 mm. The third tube may have a length of between 5.5 mm and 6 mm. The third tube may have a length of between 5.75 mm and 6 mm. 
     The third tube may have a length of between 4 mm and 5.75 mm. The third tube may have a length of between 4 mm and 5.5 mm. The third tube may have a length of between 4 mm and 5.25 mm. The third tube may have a length of between 4 mm and 5 mm. The third tube may have a length of between 4 mm and 4.75 mm. The third tube may have a length of between 4 mm and 4.5 mm. The third tube may have a length of between 4 mm and 4.25 mm. 
     The third tube may have a length of between 4.25 mm and 5.75 mm. The third tube may have a length of between 4.25 mm and 5.5 mm. The third tube may have a length of between 4.5 mm and 5.75 mm. The third tube may have a length of between 4.5 mm and 5.5 mm. The third tube may have a length of between 4.75 mm and 5.5 mm. The third tube may have a length of between 4.5 mm and 5.25 mm. The third tube may have a length of between 4.75 mm and 5.25 mm. In one example, the third tube has a length of 5 mm. 
     The aerosol-generating article may have a length of at least 35 mm. The aerosol-generating article may have a length of at least 40 mm. The aerosol-generating article may have a length of at least 45 mm. The aerosol-generating article may have a length of at least 50 mm. The aerosol-generating article may have a length of at least 55 mm. 
     The aerosol-generating article may have a length of less than or equal to 60 mm. The aerosol-generating article may have a length of less than or equal to 55 mm. The aerosol-generating article may have a length of less than or equal to 50 mm. The aerosol-generating article may have a length of less than or equal to 45 mm. The aerosol-generating article may have a length of less than or equal to 40 mm. 
     The aerosol-generating article may have a length of between 35 mm and 60 mm. The aerosol-generating article may have a length of between 35 mm and 55 mm. The aerosol-generating article may have a length of between 35 mm and 50 mm. The aerosol-generating article may have a length of between 35 mm and 45 mm. The aerosol-generating article may have a length of between 35 mm and 40 mm. 
     The aerosol-generating article may have a length of between 40 mm and 60 mm. The aerosol-generating article may have a length of between 45 mm and 60 mm. The aerosol-generating article may have a length of between 50 mm and 60 mm. The aerosol-generating article may have a length of between 55 mm and 60 mm. 
     The aerosol-generating article may have a length of between 35 mm and 55 mm. The aerosol-generating article may have a length of between 40 mm and 55 mm. The aerosol-generating article may have a length of between 40 mm and 50 mm. The aerosol-generating article may have a length of between 45 mm and 50 mm. The aerosol-generating article may have a length of between 40 mm and 45 mm. 
     In one example, the aerosol-generating article has a length of 45 mm. 
     A ratio of the internal diameter of the first tube to the external diameter of the first tube may be between 0.4 and 0.8. A ratio of the internal diameter of the first tube to the external diameter of the first tube may be between 0.5 and 0.7. 
     A ratio of the internal diameter of the first tube to the external diameter of the first tube may be 0.6. 
     A ratio of the internal diameter of the second tube to the external diameter of the second tube may be between 0.1 and 0.5. A ratio of the internal diameter of the second tube to the external diameter of the second tube may be between 0.2 and 0.4. 
     A ratio of the internal diameter of the second tube to the external diameter of the second tube may be 0.3. 
     A ratio of the internal diameter of the third tube to the external diameter of the third tube may be between 0.4 and 0.8. A ratio of the internal diameter of the third tube to the external diameter of the third tube may be between 0.5 and 0.7. 
     A ratio of the internal diameter of the third tube to the external diameter of the third tube may be 0.6. 
     The first tube may have an external diameter of at least 6 mm. The first tube may have an external diameter of at least 6.25 mm. The first tube may have an external diameter of at least 6.5 mm. The first tube may have an external diameter of at least 6.75 mm. The first tube may have an external diameter of at least 7 mm. 
     The first tube may have an external diameter of less than or equal to 8 mm. The first tube may have an external diameter of less than or equal to 7.75 mm. The first tube may have an external diameter of less than or equal to 7.5 mm. The first tube may have an external diameter of less than or equal to 7.25 mm. The first tube may have an external diameter of less than or equal to 7 mm. 
     The first tube may have an external diameter of between 6 mm and 8 mm. The first tube may have an external diameter of between 6.25 mm and 8 mm. The first tube may have an external diameter of between 6.5 mm and 8 mm. The first tube may have an external diameter of between 6.75 mm and 8 mm. The first tube may have an external diameter of between 7 mm and 8 mm. The first tube may have an external diameter of between 7.25 mm and 8 mm. The first tube may have an external diameter of between 7.5 mm and 8 mm. The first tube may have an external diameter of between 7.75 mm and 8 mm 
     The first tube may have an external diameter of between 6 mm and 7.75 mm. The first tube may have an external diameter of between 6 mm and 7.5 mm. The first tube may have an external diameter of between 6 mm and 7.25 mm. The first tube may have an external diameter of between 6 mm and 7 mm. The first tube may have an external diameter of between 6 mm and 6.75 mm. The first tube may have an external diameter of between 6 mm and 6.5 mm. The first tube may have an external diameter of between 6 mm and 6.25 mm. 
     The first tube may have an external diameter of between 6.25 mm and 7.75 mm. The first tube may have an external diameter of between 6.5 mm and 7.5 mm. The first tube may have an external diameter of between 6.75 mm and 7.25 mm. 
     The first tube may have an external diameter of 7 mm. 
     A ratio of the length of the first tube to the internal diameter of the first tube may be between 0.75 and 1.75. A ratio of the length of the first tube to the internal diameter of the first tube may be between 1 and 1.5 
     A ratio of the length of the first tube to the internal diameter of the first tube may be 1.25. 
     A ratio of the length of the second tube to the internal diameter of the second tube may be between 2 and 3. A ratio of the length of the second tube to the internal diameter of the second tube may be between 2.25 and 2.75. 
     A ratio of the length of the second tube to the internal diameter of the second tube may be 2.5. 
     A ratio of the length of the third tube to the internal diameter of the third tube may be between 0.75 and 1.75. A ratio of the length of the third tube to the internal diameter of the third tube may be between 1 and 1.5. 
     A ratio of the length of the third tube to the internal diameter of the third tube may be 1.25. 
     The first tube and the second tube may be arranged such that they are coaxial with one another along their longitudinal axes. The first tube and the third tube may be arranged such that they are coaxial with one another along their longitudinal axes. The second tube and the third tube may be arranged such that they are coaxial with one another along their longitudinal axes. In one example, the first tube, the second tube and the third tube may be arranged such that they are coaxial with one another along their longitudinal axes. 
     The aerosol-generating substrate may comprise a gel. The gel composition may comprise at least one gelling agent. The gel composition may comprise at least one alkaloid compound. The gel composition may comprise at least one cannabinoid compound. The gel composition may comprise at least one aerosol former. In one example, the gel composition comprising at least one gelling agent, at least one of an alkaloid compound and a cannabinoid compound, and an aerosol former. 
     Advantageously, the provision of an aerosol-forming substrate comprising a gel composition may be desirable since it may provide a uniform substrate that can generate a highly consistent aerosol. Additionally, the gel aerosol-forming substrate may be capable of generating an aerosol at a lower temperature than an aerosol-forming substrate comprising tobacco. This may provide more efficient generation of aerosol. Furthermore, this may advantageously reduce the need to cool the aerosol before it reaches the consumer. 
     The aerosol-forming substrate may comprise and annular plug of a porous medium loaded with the gel composition. The porous medium may comprise at least one of cellulose acetate tow, crimped viscose, and crimped cotton. 
     The aerosol-generating article may comprise an upstream element. The upstream element may be located upstream of the aerosol-forming substrate. The upstream element may abut with the aerosol-forming substrate. The upstream element may have a high resistance to draw (RTD). The upstream element may be formed from a material that provides a high RTD. The upstream element may comprise filtration material. The upstream element may comprise an annular plug. The upstream element may comprise an annular plug of fibrous filtration material. 
     The provision of an upstream element may advantageously protect the aerosol-forming substrate and prevent a user coming into direct contact with the aerosol-forming substrate. 
     The upstream element can be used to provide greater control over the overall resistance to draw (RTD) of the aerosol generating article. In particular, the upstream element can advantageously be used to compensate for potential reductions in RTD due to vaporisation of the aerosol-forming substrate during use, or due to the inclusion of other elements in the aerosol-generating article having a relatively low resistance to draw. For example, in embodiments of the present invention including an intermediate space which contributes virtually no RTD to the overall aerosol-generating article, the upstream element can be used to add RTD to the aerosol-generating article such that an acceptable level of RTD can still be provided. 
     Advantageously, the upstream element can provide an increase in the overall RTD without impacting the properties of the aerosol, due to the location of the upstream element upstream of the aerosol-forming substrate. If the desired level of RTD can be provided in large part due to the upstream element, this enables downstream elements to be used that provide minimal filtration of the aerosol. The aerosol-generating article can therefore optimise aerosol delivery from the gel composition to the consumer whilst still retaining an optimal level of RTD throughout the smoking experience. 
     Alternatively or in addition, the upstream element can advantageously be adapted to compensate for reduction in length of other elements of the aerosol-generating article so that an overall consistent length of the aerosol-generating article can be retained. This may advantageously allow the aerosol-forming substrate to be located at the optimum position for heating when the aerosol-generating article is inserted into an aerosol-generating device. This compensation in length can be provided without impacting the properties of the aerosol. 
     Furthermore, the upstream element may advantageously provide a more uniform appearance at the upstream end of the aerosol-generating article. 
     In an example that includes a recess extending from the upstream end of the aerosol generating article, through the upstream element, the upstream element may include a longitudinal opening to accommodate the recess. For example, the upstream element may have an annular shape. 
     The aerosol-generating article may comprise a ventilation zone. 
     The ventilation zone may comprise one or more rows of ventilation perforations. The one or more rows of ventilation perforations may be formed through the wall of at least one of the first tube, the second tube and the third tube. In one example, the one or more rows of ventilation perforations are formed through the wall of the third tube. In embodiments that include a wrapper, the one or more rows of ventilation perforations are formed through the wrapper. Advantageously, the one or more rows of ventilation perforations provide for a ventilation effect that can enhance cooling of vaporised volatile compounds of the aerosol-forming substrate, which improves aerosol nucleation. 
     The ventilation zone may comprise only one row of ventilation perforations. 
     Advantageously, by concentrating the cooling effect brought about by ventilation over a short portion of the aerosol-generating article, it may be possible to further enhance aerosol nucleation. This is because a faster and more drastic cooling of the stream of volatilised compounds is expected to particularly favour the formation of new nuclei of aerosol particles 
     The one or more rows of ventilation perforations may be arranged circumferentially around the wall of at least one of the first tube, the second tube and the third tube. Where the ventilation zone comprises two or more rows of ventilation perforations, the rows of ventilation perforations may be longitudinally spaced apart from one another along at least one of the first tube, the second tube and the third tube. As an example, adjacent rows of ventilation perforations may be longitudinally spaced apart from one another by a distance of between about 0.25 mm and 0.75 mm. 
     At least one of the ventilation perforations may have an equivalent diameter of at least 100 μm. At least one of the ventilation perforations may have an equivalent diameter of at least 150 μm. At least one of the ventilation perforations may have an equivalent diameter of at least 200 μm. 
     At least one of the ventilation perforations may have an equivalent diameter of less than 500 μm. At least one of the ventilation perforations may have an equivalent diameter of less than 450 μm. The term “equivalent diameter” is used herein to denote the diameter of a circle having the same surface area of a cross-section of the ventilation perforation. A cross-section of the ventilation perforations may have any suitable shape. In one example, the ventilation perforations have a circular cross sectional shape. 
     The ventilation perforations may be of uniform size. As an alternative, the ventilation perforations may vary in size. By varying the number and size of the ventilation perforations, it is possible to adjust the amount of external air admitted into the first tube, the second tube and/or the third tube when the consumer draws on the aerosol-generating article during use. As such, it is advantageously possible to adjust the ventilation level of the aerosol-generating article. 
     The ventilation perforations can be formed using any suitable technique, for example by laser technology, mechanical perforation of the first tube, the second tube and/or the third tube as part of the aerosol-generating article or pre-perforation of the first tube, the second tube and/or the third tube before it is combined with the other elements to form the aerosol-generating article. Preferably, the ventilation perforations are formed by online laser perforation. 
     A distance between the ventilation zone and an upstream end of the aerosol-generating article may be less than 50 mm. A distance between the ventilation zone and an upstream end of the aerosol-generating article may be less than 45 mm. A distance between the ventilation zone and an upstream end of the aerosol-generating article may be less than 40 mm. 
     A distance between the ventilation zone and an upstream end of the aerosol-generating article may be preferably at least 12 mm. A distance between the ventilation zone and an upstream end of the aerosol-generating article may be at least 15 mm. A distance between the ventilation zone and an upstream end of the aerosol-generating article may be at least 20 mm. A distance between the ventilation zone and an upstream end of the aerosol-generating article may be preferably at least 25 mm. 
     A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be least 2 mm. A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be at least 4 mm. A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be at least 5 mm. A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be at least 10 mm. A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be at least about 15 mm. 
     A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be less than 35 mm. A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be less than 30 mm. A distance between the ventilation zone and a downstream end of the aerosol-forming substrate may be less than 25 mm. 
     In practice, the ventilation zone may divide a cavity internally defined in the aerosol-generating article into an upstream sub-cavity, which extends longitudinally from an upstream end of the aerosol-generating article to the location of the ventilation zone, and a downstream sub-cavity, which extends longitudinally from the location of the ventilation zone to the downstream end of aerosol-generating article. Without wishing to be bound by theory, it is understood that in the upstream sub-cavity the volatilised compounds of the aerosol stream advance downstream along the cavity slowly and cool down by yielding some of the heat to the peripheral wall of for example the third tube. Thus, aerosol particles begin to nucleate. On the other hand, in the downstream sub-cavity, the aerosol stream and the ventilation air rapidly mix up, which causes a quick cooling of the volatilised compounds of the aerosol stream and so favours the nucleation of new aerosol particles and the growth of already existing aerosol particles as the aerosol advances downstream. 
     The aerosol-generating article my comprise a recess extending from the upstream end of the aerosol-generating article. The recess may extend through the upstream element. The recess may extend through at least a portion of the aerosol-forming substrate. 
     The provision of the recess may advantageously allow the aerosol-generating article to be heated using an internal heater such as a pin or a blade heater. This may facilitate more efficient heating of the aerosol-forming substrate. The inclusion of a recess is particularly advantageous since aerosol-forming substrates comprising a gel typically have a higher density than aerosol-forming substrates comprising tobacco. As a result, it will be less practical for a pin or a blade heater to be directly inserted into an aerosol-forming substrate comprising a gel compared to an aerosol-forming substrate comprising tobacco. In addition, the provision of a recess may prevent a heater coming into contact with the gel aerosol-forming substrate which may help to keep the heater clean. 
     The recess may be defined by a longitudinal opening extending though the upstream element and a longitudinal opening extending though at least a portion of the aerosol-forming substrate. The longitudinal opening extending though the upstream element and a longitudinal opening extending though at least a portion of the aerosol-forming substrate may have substantially the same diameter and be substantially aligned. 
     The recess may have any cross sectional shape. The recess may have a constant cross sectional shape. The shape of the recess may be configured to correspond to the shape of a heater of an aerosol generating device to be used with the aerosol generating article. The recess may have a circular cross sectional shape. A recess having a circular cross sectional shape may be appropriate where the heater is a pin heater. The recess may have an oblong or rectangular shape. A recess having an oblong or rectangular cross sectional shape may be appropriate where the heater is a blade heater. Preferably, the recess has a circular cross sectional shape. 
     The recess may be arranged centrally along the longitudinal axis of the aerosol generating article. This may advantageously simplify inserting the aerosol-generating article into an aerosol-generating device since the orientation of the aerosol generating device may not matter. Additionally, locating the recess centrally may advantageously ensure even heating of the aerosol-forming substrate. 
     The recess may have any diameter. Preferably, the recess has a diameter the same as, or slightly larger than, the diameter of a heater of an aerosol generating device to be used with the aerosol-generating article. 
     The diameter of the recess may be between about 0.5 mm and about 10 mm. For example, the diameter of the recess may be between about 1 mm and about 8 mm, or between about 2 mm and about 6 mm. 
     The recess may have any length. Preferably, the recess has a length the same as, or slightly larger than, the length of a heater of an aerosol generating device to be used with the aerosol-generating article. 
     The length of the recess may be between about 5 millimetres and about 30 millimetres. For example, the length of the recess may be between about 10 millimetre and about 25 millimetres, or between about 15 millimetres and about 20 millimetres. 
     The recess may extend through the full length of the aerosol-forming substrate. Where this is the case, the recess may extend further downstream of the downstream end of the aerosol-forming substrate. Alternatively, where this is the case, the recess may extend to the downstream end of the aerosol-forming substrate but not extend any further downstream. 
     Preferably, the gel composition comprises an alkaloid compound, or a cannabinoid compound, or both an alkaloid compound and a cannabinoid compound; an aerosol former; and at least one gelling agent. Preferably, the at least one gelling agent forms a solid medium and the glycerol is dispersed in the solid medium, with the alkaloid or cannabinoid dispersed in the glycerol. Preferably, the gel composition is a stable gel phase. 
     Advantageously, a stable gel composition comprising nicotine provides predictable composition form upon storage or transit from manufacture to the consumer. The stable gel composition comprising nicotine substantially maintains its shape. The stable gel composition comprising nicotine substantially does not release a liquid phase upon storage or transit from manufacture to the consumer. The stable gel composition comprising nicotine may provide for a simple consumable design. This consumable may not have to be designed to contain a liquid, thus a wider range of materials and container constructions may be contemplated. 
     The gel composition described herein may be combined with an aerosol-generating device to provide a nicotine aerosol to the lungs at inhalation or air flow rates that are within conventional smoking regime inhalation or air flow rates. The aerosol-generating device may continuously heat the gel composition. A consumer may take a plurality of inhalations or “puffs” where each “puff” delivers an amount of nicotine aerosol. The gel composition may be capable of delivering a high nicotine/low total particulate matter (TPM) aerosol to a consumer when heated, preferably in a continuous manner. 
     The phrase “stable gel phase” or “stable gel” refers to gel that substantially maintains its shape and mass when exposed to a variety of environmental conditions. The stable gel may not substantially release (sweat) or absorb water when exposed to a standard temperature and pressure while varying relative humidity from about 10 percent to about 60 percent. For example, the stable gel may substantially maintain its shape and mass when exposed to a standard temperature and pressure while varying relative humidity from about 10 percent to about 60 percent. 
     The gel composition includes an alkaloid compound, or a cannabinoid compound, or both an alkaloid compound and a cannabinoid compound. The gel composition may include one or more alkaloids. The gel composition may include one or more cannabinoids. The gel composition may include a combination of one or more alkaloids and one or more cannabinoids. 
     The term “alkaloid compound” refers to any one of a class of naturally occurring organic compounds that contain one or more basic nitrogen atoms. Generally, an alkaloid contains at least one nitrogen atom in an amine-type structure. This or another nitrogen atom in the molecule of the alkaloid compound can be active as a base in acid-base reactions. Most alkaloid compounds have one or more of their nitrogen atoms as part of a cyclic system, such as for example a heterocylic ring. In nature, alkaloid compounds are found primarily in plants, and are especially common in certain families of flowering plants. However, some alkaloid compounds are found in animal species and fungi. In this disclosure, the term “alkaloid compound” refers to both naturally derived alkaloid compounds and synthetically manufactured alkaloid compounds. 
     The gel composition may preferably include an alkaloid compound selected from the group consisting of nicotine, anatabine, and combinations thereof. 
     Preferably the gel composition includes nicotine. 
     The term “nicotine” refers to nicotine and nicotine derivatives such as free-base nicotine, nicotine salts and the like. 
     The term “cannabinoid compound” refers to any one of a class of naturally occurring compounds that are found in parts of the  cannabis  plant—namely the species  Cannabis sativa, Cannabis indica , and  Cannabis ruderalis . Cannabinoid compounds are especially concentrated in the female flower heads. Cannabinoid compounds naturally occurring in the  cannabis  plant include cannabidiol (CBD) and tetrahydrocannabinol (THC). In this disclosure, the term “cannabinoid compounds” is used to describe both naturally derived cannabinoid compounds and synthetically manufactured cannabinoid compounds. 
     The gel may include a cannabinoid compound selected from the group consisting of cannabidiol (CBD), tetrahydrocannabinol (THC), tetrahydrocannabinolic acid (THCA), cannabidiolic acid (CBDA), cannabinol (CBN), cannabigerol (CBG), cannabichromene (CBC), cannabicyclol (CBL), cannabivarin (CBV), tetrahydrocannabivarin (THCV), cannabidivarin (CBDV), cannabichromevarin (CBCV), cannabigerovarin (CBGV), cannabigerol monomethyl ether (CBGM), cannabielsoin (CBE), cannabicitran (CBT), and combinations thereof. 
     The gel composition may preferably include a cannabinoid compound selected from the group consisting of cannabidiol (CBD), THC (tetrahydrocannabinol) and combinations thereof. 
     The gel may preferably include cannabidiol (CBD). 
     The gel composition may include nicotine and cannabidiol (CBD). 
     The gel composition may include nicotine, cannabidiol (CBD), and THC (tetrahydrocannabinol). 
     The gel composition preferably includes about 0.5 percent by weight to about 10 percent by weight of an alkaloid compound, or about 0.5 percent by weight to about 10 percent by weight. of a cannabinoid compound, or both an alkaloid compound and a cannabinoid compound in a total amount from about 0.5 percent by weight to about 10 percent by weight. The gel composition may include about 0.5 percent by weight to about 5 percent by weight of an alkaloid compound, or about 0.5 percent by weight to about 5 percent by weight of a cannabinoid compound, or both an alkaloid compound and a cannabinoid compound in a total amount from about 0.5 percent by weight to about 5 percent by weight. Preferably the gel composition includes about 1 percent by weight to about 3 percent by weight of an alkaloid compound, or about 1 percent by weight to about 3 percent by weight of a cannabinoid compound, or both an alkaloid compound and a cannabinoid compound in a total amount from about 1 percent by weight to about 3 percent by weight. The gel composition may preferably include about 1.5 percent by weight to about 2.5 percent by weight of an alkaloid compound, or about 1.5 percent by weight to about 2.5 percent by weight of a cannabinoid compound, or both an alkaloid compound and a cannabinoid compound in a total amount from about 1.5 percent by weight to about 2.5 percent by weight. The gel composition may preferably include about 2 percent by weight of an alkaloid compound, or about 2 percent by weight of a cannabinoid compound, or both an alkaloid compound and a cannabinoid compound in a total amount of about 2 percent by weight. The alkaloid compound component of the gel formulation may be the most volatile component of the gel formulation. In some aspects water may be the most volatile component of the gel formulation and the alkaloid compound component of the gel formulation may be the second most volatile component of the gel formulation. The cannabinoid compound component of the gel formulation may be the most volatile component of the gel formulation. In some aspects water may be the most volatile component of the gel formulation and the alkaloid compound component of the gel formulation may be the second most volatile component of the gel formulation. 
     Preferably nicotine is included in the gel compositions. The nicotine may be added to the composition in a free base form or a salt form. The gel composition includes about 0.5 percent by weight to about 10 percent by weight nicotine, or about 0.5 percent by weight to about 5 percent by weight nicotine. Preferably the gel composition includes about 1 percent by weight to about 3 percent by weight nicotine, or about 1.5 percent by weight to about 2.5 percent by weight nicotine, or about 2 percent by weight nicotine. The nicotine component of the gel formulation may be the most volatile component of the gel formulation. In some aspects water may be the most volatile component of the gel formulation and the nicotine component of the gel formulation may be the second most volatile component of the gel formulation. 
     The gel composition additionally includes an aerosol-former. Ideally the aerosol-former is substantially resistant to thermal degradation at the operating temperature of the associated aerosol-generating device. Suitable aerosol-formers include, but are not limited to: polyhydric alcohols, such as triethylene glycol, 1, 3-butanediol and glycerine; esters of polyhydric alcohols, such as glycerol mono-, di- or triacetate; and aliphatic esters of mono-, di- or polycarboxylic acids, such as dimethyl dodecanedioate and dimethyl tetradecanedioate. Polyhydric alcohols or mixtures thereof, may be one or more of triethylene glycol, 1, 3-butanediol and, glycerine (glycerol or propane-1,2,3-triol) or polyethylene glycol. The aerosol-former is preferably glycerol. 
     The gel composition may include a majority of an aerosol-former. The gel composition may include a mixture of water and the aerosol-former where the aerosol-former forms a majority (by weight) of the gel composition. The aerosol-former may form at least about 50 percent by weight of the gel composition. The aerosol-former may form at least about 60 percent by weight or at least about 65 percent by weight or at least about 70 percent by weight of the gel composition. The aerosol-former may form about 70 percent by weight to about 80 percent by weight of the gel composition. The aerosol-former may form about 70 percent by weight to about 75 percent by weight of the gel composition. 
     The gel composition may include a majority of glycerol. The gel composition may include a mixture of water and the glycerol where the glycerol forms a majority (by weight) of the gel composition. The glycerol may form at least about 50 percent by weight of the gel composition. The glycerol may form at least about 60 percent by weight or at least about 65 percent by weight or at least about 70 percent by weight of the gel composition. The glycerol may form about 70 percent by weight to about 80 percent by weight of the gel composition. The glycerol may form about 70 percent by weight to about 75 percent by weight of the gel composition. 
     The gel composition additionally includes at least one gelling agent. Preferably, the gel composition includes a total amount of gelling agents in a range from about 0.4 percent by weight to about 10 percent by weight. More preferably, the composition includes the gelling agents in a range from about 0.5 percent by weight to about 8 percent by weight. More preferably, the composition includes the gelling agents in a range from about 1 percent by weight to about 6 percent by weight. More preferably, the composition includes the gelling agents in a range from about 2 percent by weight to about 4 percent by weight. More preferably, the composition includes the gelling agents in a range from about 2 percent by weight to about 3 percent by weight. 
     The term “gelling agent” refers to a compound that homogeneously, when added to a 50 percent by weight water/50 percent by weight glycerol mixture, in an amount of about 0.3 percent by weight, forms a solid medium or support matrix leading to a gel. Gelling agents include, but are not limited to, hydrogen-bond crosslinking gelling agents, and ionic crosslinking gelling agents. 
     The gelling agent may include one or more biopolymers. The biopolymers may be formed of polysaccharides. 
     Biopolymers include, for example, gellan gums (native, low acyl gellan gum, high acyl gellan gums with low acyl gellan gum being preferred), xanthan gum, alginates (alginic acid), agar, guar gum, and the like. The composition may preferably include xanthan gum. The composition may include two biopolymers. The composition may include three biopolymers. The composition may include the two biopolymers in substantially equal weights. The composition may include the three biopolymers in substantially equal weights. 
     Preferably, the gel composition comprises at least about 0.2 percent by weight hydrogen-bond crosslinking gelling agent. Alternatively or in addition, the gel composition preferably comprises at least about 0.2 percent by weight ionic crosslinking gelling agent. Most preferably, the gel composition comprises at least about 0.2 percent by weight hydrogen-bond crosslinking gelling agent and at least about 0.2 percent by weight ionic crosslinking gelling agent. The gel composition may comprise about 0.5 percent by weight to about 3 percent by weight hydrogen-bond crosslinking gelling agent and about 0.5 percent by weight to about 3 percent by weight ionic crosslinking gelling agent, or about 1 percent by weight to about 2 percent by weight hydrogen-bond crosslinking gelling agent and about 1 percent by weight to about 2 percent by weight ionic crosslinking gelling agent. The hydrogen-bond crosslinking gelling agent and ionic crosslinking gelling agent may be present in the gel composition in substantially equal amounts by weight. 
     The term “hydrogen-bond crosslinking gelling agent” refers to a gelling agent that forms non-covalent crosslinking bonds or physical crosslinking bonds via hydrogen bonding. Hydrogen bonding is a type of electrostatic dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N,  0 , or F atom and another very electronegative atom. 
     The hydrogen-bond crosslinking gelling agent may include one or more of a galactomannan, gelatin, agarose, or konjac gum, or agar. The hydrogen-bond crosslinking gelling agent may preferably include agar. 
     The gel composition preferably includes the hydrogen-bond crosslinking gelling agent in a range from about 0.3 percent by weight to about 5 percent by weight. Preferably the composition includes the hydrogen-bond crosslinking gelling agent in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the composition includes the hydrogen-bond crosslinking gelling agent in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include a galactomannan in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the galactomannan may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the galactomannan may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the galactomannan may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include a gelatin in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the gelatin may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the gelatin may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the gelatin may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include agarose in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the agarose may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the agarose may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the agarose may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include konjac gum in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the konjac gum may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the konjac gum may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the konjac gum may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include agar in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the agar may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the agar may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the agar may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The term “ionic crosslinking gelling agent” refers to a gelling agent that forms non-covalent crosslinking bonds or physical crosslinking bonds via ionic bonding. Ionic crosslinking involves the association of polymer chains by noncovalent interactions. A crosslinked network is formed when multivalent molecules of opposite charges electrostatically attract each other giving rise to a crosslinked polymeric network. 
     The ionic crosslinking gelling agent may include low acyl gellan, pectin, kappa carrageenan, iota carrageenan or alginate. The ionic crosslinking gelling agent may preferably include low acyl gellan. 
     The gel composition may include the ionic crosslinking gelling agent in a range from about 0.3 percent by weight to about 5 percent by weight. Preferably the composition includes the ionic crosslinking gelling agent in a range from about 0.5 percent by weight to about 3 percent by weight by weight. Preferably the composition includes the ionic crosslinking gelling agent in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include low acyl gellan in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the low acyl gellan may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the low acyl gellan may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the low acyl gellan may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include pectin in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the pectin may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the pectin may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the pectin may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include kappa carrageenan in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the kappa carrageenan may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the kappa carrageenan may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the kappa carrageenan may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include iota carrageenan in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the iota carrageenan may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the iota carrageenan may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the iota carrageenan may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include alginate in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the alginate may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the alginate may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the alginate may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include the hydrogen-bond crosslinking gelling agent and ionic crosslinking gelling agent in a ratio of about 3:1 to about 1:3. Preferably the gel composition may include the hydrogen-bond crosslinking gelling agent and ionic crosslinking gelling agent in a ratio of about 2:1 to about 1:2. Preferably the gel composition may include the hydrogen-bond crosslinking gelling agent and ionic crosslinking gelling agent in a ratio of about 1:1. 
     The gel composition may further include a viscosifying agent. The viscosifying agent combined with the hydrogen-bond crosslinking gelling agent and the ionic crosslinking gelling agent appears to surprisingly support the solid medium and maintain the gel composition even when the gel composition comprises a high level of glycerol. 
     The term “viscosifying agent” refers to a compound that, when added homogeneously into a 25° C., 50 percent by weight water/50 percent by weight glycerol mixture, in an amount of 0.3 percent by weight, increases the viscosity without leading to the formation of a gel, the mixture staying or remaining fluid. Preferably the viscosifying agent refers to a compound that when added homogeneously into a 25° C. 50 percent by weight water/50 percent by weight glycerol mixture, in an amount of 0.3 percent by weight, increases the viscosity to at least 50 cPs, preferably at least 200 cPs, preferably at least 500 cPs, preferably at least 1000 cPs at a shear rate of 0.1 s−1, without leading to the formation of a gel, the mixture staying or remaining fluid. Preferably the viscosifying agent refers to a compound that when added homogeneously into a 25° C. 50 percent by weight water/50 percent by weight glycerol mixture, in an amount of 0.3 percent by weight, increases the viscosity at least 2 times, or at least 5 times, or at least 10 times, or at least 100 times higher than before addition, at a shear rate of 0.1 s−1, without leading to the formation of a gel, the mixture staying or remaining fluid. 
     The viscosity values recited herein can be measured using a Brookfield RVT viscometer rotating a disc type RV #2 spindle at 25° C. at a speed of 6 revolutions per minute (rpm). 
     The gel composition preferably includes the viscosifying agent in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the composition includes the viscosifying agent in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the composition includes the viscosifying agent in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the composition includes the viscosifying agent in a range from about 1 percent by weight to about 2 percent by weight. 
     The viscosifying agent may include one or more of xanthan gum, carboxymethyl-cellulose, microcrystalline cellulose, methyl cellulose, gum Arabic, guar gum, lambda carrageenan, or starch. The viscosifying agent may preferably include xanthan gum. 
     The gel composition may include xanthan gum in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the xanthan gum may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the xanthan gum may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the xanthan gum may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include carboxymethyl-cellulose in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the carboxymethyl-cellulose may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the carboxymethyl-cellulose may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the carboxymethyl-cellulose may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include microcrystalline cellulose in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the microcrystalline cellulose may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the microcrystalline cellulose may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the microcrystalline cellulose may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include methyl cellulose in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the methyl cellulose may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the methyl cellulose may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the methyl cellulose may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include gum Arabic in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the gum Arabic may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the gum Arabic may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the gum Arabic may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include guar gum in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the guar gum may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the guar gum may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the guar gum may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include lambda carrageenan in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the lambda carrageenan may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the lambda carrageenan may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the lambda carrageenan may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include starch in a range from about 0.2 percent by weight to about 5 percent by weight. Preferably the starch may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the starch may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the starch may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may further include a divalent cation. Preferably the divalent cation includes calcium ions, such as calcium lactate in solution. Divalent cations (such as calcium ions) may assist in the gel formation of compositions that include gelling agents such as the ionic crosslinking gelling agent, for example. The ion effect may assist in the gel formation. The divalent cation may be present in the gel composition in a range from about 0.1 to about 1 percent by weight, or about 0.5 percent by weight to about 1 percent by weight. 
     The gel composition may further include an acid. The acid may comprise a carboxylic acid. The carboxylic acid may include a ketone group. Preferably the carboxylic acid may include a ketone group having less than about 10 carbon atoms, or less than about 6 carbon atoms or less than about 4 carbon atoms, such as levulinic acid or lactic acid. Preferably this carboxylic acid has three carbon atoms (such as lactic acid). Lactic acid surprisingly improves the stability of the gel composition even over similar carboxylic acids. The carboxylic acid may assist in the gel formation. The carboxylic acid may reduce variation of the alkaloid compound concentration, or the cannabinoid compound concentration, or both the alkaloid compound concentration and the cannabinoid compound within the gel composition during storage. The carboxylic acid may reduce variation of the nicotine concentration within the gel composition during storage. 
     The gel composition may include a carboxylic acid in a range from about 0.1 percent by weight to about 5 percent by weight. Preferably the carboxylic acid may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the carboxylic acid may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the carboxylic acid may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include lactic acid in a range from about 0.1 percent by weight to about 5 percent by weight. Preferably the lactic acid may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the lactic acid may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the lactic acid may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition may include levulinic acid in a range from about 0.1 percent by weight to about 5 percent by weight. Preferably the levulinic acid may be in a range from about 0.5 percent by weight to about 3 percent by weight. Preferably the levulinic acid may be in a range from about 0.5 percent by weight to about 2 percent by weight. Preferably the levulinic acid may be in a range from about 1 percent by weight to about 2 percent by weight. 
     The gel composition preferably comprises some water. The gel composition is more stable when the composition comprises some water. Preferably the gel composition comprises at least about 1 percent by weight, or at least about 2 percent by weight, or at least about 5 percent by weight of water. Preferably the gel composition comprises at least about 10 percent by weight or at least about 15 percent by weight water. 
     Preferably the gel composition comprises between about 8 percent by weight to about 32 percent by weight water. Preferably the gel composition comprises from about 15 percent by weight to about 25 percent by weight water. Preferably the gel composition comprises from about 18 percent by weight to about 22 percent by weight water. Preferably the gel composition comprises about 20 percent by weight water. 
     Preferably, the aerosol-forming substrate comprises between about 150 mg and about 350 mg of the gel composition. 
     The invention is defined in the claims. However, below there is provided a non-exhaustive list of non-limiting examples. Any one or more of the features of these examples may be combined with any one or more features of another example, embodiment, or aspect described herein. 
     Example Ex1. An aerosol-generating article for producing an inhalable aerosol upon heating, the aerosol-generating article comprising: a mouthpiece assembly comprising: 
     a first tube, a second tube and a third tube; and an aerosol-forming substrate; wherein the first tube abuts a downstream end face of the second tube, and the third tube abuts an upstream end face of the second tube; wherein an internal diameter of the second tube is smaller than an internal diameter of the first tube; wherein the internal diameter of the second tube is smaller than an internal diameter of the third tube; wherein the internal diameter of the first tube is at least 3 mm. 
     Example Ex2. An aerosol-generating article according to example Ex1, wherein the internal diameter of the first tube is larger than the internal diameter of the third tube. 
     Example Ex3. An aerosol-generating article according to example Ex1 or example Ex2 wherein the first tube is a cellulose acetate tube. 
     Example Ex4. An aerosol-generating article according to any preceding example, wherein the second tube is a cellulose acetate tube. 
     Example Ex5. An aerosol-generating article according to any preceding example, wherein the third tube is a cellulose acetate tube. 
     Example Ex6. An aerosol-generating article according to any preceding example, comprising a fourth tube located in an opening defined by the third tube. 
     Example Ex7. An aerosol-generating article according to example Ex6, wherein the fourth tube is formed from a material that is substantially impermeable to air. 
     Example Ex8. An aerosol-generating article according to example Ex7, wherein the fourth tube is formed from cardboard. 
     Example Ex9. An aerosol-generating article according to any preceding example, comprising a wrapper provided on the external surface area of the first tube, the second tube and the third tube. 
     Example Ex10. An aerosol-generating article according to example Ex9, wherein the wrapper is formed from a non-porous material. 
     Example Ex11. An aerosol-generating article according to any preceding example, wherein a ratio of the internal diameter of the first tube to the internal diameter of the second tube is between 1.2 and 5. 
     Example Ex12. An aerosol-generating article according to example Ex11, wherein a ratio of the internal diameter of the first tube to the internal diameter of the second tube is between 1.8 and 2.5. 
     Example Ex13. An aerosol-generating article according to any preceding example, wherein a ratio of the internal diameter of the first tube to the internal diameter of the third tube is between 0.5 and 2. 
     Example Ex14. An aerosol-generating article according to example Ex13, wherein a ratio of the internal diameter of the first tube to the internal diameter of the third tube is between 0.8 and 1.2. 
     Example Ex15. An aerosol-generating article according to any preceding example, wherein a ratio of the internal diameter of the third tube to the internal diameter of the second tube is between 1.5 and 5. 
     Example Ex16. An aerosol-generating article according to example Ex15, wherein a ratio of the internal diameter of the third tube to the internal diameter of the second tube is between 1.8 and 2.5. 
     Example Ex17. An aerosol-generating article according to any preceding example, wherein the first tube has an internal diameter of between 3 mm and 8 mm. 
     Example Ex18. An aerosol-generating article according to example Ex17, wherein the first tube has an internal diameter of 4 mm. 
     Example Ex19. An aerosol-generating article according to any preceding example, wherein the first tube has a length of between 4 mm and 6 mm. 
     Example Ex20. An aerosol-generating article according to example Ex19, wherein the first tube has a length of 5 mm. 
     Example Ex21. An aerosol-generating article according to any preceding example, wherein the second tube has an internal diameter of between 1 mm and 3 mm. 
     Example Ex22. An aerosol-generating article according to example Ex21, wherein the second tube has an internal diameter of 2 mm. 
     Example Ex23. An aerosol-generating article according to any preceding example, wherein the second tube has a length of between 4 mm and 6 mm. 
     Example Ex24. An aerosol-generating article according to example Ex23, wherein the second tube has a length of 5 mm. 
     Example Ex25. An aerosol-generating article according to any preceding example, wherein the third tube has an internal diameter of between 3 mm and 8 mm. 
     Example Ex26. An aerosol-generating article according to example Ex25, wherein the third tube has an internal diameter of 4 mm. 
     Example Ex27. An aerosol-generating article according to any preceding example, wherein the third tube has a length of between 4 mm and 6 mm. 
     Example Ex28. An aerosol-generating article according to example Ex27, wherein the third tube has a length of 5 mm. 
     Example Ex29. An aerosol-generating article according to any example, comprising a ventilation zone. 
     Example Ex30. An aerosol-generating article according to example Ex29, wherein the ventilation zone comprises one or more rows of ventilation perforations. 
     Example Ex31. An aerosol-generating article according to example Ex30, wherein the one or more rows of ventilation perforations are formed through a wall of at least one of the first tube, the second tube and the third tube. 
     Example Ex32. An aerosol-generating article according to example Ex31, wherein the one or more rows of ventilation perforations are formed through a wall of the third tube. 
    
    
     
       Specific embodiments will now be described, by way of example only, with reference to the following examples and the accompanying drawings, in which: 
         FIG.  1    shows schematically an exploded sectional side view of a mouthpiece assembly for an aerosol-generating article according to the invention; 
         FIG.  2    shows schematically a sectional side view of the mouthpiece assembly of  FIG.  1   ; 
         FIG.  3    shows schematically a sectional side view of a mouthpiece assembly for an aerosol-generating article according to the invention; 
         FIG.  4    shows schematically a sectional side view of a mouthpiece assembly for an aerosol-generating article according to the invention; 
         FIG.  5    shows schematically a sectional side view of an aerosol-generating article according to the invention; and 
         FIG.  6    shows schematically a sectional side view of an aerosol-generating article according to the invention. 
     
    
    
     Some aerosol-generating articles heat rather than combust an aerosol-generating substrate, such as a tobacco-containing substrate. An aerosol may be generated in such aerosol-generating articles through the transfer of heat from a heat source to a physically separate aerosol-generating substrate or material, which may be located in contact with, within, around, or downstream of the heat source. During use of the aerosol-generating article, heat is transferred from the heat source to the aerosol-generating substrate, which may release volatile compounds. These volatile compounds are entrained in air drawn through the aerosol-generating article by a user. As the released volatile compounds cool, they condense to form an aerosol. The aerosol may be inhaled by a user through a mouthpiece. 
     It would be desirable to provide an aerosol-generating article that can cause more of the released volatile compounds to condense, which may provide for increased flow of aerosol through the mouthpiece. This may provide a better user experience. 
       FIGS.  1  and  2    show schematically an example of a mouthpiece assembly  110  for an aerosol-generating article  112  for producing an inhalable aerosol upon heating. 
     The mouthpiece assembly  110  has a downstream end  114  and an upstream end  116 . The downstream end  114  of the mouthpiece assembly  110  is the end region of the mouthpiece assembly  110  towards which the aerosol flows after it has been produced. The upstream end  116  of the mouthpiece assembly  110  is the end region of the mouthpiece assembly  110  that is opposite to and/or distal from the downstream end  114 . In some examples, the upstream end  116  of the mouthpiece assembly  110  is the end region of the mouthpiece assembly through which the produced aerosol flows before it flows through the downstream end  114 . 
     In other words, in use, aerosol that has been generated flows from the upstream end  116  of the mouthpiece assembly  110  and towards the downstream end  114  of the mouthpiece assembly  10 . 
     In the example shown in  FIGS.  1  and  2   , the mouthpiece assembly  110  includes a first tube  118 , a second tube  120  and a third tube  122 . As is shown in  FIGS.  1  and  2   , in this example, the second tube  120  is positioned in between the first tube  118  and the third tube  122 . 
     In this example, the first tube  118 , the second tube  120  and the third tube  122  are each formed from cellulose acetate. In other words, in the example of  FIGS.  1  and  2   , the first tube  118 , the second tube  120  and the third tube  122  are each a cellulose acetate tube. 
     In  FIG.  1   , the first tube  118 , the second tube  120  and the third tube  122  are separated from one another. In  FIG.  2   , the first tube  118 , the second tube  120  and the third tube  122  are provided in a configuration in which the first tube  118  and the third tube  122  abut the second tube  120 , as will be explained. 
     Each of the first tube  118 , the second tube  120  and the third tube  122  has a downstream end face and an upstream end face. The downstream end face of a tube  118 ,  120 ,  122  is the end face that is located towards the downstream end  114  of the mouthpiece assembly  10 . The upstream end face of a tube  118 ,  120 ,  122  is the end face that is located towards the upstream end  116  of the mouthpiece assembly  10 . 
     The first tube  118  has a downstream end face  124  and an upstream end face  126 . The second tube  120  has a downstream end face  128  and an upstream end face  130 . The third tube  122  has a downstream end face  132  and an upstream end face  134 . 
     In the example shown in  FIGS.  1  and  2   , the first tube  118  and the second tube  120  are arranged such that the upstream end face  124  of the first tube  118  abuts the downstream end face  126  of the second tube  120 . 
     In the example shown in  FIGS.  1  and  2   , the second tube  120  and the third tube  122  are arranged such that the downstream end face  134  of the third tube  122  abuts the upstream end face  128  of the second tube  120 . 
     In this example, the first tube  118  abuts the second tube  120 , and the third tube  122  abuts the second tube  120 . In other examples, the first, second and third tubes  118 ,  120 ,  122  may be connected or attached to one another. The first, second and third tubes  118 ,  120 ,  122  may be attached to one another by, for example, one or more fixing elements, or an adhesive. 
     The first tube  118  may be considered as a “mouthpiece” tube because, in use, the first tube  118  is the component of the mouthpiece assembly  110  that may come into contact with a user&#39;s mouth. 
     The second tube  120  may be considered as a “venturi” tube because, in use, the second tube  120  may provide a constriction in the flow path of the produced aerosol, as will be explained. 
     The third tube  122  may be considered as a “diffuser” tube because, in use, the third tube  122  may provide space for generated aerosol to combine with air. 
     The first tube  118  has a first tube internal diameter  136 . The second tube has a second tube internal diameter  138 . The third tube  122  has a third tube internal diameter  140 . In the example shown in  FIGS.  1  and  2   , each of the first tube  118 , the second tube  120  and the third tube  122  has a uniform internal diameter that is the same along the whole length of each tube. 
     It is to be understood that an internal diameter of the first tube  118 , the second tube  120  or the third tube  122  is the diameter or distance between the internal walls of the tube. 
     The internal diameter of the second tube  120  is smaller than the internal diameter of the first tube  118 . In other words, the second tube internal diameter  138  is smaller than the first tube internal diameter  136 . 
     The internal diameter of the second tube  120  is smaller than the internal diameter of the third tube  122 . In other words, the second tube internal diameter  138  is smaller than the third tube internal diameter  140 . 
     In some examples, such as in the example shown in  FIGS.  1  and  2   , the internal diameter of the first tube  118  is larger than the internal diameter of the third tube  122 . In other words, in some examples, the first tube internal diameter  136  is larger than the third tube internal diameter  140 . 
     In the example of the mouthpiece assembly  110  shown in  FIGS.  1  and  2   , the first tube internal diameter  136  is 4 mm, the second tube internal diameter  138  is 2.5 mm, and the third tube internal diameter is 3.5 mm. 
     In the example of the mouthpiece assembly  110  shown in  FIGS.  1  and  2   , the first tube  118  has a length of 5 mm, the second tube  120  has a length of 5 mm, and the third tube  122  has a length of 5 mm. 
       FIG.  3    shows schematically another example of a mouthpiece assembly  210  for an aerosol-generating article  112  for producing an inhalable aerosol upon heating. 
     The example of  FIG.  3    has the same components as the example shown in  FIGS.  1  and  2   , and the components are numbered correspondingly. 
     However, the mouthpiece assembly  210  shown in  FIG.  3    has two differences over the mouthpiece assembly  110  shown in  FIGS.  1  and  2   . 
     Firstly, in the mouthpiece assembly  210  of  FIG.  3   , the internal diameter of the first tube  118  is not uniform along its whole length. Instead, the first tube internal diameter  136  changes along the length of the first tube  118 . The internal diameter of the first tube  118  increases from the upstream end face  124  of the first tube  118  to the downstream end face  126  of the first tube  118 . In other words, in the example shown in  FIG.  3   , the first tube internal diameter  136  is larger at the downstream end face  126  of the first tube  118  than it is at the upstream end face  124  of the first tube  118 . 
     Secondly, in the mouthpiece assembly  210  of  FIG.  3   , the internal diameter of the third tube  122  is not uniform along its whole length. Instead, the third tube internal diameter  140  changes along the length of the third tube  122 . The internal diameter of the third tube  122  decreases from the upstream end face  132  of the third tube  122  to the downstream end face  134  of the third tube  122 . In other words, in the example shown in  FIG.  3   , the third tube internal diameter  140  is smaller at the downstream end face  134  of the third tube  122  than it is at the upstream end face  132  of the third tube  122 . 
     In the example of  FIG.  3   , the internal diameter of the first tube  118  is the mean diameter or distance between the internal walls of the first tube  118 . The internal diameter of the third tube  122  is the mean diameter or distance between the internal walls of the third tube  122 . The internal diameter of the second tube  120  is the diameter or distance between the internal walls of the second tube  120 . 
       FIG.  4    shows schematically another example of a mouthpiece assembly  310  for an aerosol-generating article  112  for producing an inhalable aerosol upon heating. 
     The example of  FIG.  4    has the same components as the example shown in  FIGS.  1  and  2   , and the components are numbered correspondingly. 
     However, the mouthpiece assembly  310  shown in  FIG.  4    has a difference over the mouthpiece assembly  110  shown in  FIGS.  1  and  2   . 
     In the mouthpiece assembly  310  of  FIG.  4   , the internal diameter of the first tube  118  is not uniform along its whole length. Instead, the first tube internal diameter  136  changes along the length of the first tube  118 . The internal diameter of the first tube  118  increases from the upstream end face  124  of the first tube  118  to the downstream end face  126  of the first tube  118 . In other words, in the example shown in  FIG.  4   , the first tube internal diameter  136  is larger at the downstream end face  126  of the first tube  118  than it is at the upstream end face  124  of the first tube  118 . 
     In the example of  FIG.  4   , the internal diameter of the first tube  118  is the mean diameter or distance between the internal walls of the first tube  118 . The internal diameter of the second tube  120  is the diameter or distance between the internal walls of the second tube  120 . The internal diameter of the third tube  122  is the diameter or distance between the internal walls of the second tube  120 . 
       FIG.  5    shows schematically an example of an aerosol-generating article  112 . In the example of  FIG.  5   , the aerosol-generating article  112  includes the mouthpiece assembly  110  that is shown schematically in  FIGS.  1  and  2   . 
     The aerosol-generating article  112  also includes an aerosol-forming substrate  142 . The aerosol-forming substrate includes components that are vaporisable to form an aerosol. In the example of  FIG.  5   , the aerosol-forming substrate  142  is a liquid nicotine formulation. In another example, the aerosol-forming substrate  142  may be a different formulation. In some examples, the aerosol-forming substrate  142  may be a gel formulation. 
     In the example shown in  FIG.  5   , the aerosol-generating article  112  includes a fourth tube  144 . The example of Figure also includes a fifth tube  146 . The fourth tube  144  is arranged concentrically with the first tube  118 , the second tube  120  and the third tube  122 . The fifth tube  146  is arranged concentrically with the first tube  118 , the second tube  120  and the third tube  122 . 
     In this example, the fourth tube  144  is provided in a central opening of the third tube  122  and the fifth tube  146 . In another example, the fourth tube  144  may be provided only in a central opening of the third tube  122 . The fourth tube  144  abuts the third tube  122  and the fifth tube  146 . In another example, the fourth tube  144  is affixed to the third tube  122  and the fifth tube  146  by a one or more fixing elements, or an adhesive. 
     In some examples, the fourth tube  144  may be formed from a material that is substantially impermeable to air. For example, the fourth tube  144  may be formed from cardboard. 
     In the example shown in  FIG.  5   , the fifth tube  146  abuts the third tube  122 . In another example, the third tube  122  and the fifth tube  146  may be connected or attached to one another. The third tube  122  and the fifth tube  146  may be attached to one another by, for example, one or more fixing elements, or an adhesive. The third tube  122  and the fifth tube  146  can be arranged such that an upstream end face of the fifth tube  146  abuts the downstream end face  132  of the third tube  122 . In the example of  FIG.  5   , the fifth tube  146  has the same internal diameter as the third tube  122 . Therefore, the fifth tube  146  has an internal diameter of 3.5 mm. In another example, the fifth tube  146  may have an internal diameter that is different than the internal diameter of the third tube  122 . 
     In the example of  FIG.  5   , a space  148  is defined in the aerosol-generating article  112 . The space  148  is located between the fifth tube  146  and the aerosol-forming substrate  142 . In an example that doesn&#39;t include a fifth tube  146 , the space  148  may be defined between the third tube  122  and the aerosol-forming substrate  142 . In some examples, the space  148  may provide an area that allows vaporised volatile compounds of the aerosol-forming substrate  142  to cool and nucleate into an aerosol. 
     The aerosol-generating article  112  shown in  FIG.  5    also includes an upstream element  150 . The upstream element  150  is positioned upstream of the aerosol-forming substrate  142 . In this example, the upstream element  150  abuts the aerosol-forming substrate  142 . In the example of  FIG.  5   , the upstream element  150  is an annular plug of fibrous filtration material. The upstream element  150  of  FIG.  5    has a length of 5 mm. The RTD of the upstream element  150  of  FIG.  5    is about 130 millimetres H2O. 
     In the example of  FIG.  5   , the aerosol-generating article  112  also includes a wrapper  152 . The wrapper  152  is provided on the external surface area of the components of the aerosol-generating article  112 . The wrapper  152  partially encloses at least some of the components of the aerosol-generating article  112 . In this example, the wrapper  152  partially encloses all of the components of the aerosol-generating article  112 . As is shown in  FIG.  5   , in some examples, the wrapper  152  fully encloses all of the components of the aerosol-generating article  112  except for the downstream end face of the first tube  118  and an upstream end face of the upstream element  150 . 
     The example of the aerosol-generating article  112  shown in  FIG.  5    also includes a ventilation zone. The ventilation zone is provided at a location along the aerosol-generating article  112 . In this example, the ventilation zone is provided in the region of the third tube  122 . In this example, the ventilation zone is a row of circumferential perforations  154  formed through the wrapper  152  and the third tube  122 . The perforations  154  allow for air to flow from outside of the aerosol-generating article  112 , through the perforations  154 , and into the opening defined by the third tube  122 . 
       FIG.  6    shows schematically an alternative example of an aerosol-generating article  212 . In the example of  FIG.  6   , the aerosol-generating article  212  includes the mouthpiece assembly  110  that is shown schematically in  FIGS.  1  and  2   . 
     In the example of the aerosol-generating article  212  shown in  FIG.  6   , the structure of the upstream end  116  is different to the structure of upstream end  116  of the example of the aerosol-generating article  112  shown in  FIG.  5   , as will be explained. 
     In the example of  FIG.  6   , the upstream element  150  is disposed immediately upstream of the aerosol-forming substrate  142  and abuts the aerosol-forming substrate  142 . The upstream element  150  comprises an annular plug comprising fibrous filtration material. In this example, the upstream element  150  comprises an annular plug of cellulose acetate circumscribed by a stiff wrapper. In the example of  FIG.  6   , the upstream element  150  has a length of 5 mm and the RTD of the upstream element  150  is 30 millimetres H2O. 
     In the example of  FIG.  6   , the aerosol generating article  212  also includes a recess  156 . The recess  156  extends from the upstream end  116  of the aerosol-generating article  212 , through the upstream element  150  and through at least a portion of the aerosol-forming substrate  142 . 
     The recess  156  is located along the central axis of the aerosol-generating article  212 . In the example of  FIG.  6   , the recess  156  has a circular cross sectional shape. In the example shown, the recess  156  extends the full length of both the upstream element  150  and the aerosol-forming substrate  142  by passing through both the annular plug comprising fibrous filtration material of the upstream element  150 , and the annular plug of porous medium of the aerosol-forming substrate  142 . In this example, the recess  156  has a length of 15 mm, corresponding to the combined length of the upstream element  150  and the aerosol-forming substrate  142 . In the example of  FIG.  6   , the recess has a diameter of 4 mm. 
     The aerosol generating article  212  of  FIG.  6    has a wrapper  152 . The wrapper  152  is provided on the longitudinal inner surface of the recess. The wrapper  152  extends the full length of the recess  156  and is provided on the entire longitudinal inner surface of the recess  156 . 
     In the example shown in  FIG.  6   , the downstream end of the recess  156  is defined by the wrapper  152 . This is achieved by mechanically folding the wrapper  152  at the downstream end of the recess  156 . 
     The wrapper  152  extends out of the upstream end of the recess  156  and over the upstream end of the aerosol-generating article  212 . The wrapper  152  also extends over the entire outer surface of the aerosol generating article  212 . In this way, the wrapper  212  acts to connect the various components of the aerosol-generating article  212 . 
     The wrapper  152  may include a cellulose based paper layer co-laminated with a layer of aluminium foil. In this example, the wrapper  152  is arranged so that the paper layer is on the outer surface of the aerosol-generating article  212 . 
     Use of the aerosol-generating article  112  shown in  FIG.  5    will now be described. 
     In use, the aerosol-generating article  112  is inserted into an aerosol-generating device. When the aerosol-generating article  112  is in its inserted position in the aerosol-generating device, a heating element of the aerosol-generating device is adjacent the aerosol-forming substrate  142 . When the aerosol-generating device is activated, the heating element heats up. The increased temperature of the heating element heats the aerosol-forming substrate  142 . Volatile compounds in the aerosol-forming substrate  142  then vaporise to form a vapour, which cools and nucleates into an aerosol in the space  148  between the fifth tube  146  and the aerosol-forming substrate  142 . 
     As the user draws on (i.e. inhales from) the downstream end  114  of the aerosol-generating device  112 , air is sucked into the fourth tube  144  through the perforations  154  due to the resulting pressure change inside the aerosol-generating device  112 . The air that is drawn into the aerosol-generating article  112  though the perforations  154  entrains aerosol from the space  148 . The fourth tube  144  prevents any entrained aerosol from being lost into the structure of the third tube  122  or the fifth tube  146 . 
     The air with entrained aerosol then passes through the third tube  122  and into the second tube  120  due to the pressure regime inside the aerosol-generating article  112 . The narrower internal diameter of the second tube  120  provides a Venturi effect, which causes the air with entrained aerosol to be compressed whilst the air is in the second tube  120 . 
     Due to the user inhaling from the downstream end  114  of the aerosol-generating device  112 , the air with entrained aerosol then flows out of the second tube  120  and into the first tube  118 . When the air with entrained aerosol is drawn out of the second tube  120  and into the first tube  118 , the wider diameter of the first tube  118  allows the air to expand and cool down, which causes more droplets to form in the aerosol. The aerosol can then subsequently be inhaled by the user through the downstream end of the first tube  118 . 
     In normal use of the aerosol-generating article  112 , the user inhales aerosol by engaging their mouth with the first tube  118  at the downstream end  114  of the aerosol-generating article  112 . Since, in the example of  FIG.  5   , the first tube  118  is tube that is formed from cellulose acetate, the first tube  118  is firm and resilient, which provides an improved user experience. 
     The first tube  118  being formed from a stiff material such as cellulose acetate also ensures that the user can properly handle the aerosol-generating article  112 . 
     The first tube  118  being formed from a material that is substantially impermeable to water, such as cellulose acetate, is also less sensitive to humidity from a user&#39;s mouth. 
     For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about”. Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. In this context, therefore, a number A is understood as A±10% of A. Within this context, a number A may be considered to include numerical values that are within general standard error for the measurement of the property that the number A modifies. The number A, in some instances as used in the appended claims, may deviate by the percentages enumerated above provided that the amount by which A deviates does not materially affect the basic and novel characteristic(s) of the claimed invention. Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.