Patent Publication Number: US-9895707-B2

Title: Method and apparatus for generating monodisperse aerosols

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and an apparatus for generating monodisperse aerosols for laboratory research and experimentation. 
     BACKGROUND OF THE INVENTION 
     The field of aerosol science and technology is concerned with the study of small particles suspended in a gas. The gas is usually air. However, particles suspended in other gaseous media, such as helium, argon, hydrogen, etc. are also considered as an aerosol. 
     The study of properties and behavior of small airborne particles is facilitated by the use of monodisperse aerosols, i.e. aerosol comprised of particles that are substantially uniform in size. This invention relates to a method and apparatus for generating monodisperse aerosols which may be subsequently processed to carry a specific charge or charge distribution for laboratory research and experimentation. 
     Many methods and apparatus have been developed by scientists and engineers working in the field of aerosol science and related fields for generating monodisperse aerosols. Examples include those described in U.S. Pat. Nos. 3,790,079 and 8,272,576 B2. 
     SUMMARY OF THE INVENTION 
     An aspect of the present disclosure relates to an apparatus for generating aerosol particles that are substantially uniform in size. The apparatus includes a droplet generator comprised of a metal capillary for a liquid to flow through to form a liquid stream. The liquid stream flows through the capillary and joins with a gas stream. The metal capillary is vibrated by a piezoelectric ceramic at a substantially constant frequency causing the liquid stream to breakup into droplets of a substantially uniform size in the gas stream, with the gas stream being maintained at a velocity in the range between approximately 10% to 100% of the speed of sound. 
     Another aspect of the present disclosure relates to a method for generating monodisperse aerosol particles, which includes flowing a liquid at a selected liquid flow rate through a vibrating metal capillary, the metal capillary being vibrated by a piezoelectric ceramic at a substantially constant frequency. Flowing a gas stream proximate an exit of the metal capillary such that gas stream joins the liquid exiting the metal capillary allows droplets to form of a substantially uniform size. The method further comprises adjusting said liquid flow rate to a selected set-point value and adjusting the gas flow rate to a range between approximately 10% to 100% of the speed of sound. 
     Yet another aspect of the present disclosure relates to a method for generating monodisperse aerosol particles by combining a liquid stream exiting a vibrating capillary with a gas stream flowing at a velocity in a range between approximately 10% to 100% of the speed of sound to generate droplets in the gas stream. The vibrations are produced by a piezoelectric source and the droplets produced are substantially uniform size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the system for generating monodisperse aerosols described in the present disclosure 
         FIG. 2  is a schematic diagram of the aerosol charging apparatus described in the present disclosure 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic vertical sectional view of the apparatus (also referred to as a system) for generating monodisperse droplets described in the present disclosure. The apparatus may have a cylindrical cross section and is generally illustrated at  10 . The apparatus  10  includes a housing  24  comprising a metal cover cap piece  25  and a metal base piece  35 , the pieces being sealed at a connection point with an O-ring  40  between the two pieces. The apparatus  10  is provided with a source of compressed gas  20  and a source of liquid  30 . The liquid from source  30  flows into droplet generating head  65  through liquid flow controller  55 , at a specific selected set-point value. The liquid then flows into flow channel  60  and into capillary flow channel  45  in the droplet generating head  65 . At substantially the same time, compressed gas from source  20  flows through gas flow controller  46  at a specific selected set point value through a hole, preferably a cylindrical hole  80  and into the gap space  110  between the metal cap piece  25  and an internal metal support  100  as shown by arrows  120  and  130 . The metal support  100  comprises a top annular section  102  and a cylindrical lower section  104 . All metal pieces, including the cap  25 , the metal support  100  and base  35 , are typically made of a metal such as stainless steel. The metal base  35 , is provided with an O-ring seal  145  to prevent liquid from source  30  from leaking out of liquid flow channel  60 . 
     Attached to metal support  100  is a cylindrical shaped-piezoelectric ceramic  140 . The piezoelectric ceramic  140  is attached to a bottom surface of the section  102  at a top end and to a bottom metal electrode  150  at a bottom surface, the top and bottom surfaces of the piezoelectric ceramic being attached to the section  102  and metal electrode  150  respectively by a suitable adhesive cement. An AC voltage, from voltage source  160  is provided by a metal wire  170  through insulator  180  to the bottom electrode  150  to cause the piezoelectric ceramic  140  to vibrate at substantially the same frequency as the AC voltage from the voltage source  160 . Since the AC voltage is at a substantially constant frequency the vibrations in the piezoelectric ceramic occur at a substantially constant frequency which causes droplets forming to be of a substantially uniform size. The metal support  100  is threaded and is screwed onto the base with threads  105 . The vibrations from the piezoelectric source ceramic  140  are transmitted at a substantially constant frequency through the support  100  to the droplet generating head  65  and to the liquid stream flowing out of the droplet generating head  65 , which forms a stream of uniformly sized monodisperse droplets flowing out of the droplet generating head  65 . 
     The use of a piezoelectric ceramic material to create mechanical vibration for the controlled disintegration of a liquid jet to form uniform droplets is well known. Such an approach is described in U.S. Pat. Nos. 3,790,079 and 8,272,576 B2, which are hereby incorporated by reference and will not be further explained. 
     The gas identified by arrows  120  and  130  flows through and out of cap  25  through outlet  182  which at least partially surrounds the droplet generating head  65  as the droplet generating head  65  extends upwardly and at least partially into the outlet opening  182  as illustrated in  FIG. 1 . When the gas flowing out of the outlet  182  is at a velocity that is lower than approximately 10% of the speed of sound in the gas, the liquid flowing out of the capillary flow passage way  45  will form a jet with a diameter that is of the same order of magnitude as the diameter of the capillary as the gas flow and the liquid flow are directed out of the outlet  182  and droplet generating head  65  in parallel directions. This means that a large diameter capillary passage way  45 , will result in formation of large diameter droplets of the same order of magnitude. Using a capillary diameter of for example, approximately 100 μm, the droplet diameter will typically be on the order of approximately 200 μm. 
     The apparatus and method of the present disclosure achieve a liquid-jet and droplet diameter as small as possible. This is accomplished using the flow focusing effect by increasing the gas flow velocity in the gas flow passageway in the outlet  182  in the cap  25 . 
     Flow focusing refers to the effect of a high gas flow velocity surrounding a liquid jet traveling at a slower velocity to cause the liquid jet diameter to become narrower and thus more sharply focused. The maximum gas velocity achievable in the gas outlet  182  is the speed of sound. Thus, the maximum flow focusing effect is achieved when the gas flow becomes sonic. For air, which is comprised mainly of diatomic gas molecules of oxygen and nitrogen, the speed of sound at normal atmospheric temperature of approximately 23° C. and a pressure of approximately one atmosphere is approximately 343 meters per second. For the purpose of creating flow focusing in this disclosure, the mean velocity gas flowing out of the outlet  182 , which is a narrow gap space, can be set in the range between approximately 10% and 100% of the speed of sound. Therefore the nominal gas velocity used for flow focusing is usually between the limit of approximately 34.3 meters per second and approximately 343 meters per second. Generally, the smaller the droplet diameter desired, the higher is the gas flow velocity needed to achieve the smaller diameter. To achieve a droplet diameter of approximately 0.1 μm, the gas flow velocity generally will need to be set to close to the speed of sound in the gas. 
     For generating monodisperse aerosol particles comprised of small, stable, non-volatile material suspended in air or other gases for laboratory experimentation, a non-volatile material can be dissolved in a volatile solvent to form a solution. The solution droplets created by the droplet generation apparatus described in this disclosure can then be allowed to evaporate to form non-volatile monodisperse particles of a much smaller diameter. 
     For example, in order to generate a monodisperse sodium chloride (NaCl) aerosol, an aqueous solution of NaCl can be prepared by dissolving the non-volatile NaCl solid in water. When the water evaporates from the NaCl solution droplets, monodisperse NaCl particles are formed as residue particles of the solution droplets. Using this approach, it is possible to generate monodisperse NaCl particles as small as approximately 20 nm, i.e. 0.02 μm, if the solution droplet diameter is approximately 1 μm. Other aerosol materials of interest can similarly be generated by the solvent evaporation technique. 
       FIG. 2  is a schematic diagram of a charging apparatus for placing an electrical charge on the monodisperse aerosol generated by the methods described herein. The droplet generator  10  is used as an aerosol generator to create a monodisperse aerosol in the submicron size range, typically in the range between approximately 20 nm, i.e. 0.02 μm, to approximately 1 μm in diameter. The aerosol then flows into aerosol charging apparatus  230 , which is approximately cubical in shape with an inlet  240  and outlet  250  for the aerosol to enter and exit respectively. Both inlet  240  and outlet  250  are in the form of circular holes machined or drilled into the cubical-shaped charging apparatus. The charging apparatus is typically made of a metal, for example, aluminum. 
     The charging apparatus  230  includes a metal needle  260  with a sharp tip. Needle  260  is held on a support,  270 , which is made of an insulating material. Needle  260  is connected to a high-voltage power supply  280  in order to generate gaseous ions in the gaseous medium of the aerosol. 
     In some applications, unipolar corona ions of either a positive or a negative polarity are desired. In the embodiment illustrated in  FIG. 2 , a DC high-voltage power supply  280  capable of generating the specific polarity of the DC voltage will be needed. In some embodiments, it is desired to generate corona ions of both a positive and negative polarity in the gaseous medium. In these embodiments, an AC power supply can be used. Typically the voltage needed to generate a self-sustaining corona discharge is on the order of a few thousand volts. The art of designing corona discharge systems for generating corona ions is well known to those skilled in the art of designing corona generation apparatus, and will not be further discussed. 
     When using a DC high-voltage power supply to generate unipolar ions of either a positive or a negative polarity, corona ions of either positive or negative polarity will collide with the aerosol particles to transfer a charge to the particles. The resulting charge on the particles will also be unipolar, i.e. all having the same polarity. For a small particle size, only a fraction of the particles will be charged. The fraction of particles carrying a charge is a complex function of the particle size, the concentration of corona ions in the gas phase, and the total residence time of the aerosol in the charging apparatus. 
     When an AC voltage is used to generate corona ions in the aerosol charging apparatus, particles of both polarities will appear in the aerosol. In these cases, approximately equal concentration of positively and negatively charges particles will appear in the aerosol. The overall charge on the aerosol cloud will be substantially equal to zero. 
     When making measurement in aerosols with a small particle size, using a unipolar charge will generally lead to greater sensitivity of measurement. As a result using a DC power supply to generate unipolar ions will generally give rise to greater measurement sensitivity and may be preferred in some aerosol measurement applications. 
     Although the present disclosure has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.