Electrohydrodynamic atomization nozzle emitting a liquid sheet

Embodiments for producing un-agglomerated, monodisperse droplets using a liquid sheet are provided. Nozzles with exit slit openings shape a spray liquid into a thin liquid sheet as the spray liquid exits from the slit opening. Stable multi-jet operation is achieved by including notches along the edge of the slit. The notches separate the liquid sheet into multiple jets to provide anchoring and stable multi jet operation. In some embodiments, the liquid sheet electrospray techniques and nozzles described herein provide high mass throughput and versatile multiplexing spray systems while reducing the engineering effort and high manufacturing cost.

BACKGROUND

Electrohydrodynamic atomization, often called electrospray (ES), has recently attracted great attention for potential and practical particle applications in fine powder production, food processing, medicine, pharmaceutics, biology, and chemistry. The technique enables the production of un-agglomerated, monodisperse particles of various materials with sizes ranging from micro-meter to nano-meter. In known ES systems, a liquid meniscus at the exit of a capillary nozzle is subjected to an electrical stress, resulting from a divergent electrical field established between a spray head and a reference electrode. Due to the geometry of capillary nozzles, however, known ES systems have low mass throughput, which makes the feasibility of large scale implementation difficult.

Several recent electrohydrodynamic atomization techniques reportedly increase the mass throughput. For example, in a single-capillary ES nozzle, a multi jet mode can be generated when the applied voltage exceeds a voltage range for a cone-jet mode. However, multiple liquid jets initiated from a single-capillary head are unstable. Arranging a number of individual capillaries in a one-dimensional linear array also increases the total spray liquid flowrate. However, although the one-dimensional arrangement of individual capillaries is easy to construct for laboratory investigation, it is not practical for industrial scale applications. Further each individual capillary uses a separate feeding and/or distribution channel.

SUMMARY

In a first aspect, a nozzle for electrohydrodynamic atomization is provided. The nozzle includes an inner rod, an outer tube concentrically aligned with the inner rod, an annular channel defined between the inner rod and the outer tube, the annular channel forming a circular slit at a spray end of the nozzle, and at least one electrically chargeable notch located on at least one of the inner rod and the outer tube proximate the circular slit.

In another aspect, a system for electrohydrodynamic atomization is provided. The system includes a nozzle including a source end, a spray end, a first component including a first surface, a second component including a second surface, a fluid channel defined between the first surface and the second surface, the fluid channel forming an exit slit at the spray end of the nozzle (e.g., a circular exit slit, a planar or linear exit slit, etc.), and at least one electrically chargeable notch located on at least one of the first component and the second component proximate the exit slit. The system may further include a voltage source electrically coupled to the nozzle and configured to supply a voltage to the at least one electrically chargeable notch, and a syringe pump in flow communication with the source end of the nozzle, the syringe pump configured to propel a spray liquid through the nozzle.

In yet another aspect, a method for electrohydrodynamic atomization is provided that may use one or more of the nozzle configurations described herein. For example, in one aspect the method may include providing a nozzle including an inner rod, an outer tube concentrically aligned with the inner rod, an annular channel defined between the inner rod and the outer tube, the annular channel forming a circular slit at a spray end of the nozzle, and a plurality of notches located on at least one of the inner rod and the outer tube proximate the circular slit. The method further may include supplying a voltage to the plurality of notches, and pumping a spray liquid through the annular channel of the nozzle.

DETAILED DESCRIPTION

Embodiments provide electrohydrodynamic atomization using a liquid sheet to produce un-agglomerated, monodisperse droplets. In contrast to the capillaries/tubes used in known electrospray (ES) systems, nozzles with exit slit openings shape a spray liquid into a thin liquid sheet as the spray liquid exits from the slit opening. Stable multi jet operation is achieved by including notches along the edge of the slit. The notches separate the liquid sheet into multiple jets to provide anchoring and stable multi jet operation. That is, each notch anchors a corresponding jet by preventing the corresponding jet from migrating around the nozzle, substantially fixing the position of the corresponding jet. In some embodiments, the liquid sheet electrospray techniques and nozzles described herein provide high mass throughput and versatile multiplexing spray systems while reducing the engineering effort and high manufacturing cost.

A schematic diagram of an exemplary electrospray system100is shown inFIG. 1. System100includes a syringe pump102, a nozzle104, and a high-voltage source106. In an exemplary embodiment, syringe pump102is a Harvard Apparatus PHD2000 series pump. Alternatively, syringe pump102is any pump that enables system100to function as described herein.

High-voltage source106is coupled to nozzle104such that high-voltage source106enables a voltage to be applied to a spray end108of nozzle104. In an exemplary embodiment, high-voltage source106provides a voltage ranging from 13 kV to 16 kV to spray end108. Alternatively, high-voltage source may apply any voltage to nozzle104that enables system100to function as described herein.

System100also includes a monitoring system110. Monitoring system110includes a microscopic lens112, a digital camera114, a monitor116, and a computer118, which enable monitoring system110to monitor the spray produced by nozzle104.

System100also includes a current system120. Current system120includes a multimeter122electrically coupled to a ground124and electrically coupled to ring126across a resistor128. By measuring the voltage across resistor128, current system120can measure the current of the spray produced by nozzle104. Through multimeter122, ring126is also electrically coupled to ground124.

FIG. 2is a cross-sectional view of an exemplary nozzle200that may be used with system100. Nozzle200includes a source end202and a spray end204. Nozzle200includes an inner rod206coaxially aligned with an outer tube208along a longitudinal axis210of nozzle200. An annular flow channel212is defined between an inner surface214of outer tube208and an outer surface216of inner rod206. At spray end204, annular flow channel212becomes a circular slit218. In an exemplary embodiment, an inner diameter220of circular slit218is 2,950 μm, and an outer diameter222of circular slit218is 3,250 μm, such that a width224of circular slit218is 150 μm. Alternatively circular slit218may have any dimensions that enable nozzle200to function as described herein.

In operation, syringe pump102applies a pressure to a spray liquid, such that the spray liquid is pushed towards nozzle200and received at source end202of nozzle200. The spray liquid is then evenly distributed into annular flow channel212and emitted from circular slit218as a thin liquid sheet. In the exemplary embodiment shown inFIG. 2, the emitted liquid sheet is substantially cylindrical.

Nozzle200further includes one or more notches230proximate to circular slit218. As used herein, a “notch” refers to a protruding element that extends from a spray end of a nozzle, such as notches230extending from spray end204of nozzle200. For example, such notches may extend or protrude further in the direction of the spray jet than other regions of the spray end separating such notches. In an exemplary embodiment, notches230are located on both inner rod206and outer tube208(see, also, notches330ofFIG. 6GC). Alternatively, notches230may be located only on inner rod206(see, e.g., notches330ofFIG. 6GB) or outer tube208(see, e.g., notches330ofFIG. 6GA). When high-voltage source106applies a voltage to spray end204of nozzle200, the shape and configuration of notches230facilitate local enhancement of an electric field at notches230. When the spray liquid reaches the circular slit218, it exits nozzle200as a thin liquid sheet due to the shape of annular flow channel212. With a sufficiently high voltage applied to spray end204of nozzle200, the thin liquid sheet is separated into multiple jets. Each jet is located at one of notches230due to the locally intensified electric field at each notch230. Further, with a high enough voltage applied to spray end204, stable multi jet operation may be achieved. In some embodiments, “stable multi jet operation” means that a jet of spray liquid is emitted from each notch230on nozzle200.

FIGS. 3A and 3Bare perspective views of exemplary nozzles200that may be used with the system ofFIG. 1.FIGS. 4A and 4Bare plan views of the nozzles200shown inFIGS. 3A and 3B. The embodiment shown inFIGS. 3A and 4Aincludes six notches230, and the embodiment shown inFIGS. 3B and 4Bincludes twenty notches230. Notches230are circumferentially spaced apart from one another by a circumferential distance,240. In some embodiments, circumferential distance240is no less than a size of a notch230. Circumferential distance240is chosen such that the number of notches230is maximized while maintaining regions of intensified electric field. That is, if circumferential distance240between notches230is too small, the electric field generated at one notch230will interfere with the electric field generated at an adjacent notch230. However, as the circumferential distance240between notches230increases, fewer notches230can be located on nozzle200. With fewer notches230, fewer jets are created, and the overall mass throughput of nozzle200is decreased.

FIGS. 6A-6Lare cross-sectional views of exemplary nozzles300that may be used with the system ofFIG. 1. As demonstrated by the embodiments shown inFIGS. 6A-6L, several different configurations of nozzle300enable nozzle300to function as described herein. Moreover, configurations of nozzle300are not limited to those specifically described herein.

Each nozzle300inFIGS. 6A-6Lincludes a source end302and a spray end304. Further, each nozzle300includes an inner rod306coaxially aligned with an outer tube308along a longitudinal axis310of nozzle300. An annular flow channel312is defined between an inner surface314of outer tube308and an outer surface316of inner rod306. At spray end304, annular flow channel312becomes a circular slit318. Further, at spray end304, inner rod306includes a center piece320, and outer tube308includes an end portion322. Each nozzle300also includes a plurality of notches330which function substantially similar to notches230shown inFIG. 2. The embodiments ofFIGS. 6A-6Lare each discussed in detail below.

In the embodiment of nozzle300shown inFIG. 6A, both inner rod306and outer tube308includes notches330thereon. Further, center piece320is not retracted or extended with respect to end portion322. During operation of the embodiment shown inFIG. 6A, a spray liquid is emitted from circular slit318.

In the embodiment of nozzle300shown inFIG. 6B, only outer tube308includes notches330thereon. That is, inner rod306does not include notches330. Further, center piece320is retracted with respect to end portion322. During operation of the embodiment shown inFIG. 6B, the spray liquid is emitted from circular slit318.

The embodiment of nozzle300shown inFIG. 6Cis substantially similar to the embodiment shown inFIG. 6A, except that the embodiment shown inFIG. 6Cincludes a central flow channel340defined through inner rod306. Likewise, the embodiment of nozzle300shown inFIG. 6Dis substantially similar to the embodiment shown inFIG. 6B, except that the embodiment shown inFIG. 6Dincludes central flow channel340defined through inner rod306. In the embodiments ofFIGS. 6C and 6D, a stabilizing gas is emitted from central flow channel340. The emitted gas facilitates maintaining the thin liquid sheet shape of the spray liquid when the spray liquid is emitted from circular slit318.

The embodiments of nozzle300shown inFIGS. 6E and 6Fare substantially similar to the embodiments shown inFIGS. 6C and 6Drespectively, except that instead of a stabilizing gas, a stabilizing liquid is emitted from central flow channel340. Similar to the stabilizing gas, the stabilizing liquid facilitates maintaining the thin liquid sheet shape of the spray liquid when the spray liquid is emitted from circular slit318. The embodiment of nozzle300shown inFIG. 6GAis substantially similar to the embodiment shown inFIG. 6F, except that center piece320is extended with respect to end portion322.

The embodiments of nozzle300shown inFIGS. 6H and 6Iare substantially similar to the embodiments shown inFIGS. 6C and 6Drespectively, except that the embodiments ofFIGS. 6H and 6Ifurther include a second outer tube350. Second outer tube350is concentrically aligned with outer tube308such that a second annular flow channel352is defined between an outer surface354of outer tube308and an inner surface356of second outer tube350. Second annular flow channel352becomes a second circular slit360at spray end304.

In the embodiments shown inFIGS. 6H and 6I, second outer tube350includes notches330thereon. In addition to the spray liquid emitted from first circular slit318, a sheath liquid is emitted from second circular slit360in a thin liquid sheet shape. With the spray liquid and the sheath liquid both emitted from nozzle300, particle encapsulation is facilitated, such that the particles produced by nozzle300include particles of spray liquid encapsulated by particles of sheath liquid and/or particles of sheath liquid encapsulated by particles of spray liquid. Similar to the embodiments shown inFIGS. 6C and 6D, a stabilizing gas emitted from central flow channel340facilitates maintaining the thin liquid sheet shape of the spray liquid and the sheath liquid. Alternatively, similar toFIGS. 6E-6G, a stabilizing liquid may be emitted from central flow channel340. The embodiment shown inFIG. 6Jis substantially similar to the embodiment shown inFIG. 6I, except that the center piece320is extended with respect to end portion322.

The embodiment of nozzle300shown inFIG. 6Kis substantially similar to the embodiment shown inFIG. 6H, except that the embodiment ofFIG. 6Kfurther includes a third outer tube370. Third outer tube370is concentrically aligned with second outer tube350such that a third annular flow channel372is defined between an outer surface374of second outer tube350and an inner surface376of third outer tube370. Third annular flow channel372becomes a third circular slit380at spray end304.

In the embodiment shown inFIG. 6K, third outer tube370includes notches330thereon. In addition to the spray liquid emitted from first circular slit318and the sheath liquid emitted from second circular slit360, an outer liquid is emitted from third circular slit380in a thin liquid sheet shape. The emission of liquids from first circular slit318, second circular slit360, and third circular slit380facilitates particle encapsulation and the production of multi-layered particles. Similar to the embodiments shown inFIGS. 6C and 6D, a stabilizing gas emitted from central flow channel340facilitates maintaining the thin liquid sheet shape of the emitted liquids. Alternatively, similar toFIGS. 6E-6G, a stabilizing liquid may be emitted from central flow channel340.

The embodiment shown inFIG. 6Lis substantially similar to the embodiment shown inFIG. 6K, except that the circular slits318,360, and380are arranged in a stepped configuration, such that outer tube308is extended with respect to second outer tube350, and second outer tube350is extended with respect to third outer tube370. Due to the stepped configuration, less voltage may be applied to the embodiment shown inFIG. 6Lthan the embodiment shown inFIG. 6Kto achieve stable multi-jet operation. In the embodiments shown inFIGS. 6L and 6K, the outer liquid emitted from third circular slit380may be the same liquid as at least one of the spray liquid emitted from first circular slit318and the sheath liquid emitted from second circular slit360. Alternatively, the spray liquid, the sheath liquid, and the outer liquid may all be different liquids. Furthermore, while in the embodiments shown inFIGS. 6A-6L, certain flow channels are denoted as containing a gas or a liquid, alternatively, any suitable fluid (i.e., gas, liquid) may be provided in any flow channel that enables nozzle300to function as described herein.

Multiple experiments were executed utilizing the nozzles described herein. In the following examples, Isopropanol was selected as the spray liquid, and nitric acid was used as an ion additive to vary the electrical conductivity of the spray liquid from 0.0079 μS/cm (pure isopropanol) to 1,044 μS/cm. The electrical conductivity of the spray liquid was measured by a conductivity meter (Orion 162A, Thermo Electron Corporation), and the electrical resistance of pure isopropanol was measured by a lab-made liquid cell. Alternatively, those of ordinary skill in the art will understand that any spray liquid may be utilized which allows system100to function as described herein.

FIG. 7is a graph that illustrates the applied voltage for establishing stable multi-jet operation for nozzles having various numbers of notches. The graph demonstrates that the applied voltage increases with the number of notches. It was also determined that the applied voltage for establishing stable multi jet operation is also slightly proportional to the feed flowrate of the spray liquid.

In one example, to study the evolution in the formation of multiple jets, the spray current was measured, and the number of jets was counted as the applied voltage was continuously increased and then decreased.FIGS. 8A-8Care graphs illustrating the spray current as a function of the applied voltage for nozzles with six, twelve, and twenty notches, respectively. The spray current and the number of jets increased with the increase of applied voltage. The stable multi jet operation, in which the number of jets is the same as the number of notches, was achieved at a sufficiently high applied voltage. When the applied voltage was reduced, the number of jets correspondingly decreased. Further, as shown in the graphs, a hysteresis phenomenon was observed.

In another example, to determine the mass throughput of the nozzles, spray liquids with various electrical conductivities were used to find the maximum liquid flowrate (Qmax) for nozzle operation.FIG. 9is a graph that illustrates the maximum liquid flowrate as a function of the electrical conductivity of the spray liquid. The graph demonstrates that the value of Qmax significantly decreases as the electrical conductivity of the spraying liquid increases, due to the fact that spray liquids with higher electrical conductivity generally use a stronger electric field to establish stable multi-jet operation. The graph also demonstrates that Qmax increases as the number of notches increases. This is because a greater number of notches allow more jets to be established, thus increasing the spray liquid flowrate and the overall mass throughput.

FIG. 10is a graph illustrating a ratio of the maximum liquid flowrate of the liquid sheet nozzles described herein to the maximum liquid flowrate of a known single-capillary nozzle as a function of the electrical conductivity of the spray liquid. In this example, the diameter of the single-capillary nozzle and the width of the circular slit in the liquid sheet nozzle were both 150 μm. As illustrated in the graph ofFIG. 10, when using a spray liquid with an electrical conductivity of 0.0079 μS/cm, the maximum liquid flowrate, Qmax, of the liquid sheet nozzle with 20 notches was one hundred and sixty-six times greater than the maximum liquid flowrate for the single-capillary nozzle. Further, when using a spray liquid with an electrical conductivity of 1,044 μS/cm μS/cm, the maximum liquid flowrate of the liquid sheet nozzle with twenty notches was seventy times greater than the maximum liquid flowrate for the single-capillary nozzle.

Notably, the value of Qmax for a liquid sheet nozzle with twenty notches is much higher than the sum of the total liquid flowrates for twenty single-capillary nozzles. This same phenomenon was also observed using liquid sheet nozzles with six and twelve notches. This indicates that, as compared to existing one-dimensional and two-dimensional arrays of single-capillary nozzles, liquid sheet nozzles have potential to drastically increase the mass throughput for spray liquids having a wide range of electrical conductivity.

FIG. 11is a plan view of an alternative nozzle500that may be used with system100.FIG. 12is a perspective view of nozzle500shown inFIG. 11. In an exemplary embodiment, nozzle500includes a first plate502, a second plate504, a source end506, and a spray end508. A planar flow channel510is defined between first plate502and second plate504. Planar flow channel510becomes a linear slit512at spray end508. In operation, a spray liquid is emitted from nozzle500from linear slit512. While the thin liquid sheet emitted from nozzle200is substantially cylindrical, the thin liquid sheet emitted from nozzle500is substantially planar.

In an exemplary embodiment, a plurality of notches514are located on both first plate502and second plate504, and staggered with respect to one another. Alternatively, notches514may only be located on one of first plate502and second plate504. With a voltage applied to notches514, notches514facilitate separating the thin liquid sheet into a plurality of jets, substantially similar to notches230of nozzle200.

The nozzles illustrated inFIGS. 1-6L, 11, and 12constitute exemplary means for emitting a liquid sheet. Further, the notches illustrated inFIGS. 2-6L, 11, and 12constitute exemplary means for separating a liquid sheet emitted by a nozzle into multiple jets. Moreover, nozzles illustrated inFIGS. 6C-6Lconstitute exemplary means for maintaining a shape of a liquid sheet emitted from a nozzle.

In addition to the cylindrical and planar nozzles specifically described herein, those of ordinary skill in the art will understand that any nozzle shape and/or configuration may be utilized which allows system100to function as described herein.

Embodiments described herein enable electrohydrodynamic atomization, or electrospray (ES), using nozzles that produce a thin liquid sheet. The methods and systems described herein increase the mass throughput of ES systems while decreasing the design and manufacturing costs as compared to known ES systems utilizing multiple single-capillary nozzles. The nozzles described herein include annular and/or planar slits designed emit a thin liquid sheet of spray liquid. To separate the thin liquid sheet into multiple jets and to anchor the jets for stable operation, a plurality of notches are included at the annular and/or planar slits. That is, each notch anchors a corresponding jet by preventing the corresponding jet from migrating around the nozzle, substantially fixing the position of the corresponding jet. When a voltage is applied to the nozzles described herein, these notches enable local enhancement of an electric field. Further, stable multi jet operation of the nozzles described herein can be established for a wide range of spray liquids having various electrical conductivities.

Moreover, as compared to known ES systems utilizing arrays of single-capillary nozzles, multiple liquid flow feeding and/or distribution channels are no longer necessary for the nozzles described herein. As such, the design concept and fabrication of the nozzles described herein is simpler than known nozzles, enabling flexibility in the design and/or geometry of the nozzles.

Through experimentation utilizing the nozzles described herein, it was demonstrated that the applied voltage for establishing stable multi-jet operation increased as both the number of jets and the liquid flowrate increased. Further, the maximum operational flowrate through the nozzles described herein was a function of the electrical conductivity of the spray liquid. Moreover, the maximum flowrate for liquid sheet nozzles with various numbers of notches was consistently greater than the total flowrate sum of an array of an equivalent number of single-capillary nozzles. Accordingly, the liquid sheet nozzles described herein enable superior ES techniques. Further, the liquid sheet shape of the spray liquid, as opposed to the cone jet emitted from known single-capillary nozzles, enables the nozzle design to have various geometries, including, but not limited to annular and/or planar slits.