Technical Field

Embodiments relate to an electrohydrodynamic atomizer and related method of atomizing a liquid.

Background

Electrohydrodynamic atomization, also commonly referred to as electrospraying, is the process of using an electric field in conjunction with a nozzle from which liquid is dispensed to atomize the liquid by the electric field. Generally, this involves the use of a nozzle electrode (often the electrode itself) and a reference electrode.

Under the correct conditions, which depend on the properties of the liquid and a threshold voltage for the potential difference forming the electric field, the liquid will form a cone and as the apex of the cone becomes smaller it reaches a point where the surface tension of the liquid breaks down forming a fine mist of liquid particles. Since the particles are charged, they will be attracted to the reference electrode which may then be attached to a surface, effectively turning the entire surface into the reference electrode, providing a simple arrangement for applying a coating, for example.

Electrohydrodynamic atomization has found applications not only in coating surfaces but in many other fields such as an ionization source for mass spectrometry.

These devices take advantage of the fact that the atomized liquid is attracted to the reference electrode. However, in many applications it is undesirable for the atomized liquid to interact with the reference electrode; instead it is desirable to create a stream of the atomized liquid which may, for example, be directed.

For these reasons it may be desirable to produce a stream of neutrally charged particles whilst retaining the relative simplicity of the electrohydrodynamic atomization arrangements.

Known arrangements for producing a stream of neutrally charged particles, or for directing a stream of charged particles use a so called ion-wind which involves interacting a directed stream of charged particles with the charged stream of atomized liquid to produce a neutral stream (or a charged stream) which can be directed. Summary of the Disclosure

An embodiment provides an electrohydrodynamic atomization device comprising: a nozzle for emitting a stream of atomized liquid, the nozzle being connected to a liquid source; a nozzle electrode associated with the nozzle; and a reference electrode; wherein the nozzle and reference electrodes co-operate to generate an electric field therebetween; a source of alternating electric current connected to the nozzle electrode and the reference electrode; and wherein the nozzle, nozzle electrode, reference electrode, and source of alternating current are arranged so that a liquid emitted by the nozzle is atomised into positively and negatively charged particles, the positively and negatively charged particles combining downstream of the nozzle to form a stream of neutral liquid particles.

Since the stream is neutral, it will not interact with the electrodes and therefore, there may be reduced loss of liquid during the atomisation process. Furthermore, since the resultant stream is neutral, it has applications outside of those of other atomization arrangements. A particularly significant application may be in the medical field and, in particular, in the field of nebulisation or vaccination where it is important that the stream of atomized liquid is not charged.

Embodiments of the invention may further find application in deposition whereby the neutral particles are deposited on a substrate to form a structure.

There are a number of factors which interact to produce the neural particles. The properties of the liquid including the surface tension, density, viscosity, conductivity or relative permittivity of the liquid may determine aspects of the device.

Aspects of the device which may be adapted to produce the neutral liquid particles include design aspects of the device such as a shape and size of the outlet of the nozzle, the size and shape of the reference and nozzle electrodes, the arrangement and spacing of the reference electrode relative to the nozzle electrode, and the number of nozzles, nozzle outlets, nozzle electrodes and reference electrodes.

Other aspects of the devices which may be adapted to produce the neutral liquid particles relate to the operation of the device such as varying a frequency for the alternating current, changing a maximum potential difference for the electric field, and changing a flow rate of the liquid.

In an embodiment, the nozzle is a capillary nozzle, the reference electrode is an annular ring situated upstream of an outlet of the nozzle at a distance between 2 and 10 mm from the outlet, the frequency of the alternating current is between 50 and 500 Hz and a maximum potential difference of the electric field is between 3 kV and 6kV.

It has been found that such an arrangement may produce a stream of neutral particles for many liquids which may be used as a carrier liquid for nebulised pharmaceuticals. However, it is to be realised that each of these parameters may be tuned according to design criteria and the characteristics of the liquid involved to produce the neutral liquid particles.

The nozzle may comprise an outlet and the reference electrode may be situated upstream of the outlet. The reference electrode may be situated between 2 and 10 mm upstream of the outlet. In a further embodiment, the reference electrode is situated between 0.5 and 50 mm upstream of the outlet.

The reference electrode may be at least partially annular. The reference electrode may encircle the nozzle. The reference electrode may be a ring. The ring may be a hexagon, a rectangle, or may have multi-edges. The ring may be either open or closed. The ring need not be symmetrically arranged about the nozzle.

The ring may have a diameter of between 3 mm and 20 mm. In a further application the diameter of the ring may be between 100 pm and 1 mm. This application may be deposition.

The nozzle electrode may be incorporated into the nozzle. Alternatively, the nozzle may comprise the nozzle electrode or the nozzle electrode is provided separate to the nozzle.

The source of alternating current may produce an alternating current with a frequency of between 10 Hz and 2,000 kHz. The frequency may be chosen in dependence on properties of a liquid to be atomised. The properties may be one or more of: surface tension, density, viscosity, conductivity or relative permittivity. The frequency may be between 10 Hz and1,500 Hz. In an embodiment, the frequency is between 50 Hz and 500 Hz. The maximum potential difference between the nozzle electrode and the reference electrode may be between 100 V and 30 kV. In an embodiment, the voltage depends on a distance between the nozzle electrode and the reference electrode. The voltage may depend on properties of a liquid to be atomised. The properties may be one or more of: surface tension, density, viscosity, conductivity or relative permittivity. The voltage may be between 1kV and 10kV. Alternatively, the voltage may be between 3kV and 6kV

The nozzle may be a capillary nozzle. The nozzle may have an output with a radius of less than 2 mm. A radius of an output of the nozzle may be between 25pm and 2 mm.

A flow rate of liquid from the liquid source to the nozzle may be variable. In an embodiment, the flow rate is between 1 and 2 ml/h.

In an embodiment, a desired flow rate is estimated by:

Q ~(geeo/rK where eo ~ 5.85 pF/m (permittivity of vacuum) and g is the surface tension, e is the relative permittivity, p is the density and K is the conductivity of the spray liquid.

The flow rate of the liquid may vary between one nanolitre per minute up to 50 microlitres per minute. A further embodiment comprises multiple nozzles and/or multiple reference electrodes preferably arranged as rings.

A further embodiment extends to a method of electrohydrodynamic atomization comprising providing: a nozzle for emitting a stream of atomized liquid, the nozzle being connected to a liquid source; a nozzle electrode associated with the nozzle; a reference electrode; a source of alternating electric current connected to the nozzle electrode and the reference electrode; the method comprising: arranging the nozzle, nozzle electrode, reference electrode, and source of alternating current so that an alternating electric field is generated between the electrodes and so that a liquid emitted by the nozzle is atomised into positively and negatively charged particles, the positively and negatively charged particles combining downstream of the nozzle to form neutral liquid particles. The nozzle may comprise an outlet and the reference electrode may be situated upstream of the outlet.

The reference electrode may be at least partially annular. The reference electrode may be a ring.

The nozzle electrode may be incorporated into the nozzle.

The method may further comprise operating the source of alternating current to produce an alternating current with a frequency of between 10 Hz and 2,000 kHz.

The nozzle may be a capillary nozzle.

The method may further comprise adapting a flow rate of the liquid to form the neutral liquid particles.

The method may further comprise the step of auto-tuning to produce a stream of neutral liquid particles.

The step of auto-tuning may comprise varying one or more of: a frequency of the alternating current; a maximum voltage of a potential difference between the nozzle electrode and the reference electrode; or a distance between the reference electrode and the nozzle electrode to produce the stream of neutral liquid particles.

The step of auto-tuning may comprise measuring a tuning current between a position at the reference electrode and a position at the nozzle electrode and determining that a stream of neutral liquid particles is produced when the tuning current is reduced.

The step of auto-tuning may comprise the step of increasing a frequency of the alternating current and determining that a stream of neutral particles is produced when a tuning current between a position at the reference electrode and a position at the nozzle electrode reaches a minimum.

The method may further comprise form a stream of ion particles and combining the stream of ion particles with the neutral particles.

The method may further comprise providing at least one pin electrode arranged at or close to the outlet of the nozzle.

The method may further comprise providing a second reference electrode arranged downstream of the outlet of the nozzle. The method may further comprise providing a housing situated between the reference electrode and the second reference electrode for directing airflow. The housing may be formed with one or more voids.

The method may further comprise providing a collimating electrode situated downstream from an outlet of the nozzle. Either or both the second reference electrode and the collimating electrode may be: annual, partially annular or formed as a ring.

A further embodiment extends to a pharmaceutical nebuliser incorporating a device as herein described.

Description of the Drawings

Embodiments are herein described, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a electrohydrodynamic atomization device according to an embodiment;

Figure 2 is a cross-sectional view of a nozzle for use with the electrohydrodynamic atomization device of Figure 1;

Figure 3 is a schematic diagram illustrating use of the device of Figure 1 under a first set of operating conditions;

Figure 4 is a schematic diagram illustrating use of the device of Figure 1 under a second set of operating conditions;

Figure 5 illustrates graphs of certain parameters of the device of Figure 1;

Figure 6 illustrates the operation of the device of Figure 1; and

Figures 7 through 10 illustrate alternate embodiments of electrohydrodynamic atomization devices.

Detailed Description of Specific Embodiment

Figure 1 illustrates a electrohydrodynamic atomization device 10 which includes a nozzle 12 having a capillary tube 14 with an outlet 16. In this embodiment, the capillary tube 14 is constructed from an electrically conductive material and therefore forms a nozzle electrode. A ring electrode 18 is mounted downstream of the outlet 16 and is situated symmetrically about the capillary tube 14.

The nozzle 14 and the ring electrode are connected to a high voltage power supply 20 which is, in turn, connected to a function generator 22. The high voltage power supply 20 supplies a potential difference between the nozzle 14 and the reference electrode 18. Furthermore, the function generator 22 causes that potential difference to vary thereby producing an alternating current.

In use, the nozzle is provided with a fluid at a constant flow rate by piston 24 and, as described below, produces a droplet jet 26.

Figure 2 illustrates a nozzle 38 for use with the device 10 of Figure 1. The nozzle 38 includes a plastic body 44 housing a ring electrode 40 covered by a ring electrode cover 42. The capillary electrode 46 with outlet 48 extends through the centre of the plastic body 44 and is situated so that when liquid is ejected from the outlet 48, the ring electrode 40 is situated upstream of the outlet 48. In other words, if the outlet 48 is considered the front of the nozzle 38, then the ring electrode 40 is situated behind the outlet 48.

Although many different arrangements are possible, the following discussion of embodiments are based on a setup with the nozzle 38 of Figure 2 having a capillary outlet with a diameter of 0.2 mm, a ring reference electrode 40 in the form of a ring symmetrically disposed about a longitudinal axis of the nozzle. The ring of the electrode is comprised of a round conducting annulus (a washer) with an inner diameter of 4 mm and an outer dimeter of 8 mm. The distance (d) between the outlet 48 of the nozzle and the ring electrode 40 is 4 mm.

Although the illustrations of Figures 3 and 4 are slightly different from that of Figure 1, it is to be realised that these Figures are schematic and therefore the same reference numbering is used in Figures 3 and 4 as was used in Figure 1. Figure 3 is a schematic diagram illustrating use of the device of Figure 1 under a first set of operating conditions.

In this set of operating conditions, the maximum potential difference between the reference electrode and the ring electrode is 3.8 kV and the frequency of the applied function generator is 10 Hz (similar observations are applicable, for this setup for frequencies less than 50 Hz ).

The flow rate was set to about 1.6 ml/hour based on the approximation derived from: Q ~(geeo/rK where eo ~ 5.85 pF/m (permittivity of vacuum) and g is the surface tension, e is the relative permittivity, p is the density and K is the conductivity of the spray liquid. In this case the spray liquid was isopropyl alcohol (Sigma-Aldrich 99.5%), with surface tension g ~ 20.8 mN/m, density p ~ 0.785 g/ml, conductivity K ~ 6 pS/m and relative permittivity e ~ 18.6).

In a further embodiment, the viscosity of the liquid may also be taken into account to determine the flow rate, frequency or other operating parameters.

As the electric field switches polarity, the liquid particles (or droplets) emitted from the outlet of the nozzle will change in polarity. The result is the illustrated ‘waves’ of charged particles, with adjacent waves having opposite charges. Therefore, for example, 50 and 54 are waves of positively charged particles with a wave 52 of negatively charged particles.

Figure 4 illustrates the same setup as Figure 3 where the frequency has been increased to 400 Hz. As illustrated, under these conditions the waves of oppositely charged particles 56 combine to form neutral particles 58, thereby producing a stream of neutral particles.

In both of the operating conditions of Figures 3 and 4 the liquid was isopropyl alcohol with surface tension y- 20.8 mN/m, density p ~ 0.785 g/ml, viscosity m ~ 1.66 mPas, conductivity K ~ 6 pS/m and relative permittivity e ~ 18.6. The liquid was introduced into the capillary nozzle at a flow rate of 1 ml/h.

Without being bound by theory, the inventors’ current understanding is that the production of neutral particles occurs when the varying acceleration that the positive and negative particles undergo due to the changing potential difference between the nozzle electrode and the reference electrode allows these oppositely charged particles to combine to form neutral particles.

It is to be realised that there are a number of parameters which may influence the behaviour of the liquid particles which are produced. However, the inventors have noticed that the setup illustrated in Figures 1, 3 and 4 only produces the observed stream of neutral particles when alternating current is applied.

Figure 5(a) illustrates the difference between AC and DC for this setup. Figure 5(a) shows the charge of the particle flow measured by an electrometer probe placed 200 mm downstream from the capillary outlet. The charge is approximately 8000 fA for the DC case and is 40 times larger in magnitude than the result for the AC case at 100 Hz, which is in a range of 200 fA.

Figure 5(b) compares measurements of the particle size distributions of the setup of Figures 1, 3 and 4 with that generated by an aerosol spectrometer (“control”). In the DC case, as the majority of charged particles divert to the ring electrode and a small amount of particles are detected downstream. In the AC case, a high particle concentration sizing 100 - 500 nm are recorded.

These particle sizes are significantly smaller than other nebulising technologies and therefore, embodiments may be particularly well suited to nebulising pharmaceuticals for delivery to a patients’ lungs, since in this application, small particle size may assist in both the penetration and absorption of the nebulised pharmaceutical.

The effect produced by embodiments is further illustrated with reference to Figure 6. Figures 6(a) and 6(b) are photographs illustrating the stream of particles under various conditions: 6(a) depicts a DC case and 6(b) depicts an AC case where the applied frequency is 300 Hz. Figure 6(c) is a photograph of the AC case showing the extent of the stream relative to a scale marking of 150 mm.

Figure 6(d) is a graph illustrating the dispersion of the stream (designated by Q measuring the angle between the direction of the outlet of the nozzle and the maximum dispersion of the stream) changing as a function of frequency. For this setup, the dispersion of the stream is significantly reduced for frequencies of 100 Hz and over. However, for frequencies over 450 Hz, the stream collapses. Furthermore, as illustrated in Figure 6(d), the procedure was repeated at maximum potential difference of 3.8 kV, 3.9 kV and 4.1 kV. The results were independent of the voltage.

For alternate embodiments with different nozzles and nozzle and reference electrode arrangement, the potential difference may have a greater influence.

There are a number of factors which interact to produce the neural particles. The properties of the liquid including the surface tension, density, viscosity, conductivity or relative permittivity of the liquid may determine aspects of the device.

Aspects of the devices which may be adapted to produce the neutral liquid particles include a size of the nozzle, the arrangement and spacing of the reference electrode relative to the nozzle electrode. The shape and other physical characteristics of the reference electrode may additionally have an influence.

For a particular device, is it easiest to vary the frequency for the alternating current and the maximum potential difference for the electric field. In an embodiment, the frequency has a larger role to play than the potential difference. Therefore, for a given device, the frequency may be adjusted to adapt the device to different liquids.

Furthermore, in those arrangements where the formation of a stream of neutral particles is determined by the frequency of the applied alternating current, an auto-tuning feature may be incorporated. Since, at low frequencies, the dispersion of the stream is such that charged particles with be attracted back to the reference electrode, this will create a current between the reference electrode and the nozzle electrode.

By continuously measuring the current and adjusting the frequency the device can be auto-tuned. When the current reaches a predetermined maximum level, the frequency can the set since, at that frequency, the stream of particles with be sufficiently focused. The predetermined maximum current level can be set according to a degree of dispersion required.

Figures 7 through 10 illustrate further embodiments. In the description which follows, like numbers are used to denote like features. Figure 7 illustrates an electrohydrodynamic atomization device 100 with an AC power supply 102. For ease of reference, the power supply 102 is denoted as a single entity, but it is to be realised that this is equivalent to the high voltage amplifier 20 and function generator 22 of the embodiment of Figure 1. Liquid is emitted from a nozzle 104 having an outlet 106. The nozzle acts as a nozzle electrode and a ring electrode 108 is provided upstream of the outlet 106 of the nozzle 104.

This embodiment differs from the embodiment of Figure 1 in that two pin electrodes 110 and 112 are provided on either side of the outlet 106. As illustrated the pin electrodes 110 and 112 are connected to the AC power supply 102 and therefore will have the same potential as the nozzle 104. The device 100 further comprises two fans 112 and 114. Fan 112 is situated behind pin electrode 112 and fan 114 is situated behind electrode 110. The pin electrodes 110 and 112 generate an electric field with the ring electrode 108. An ion cloud is formed by this electric field by attracting particles emitted by the nozzle outlet 106. The ion clouds are then propelled by the corresponding fans 112 and 114 to generate an ‘ion wind’ or a stream of ions which assists in the propagation of the atomized droplets at the outlet. Figure 8 illustrates an electrohydrodynamic atomization device 130 according to a further embodiment. The embodiment of Figure 8 includes a guard electrode 130 comprised of a dielectric such as plastic situated between the ring electrode and the outlet 106. Since the guard electrode 130 is comprised of a dielectric, it will not conduct electricity and will not attract the drops being emitted by the outlet 106 of nozzle 104. However, the guard electrode is connected to the AC supply 102 to have the same polarity as the ring electrode 108. Therefore, this may provide the same, or similar effect as the ring electrode on its own for a reduced voltage.

Figure 9 illustrates an electrohydrodynamic atomization device 150 according to a further embodiment. The device 150 of Figure 9 includes a guard electrode 152 and a ring electrode 154 which operate in the same manner as the guard electrode 132 and ring electrode 108 of Figure 8. The device 150 further comprises a housing 156 made from a dielectric (such as plastic) into which the ring electrode 154 and guard electrode 152 are incorporated. The housing 156 has two voids 158 and 160 formed therein. The voids 158 and 160 allow the directed ingress of air into the interior of the housing which may assist in the movement of the atomized liquid. The device may further incorporate a fan or other source of air pressure to assist in the movement of the atomized liquid.

Figure 10 illustrates an electrohydrodynamic atomization device 180 according to a further embodiment. The device 180 includes a collimating ring 182 provided downstream of the outlet 106 of the nozzle 104. The collimating ring is connected to the AC source 102 so that the nozzle 104 and the collimating ring 182 have the same charge at the same time. This may enhance the focus of the stream of particles.

The embodiments described may be used as nebulisers or, in an alternate embodiment, to deliver vaccines.

Although specific embodiments have been described, it is to be realised that many other embodiments are possible too. The design of the device may depend on the application.

Aspects of the devices which may be adapted to produce the neutral liquid particles include design aspects of the device such as a shape and size of the outlet of the nozzle, the size and shape of the reference and nozzle electrodes, the arrangement and spacing of the reference electrode relative to the nozzle electrode, and the number of nozzles, nozzle outlets, nozzle electrodes and reference electrodes. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments. Similarly, the word “device” is used in a broad sense and is intended to cover the constituent parts provided as an integral whole as well as an instantiation where one or more of the constituent parts are provided separate to one another.