ATOMISING A CONDUCTING LIQUID

This invention relates to apparatus and a method for atomising a conducting liquid. More especially, this invention relates to apparatus and a method for atomising a conducting liquid using electrostatic forces.

When a sufficiently high voltage is applied to a conducting or semiconducting liquid droplet, emerging from a capillary tube at very low flow rates, the liquid becomes charged. Electrostatic forces counteract the surface tension and the droplet is stretched. At a critical point, the stretched droplet becomes conical and a fine stream of liquid ejects from its tip. The conical droplet is known, in the field of electro-hydrodynamics, as the Taylor cone.

This electrostatic phenomenon has found uses in the field of electrospinning. In electrospinning, a conducting and viscous polymer solution is pumped, at extremely low flow rates, through a hypodermic needle maintained at high voltage, typically 5 - 30kV. As a result of the electrostatic forces, the Taylor cone is formed and a very fine jet ejects from its tip, whipping or spinning around. As the solvent evaporates, nano and micro fibres are collected on an earthed target. These fibres are used in textile and tissue engineering and in the production of filters. The very low flow rates involved in this process make it ideal for the production of these nano and micro fibres. To increase the production of these fibres, multi hypodermic needles are used in a variety of configurations.

For low viscosity liquids, the liquids stream emerging from the Taylor cone breaks up into fine droplets. This phenomenon has found uses in the atomisation of liquids such as oils, fuels, agrochemicals and sprayable paints. However, to achieve the higher flow rates required by these uses, the liquid has to be semi-conducting to maintain the electrostatic field along the droplet and to form the Taylor cone. This is a disadvantage of these uses.

An important criterion for obtaining the Taylor cone, with a conducting liquid, is for the flow rate to be extremely low. If the liquid flow rate is increased, the surface of the droplet will reach an equipotential instantly, preventing the formation of the Taylor cone. Instead, spluttering and sparking occurs at the surface of the droplet. If the liquid flow rate is further increased so that a jet emerges from the capillary tube instead of a droplet, the electrostatic forces will act on the surface of the jet causing it to whip erratically and break up into very large droplets. If the electrified conducting jet touches the earthed target, a short circuit will result and the electrostatic field will be switched off.

It is an aim of the present invention to reduce or overcome the problems associated with the application of electrostatic fields to a conducting liquid jet emerging from a capillary tube, and to cause it to atomise into fine droplets.

Accordingly, in one non-limiting embodiment of the present invention there is provided apparatus for atomising a conducting liquid, which apparatus comprises a capillary tube, an electric motor, means for applying a high voltage, and feed means for feeding the liquid along the capillary tube, and the apparatus being such that the capillary tube has an inlet for the liquid and an outlet that is gyrated using the electric motor, the means for applying the high voltage applies the high voltage to the capillary tube, the liquid is feedable along the capillary tube from the inlet to the outlet and the feed means is operable to provide a liquid flow rate which is sufficiently high that in use the liquid emerges from the outlet in the form of a jet which is gyrated into a liquid spiral by the electric motor and which breaks up into droplets.

The apparatus of the present invention may be especially useful for atomising conducting liquids, for example wood care coatings and decorative paints, using electrostatic forces.

The apparatus may be one in which the electric motor has motor gears, in which the capillary tube is gyrated using the motor gears, in which one motor gear is attached to a motor shaft, in which another motor gear is attached to the outlet, and in which the outlet passes through an offset drive bush housed inside a gear driven bearing.

Alternatively, the apparatus may be one in which the electric motor is a hollow shaft electric motor, and in which the hollow shaft electric motor gyrates the capillary tube such that the inlet passes freely through a support bush, and the outlet passes through an offset drive bush.

The apparatus may be one in which, in use, the end of the liquid spiral whips erratically upon the application of the high voltage and breaks up into very fine electrostatically charged droplets. The feed means may be a peristaltic pump. Other types of feed means may be employed.

In another non-limiting embodiment of the present invention, there is provided a method for atomising a conducting liquid using apparatus comprising a capillary tube, the capillary tube having an inlet for the liquid and an outlet that is gyrated using an electric motor, and the method comprising applying a high voltage to the capillary tube, using feed means to feed the liquid along the capillary tube from the inlet to the outlet, and operating the feed means such that the liquid flow rate is sufficiently high that the liquid emerges from the outlet in the form of a jet which is gyrated into a liquid spiral by the electric motor and which breaks up into droplets.

The method of the present invention may be useful for atomising conducting liquids, for example wood care coatings and decorative paints, using electrostatic forces.

The method of the present invention may be one in which the capillary tube is gyrated using motor gears, in which one motor gear is attached to the motor shaft, and in which another motor gear is attached to the outlet end of the capillary tube which passes through an offset drive bush housed inside a gear driven bearing.

Alternatively, the method of the present invention may be one in which the electric motor is a hollow shaft electric motor, in which the capillary tube is gyrated using the hollow shaft electric motor, in which the inlet passes freely through a support bush, and in which the outlet passes through an offset drive bush. The method may be one in which the end of the liquid spiral whips erratically upon the application of the high voltage, and breaks up into very find droplets.

The method of the present invention may be one in which the feed means is a peristaltic pump.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 shows first apparatus of the present invention for atomising liquids;

Figure 2 illustrates part of the operation of the apparatus as shown in Figure 1 ;

Figure 3 illustrates a further part of the operation of the apparatus as shown in Figure 1 ;

Figure 4 shows second apparatus of the present invention for atomising liquids;

Figure 5 shows third apparatus of the present invention for atomising liquids;

Figure 6 illustrates two different types of capillary tips for use in apparatus of the present invention; and

Figure 7 shows apparatus of the present invention connected to a supply of liquid, with the liquid being in the form of paint.

Referring to the drawings, Figure 1 shows first apparatus 2 of the present invention for atomising a conducting liquid 4. The apparatus 2 is shown in a state when a jet 6 is emerging from a capillary tip 8 of a stationary capillary tube 10. The jet 6 breaks up into very large droplets 12. The tube 10 is connected to a high voltage source 14. In Figure 1 , the high voltage source 14 is not providing any voltage to the conducting liquid 4.

Figure 2 shows how the apparatus 2 shown in Figure 1 is able to gyrate as shown by arrow 16 to produce a spiralling jet 18 and large droplets 20. The gyration is produced as a result of an electric motor (not shown) gyrating the capillary tip 8 of the tube 10. The high voltage source 14 is still not providing any voltage to the conducting liquid 4.

Figure 3 shows how the apparatus 2 is able to operate to provide erratic break-up of the spiralling jet 18 into fine droplets 22, this being after the high voltage source 14 applies a high voltage, for example 15kV, to the conducting liquid 4.

Figure 4 shows second apparatus 24 of the present invention. More specifically, Figure 4 illustrates apparatus 24 and a method for causing a capillary tube 26 to gyrate using an electric motor 28 together with gears 30, 32. One gear 30 is attached to a shaft 34 of the electric motor 28. The other gear 32 is attached to a liquid outlet end 36 of the capillary tube 26. The liquid outlet end 36 of the capillary tube 26 is then passed through an offset drive bush and bearing arrangement 38 which is housed inside the gear 32. The offset drive bush and bearing arrangement 38 may be offset at an angle of 1 - 5°. When the electric motor 28 is switched on, the liquid outlet end 36 of the capillary tube 26 starts to gyrate as shown by the arrow 40, and the liquid emerges as a spiralling jet 42 which breaks up into droplets 44. In order to generate a liquid spray of the charged droplets 44, a conducting liquid 46 is passed through the capillary tube 26. The capillary tube 26 is gyrated by switching on the electric motor 28. A high voltage is applied using a high voltage power supply. The conducting liquid 46 is pumped through the capillary tube 26 by a pump (not shown). The conducting liquid 46 ejects from the tip 36 of the capillary tube 26 in a spiral that whips erratically at its tip, and that breaks up into the finally charged droplets 44. The capillary tube 26 is held in position as shown by a support clamp 50.

Figure 5 shows third apparatus 52 of the present invention. The apparatus 52 has a capillary tube 54. A hollow shaft motor 56 is used to rotate the capillary tube 54, with a support bush 58 being positioned at one end of the hollow shaft motor 56, and an offset drive bush 60 being positioned at the other end of the hollow drive shaft motor 56. The capillary tube 54 is allowed to pass freely through the support bush 58 at the liquid inlet end of the capillary tube 54 so that the capillary tube 54 does not rotate. At the liquid outlet end, the capillary tube 54 passes through the offset drive bush 60, for example at an angle of 1 - 5°. When the hollow shaft motor 56 is switched on, the capillary tube 54 gyrates and the liquid outlet end to produce a spiralling jet 62 and find droplets 64.

In the apparatus of the present invention, the extensional viscosity of the conducting liquid has an important role in ensuring that the tip of the spiral elongates, and becomes very thin before breaking up into fine droplets. The diameter of the capillary tube and the resulting liquid jet also have a major influence of the droplet size of the spray. In order to reduce the diameter of the jet emerging from the capillary tube, plastics inserts can be fitted to the tip of the capillary tube. The plastics inserts may have a hole or a plurality of holes of the required size, see for example the two embodiments shown in Figure 6. More specifically, in the left hand embodiment of Figure 6, there is shown a conducting liquid 66 passing along a capillary tube 68. The capillary tube 68 has a capillary tip insert 70. The capillary tip insert 70 has a single aperture 72 which forms a single liquid outlet. In the right hand embodiment shown in Figure 6, similar parts have been given the same reference numerals for ease of comparison and understanding. It will be seen that the capillary tip insert 70 has multiple apertures 72 forming multiple liquid outlets.

The projection angle of the spray depends on the offset angle of the drive bush. A small offset angle will result in a narrower spray projection. Another method for changing the spray projection for a fixed offset angle, is to pull the capillary tube backwards, or push the capillary tube forwards, in order to obtain either a narrower or a wider spray projection.

Typical apparatus of the present invention that is able to be used in the method of the invention may have a 15cm capillary tube with an outside diameter of 3mm and an inside diameter of 2mm. A plastics insert placed a the tip of the capillary tube may have a hole diameter of 0.6 - 0.9mm. The capillary tube may be connected to a high voltage power supply that can supply up to 30kV voltage and is driven by 12 or 24 volts. The capillary tube may be driven by a 12 or a 24 volt electric motor having a 12W power output, and up to 10,000rpm speed. The capillary tube may pass through an offset bush to form a 1° angle. The motor may be 32mm long, 28mm in diameter, and may have a shaft diameter of 2.5mm. If a hollow shaft motor is used, then the diameter of the hollow shaft can typically be from 10mm in diameter to allow for the bushes to be inserted.

Typical operating conditions for the apparatus and method of the present invention are for the conducting liquid to have a conductivity >10 ^"6 S/m and be pumped at 4 - 5 bar pressure through the capillary tube at a flow rate of 200ml per minute. At the application of typically 20kV high voltage to the capillary tube, and with a motor speed of 3,000rmp, a spray may be obtained having a droplet size ranging from 20 - 300pm. The liquid may be a water based sprayable paint such for example as an exterior wood coating, for example for use for wood fencing, or a sprayable masonry or decorative paint.

Referring now to Figure 7, there is shown apparatus 74 comprising a spray gun 76 having a handle 78. A high voltage generator 80 supply is housed in the handle 78. A barrel part 82 of the gun houses a capillary tube 84 and an electric motor 86. A liquid in the form of a paint spray 88 is shown emerging from the spray gun 76 and in the form of a spray. Liquid paint 90 for the spray gun 76 is shown contained in a container 92. Feed means in the form of a peristaltic pump 94 is provided on a lid 96 for the container 92. A tube 98 passes through the peristaltic pump 94 and into the container 92. The tube 98, and an electric cable 100 for providing power for the electric motor 86 and the peristaltic pump 94, may be, for example, 4m long. The peristaltic pump 94 may operate on a rechargeable 12 or 24V power supply 102 and/or a mains power supply. The peristaltic pump 94 is shown connected to a lead 104 for connection to a mains power supply.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Individual components shown in the drawings are not limited to use in their drawings and they may be used in other drawings and in all aspects of the invention.