GB2524337A - A method of forming a membrane, a membrane and an ultrasonic atomiser using the membrane - Google Patents

Abstract

A membrane (70 figure 1) for use in an ultrasonic atomiser comprising a plurality of orifices in a selected pattern with each orifice passing through the membrane from the front surface to the rear surface. The orifices are either a nozzle or a perforation, and the membrane consisting of at least 1 nozzle and at least 1 perforation; wherein the perforations have a ratio of entry diameter to exit diameter of less than 5 and a textured internal surface; and wherein the nozzles comprise; a cylindrical section having a smooth internal surface and, the diameter of the cylindrical section being the diameter of the perforations at the front surface plus at least twice the maximum peak height of the recast melt on the internal surface of the perforations. There is a neck section having a textured internal surface and which is shorter in length than the body section; and a ledge at the intersection of the body and neck sections. Also claimed is a method of forming orifices in a membrane.

Description

A METHOD OF FORMING A MEMBRANE, A MEMBRANE AND AN ULTRASONIC

ATOMISER USING THE MEMBRANE

Related art

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Aerosol generating devices of the vibrating membrane type are well known, they contain a membrane that has a number of nozzles, a piezoelectric actuator and a drive circuit. The actuator when driven at resonance by the drive circuit will vibrate the membrane. Bulk liquid in contact with the first face of the membrane will be subjected to varying hydraulic pressure and is pumped into the nozzles. When the membrane's amplitude of vibration is sufficiently large droplets will be ejected from the opposite face of the membrane creating an aerosol mist.

Membranes are typically made in a metal such as stainless steel and have nozzles created by a laser. Lasers do not produce a cylindrical hole in relatively hard metals, rather they are known to produce a tapered' hole whereby the entry hole has the largest cross sectional area and exit hole has the smallest cross sectional area. Therefore metallic membranes when in operation as part of an aerosol generation device fall into two broad categories.

Firstly those that have a forward taper whereby the nozzles decrease in cross sectional area from the liquid entry side to the liquid exit side. And secondly those that have a reverse taper whereby nozzles increase in cross sectional area from the liquid entry side to the liquid exit side.

The patent EP1152836B1 illustrates how ultrasonic atomisation can take place with a reverse taper membrane, it explains how a new lower frequency regime can take place than with the more common forward taper devices. According to this new regime the following frequency to entry hole diameter relationship holds true: f c [(3s)/(7pq3)h/2 The entry diameter of nozzles in the membrane = q The liquid to be atomised having a surface tension = 5 The liquid having a density = p The vibration frequency of the membrane = f Then the membrane is vibrated at the frequency f which is determined by the relationship so that droplets emerge from the exit surface with a diameter approximately equal to the diameter of the nozzle at the entry surface.

The patent is aimed at reducing the power consumption of a droplet generating device. The cost of droplet generator drive electronics is related to the frequency of operation, with higher frequencies associated with higher cost. The patent demonstrates how relatively large droplets can be produced at lower frequencies in comparison to a device containing a forward taper membrane. As the droplets are relatively large the device will spray the same amount of liquid as a similar forward taper membrane but at lower frequencies resulting in cheaper drive electronics.

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The invention requires the liquid to be subjected to a pressure bias such that the pressure of the bulk liquid at the container side of the membrane is less than the pressure on the air side from which droplets emerge. However we have found that adding a pressure bias is unsatisfactory in operation. In particular as the container holding the liquid empties the pressure within the container changes, this causes an alteration in the spray rate of the atomiserfor the same input of energy. Counteracting this pressure change is cumbersome adding to the complexity of the device and negating the value gained from cheaper drive electronics. It is consequently preferable to avoid using a pressure bias and alternate power saving methods found.

When spraying water at around 100KHz the equation noted above implies that there is a 40 micron entry hole diameter and exit hole greater than 40 microns. As the droplets are produced with a diameter approximately equal to the entry diameter of the nozzle the membrane can potentially produce droplets of 40 microns diameter at 1 00KHz. However with exit holes at or close to this diameter we have found that dripping becomes a problem. Liquid drips from the nozzles onto the surface of the membrane and is atomised from that surface, rather than directly from the nozzles themselves, this leads to uncontrolled droplet diameter.

In our invention this circumstance can be avoided by using smaller nozzles of below 40 microns entry and exit diameter with a preferred range of entry and exit holes of 20 -40 microns. Below 20 microns our nozzles are more difficult and expensive to produce as the reduction in size requires greater accuracy. While diameter ranges of 40-80 microns are not excluded from our invention the nozzle geometry must be adjusted to ensure dripping does not cause occur.

The patent US7316067B2 discloses a membrane and a technique to create it. The membrane can be used in a forward or reverse taper configuration in an aerosol generating device and is created via laser drilling and a subsequent electropolishing technique. A two step laser drilling process is used to create perforations in the membrane that have a larger cross sectional area called the diverging portion and a smaller cross sectional area called the throat portion. In the first step percussion laser energy strikes the first face of the membrane but is turned off before it passes right through the membrane, thus creating the diverging portion. In the second step the focal length and/or the energy level of the laser is changed, then the laser is turned on again and drills right through the membrane. In this way a ledge inside the perforation is created at the intersection between the diverging and throat sections of the nozzle. The laser drilling process is also controlled to create a low friction nozzle, this is achieved by altering the power and/or the focal distance of the laser during the creation of either or both of the portions so that they are coated with a smooth recast melt layer. Upon completion of lasering the perforations are electropolished in order to make the nozzles smoother internally and reduce burrs in the drill area. As the diameter and length of the throat portion strongly affects droplet size the process is controlled so that the throat portion of the perforations is left unaffected by electropolishing thereby protecting them from variation. This low friction regime allows the passage of liquid through the nozzles with less energy than a membrane that is coarser.

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A problem we have found with this technique is that even when recast melt is created in a controlled fashion and subsequently electropolished for the useful purpose of reducing friction and thereby power consumption it can still become dislodged from the nozzles. In the case of medical nebulisers the recast melt will contaminate medicine and this circumstance should be avoided. Therefore it is preferable for recast melt to be further removed rather than actively created.

A further problem is that the nozzle quality even after using this technique is still too low for those applications in which the visual impact of the aerosol mist is a factor. The quality of individual nozzles governs the distribution of droplet diameters in an aerosol mist, and even small variations in quality from one nozzle to the next, in particular of the cross sectional area of the throat section will produce a greater range of droplet sizes than is desirable. Passage of liquid through the nozzles is greatly affected by imperfections caused during the laser drilling process and a weakness in removing those imperfections with the subsequent electropolishing step. With a pulsed laser of the variety typically used to create nozzles in atomisers the imperfections regularly produced are broadly caused by recast melt and the tolerance of the laser itself The recast melt can cause partial blockages, burrs on the surfaces of the membrane and a ripple or undulating surface through the nozzle. Typical laser drilling techniques have a tolerance of 2-3 microns, with an exit diameter of 100 microns this represents just a 2-3% error, however with an exit diameter of 20 microns the same error is 10-15%, the preferred embodiments for our invention is in this lower nozzle diameter region. A potential 15% variation will adversely affect the visual appeal of the mist with the greatest visual appeal from the lowest variation. A further electropolishing step is intended to improve the nozzle quality, the electropolishing as outlined lasts around two minutes however even with a high current density this will only remove a small amount of material, typically 0.2 microns. With an error of 2-3 microns from laser drilling the electropolishing does not substantially improve the nozzle quality. Electropolishing can be continued for longer than two minutes however in this circumstance warping starts to occur because the membranes are typically less than 200 microns in thickness and stresses inherent in the metal start to become apparent. Consequently laser drilling and subsequent electropolishing in the fashion disclosed does not provide an aerosol mist with droplets of sufficiently consistent size.

One of the aims of this patent is to reduce power consumption, when used in a forward taper configuration the nozzles have a ledge at the interchange between the throat and diverging portions that decreases the cross sectional area of the nozzle from the liquid entry side to the droplet exit side thus restricting flow, as the ledge acts as a barrier to the passage of the fluid energy is wasted with wider ledges associated with larger energy wastage. To create the ledge a two step laser process is employed, the diverging portion of the nozzle is created first and then the focal length and/or energy of the laser is changed to create a narrower throat section. For example a membrane is 100 microns thick, laser energy is applied in pulses to create nozzles with a diverging portion of 100 microns diameter at the first surface and they taper to a diameter of 80 microns half way through the membrane before the laser is turned off. In the second stage the focal length of the laser is changed and the laser turned back on, S a throat section of 40 microns diameter is put through the membrane. This will create a ledge that is 20 microns wide. However if the same membrane has a diverging portion with a diameter of 30 microns at the first face and tapers to a diameter of 26 microns half way through, then with a throat diameter of 20 microns, the ledge shrinks to 3 microns. As the laser is only accurate to 2-3 microns ledges cannot be created in this fashion for smaller nozzles. Consequently this technique at higher nozzle diameters will produce ledges that are relatively large wasting energy in an atomiser while at lower nozzle diameters it will not produce a ledge at all.

The Current Invention The invention is defined in the accompanying claims.

This patent concerns a method of forming a membrane, a membrane and the use of that membrane in an ultrasonic atomiser. In order to solve the problems outlined above a new technique to create nozzles is required. That technique is now discussed, it begins with a laser drilling process and progresses to a reaming process. The laser process drills a plurality of perforations in a material and then the reaming process hones the perforations into the desired nozzle geometry. The reaming allows ledges to be created within the nozzles, recast melt to be removed from the nozzles and the quality of the nozzles to be optimised. The geometry in the nozzles is chosen to ensure that the smallest diameter section is not reamed and its length is minimised. The technique allows these features to be created in nozzles with exit and entry below 40 microns in diameter, which laser drilling alone would not allow. Upon creation of the membrane it is used in an ultrasonic atomiser. The atomiser allows relatively large droplets of liquid to be created in comparison to similar forward taper devices operating at the same frequency, with no dripping or pressure bias required.

Summary of the Manufacturing TechniQue

A summary of the technique to manufacture a membrane to solve the outlined problems is now noted.

1) A material with a given shape and thickness is chosen. A nozzle geometry is chosen to form a plurality of holes through the material forming a membrane, each nozzle will have 2 distinct but connected sections. Each distinct section can have a different internal shape, depth and dimensions with the section with smallest diameter chosen also to be the shortest in length.

2) Creation of perforations via laser drilling; The material is placed into a focussed and pulsed laser machine so that the machine can apply laser energy and drill through the material from the front surface to the rear surface creating a membrane with a plurality of perforations in a selected pattern. The energy and/or focal distance of the laser is changed after each pulse or a number of pulses so that the taper of the perforations from the entry to exit is reduced leaving the ratio of the entry diameter to the exit diameter less than 5 but greater than 1. The entry diameter being the diameter at the front surface and the exit diameter being the S diameter at the rear surface. The resulting perforation having a textured internal surface caused by the formation of recast melt and other imperfections.

3) Creation of nozzles via reaming; The membrane is placed into a reaming machine, the reamer is a metal rod which has micron size diamond shards bonded to its exterior. The machine rotates the reamer at high revolutions and lowers it into the perforation with accurate computer controlled force and depth. The diamond shards remove material via abrasion and hone the perforations improving nozzle quality, removing recast melt and create 2 sections, a reamed body section and an unreamed neck section with a ledge at the intersection of the 2 sections. The 2 sections comprising a body section which is cylindrical and smooth and a neck section having a textured internal surface which is shorter in length than the body section, the texture comprising leftover recast melt and other imperfections. The diameter of the reamer is chosen to be the diameter of the perforation at the front surface plus at least twice Rp the maximum peak height of the recast melt.

The laser process is now described, the laser is turned on and illuminates a spot on a membrane. If the power of the laser at the spot is sufficiently high it will cause ablation of the membrane. A percussion laser is preferentially used and over a number of pulses will progressively drill through the membrane. With each pulse not all of the laser energy causes ablation and recast melt is deposited in and around the illuminated spot. Recast melt is the re-solidification of molten material on the walls of the hole or on the surface of the membrane.

A ripple of recast melt can develop with each successive pulse of the laser, the ripple is created by recast being deposited in a ring above the ablation spot. As the laser strikes the membrane a heat affected zone surrounding the illuminated area is created, in this area the microstructure of the metal is altered due to that heating. The thermal stresses inherent in laser drilling make microcracks likely to occur, these may be confined to the recast layer but can also pass into the parent metal itself. The microcracks are particularly liable to fracture during operation in a device causing contamination of the liquid being atomised. The recast melt causes decreased accuracy and repeatability of perforation diameter and nozzle geometry, it can cause partial or full blockages of the nozzles and increases the coefficient of friction of the nozzles. The decreased reliability of perforation diameterwill feed through to decreased reliability in droplet diameter. The first laser pulse removes material at the illuminated spot on the surface of the membrane, the material removed matches that of the initial laser spot and the area below it. With further pulses the perforation becomes deeper and the area to be ablated no longer precisely in focus, has less power and becomes smaller.

Consequently the perforation drilled is not cylindrical and will tend towards a number of different tapered shapes, a champagne flute, a trumpet flute, or wine glass shape are amongst those created. In metallic materials the taper can be pronounced with dramatic reductions in diameter from one side of the membrane to the other, this is caused by the hardness of the metal and is a particular problem. Current thermal drilling techniques in the sub 30 micron range have an aspect ratio at best of around 8 in stainless steel. If a 200 micron thick stainless steel membrane is laser drilled a 25 micron entry hole may taper to an exit hole of around 5 microns. Removing stainless steel via abrasion is more time consuming and costly than via laser drilling it would therefore be preferable if the hole tapered instead from 25 microns to 15 microns. A goal of this invention is therefore to reduce the taper via S laser drilling such that the ratio of the entry diameter of the hole to the exit diameter of the hole is closer to 1 but remaining above 1 and to use the reaming to hone and re-shape the nozzles. A reduction in the taper can be achieved by changing the focal distance of the laser and/or changing the power of the laser between successive laser pulses. As an example a pulsed laser beam with a fixed power has a diameter of 10 microns at a chosen focal distance allowing 3 pulses to penetrate a membrane of 60 microns thickness. For the first pulse the laser is focused on the membrane's front surface nearest to the laser and a hole of depth 20 microns is drilled, then the membrane is raised 20 microns, now the laser is focused at a depth of 20 microns below the surface of the membrane, now the second pulse is sent and a further depth of 20 microns removed leaving the hole 40 microns deep. The membrane is now raised a further 20 microns and the third pulse sent, the laser penetrates the membrane creating an exit hole at the rear surface. The exit hole diameter is 9 microns. As a comparison the same laser is focussed on the surface of the same membrane, as before the beam is 10 microns in diameter at the surface and 3 pulses are sent from the laser, however the height of the membrane is not adjusted after each pulse, now the exit hole has a diameter of 6 microns. This result can also be obtained if the laser is initially focused at the bottom surface of membrane and then the membrane is lowered. As an alternative the focal distance can remain fixed at either surface of the membrane however after each pulse the power of the beam is increased or decreased. The two methods can be combined so that both the focal distance and the power can be changed after each pulse. This technique is integrated into the laser apparatus so that it is automated and allows the process to occur quickly and efficiently. The inventors have found that satisfactory laser drilling occurs in all circumstances wherein the perforations have a ratio of entry diameter to the exit diameter of less than 5 as they pass through the front surface to the rear surface.

Upon completion of laser drilling a next phase of micro reaming is undertaken. At its simplest a reaming tool is a cylindrical metal rod that has multiple diamond shards glued to its exterior, the reaming tool is inserted inside a hole that needs reshaping or smoothing and rotated, as diamond is a very hard material the shards in contact with the hole remove material via abrasion. An alternate technique is to create a reaming tool without bonding diamonds to the tool, however then the tool is lowered into the hole to be reshaped and an abrasive liquid containing micro sized diamond fragments is poured into the hole, as the tool is rotated the abrasive liquid removes material. A computer controlled mechanism adjusts the force, rotation speed, and depth of reaming. It also allows automation of the process for a plurality of perforations to be reamed without manual resetting. A specialist reaming service can be employed for example Microcut (RTM) Switzerland.

The inventors have found that subsequently reaming the hole can remove much of the recast melt left behind by laser drilling. Recast melt is deposited during the laser drilling process in three major ways, firstly as burring whenever the laser first strikes the membrane, secondly as a ripple on the interior of the hole and thirdly as a layer of metal that is uneven. With each pulse of laser light recast melt can be left at the hole resembling a ring torus, for the first pulse this leaves a burr around the entry hole on the surface of the membrane. For each subsequent pulse as the hole becomes deeper a new ring torus may be deposited at the S lowest depth, this leaves the completed hole with an undulating surface or ripple between the entry and exit surfaces. This recast melt can be understood as surface roughness of the hole interior. By convention every 2 dimensional surface roughness is denoted by a capitalised R followed by a lowercase letter denoting the exact parameter of roughness. The term Rp specifies the maximum peak height of a surface from its ideal form. If a hole created with a laser has a surface roughness Rp= 1 micron this signifies the recast has a maximum peak height of 1 micron, a reamer is then chosen to have a diameter that is 2 microns larger than the entry diameter of the laser drilled hole allowing for the removal of the recast. The same laser drilled hole when subsequently reamed can readily achieve a surface roughness of Rp=0.2 microns. The diameter of the reamer should be the largest diameter of the section being reamed plus at least twice the value of Rp to ensure substantial removal.

The regularity of the intemal geometry of the nozzles directly affects the volume and repeatability of ejected droplets. Atomisers can be used as medical nebulisers and droplets that are either too large or small may not be ingested correctly into the lungs leading to a reduction in the efficacy of the medicine. For other atomisers the visual appeal of the aerosol plume is important and a consistent droplet diameter improves this appeal. Passage through the nozzles is greatly affected by imperfections such as partial blockages that will reduce the volume of a particular nozzle such that less liquid will be available for atomisation in that particular nozzle. In regularising the volume of each nozzle any section with a taper can have it removed via reaming leaving that section cylindrical. Reaming will produce nozzles with much tighter diameter tolerance than laser drilling alone, laser drilling can produce perforations with a particular entry diameter tolerance of 2-3 microns however subsequent reaming can reduce the tolerance on this diameter to 0.5 microns. Removing burrs from the membrane surfaces is advantageous because the recast melt has a capillary like structure which can cause unwanted fluid flow over the surface of the membrane near each nozzle resulting in uncontrolled droplet creation in aerosol devices. The burring can be removed at the earlier laser drilling stage by the use of a mask, in this circumstance the mask is placed on the surface so that the laser energy will pass through it before striking the membrane, burring will then land on the mask which can be disposed of upon completion of laser drilling.

Alternatively a brush can be placed on the reamer above the micro diamond shards, as the reamer is lowered into the hole the brush will remove burring. The simplest method of removing burring is with a mask and this is the preferred method.

Ultrasonic atomisers can be utilised in portable devices and in order to make them easier to carry it is preferable to reduce the weight and the number of batteries they work with, consequently power saving is a goal of the present invention. Our invention utilises an ultrasonic atomiser comprising a membrane configured for traditional forward taper usage, in that the liquid entry diameter is greater than the droplet exit diameter. During our construction technique a ledge is created inside the nozzles and with each vibration cycle of the atomiser the liquid must cross this ledge which acts as a physical barrier thus wasting power.

Therefore a reduction in power consumption can be achieved by reducing the width of the ledges inside the nozzles. Larger ledges will inhibit the flow of liquid costing more energy than smaller ledges, therefore purely to reduce power consumption ledge size should be S minimised. As an example of ledge creation consider a membrane that is 150 microns thick, a laser drill creates a perforation in the membrane that tapers evenly so that it is 30 microns in diameter at the first surface, 20 microns halfway through and 10 microns at the second surface. Now a reamer with a diameter of 30 microns is lowered into the 30 microns diameter hole but only to a depth of 75 microns, material is removed and the resulting section is cylindrical. Now the membrane has a nozzle with 2 sections with one section 30 microns wide and 75 microns deep and the other section tapering from 20 microns down to 10 microns and microns deep. A ledge of width 5 microns has been formed at the interchange of the 2 sections. It is an aspect of this patent that narrow ledges can be created inside relatively deep nozzles with the exact size of the ledge adjusted on a per application basis, while smaller ledges are generally preferred larger ledges may be created to prevent dripping in some applications. A further mechanism to reduce power consumption is inherent in the reaming process, nozzles with walls that have low surface friction will also have a low viscous drag thereby increasing the volume of flow for a given power input.

The geometry of the nozzles at any point through the membrane include but not exclusively a diameter of 40 microns because above this level dripping becomes a problem. At this time reamers have a minimum diameter of 15 microns and as they approach this diameter the risk of the reamer breaking increases. A single breakage can cause the membrane to become a failure because the reamer may become lodged inside the membrane and impossible to retrieve, to mitigate this risk the smallest diameter section is left unreamed. The rate of flow in a nozzle is strongly determined by this unreamed section. This can be understood by reference to Poiseuilles's equation for capillary flow in laminar nozzles.

T= [(rrq4)/8m] [(v2-v1)Iw] Where T= flow rate of liquid q= radius of capillary/nozzle m=viscosity of fluid w= length of channel And (v2-v1)/w the average pressure gradient along the nozzle The rate of flow is affected by the length of the channel (w) and by the fourth power of the radius (q4) with shorter lengths associated with more flow. Therefore reducing the length of the smallest diameter section is an important feature for nozzle design. As the smallest diameter section is unreamed an advantage of reducing the length of this section is that a greater part of the nozzle is reamed thus removing more recast melt and regularising a greater length of the overall nozzle.

The term orifices is used in this patent to describe both perforations and nozzles, therefore as an example a membrane has a total of 1000 orifices; comprising 500 laser dhlled and reamed nozzles; and 500 laser drilled perforations that are not reamed. Each orifice passes through the membrane from the front surface of the membrane to the rear surface of the membrane S allowing the passage of liquid.

Membranes contain a plurality of orifices typically in the range of 1,000 to 3,000 in a selected pattern typically comprising an array of equilateral triangles. While each laser drilling step can be completed relatively quickly each reaming step takes longer. Reamers will fracture if the abrasion process is performed in a fashion that is too robust, this can be mitigated by removing material more slowly but this increases the machining time and cost. Consequently as a cost saving measure while all of the orifices in a membrane will be laser drilled some or all will be subsequently reamed. The extent of reaming will be determined by the final application, a medical nebuliser may require each nozzle to be reamed to remove recast melt, however an inexpensive air freshener may not have each nozzle reamed. In practise flexibility is required in the choice of the number of laser drilled perforations that are reamed in order to create nozzles and a completed membrane will consist of at least 1 nozzle and at least 1 perforation.

The membrane formed is for use in an ultrasonic atomiser in a traditional forward taper configuration whereby all of the orifices decrease in cross sectional area from the liquid entry side to the droplet exit side.

Summary of Diagrams

Fig 1: Diagrams illustrating the laser drilling of a membrane to create perforations.

Fig 2: Diagrams illustrating the subsequent reaming of the perforations to create a nozzle in the membrane.

Fig 3: Diagram illustrating a completed nozzle Fig 4a: Illustrates a top down view of a nozzle after laser drilling.

Fig 4b: Illustrates a top down view of a nozzle after reaming is completed.

Fig 5 shows a close up view of an array of nozzles in the membrane after reaming is completed.

Fig 6: Illustrates an ultrasonic atomiser.

Fig 7 & 8: Shows a membrane used in an ultrasonic atomiser from the front and also from the side.

Fig 9: Shows a close up of a membrane in operation highlighting the nozzles.

Detailed Example of the Manufacturing Technique The manufacturing technique to create a membrane with nozzles will now be described with reference to the diagrams. The diagrams are not held to scale in order to aid the description.

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Fig 1 a) shows a picture of a membrane [70], it is flat, 150 microns (um) thick and made of stainless steel. A laser beam [21] is shown prior to the beam striking the membrane for the first time, an arrow indicates the direction of the laser beam which is perpendicular to the front surface [72] of the membrane, the laser beam has a diameter of 34 microns at the point of contact, the laser is focused at the membrane surface so the maximum laser energy is at this surface. Fig 1 b) Shows the hole after 4 pulses of laser energy have struck the membrane. A partial hole [22] has been created and recast melt [23] has been deposited on the surface of the interior of the hole. The recast melt is indicated by the thick and uneven markings on the surface of the hole. The hole has an entry diameter at the front surface [72] of 34 microns and it is 50 microns deep. Burring has not occurred, however it can appear as deposits of stainless steel in a ring around the hole on the front surface, a mask can be placed on the surface to be drilled prior to drilling, such that the burring is then captured on the mask which is subsequently removed and disposed of, this does not alter the performance of the drilling, the mask is optional and not shown. Fig ic) shows the laser [21] just about to strike the membrane hole [22], the focus of the laser has been changed by raising the membrane in height by 50 microns, the new focal distance is indicated by the line [29] and is known by prior experimentation to be the lowest depth of the first hole. The laser will therefore offer the maximum power to the bottom of the hole. The diameter of the laser beam is unchanged. Fig ld) shows the hole created after a further 4 pulses of laser energy, the hole is now 100 microns deep and a ripple has been formed, this is noted in the recast melt [23] as four curves, hereafter the ripples are not shown. Fig 1 e) shows the laser just prior to striking the membrane once again, the focal distance of the laser [29] has been changed by raising the membrane by a further 50 microns, the maximum power density will now be incident on the bottom of the hole, the laser apparatus is otherwise unchanged. Fig if) shows the hole after a further 4 pulses of laser energy, the membrane has been penetrated so that the hole passes right through from the front surface to the rear surface and at this point can be described as a perforation. The recast melt [23] covers the entire internal surface of the perforation and is 1 micron thick at its maximum peak height. The exit diameter at the rear surface [71] is 22 microns giving a ratio of entry diameter to exit diameter of 1.55.

For comparison Fig ii) shows the circumstance whereby the laser remains focussed at the front surface to drill through the membrane with successive pulses, all other aspects of the process remaining the same, the entry diameter is still 34 microns however the exit diameter is 10 microns giving a ratio of entry diameter to exit diameter of 3.4.

Fig 2a) shows the membrane and perforation from Fig if) prior to reaming. The reamer [30] is shown above the perforation [22] and has a diameter chosen to be the diameter of the perforations at the front surface plus at least twice the maximum peak height of the recast melt on the internal surface of the perforations, in this case 36 microns, allovving for the removal of the recast melt layer. The reamer is centred to accurately match the perforation previously drilled by laser in the membrane [70]. The reaming tool has micron sized diamond shards [31] bonded to its surface. The reaming tool rotates at 1,000 revolutions per minute in the direction shown by the arrow [32]. Fig 2b) shows the reaming tool after it has been lowered normal to the front surface [72] into the perforation, it is lowered to a depth (a) that is 100 microns and left in place until abrasion is complete and then removed.

Fig 3) shows the completed nozzle. It has 2 sections a body section [73] and a neck section [75]. Internally the body section is smooth, cylindrical and symmetrical about the imaginary line [25] with a length (a) of 100 microns with its recast melt removed as indicated by the flat surfaces and thus offering a high degree of nozzle quality. The cylindrical section being the diameter of the perforations at the front surface plus at least twice the maximum peak height of the recast melt. A neck section of length (c) of 50 microns has been created having a textured internal surface comprising recast melt or other imperfections. The neck tapers from 26 microns in diameter at the depth (a) to 22 microns at the rear surface [71]. A ledge [24] has been created at the intersection of the body and neck sections that is 5 microns wide.

The body section is 36 microns in diameter and longer in the length than the neck section.

Fig 4a) shows a top down and close up view of the front surface [72] of the membrane after laser drilling has taken place so that the single perforation in Fig lfl is viewed. The perforation at the front surface is represented by the uneven and bounded line [27] and is 34 microns in diameter. The ripple and taper through the perforation is represented by two uneven and bounded lines [28]. The 22um exit hole of the laser at the rear surface is represented by the bounded line [26]. Variation in the laser drilling has left the perforation unsymmetrical, it has a taper, recast melt and it is not cylindrical. Fig 4b) shows a top down view of the front surface [72]. Both laser drilling and reaming of the hole has taken place to leave a completed nozzle allowing comparison with the perforation in Fig 4a) and demonstrating the effect of reaming. Notice the bounded line [27] has now become circular.

This indicates that internally the body section has become smooth, regular and cylindrical with a new diameter of 36 microns, the taper, ripple and recast melt have been removed while nozzle quality has been optimised. The bounded lines [25] are no longer present indicating a ledge has been created between the newly created body section and the neck section that has a width of 5 microns.

Fig 5) shows a close up of the front surface [72] of the membrane that illustrates an array of nozzles. The array is formed from a plurality of nozzles [80] placed to form an equilateral triangle, an equilateral triangle pattern is chosen because it allows the nozzle diameter [81] to be increased close to half of the distance between nozzles [82] thus maximising the potential number of nozzles that can be packed into the array. Nozzle diameter is set at 36 microns and one side of the equilateral triangle [82] has a length of 150 microns.

In order to create the membrane a plurality of orifices must be created, the orifices comprise either a perforation created by laser energy alone or nozzles created by laser energy and the subsequent reaming of the perforations. The membrane is formed by reaming some or all of the perforations. In this example laser energy is applied to the material in order to laser drill 1,500 perforations arranged in an array central to the membrane, the number of nozzles required is 900 which are then reamed centrally to the membrane leaving 1,500 orifices comprising 900 nozzles and 600 perforations. Both the laser drilling and reaming are controlled electronically to automate the production process. The laser drills all 1,500 S perforations as noted in fig la) to it) consecutively. The reaming is then carried out with 900 perforations reshaped consecutively as noted in Fig 2) leaving 900 complete nozzles as indicated in Fig 3) and 600 perforations as indicated in Fig if). For the membrane as produced and used in an ultrasonic atomiser flexibility is required in that amongst a plurality of orifices there will be at least 1 nozzle and at least 1 perforation. In those membranes in which perforations are large in number the flow characteristics of the membrane will be altered in comparison to a similar membrane with a low number of perforations because the perforations are textured internally from the front to the rear surface, the texture comprising recast melt, ripple, partial blockages and other imperfections.

The preferred range of diameters for orifices in the membrane is between 20 and 40 microns for both entry and exit diameters. Orifices with both an entry and exit diameter below 20 microns have a particular problem to overcome in that during the reaming process the reamer with a known minimum diameter of 15 microns is more likely to break and may become lodged in the membrane causing failure of the part. For example a membrane is 50 microns thick and has been laser drilled to create a perforation that has an entry diameter of 13 microns and an exit diameter of 10 microns, a 15 microns reamer is chosen to ream the 13 microns entry diameter and create a nozzle, the reamer in this case is more likely to break than a perforation that has an entry diameter of 28 microns and an exit diameter of 20 microns with a 30 microns reamer chosen to create the nozzle in the same membrane. This problem becomes acute with increasing material hardness or thickness. The solution is to ensure that the laser drilling removes much more material than with perforations that are in the higher diameter range. The inventors have found that the ratio of entry to exit diameter of the perforations when below 1.2 after the application of laser energy is satisfactory. In this circumstance the completed nozzles are different from those in the region of 20 to 40 microns. Their geometry approaches that of a tunnel and can be described as tunnel like, in that the droplet exit diameter may be just 1 micron smaller than the liquid entry diameter. The body of the nozzle named because it contains the majority of fluid in transit is cylindrical, smooth and symmetrical and after reaming has a diameter as with the other orifice diameter range embodiments that is the diameter of the perforations at the front surface plus at least twice the maximum peak height of the recast melt on the internal surface of the perforations.

The neck section of the nozzle remains unreamed and is textured internally in that it contains, imperfections, such as recast melt, uneven metal grains, partial blockages and ripple, the neck tapers in that the cross sectional area decreases from the intersection with the body to the droplet exit surface. In these tunnel like nozzles the ledge at the intersection of the neck section and body section while present and not zero is less than 1 micron wide.

Membranes with orifice entry and exit diameters between 40 and 80 microns can have a problem with dripping, however we have found that operation is satisfactory when nozzles have a ledge at the intersection of the body and the neck sections that is at least 5 microns wide when the ratio of entry to exit diameters of the laser drilled perforations is less than 1.5.

With perforations above 1.5 and below 5 it is found the rapid change in the ratio of entry to exit diameter can act as a barrier helping prevent dripping and a specified minimum ledge width is not required. With a ledge of at least 5 microns width the nozzles created are syringe like in shape (with plunger and handles removed) in that they have a cylindrical body section then a pronounced reduction in diameter constituting the ledge and an attached neck section that reduces in cross sectional area from the ledge to the rear surface. Dripping is found not to be a problem because the ledge acts as a barrier preventing the excessive flow of liquid.

The ledge inhibits the flow of liquid thus reducing the electrical efficiency of an atomiser incorporating such nozzles, however there are advantages in working at higher diameters.

The advantages include laser drilling that is quicker, utilising less expensive laser apparatus and fewer breakages of the reamer.

Creation of an Ultrasonic Atomiser using the Invented Membranes The membranes created via the techniques noted in Fig 1 to Fig 5 above are to be used in an ultrasonic atomiser. This can include aerosol generators, humidifiers, inkjet devices and other apparatus. The atomiser converts liquid in contact with it into liquid droplets emergent from it.

Liquid is understood to include pure liquids, mixtures of liquids, solutions, and suspensions of particles in liquids. All of the membranes are used in the forward taper configuration in that the resulting ultrasonic atomiser has orifices that decrease in cross sectional area from the liquid entry side to the droplet exit side. The resulting atomiser has a number of advantages over prior art devices. There is no pressure bias needed during the operation of our atomiser and there is no dripping from the nozzles with its consequent uncontrolled atomisation. If desired an atomiser apparatus can have relatively large droplets up to 40 microns diameter or a majority of droplets can have a diameter of over 30 microns while operating at frequencies below 150KHz, something not hitherto observed in forward taper devices.

Relatively larger droplets allow a greater rate of spray for a given power level and a small internal ledge helps reduce the inhibition of liquid flow, this results in fewer batteries and a more portable device. The lower operational frequency allows for cheaper drive electronics with a consequent reduction in per unit manufacturing cost. The regularised nozzle geometry allows for a more consistent droplet diameter which improves the visual appeal of the aerosol mist. Recast melt can be removed such that it poses no risk of medicine contamination.

Detailed example of an Ultrasonic Atomiser using the invented membrane One example of how the membranes created in Fig 1 to Fig 5 can be used in an ultrasonic atomiser is now detailed. The present invention is not intended to be limited to the exact construction technique described below.

Fig 6 shows a droplet generating apparatus [60] comprising an ultrasonic atomiser [50] that is held in a container [61] by a soft silicone rubber (not shown), the atomiser is able to vibrate and is not clamped, the container has 3 solid sides and holds a liquid [62] that is in contact with the ultrasonic atomiser, liquid is only able to exit the container through the atomiser. The drive electronics with integrated power supply [63] supply a sine wave of 100V peak to peak amplitude and frequency 100KHz through the power leads [64] to the atomiser. The atomiser is operated in the bending mode at a frequency just below its resonant frequency producing a vibration known to produce the highest rate of atomisation. There are a range of frequencies S from which atomisation can take place, for the part described they are 90-110KHz. Liquid enters the atomiser and exits as droplets [65]. No pressure bias is applied.

Fig 7 shows the ultrasonic atomiser from the front while Fig 8 shows the same part from the side. The ultrasonic atomiser [50] is comprised of a circular stainless steel ring [51], the stainless steel membrane [70] is given two arrows that are used to illustrate that it is a single concentric disc of stainless steel however the central part has been machined by laser drilling and reaming and contains an array of nozzles [54] through which liquid is atomised. The membrane will be understood to comprise nozzles and perforations centred around the middle of the membrane. The membrane and the stainless steel ring are bonded to a piezoelectric ring [52] using an electrically conducting epoxy resin. The stainless steel ring and membrane are not in physical contact. Power from the drive circuitry passes through the power leads (not shown) and enters the piezoelectric ring via the electrode [53] and via the stainless steel ring to a second electrode on the underside of the piezoelectric ring (not shown). The stainless steel ring acts as a mount allowing relatively unrestricted movement of the membrane while being vibrated by the piezoelectric ring. The stainless steel ring has an outside diameter of 30mm and an inside diameter of 18mm. The piezoelectric ring is made from PZT type Navy VI and has an outside diameter of 23mm and an inside diameter of 15mm. The membrane consists of a stainless steel disc with a diameter of 17mm with the array of orifices having a central area covering a 6mm diameter. Both the stainless steel ring and membrane are 0.15mm thick.

Fig 9 shows a close up and cut away diagram of Fig 6 focusing on the array of the orifices in the membrane. For illustrative purposes the membrane contains three nozzles although it will be understood that in actuality it would contain many orifices. The nozzle is that shown in Fig 3) with the dimensions noted in the discussion of Fig 3. Fluid [62] enters the front surface [72] of the membrane via the nozzles [80]. The membrane is subjected to a bending mode vibration by the piezoelectric ring during operation and the nozzles empty in a cyclic fashion.

The droplets [65] have a mean diameter of 35 microns and exit the nozzles from the rear surface [71] and are propelled away from the membrane forming an aerosol mist.