Droplet formation device and method for driving discharge head

Abstract

The present invention provides a droplet formation device comprising a discharge head that discharges a droplet of a liquid and a control unit that supplies an electric signal so as to control the operation of the discharge head, wherein: the discharge head has a tubular liquid holding part that holds the liquid and a vibration part that discharges the droplet on the basis of the electric signal; the vibration part has a film-like member that has a discharge port through which the droplet is discharged and a vibration means that vibrates the film-like member on the basis of the electric signal; the vibration part covers one end side of the liquid holding part and forms, together with the liquid holding part, a liquid chamber that holds the liquid; in the liquid chamber, the other end side of the liquid holding part is open to the atmosphere; and the control unit supplies, to the vibration means, the electric signal having a driving frequency represented by formula (1). (1): [Driving frequency]=([resonance frequency of structural vibration of vibration means] / n)×a (where n is an integer of 1-100, and a represents 0.9-1.1)

Description

Droplet forming device and ejection head driving method

[0001] The present invention relates to a droplet forming device and a method for driving a discharge head.This application claims priority to Japanese Patent Application No. 2023-123889, filed on July 28, 2023, the contents of which are incorporated herein by reference.

[0002] 2. Description of the Related Art Conventionally, inkjet droplet forming devices are known as a technique for ejecting a liquid such as ink onto a desired position.

[0003] In recent years, droplet generation devices have been required to eject various liquids in place of the inks used in conventional two-dimensional printing. For example, liquids to be ejected include dispersions as well as solutions. Dispersoids (particles) contained in dispersions include organic materials such as resin materials, inorganic materials such as metal particles and oxide particles, and biomaterials such as cells and genes.

[0004] Non-Patent Document 1 discloses the configuration of a droplet ejection head that has an ink storage section that is open to the atmosphere, and that forms droplets while stirring the dispersoid in the ink by vibrating a film-like member that is provided at the bottom of the storage section and has an ejection port.

[0005] In the configuration described in Non-Patent Document 1, droplets are formed by vibrating a film member in accordance with the resonance frequency of the ink stored in the storage section. However, in this case, if the amount of ink stored in the storage section changes, the resonance frequency of the ink changes, making it difficult to form droplets. To continuously form droplets with the configuration described in Non-Patent Document 1, it is necessary to change the drive signal (waveform) that vibrates the film member depending on the amount of ink, making control difficult.

[0006] The present invention has been made in view of the above circumstances, and has an object to provide a droplet forming device that can easily and stably form droplets, and a method for driving a discharge head that can easily and stably form droplets.

[0007] In order to solve the above-mentioned problems, one aspect of the present invention provides a droplet forming device including a discharge head that discharges droplets of liquid and a control unit that supplies an electric signal to control the operation of the discharge head, wherein the discharge head includes a cylindrical liquid holding unit that holds the liquid and a vibration unit that discharges the droplets based on the electric signal, the vibration unit including a film-like member having an outlet through which the droplets are discharged and a vibration unit that vibrates the film-like member based on the electric signal, the vibration unit covering one end of the liquid holding unit and forming, together with the liquid holding unit, a liquid chamber that holds the liquid, the other end of the liquid holding unit being open to the atmosphere, and the control unit supplying the electric signal to the vibration unit having a drive frequency expressed by the following formula (1): [Drive frequency] = ([Resonant frequency of structural vibration of the vibration unit] / n) × a (1), where n is an integer from 1 to 100 and a is 0.9 to 1.1.

[0008] According to the present invention, it is possible to provide a droplet forming device capable of easily and stably forming droplets, and also to provide a method of driving a discharge head capable of easily and stably forming droplets.

[0009] FIG. 1 is a schematic diagram of a droplet forming device 1. FIG. 2 is a schematic diagram of a discharge head 110. FIG. 3 is a partial enlarged view of the discharge head 110. FIG. 4 is an explanatory diagram of a method for measuring the resonance frequency of the structural vibration of the vibration means. FIG. 5 is an example of a vibration spectrum obtained by measurement. FIG. 6 is a diagram showing an example of a drive waveform set from the measurement results of FIG. 5.

[0010] The droplet forming device and the method for driving the ejection head according to this embodiment will be described below with reference to Figures 1 to 6. Note that in all of the following figures, the dimensions and proportions of the components have been changed as appropriate to make the drawings easier to understand.

[0011] 1 is a schematic diagram of a droplet forming device 1 according to this embodiment. As shown in FIG. 1, the droplet forming device 1 includes a discharge unit 10, an attachment unit 30, a placement unit 40, and a control unit 50.

[0012] In the following explanation, an xyz Cartesian coordinate system is set, and the positional relationships of the components are explained with reference to this xyz Cartesian coordinate system. Here, a predetermined direction in a horizontal plane is defined as the x-axis direction, a direction perpendicular to the x-axis direction in the horizontal plane is defined as the y-axis direction, and a direction perpendicular to both the x-axis and y-axis directions (i.e., the vertical direction) is defined as the z-axis direction.

[0013] In addition, the vertically upward direction is the +z direction, and the vertically downward direction is the -z direction. In the following description, the "upper" in "upper" and "upper surface" and the "lower" in "lower" and "lower surface" have the same meaning.

[0014] Furthermore, in the following description, "planar view" refers to viewing an object from above (+z direction), and "planar shape" refers to the shape of an object viewed from above.

[0015] <<Discharge Section>> As shown in FIG. 1 , the discharge section 10 includes a discharge head 110 and a conveying means 120 .

[0016] <Ejection Head> The ejection head 110 ejects droplets L1. The ejection section 10 may have only one ejection head 110, or may have multiple ejection heads 110. The ejection section 10 shown in the figure has three ejection heads 110a, 110b, and 110c. The three ejection heads 110a, 110b, and 110c are collectively referred to as an ejection unit 110L.

[0017] The ejection heads 110a, 110b, and 110c may have the same configuration or may have different configurations.

[0018] The three ejection heads 110a, 110b, and 110c are arranged in a direction (x direction in the drawing) that intersects with the ejection direction of the liquid ejected from the ejection head 110 (-z direction in the drawing).

[0019] 2 is a schematic diagram of the ejection head 110. The ejection head 110 includes a liquid holding portion 111, a vibration unit 115, and a fixing member 117.

[0020] The space surrounded by the liquid holding portion 111 and the vibration portion 115 is a liquid chamber 110A of the ejection head 110. The liquid chamber 110A holds a liquid (liquid L) that is the source of the droplets L1.

[0021] The amount of liquid L held in the liquid chamber 110A is not particularly limited. For example, the amount of liquid L held in the liquid chamber 110A can be approximately 1 μl to 1 ml. When an expensive liquid such as a cell suspension is ejected from the droplet forming device 1, the amount of liquid L held in the liquid chamber 110A should be approximately 1 μl to 200 μl.

[0022] There are no particular limitations on the liquid L that is discharged by the droplet forming device 1, and it can be appropriately selected depending on the purpose. Examples of the liquid L include pure water (ion-exchanged water, distilled water), physiological saline, alcohol, various organic solvents such as mineral oil, and vegetable oil.

[0023] The liquid L may be a dispersion liquid in which particles are dispersed.

[0024] Examples of particles contained in the liquid L include organic materials such as polymer particles, metal fine particles, and inorganic materials such as inorganic oxide particles. Examples of metal fine particles include silver particles and copper particles. Examples of inorganic fine particles include titanium oxide particles and silicon oxide particles.

[0025] Cells can also be used as particles. Plant cells and animal cells can be used as cells. Animal cells can particularly include cells of human origin.

[0026] When the liquid L is a dispersion liquid, examples of the dispersion medium include water, alcohol, etc. The dispersion medium may contain a wetting agent to suppress evaporation or a surfactant to reduce surface tension. When the particles are cells, examples of the dispersion medium include known buffer solutions such as phosphate buffered saline and Hank's balanced salt solution, and culture media for various cells.

[0027] The ejection heads 110a, 110b, and 110c may hold the same liquid L, or may hold different liquids L from each other.

[0028] (Liquid Retaining Portion) The liquid retaining portion 111 is a tubular member with both ends in the z-axis direction open. The liquid retaining portion 111 may be, for example, a cylindrical member. Examples of materials for the liquid retaining portion 111 include metals such as stainless steel, nickel, and aluminum; plastics (resin materials) such as ABS, polycarbonate, and fluororesin; ceramics such as silicon dioxide, alumina, and zirconia; and silicon.

[0029] One end of the liquid holding part 111, that is, the lower end, is covered and blocked by the vibration part 115. The other end of the liquid holding part 111, that is, the upper end, is open to the atmosphere. When the upper part of the liquid holding part 111 is open to the atmosphere, the liquid L held in the liquid holding part 111 is less likely to be pressurized when droplets are ejected, and damage to cells can be suppressed.

[0030] (Vibration Unit) The vibration unit 115 has a nozzle plate (film-like member) 112 and vibration means 113. The vibration unit 115 shown in Fig. 2 has the vibration means 113 located above and the nozzle plate 112 located below, but this is not limited thereto, and the nozzle plate 112 may be located above and the vibration means 113 below.

[0031] (Nozzle Plate (Film-Like Member)) The nozzle plate 112 is a film-like member having ejection ports 112x. The nozzle plate 112 closes the lower end of the liquid holding portion 111, and together with the liquid holding portion 111 forms a liquid chamber 110A that holds the liquid L. The ejection ports 112x are in communication with the liquid holding portion 111.

[0032] There are no particular limitations on the planar shape, size in plan view, material, and structure of the nozzle plate 112, and they can be selected appropriately depending on the purpose.

[0033] Examples of the planar shape of the outer edge of the nozzle plate 112 include a circle, an ellipse, a rectangle, a square, a diamond, etc. For example, when the shape of the outer edge of the nozzle plate 112 is a circle, the nozzle plate 112 becomes an annular member.

[0034] As an example, the nozzle plate 112 may be a circular member having a diameter of 20 mm and an average thickness of 0.05 mm.

[0035] The nozzle plate 112 has an end portion on the ejection port 112x side that is not supported. The end portion on the ejection port 112x side can vibrate up and down. When the end portion on the ejection port 112x side of the nozzle plate 112 vibrates, a downward force is applied to the liquid L near the ejection port 112x, causing the liquid L to be ejected as droplets L1 from the ejection port 112x.

[0036] If the nozzle plate 112 is made of a material that is too soft, it will vibrate easily and it will be difficult to immediately stop the vibrations when no ink is being ejected. Therefore, it is preferable to use a material that has a certain degree of hardness.

[0037] Furthermore, when the liquid L to be ejected is a cell dispersion liquid, a material to which cells are less likely to adhere is preferable for the material of the nozzle plate 112. Such a material is preferably a highly hydrophilic material.

[0038] Such materials include, for example, metals, ceramics, and polymeric materials.

[0039] More specifically, examples of the material for the nozzle plate 112 include stainless steel, nickel, aluminum, silicon dioxide, alumina, zirconia, ABS, polycarbonate, fluororesin, etc. Furthermore, a composite material can be used in which the surface of the nozzle plate 112 formed from a material other than the above-mentioned materials is coated with the above-mentioned metal, ceramics, or synthetic phospholipid polymer that mimics a cell membrane (for example, Lipidure manufactured by NOF Corporation).

[0040] The number of the discharge ports 112x arranged, the arrangement pattern, the interval (pitch), the opening shape, the opening size, etc. are not particularly limited and can be appropriately selected depending on the purpose.

[0041] The opening shape of the discharge port 112x can be appropriately selected depending on the purpose. Examples of the opening shape of the discharge port 112x include a circle, an ellipse, a rectangle, etc. Among these, a circle is preferable as the opening shape of the discharge port 112x.

[0042] The average opening diameter of the discharge port 112x is not particularly limited and can be appropriately selected depending on the purpose. When the discharged liquid L is a dispersion liquid, the opening shape of the discharge port 112x is preferably at least twice the maximum diameter of the dispersoids, in order to prevent the dispersoids, such as cells, dispersed in the liquid L from clogging the discharge port 112x.

[0043] When the dispersoid is an animal cell, particularly a human cell, the average opening diameter of the discharge port 112x is preferably 10 μm or more and 1000 μm or less. The size of human cells varies depending on the cell type, but is generally 5 μm or more and 50 μm or less. Furthermore, when the dispersoid is a cell cluster (spheroid), the size of the cell cluster is several tens of μm to several mm. Therefore, by setting the discharge port 112x to the above size and providing the discharge port 112x with an average opening diameter appropriate for the cells to be discharged, clogging of the discharge port can be suppressed.

[0044] While increasing the opening diameter of the outlet 112x allows for larger cell aggregates to be ejected, the larger the opening diameter, the more difficult it becomes to eject the larger the opening diameter. By setting the average opening diameter of the outlet 112x to 1000 μm or less, it becomes possible to eject a large number of cell aggregates stably. Furthermore, to achieve stable ejection, it is preferable that the upper limit of the average opening diameter of the outlet 112x be 200 μm or less.

[0045] The position of the ejection opening 112x in the nozzle plate 112 is not particularly limited and can be appropriately selected depending on the purpose. For example, the ejection opening 112x may be located at the center of the nozzle plate 112 when viewed in a plan view, or at a position other than the center of the nozzle plate 112 when viewed in a plan view.

[0046] Furthermore, the number of ejection ports 112x in the nozzle plate 112 may be one or more. A nozzle plate 112 having multiple ejection ports 112x can be suitably employed in a cylindrical liquid holding portion 111. In the nozzle plate 112 exposed to the internal space of the liquid holding portion 111, the multiple ejection ports 112x are preferably arranged at an equal distance from the central axis of the liquid holding portion 111. By arranging them in this manner, the vibration state of each ejection port 112x in the nozzle plate 112 becomes equivalent, and it becomes possible to eject droplets from the multiple ejection ports 112x simultaneously.

[0047] Note that the nozzle plate 112 having the plurality of ejection ports 112x can be used in liquid holding portions other than cylindrical ones. Various shapes of nozzle plate 112 can be used as long as the vibration state of the nozzle plate 112 at the plurality of ejection ports 112x is equivalent. For example, in the case of an elliptical cylindrical liquid holding portion, if ejection ports are provided at positions that overlap with the focal points in the xy cross section (rectangle) of the liquid holding portion, the vibration state at each ejection port becomes equivalent, allowing droplets to be ejected simultaneously.

[0048] Similarly, when the liquid holding portion is a rectangular tube, an xy cross section of the liquid holding portion is imagined, an ejection outlet is provided at an arbitrary point on the cross section, and an ejection outlet is provided at a position that is symmetrical (line symmetric, point symmetric) to the arbitrary point on the cross section, thereby making the vibration state at each ejection outlet equivalent.

[0049] (Vibration Unit) The vibration unit 113 vibrates the nozzle plate 112 based on an input electrical signal, causing the droplets L1 to be ejected from the ejection ports 112x.

[0050] The vibration means 113 is installed on the upper surface of the nozzle plate 112 .

[0051] There are no particular limitations on the shape, size, material, and structure of the vibration means 113, and they can be selected appropriately depending on the purpose.

[0052] There are no particular limitations on the shape or arrangement of the vibration means 113 as long as it does not impair the effects of the invention, and it can be designed appropriately to match the shape of the nozzle plate 112. For example, if the nozzle plate 112 has a circular planar shape, it is preferable to provide the vibration means 113 concentrically around the ejection port 112x.

[0053] Examples of the vibrating means 113 include a piezoelectric element and an electromagnetic solenoid, with a piezoelectric element being preferred. The piezoelectric element may have a structure in which electrodes for applying a voltage are provided on the upper and lower surfaces of a piezoelectric material, for example. In this case, by applying a voltage between the upper and lower electrodes of the piezoelectric element from the control unit 50, a compressive stress is applied in the lateral direction of the film surface, causing the nozzle plate 112 to vibrate in the vertical direction of the film surface.

[0054] The piezoelectric material is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include lead zirconate titanate (PZT), bismuth iron oxide, metal niobate, barium titanate, or any of these materials to which a metal or a different oxide has been added. Among these, lead zirconate titanate (PZT) is preferred.

[0055] The vibration mode of the piezoelectric element is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include a longitudinal mode, a bend mode, etc. A longitudinal mode piezoelectric element is, for example, a stacked type piezoelectric element stacked in the z direction, which expands in the vertical direction (z direction) and contracts in the horizontal direction (x and y directions) when a voltage is applied.

[0056] Furthermore, as a bend mode piezoelectric element, for example, a bimorph type piezoelectric element can be used, in which the piezoelectric element is deformed and bent by applying a voltage, and the position of one end of the piezoelectric element is displaced.

[0057] (Fixing Member) The fixing member 117 is a cylindrical member that surrounds the periphery of the liquid holding portion 111. The fixing member 117 holds the vibration unit 115 at its lower end. The fixing member 117 is also used to attach the ejection head 110 to the conveying means 120.

[0058] The shape of the fixing member 117 is not limited to a cylindrical shape, and various shapes can be adopted as long as it can hold the vibration unit 115 and can attach the discharge head 110 to the conveying means 120.

[0059] 3 is a partially enlarged view of the ejection head 110. As shown in FIG. 3, the fixing member 117 is bonded to the vibration unit 115 via a first adhesive layer 118.

[0060] The first adhesive layer 118 is made of an adhesive having a hardness that does not hinder the displacement of the vibration means 113 of the vibration unit 115 and can follow the displacement of the vibration means 113. The elastic modulus of the material for the first adhesive layer 118 is preferably in the range of the elastic modulus of ordinary rubber, from 1 MPa to 100 MPa, and more preferably from 10 MPa to 100 MPa.

[0061] On the other hand, the vibration unit 115 has a second adhesive layer 119. The second adhesive layer 119 is sandwiched between the nozzle plate 112 and the vibration means 113, and bonds the nozzle plate 112 and the vibration means 113 together.

[0062] The material forming the second adhesive layer 119 is preferably harder than the material forming the first adhesive layer 118 in order to facilitate transmission of the displacement of the vibration means 113 to the nozzle plate 112 .

[0063] For example, a silicone-based elastic adhesive can be suitably used as the material for the first adhesive layer 118. Furthermore, an epoxy-based adhesive can be suitably used as the material for the second adhesive layer 119.

[0064] The above-mentioned "material for forming first adhesive layer 118" and "material for forming second adhesive layer 119" refer to the cured products of these silicone-based elastic adhesives and epoxy-based adhesives. The magnitude relationship of the elastic modulus of the material for forming first adhesive layer 118 and the material for forming second adhesive layer 119 may be compared using the manufacturer's published value for the elastic modulus of the cured product of the adhesive used, or may be compared using the measured value of the elastic modulus of each cured product.

[0065] When a predetermined electric signal (voltage pulse) is applied to the vibration means 113, the central portion of the vibration means 113, which is not fixed to the fixing member 117, deforms up and down. This deformation action applies local pressure to the liquid L near the nozzle plate 112 in the liquid chamber 110A, causing a flow of the liquid L toward the ejection port 112x. Part of this flow is ejected from the ejection port 112x as droplets.

[0066] In the droplet forming device 1, the above operation prevents a large pressure from being applied to the entire liquid chamber, as is the case with known inkjet heads that have closed liquid chambers. Therefore, when ejecting a dispersion liquid in which cells are dispersed, it is possible to suppress damage to the cells in the dispersion liquid.

[0067] <Transportation Means> The transporting means 120 has a first moving section 121 and a second moving section 122 .

[0068] (First Moving Section) The first moving section 121 has a support member 121 a and a linear moving section 121 b. The first moving section 121 is a pair of members provided at the end of the second moving section 122 on the +x side and the end of the second moving section 122 on the −x side.

[0069] The support member 121 a is a rectangular member when viewed from the +y direction, and supports the second moving part 122 .

[0070] The linear motion unit 121b is a long member extending in the z-axis direction. The linear motion unit 121b moves the support member 121a up and down in the z-axis direction. For example, a known linear actuator equipped with a stepping motor as a drive source can be used as the linear motion unit 121b.

[0071] The first moving part 121 moves the support member 121a in the z-axis direction, thereby moving the discharge unit 110L supported by the second moving part 122 in the z-axis direction.

[0072] (Second Moving Section) The second moving section 122 has a support member 122a and a linear moving section 122b.

[0073] The support member 122a is a rectangular member when viewed from the +y direction, and supports the discharge unit 110L.

[0074] The linear movement portion 122b is a long member extending in the x-axis direction. The linear movement portion 122b moves the support member 122a horizontally in the x-axis direction. Both ends of the linear movement portion 122b are supported by the support member 121a of the first moving portion 121.

[0075] The linear movement portion 122b may be, for example, a known linear actuator equipped with a stepping motor as a drive source.

[0076] The second moving section 122 moves the support member 122a in the x-axis direction, thereby moving the discharge unit 110L supported by the support member 122a in the x-axis direction.

[0077] The adhesion portion 30 is disposed in the ejection direction of the droplet L1 ejected from the ejection portion 10, and the droplet L1 adheres to the adhesion portion 30. The adhesion portion 30 can be made of a structure having various materials and shapes according to the purpose of ejecting the liquid.

[0078] The mounting portion 40 mounts the attachment portion 30. The mounting portion 40 has an x-stage 41, a y-stage 42, and a base 43.

[0079] The x-stage 41 supports and fixes the attachment portion 30. The x-stage 41 also moves the attachment portion 30 horizontally in the x-axis direction.

[0080] The y-stage 42 moves the x-stage 41 horizontally in the y-axis direction. The base 43 supports the y-stage 42.

[0081] The mounting unit 40 can employ a known structure as an xy stage.

[0082] The control unit 50 generates electrical signals to operate each part of the droplet forming device 1 and supplies the signals to each part to control them. For example, the control unit 50 generates drive signals to be supplied to the discharge unit 10 and the placement unit 40 and supplies the signals to each part to control the operation of each part.

[0083] The control unit 50 supplies an electric signal having a drive frequency of the following formula (1) to the vibration means 113 of the vibration unit 115. That is, as a method of driving the ejection head, an electric signal having a drive frequency of the following formula (1) is supplied to the vibration means 113. [Drive frequency]=([Resonance frequency of structural vibration of vibration means] / n)×a (1) (where n is an integer from 1 to 100, and a is 0.9 to 1.1.)

[0084] The above n is preferably an integer of 1 to 10.

[0085] The above formula (A) indicates that the drive frequency of the vibration means 113 is approximately an integer fraction of the resonance frequency of the structural vibration of the vibration means 113. The drive frequency does not need to be an exact integer fraction of the resonance frequency, and a coefficient a is set as a tolerance.

[0086] Here, the "resonant frequency of the structural vibration of the vibrating means" can be measured as follows: Figure 4 is an explanatory diagram of a method for measuring the resonant frequency of the structural vibration of the vibrating means.

[0087] As shown in Fig. 4, a white noise voltage is applied as an electrical signal S from the control unit 50 to the vibration applying means 113 of the ejection head 110. In this state, the vibration of the nozzle plate 112 in the vicinity of the ejection orifice 112x is measured using a vibrometer 55. A known laser Doppler vibrometer can be used as the vibrometer 55. With the white noise voltage applied to the vibration applying means 113, the nozzle plate 112 is irradiated with a measurement laser beam La from the vibrometer 55, and the vibration caused in the vibration applying means 113 by the white noise is measured.

[0088] The above measurement is performed when (a) the liquid chamber 110A is filled with water and (b) the liquid chamber 110A is empty.

[0089] Fig. 5 shows an example of the vibration spectrum obtained by the above measurement. In Fig. 5, the solid line shows an example of the vibration spectrum when the liquid chamber 110A is filled with water, and the dotted line shows an example of the vibration spectrum when the liquid chamber 110A is empty. In Fig. 5, the horizontal axis shows frequency (unit: kHz) and the vertical axis shows amplitude (arbitrary units).

[0090] As is clear from Figure 5, peak P1 of the vibration spectrum shown by the solid line and peak P2 of the vibration spectrum shown by the dotted line are resonance points that are observed at the same frequency (near 18 kHz) regardless of the amount of water in the liquid chamber 110A. The vibrations at these peaks coincide with the resonance frequency of the vibration means 113 used in an unloaded state, and can be understood to be caused by structural vibration of the vibration means 113. The resonance frequency of the vibration means 113 in an unloaded state can be determined using the value published by the manufacturer.

[0091] In contrast, peak P3 of the vibration spectrum indicated by the solid line and peak P4 of the vibration spectrum indicated by the dotted line have different resonance frequencies depending on the amount of water in the liquid chamber 110 A. It is known that these resonance points are mainly determined by the elasticity (spring) of the nozzle plate 112 and the mass (mass) of the water held on the nozzle plate 112, and are the so-called spring-mass vibration resonances.

[0092] From the above measurement, it can be determined that the [resonance frequency of the structural vibration of the vibrating means] in the ejection head 110 used in the measurement is approximately 18 kHz. Therefore, by supplying an electrical signal having a drive frequency of approximately one integer fraction of 18 kHz (([resonance frequency of the structural vibration of the vibrating means] / n)×a) from the control unit 50 to the vibrating means 113, it is possible to eject droplets suitably. The resonance frequency used in the calculation is the measured value (to three significant digits) from the above measurement.

[0093] It is preferable that the drive frequency of the nozzle plate 112 and the resonance frequency of the structural vibration of the nozzle plate 112 are approximately equal (n=1 in equation (1)).

[0094] Fig. 6 is a diagram showing an example of a drive waveform set from the measurement results of Fig. 5. The drive waveform shown in Fig. 6 is a sine wave with a period set to 55.6 μs, which corresponds to the vibration period of a frequency of 18 kHz. The drive waveform does not necessarily have to be a continuous sine wave, but may be a composite wave in which waveforms intended to stabilize the ejection of droplets are combined, and the frequency of the composite wave approximately matches the resonant frequency.

[0095] The inventors have confirmed that when a drive waveform with a drive frequency that roughly matches the resonance frequency of peak P3 in Fig. 5 is set and an electrical signal with the same drive waveform is supplied to the vibration means 113, droplets can only be ejected up to 89% of the initial volume from the state in which the liquid chamber 110A is filled with water. This is thought to be because the amount of water in the liquid chamber 110A decreases as droplets are continued to be ejected, causing the frequency of peak P3 to shift to a higher frequency, resulting in a mismatch between the resonance frequency and the drive frequency.

[0096] In contrast, when an electrical signal having the drive waveform shown in Figure 6 is supplied to the vibration means 113 of the ejection head 110 for which the vibration spectrum shown in Figure 5 is measured, it was confirmed that droplets can be ejected continuously from a state in which the liquid chamber 110A is filled with water until the liquid chamber 110A is emptied.

[0097] The drive frequency of the nozzle plate 112 may be an integer fraction of the resonance frequency of the structural vibration of the nozzle plate 112. By using such a drive frequency, it is expected that residual vibrations from one ejection will be less likely to affect the next ejection.

[0098] According to the droplet forming device 1 configured as above, droplets can be formed easily and stably.

[0099] While the preferred embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to these examples. The shapes and combinations of the components shown in the above examples are merely examples, and various modifications can be made based on design requirements, etc., without departing from the spirit of the present invention.

[0100] In order to solve the above problems, one aspect of the present invention includes the following aspects.

[0101] [1] A droplet forming device comprising: a discharge head that discharges droplets of liquid; and a control unit that supplies an electric signal to control the operation of the discharge head, wherein the discharge head has a cylindrical liquid holding unit that holds the liquid; and a vibration unit that discharges the droplets based on the electric signal, wherein the vibration unit has a film-like member having an outlet through which the droplets are discharged; and a vibration unit that vibrates the film-like member based on the electric signal, wherein the vibration unit covers one end of the liquid holding unit and, together with the liquid holding unit, forms a liquid chamber that holds the liquid, and the other end of the liquid holding unit is open to the atmosphere, and the control unit supplies the vibration unit with the electric signal having a drive frequency expressed by the following formula (1): [Drive frequency] = ([Resonant frequency of structural vibration of the vibration unit] / n) × a (1), where n is an integer from 1 to 100, and a is 0.9 to 1.1.

[0102] [2] The droplet forming device according to [1], wherein n is 1.

[0103] [3] The droplet forming device according to [1] or [2], wherein the vibration means is a piezoelectric element.

[0104] [4] A droplet forming device described in any one of [1] to [3], further comprising a cylindrical fixing member surrounding the liquid holding portion, the fixing member being adhered to the vibration portion via a first adhesive layer, and the elastic modulus of the material forming the first adhesive layer being 1 MPa or more and 100 MPa or less.

[0105] [5] The vibration means and the film-like member are bonded via a second adhesive layer, and the material forming the second adhesive layer is harder than the material forming the first adhesive layer. [4] A droplet forming device as described in [4].

[0106] [6] A method for driving a discharge head that discharges droplets of liquid, the discharge head comprising: a cylindrical liquid holding portion that holds the liquid; and a vibration unit that discharges the droplets based on an electric signal, the vibration unit comprising: a film-like member having an ejection port through which the droplets are ejected; and vibration means that vibrates the film-like member based on an electric signal, the vibration unit covering one end of the liquid holding portion and forming, together with the liquid holding portion, a liquid chamber that holds the liquid, the other end of the liquid holding portion of the liquid chamber being open to the atmosphere, the method for driving a discharge head comprising: [driving frequency]=([resonance frequency of structural vibration of vibration means] / n)×a...(1), where n is an integer from 1 to 100, and a is 0.9 to 1.1.

[0107] 1... droplet forming device, 50... control unit, 110... ejection head, 110A... liquid chamber, 111... liquid holding unit, 112... nozzle plate (film-like member), 112x... ejection port, 113... vibration means, 115... vibration unit, 117... fixing member, 118... first adhesive layer, 119... second adhesive layer, L... liquid, L1... droplet, S... electrical signal

[0108] Gokhan P., et al. "Piezoelectric droplet ejector for ink-jet printing of fluids and solid particles", REVIEW OF SCIENTIFIC INSTRUMENTS, VOLUME 74, NUMBER 2, p.1120-1127, 2003

Claims

1. A droplet forming device comprising: a discharge head for discharging droplets of liquid; and a control unit for supplying an electric signal to control the operation of the discharge head, wherein the discharge head comprises a cylindrical liquid holding unit for holding the liquid; and a vibration unit for discharging the droplets based on the electric signal, wherein the vibration unit comprises a film-like member having an outlet for discharging the droplets; and a vibration means for vibrating the film-like member based on the electric signal, wherein the vibration unit covers one end of the liquid holding unit and forms a liquid chamber for holding the liquid together with the liquid holding unit, wherein the other end of the liquid holding unit of the liquid chamber is open to the atmosphere, and wherein the control unit supplies the vibration means with the electric signal having a drive frequency of the following formula (1): [Drive frequency] = ([Resonant frequency of structural vibration of vibration means] / n) x a ... (1) (where n is an integer from 1 to 100, and a is 0.9 to 1.1.) 2. A droplet forming device according to claim 1, wherein n is 1.

3. A droplet forming device according to claim 1 or 2, wherein the vibration means is a piezoelectric element.

4. A droplet forming device as described in claim 1 or 2, further comprising a cylindrical fixing member surrounding the liquid holding portion, the fixing member being adhered to the vibration portion via a first adhesive layer, and the elastic modulus of the material forming the first adhesive layer being 1 MPa or more and 100 MPa or less.

5. A droplet forming device as described in claim 4, wherein the vibration means and the film-like member are bonded via a second adhesive layer, and the material forming the second adhesive layer is harder than the material forming the first adhesive layer.

6. A method for driving a discharge head that discharges droplets of a liquid, the discharge head comprising: a cylindrical liquid holding portion that holds the liquid; and a vibration unit that discharges the droplets based on an electric signal, the vibration unit comprising: a film-like member having an ejection port for ejecting the droplets; and vibration means for vibrating the film-like member based on an electric signal, the vibration unit covering one end of the liquid holding portion and forming a liquid chamber for holding the liquid together with the liquid holding portion, the other end of the liquid holding portion of the liquid chamber being open to the atmosphere, the method for driving a discharge head supplying the electric signal having a drive frequency of the following formula (1) to the vibration means: [Drive frequency] = ([Resonant frequency of structural vibration of vibration means] / n) x a ... (1) (where n is an integer from 1 to 100, and a is 0.9 to 1.1.)

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