EP4656877A1

EP4656877A1 - An acoustic micropump device using piezoelectric transducers to create a directional fluid flow

Info

Publication number: EP4656877A1
Application number: EP24178366.1A
Authority: EP
Inventors: Chen Wu; Grim Keulemans; Xavier Rottenberg; Veronique Rochus; Benjamin Jones
Original assignee: Katholieke Universiteit Leuven; Interuniversitair Microelektronica Centrum vzw IMEC
Current assignee: Katholieke Universiteit Leuven; Interuniversitair Microelektronica Centrum vzw IMEC
Priority date: 2024-05-28
Filing date: 2024-05-28
Publication date: 2025-12-03
Also published as: US20250369434A1

Abstract

The present disclosure relates to a micropump device for creating a net fluid flow along a flow direction. The device comprises a fluid channel and a plurality of piezoelectric transducers arranged adjacent to the fluid channel. The piezoelectric transducers are organized into a set of at least three groups, the groups being consecutively arranged along the flow direction, and each comprising at least one piezoelectric transducer. A controller of the device is configured to actuate the piezoelectric transducers using at least three periodic electrical control signals, each electrical control signal being associated with one group of the set. The controller is configured to consecutively delay the at least three electrical control signals to another, in accordance with the consecutively arranged groups of the set, to create the net flow of fluid along the flow direction.

Description

TECHNICAL FIELD

The present disclosure relates to a micropump device. The micropump device is configured to create a net flow of a fluid inside a fluid channel along a flow direction. For example, the net flow may be created between an inlet port and an outlet port of the fluid channel. For creating the net fluid flow, the micropump device employs piezoelectric transducers, which are controlled in at least three groups.

BACKGROUND

Micro fabrication has the ability to integrate various microscopic electromechanical components within a microelectromechanical system (MEMS). MEMS has various advantages, for example, high functionality and performance, small size, low cost, and ease of fabrication. With these advantages, micro fabrication has been increasingly popular in microfluidics systems, for example, for bioassays applications, wherein all functional modules are miniaturized and integrated into one single micro-sized chip. Such systems are cheap, rapid in transportation of bio samples, and highly reproducible. Of all integrated components, micropumps or micropump devices play an essential role to transport the bio samples and reagents.
Various types of micropump devices have been developed based on different actuation mechanisms. One of the most representative micropump devices is the silicon-based diaphragm piezoelectric micropump device, which relies on the piezoelectric effect to deform a membrane to drive a fluid in a channel between an inlet and an outlet valve. This kind of micropump device can achieve a high pumping flow rate, however, it has some disadvantages. For instance, the micropump device has a rather complicated design, and requires the complex fabrication of the inlet and outlet valves. Additionally, the micropump device works in a low frequency range (normally below 100 Hz), and requires a very high input power to pump the fluid (e.g., an AC signal having an amplitude of around 100 V is needed). Moreover, once the micropump device is fabricated, the flow direction of the fluid is fixed, which results in a limited flexibility of application.
In order to eliminate the impact of mechanical loss caused by moving parts, and to drive the fluid in a miniaturized microchannel, a phase shift vibrational membrane micropump device has been proposed. In this kind of micropump device, a vibrational wall composed of a thin piezoelectric film and square electrodes are fabricated on top of a straight fluid microchannel. By applying a phase delayed AC signal to the adjacent electrodes, a peristaltic mechanical vibrational wall motion is induced, which moves the fluid inside the microchannel unidirectionally. This kind of micropump device works in a moderate frequency range (e.g. at around 100 kHz), and with an AC signal that has a moderate amplitude (e.g. of around 20 V). However, the membrane displacement is only in the order of a hundred nanometers, which results in a rather low pumping flow rate.
Apart from the above-mentioned mechanical actuation mechanisms, the development of wave-based acoustic streaming micropump devices has been carried out as well. Acoustic streaming refers to a fluid flow, which is induced by the force arising from the presence of a gradient in the time-averaged acoustic momentum flux inside the fluid domain. The acoustic streaming flow velocity is generally proportional to the acoustic wave amplitude and the wave frequency. Compared to the discrete mechanical micropump devices, the elastic wave motion is distributed along the length of the fluidic microchannel. Additionally, most of the acoustic energy of the elastic wave accumulates at the fluid-solid interface, which makes it more promising for fluid transportation inside a microchannel with a low channel height (e.g., of only a few hundred micrometers). Furthermore, the fabrication process of this kind of micropump device is much less complicated, as it does not have moving parts, which are also prone to mechanical failure. With these advantages, acoustic streaming micropump devices have been widely used in various microfluidics applications including pumping, jetting, and manipulation of droplets or bio particles.
Surface acoustic wave (SAW) is a promising candidate for realizing acoustic steaming micropump devices. For example, an interdigitated transducer (IDT) composed of multiple comb-liked electrode finger pairs can be deposited on top of a piezoelectric film, and can be used to generate a SAW. The SAW propagates non-directionally at the electrode finger region and travels directionally to either side outside of the transducer region. A fluid chip with a bent channel may be bonded to one side of the IDT at a certain distance. In this design, the SAW first propagates along a substrate, through the microfluidic channel wall, and subsequently meets with the fluid inside the channel to induce acoustic streaming. The advantage of a micropump device with this design is that it does not have mechanical moving parts, but uses only the SAW, which is easily generated, as the actuation force to pump the fluid.
However, there are still several drawbacks. Firstly, the amplitude of the SAW is typically extremely low (only in the sub nanometer range), which results in an extremely low pumping flow rate. Secondly, since the SAW amplitude is extremely low, a high input power is needed. Thirdly, since a directional SAW is needed to drive the fluid motion, the microchannel has to be placed at one side of the channel. Consequently, only half of the acoustic energy generated by the IDT is used, which is energy inefficient and increases the energy consumption of the micropump device. Lastly, the SAW has a severe acoustic attenuation inside the necessarily elastic fluid channel material before it meets the fluid, wherein over 90% of the acoustic energy may be lost.

SUMMARY

In view of the above, an objective of this disclosure is to provide an acoustic micropump device, which overcomes the disadvantages described. An objective is, in particular, to achieve a high pumping velocity and thus a large pumping flow rate with the micropump device. Another objective is to enable low input power for the micropump device.
These and other objectives are achieved by the solutions provided in the independent claims. Advantageous implementations are described in the dependent claims.
A first aspect of this disclosure provides an acoustic micropump device for creating a net flow of fluid along a flow direction, the acoustic micropump device comprising: a fluid channel for the fluid; a plurality of piezoelectric transducers arranged adjacent to the fluid channel; wherein the piezoelectric transducers are organized into a set comprising at least three groups, the groups being consecutively arranged along the flow direction, and each group comprising at least one piezoelectric transducer; wherein each piezoelectric transducer comprises a respective membrane, which is configured to vibrate when the piezoelectric transducer is electrically actuated and to acoustically couple to the fluid channel; and a controller configured to actuate the plurality of piezoelectric transducers using at least three periodic electrical control signals, each electrical control signal being associated with one group of the at least three groups of piezoelectric transducers of the set; wherein the controller is configured to consecutively delay the at least three electrical control signals to another, in accordance with the consecutively arranged groups of the set associated with the electrical control signals, to create the net flow of fluid along the flow direction.
The fluid channel may be configured to hold and enable transport the fluid. The fluid channel may have at least two ports, wherein one port may act as inlet port, the other port may act as outlet port, and the flow direction may be defined from inlet to outlet port. The function of inlet and outlet port may be reversed. In this case, the controller may be configured to change the delay between the at least three control signals to reverse the net fluid flow. The controller may be configured to set the delay(s) between respectively the at least three control signals in a variable manner, so as to achieve different flow directions of the net fluid flow in the fluid channel. For instance, changing a sign of the respective delays between the control signals may reverse the flow direction. The fluid channel may be straight and the flow direction may be along the fluid channel length. The fluid channel may also be bent or curved, and the flow direction may lie arbitrarily within fluid channel or may follow the bend or curve of the fluid channel.
The fluid that is usable with the micropump device may be a liquid, in particular, maybe water or an aqueous solution. Since the micropump device maybe used for bio-applications, the liquid may also be or comprise blood, pharmaceutical compounds, cell culture media, and various buffers as used in diagnostic and therapeutic procedures. The fluid is notably not a part of the micropump device, and may be supplied externally to the fluid channel of the micropump device.
The delayed electrical control signals provided by the controller may be time-delayed with respect to each other, and/or may be phase-delayed with respect to each other. For instance, the electrical control signals may be identical or similar pulsed signals, wherein the pulses of different electrical control signals appear at different time instances, i.e., with a time delay. The electrical control signals may also be sinusoidal signals and/or AC signals with a phase difference between different electrical control signals. For example, each electrical control signal may have a phase shift with respect to a common periodic electrical control signal (providing a reference phase). The common electrical control signal may be one of the electrical control signals used. The electrical control signals may all have the same shape and/or amplitude and/or frequency. The delay may be the same between any two consecutively arranged groups of piezoelectric transducers of the set. However, the delays between respective pairs of consecutively arranged groups of the set may also differ. That is, equidistant or non-equidistant delays may be used.
The vibration of the respective membranes of the piezoelectric transducers, which can be coupled acoustically to the fluid channel, may create a traveling acoustic wave, for example a SAW like flexural plate wave, at the interface of the fluid and the fluid channel (wall), and may cause the directional net flow of the fluid in the fluid channel. A flexural plate wave is a type of acoustic wave that travels along, for instance, a thin plate, bending the plate as it goes. In the present case, this plate may be a wall and/or elastic layer of the fluid channel. In this way a pumping of the fluid can be achieved along the fluid channel. Since the travelling wave displacement (amplitude), for example, the flexural plate wave displacement, may be in the sub-micrometer range, i.e. can be much larger than that of the SAW, which is typically only in the sub-nanometer range, a much higher acoustic pressure and acoustic streaming velocity can be achieved as for conventional actuation mechanisms based on SAW. Thereby, also a relatively low input power is needed for the electrical control signals. The piezoelectric transducers may be PMUTs, piezoelectric diaphragm transducers, or the like.
In an implementation of the acoustic micropump device, the piezoelectric transducers are organized into two or more sets, the sets being consecutively arranged along the flow direction; wherein each set comprises at least three groups of piezoelectric transducers, which are consecutively arranged along the flow direction; and wherein the controller is configured to actuate the piezoelectric transducers of each set using the same at least three electrical control signals, each electrical control signal being associated with one group of the set, to create the net flow of fluid along the flow direction.
That is, there is more than one set of piezoelectric transducers in the plurality of piezoelectric transducers. Using more than one set allows transporting the fluid along the fluid direction over a longer distance, that is allows longer fluid channels. Thereby, the electrical control signals used for the first set can be re-used for other sets, and little extra power is required. The multiple sets of piezoelectric transducers can be arranged one after the other parallel to the flow direction, and within each of the sets the at least three groups of piezoelectric transducers (each having one or more piezoelectric transducers) can be arranged parallel to the flow direction as well.
In an implementation of the acoustic micropump device, the at least three groups of the set comprise a first group, a second group, and a third group, which are arranged in this order along the flow direction, and the at least three electrical control signals are phase-shifted versions of a common periodic electrical control signal, wherein the at least three electrical control signals are phase-shifted to another in accordance with the consecutively arranged groups associated with the electrical control signals and comprise: a first periodic electrical control signal having a phase shift in a range of -120° to -70°, and being associated with the first group; a second periodic electrical control signal having a phase shift of 0°, and being associated with the second group; and a third periodic electrical control signal having a phase shift in a range of +70° to +120°, and being associated with the third group.
For example, the phase shift of the first control signal may be around -120°, the phase shift of the second control signal may be around 0° (e.g. ± 10°), wherein the second control signal may be used as a reference for the other control signals, and the phase shift of the third control signal may be around +120°. That is the phase difference between the first control signal and the second control signal is 120°, and is the same as the phase difference between the second control signal and the third control signal. The phase shifts maybe relative to a common electrical control signal, which may be identical to the second electrical control signal. The amplitude and frequency maybe the same for each electrical control signal. If there are more than three groups in a set of piezoelectric transducers, then the phase shift may consecutively change by up to 90° between adjacent groups e.g. in case of four groups or more, or only by up to 60° between adjacent groups e.g. in case of six groups or more, or even only by up to 30° between adjacent groups e.g. in case of twelve groups or more.
However, also non-equidistant phase shifts are possible. For instance, the phase shift of the first control signal could be around -120°, the phase shift of the second control signal could be in a range of -60° to --10°, and the phase shift of the third control signal could be in a range of +130° to +180°. In this case, the phase difference between the first control signal and the second control signal may be in a range of 60-110°, and the phase difference between the second control signal and the third control signal is in a range of 140°-240°. It is further possible that the phase shift of the first control signal is about 0°, and the phase shifts of the second control signal, the third control signal, and potentially further control signals for the set are consecutively increased in an equidistant or in a non-equidistant manner. Notably, in the end the phase differences between the control signals may be decisive, not the "absolute" phase shifts regarding a certain reference. For instance, the situation -120°, 0°, and +120° for the three control signals, as described above, may be identical to the situation 0°, +120°, and +240° for the same three control signals.
In an implementation of the acoustic micropump device, the piezoelectric transducers are organized into an array of rows and columns; wherein each row of piezoelectric transducers is one group of piezoelectric transducers; or wherein each row of piezoelectric transducers comprises at least three groups of piezoelectric transducers.
An array of piezoelectric transducers enables flexible selection of the flow direction by the controller, i.e., by controlling the transducers with suitable electrical control signals and delays. In this implementation, a horizontal or vertical flow direction may be achieved (vertical, e.g., being along the column direction). It may be possible that each piezoelectric transducer of the array can be controlled individually. The controller would thus be able to selectively control certain piezoelectric transducers as groups with the same electrical control signal, and could "regroup" the piezoelectric transducers if needed.
In an implementation of the acoustic micropump device, the piezoelectric transducers are organized into an array of rows and columns; wherein the at least one piezoelectric transducer of each group of the at least three groups of the set is arranged in a different row and in a different column than the at least one piezoelectric transducer of the other groups of the at least three groups of the set.
In this implementation, a diagonal flow direction through the array may be achieved.
In an implementation of the acoustic micropump device, the fluid channel is bonded to the array of piezoelectric transducers.
For instance, the fluid channel may be attached to or coupled to the vibrating membranes of the piezoelectric transducers, so that membrane vibrations can be transferred to vibration(s) of the fluid channel.
In an implementation of the acoustic micropump device, the fluid channel extends at least predominantly along the flow direction.
This is of particular advantage, if a fixed flow direction is desired, for instance, from a dedicated inlet port to a dedicated outlet port.
In an implementation of the acoustic micropump device the respective membrane of each piezoelectric transducer comprises a piezoelectric layer, which is sandwiched between a bottom electrode and a top electrode; and wherein the fluid channel comprises an elastic layer attached to the respective membrane and/or to the top electrode.
In an implementation of the acoustic micropump device, at least the piezoelectric transducers of the same group have a common piezoelectric layer.
The piezoelectric layer may also be shared by more or even all piezoelectric transducers.
In an implementation of the acoustic micropump device, the electrical control signals are applied to the top electrodes of the piezoelectric transducers and the bottom electrodes of the piezoelectric transducers are grounded; or the electrical control signals are applied to the bottom electrodes of the piezoelectric transducers and the top electrodes of the piezoelectric transducers are grounded.
In an implementation of the acoustic micropump device, the top electrodes or the bottom electrodes of two or more piezoelectric transducers of the same group are commonly connected to the controller for receiving the same electrical control signal.
In an implementation of the acoustic micropump device, the electrical control signals are applied to the top electrodes of the piezoelectric transducers, and the acoustic micropump device further comprises a grounded shielding layer arranged between the top electrodes and the fluid channel.
The shielding layer allows reducing dielectrophoresis forces, which may be induced by applying the control signals to the top electrodes.
In an implementation of the acoustic micropump device, the respective membrane of each piezoelectric transducer is suspended in a cavity formed in a substrate of the piezoelectric transducer; wherein the cavity has a width in a range of 10-150 µm; and/or wherein a spacing between any two adjacent piezoelectric transducers is at least 50-200 µm.
For instance, the membranes may be formed from the piezoelectric layer, which may be shared by two or more piezoelectric transducers. The membrane is in this case defined by the part of the piezoelectric layer being suspended in the cavity. The substrate in which the cavities are formed may be rigid. The piezoelectric transducers may share the rigid substrate. Due to the rigid substrate, only the membranes suspended over the cavities are able to vibrate, which leads to a discontinuous membrane vibration. The membranes' vibrations may correspond to local deformations of the parts of the piezoelectric layer above the cavities.
The width of the cavity may be the maximum extension (can be shape-dependent) of the cavity along the direction in which multiple piezoelectric transducers are arranged one after the other, e.g., along the flow direction. The spacing between the two adjacent piezoelectric transducers is the spacing between the cavities of these two piezoelectric transducers. The spacing may be in a range of 50-200 µm, or may be larger.
In an implementation of the acoustic micropump device, the piezoelectric transducers are piezoelectric micromachined ultrasonic transducers (PMUTs).
A PMUT works by utilizing the piezoelectric effect, where the application of an electrical voltage to a piezoelectric material causes it to deform, emitting ultrasonic waves. The piezoelectric material may be the membrane and the electrical voltage may be the electrical control signal applied by the controller.
A second aspect of this disclosure provides a method of operating an acoustic micropump device according to the first aspect or any of its implementations, so as to create the net flow of the fluid along the flow direction, the method comprising: actuating the plurality of piezoelectric transducers using the at least three periodic electrical control signals, wherein each electrical control signal is used to actuate all piezoelectric transducers of one group of the at least three groups of the set; and consecutively delaying the at least three electrical control signals to another in accordance with the consecutively arranged groups of the set associated with the electrical control signals.
In an implementation of the acoustic micropump device, the at least three electrical control signals are phase-shifted versions of a common periodic electrical control signal, a first electrical control signal of the at least three electrical control signals has a phase shift in a range of -120° to -70°, and is used to actuate all piezoelectric transducers of a first group of the at least three groups of piezoelectric transducers of the set; a second electrical control signal of the at least three electrical control signals has a phase shift of 0°, and is used to actuate all piezoelectric transducers of a second group of the at least three groups of piezoelectric transducers of the set; and a third electrical control signal of the at least three electrical control signals has a phase shift in a range of +70° to +120°, and is used actuate all piezoelectric transducers of a third group of the at least three groups of piezoelectric transducers of the set; and the first group, the second group, and the third group are arranged in this order along the flow direction.
In an implementation of method, the first electrical control signal is further used to actuate all piezoelectric transducers of a fourth group of piezoelectric transducers of a further set; the second electrical control signal is further used to actuate all piezoelectric transducers of a fifth group of piezoelectric transducers of the further set; and the third electrical control signal is further used actuate all piezoelectric transducers of a sixth group of piezoelectric transducers of the further set; wherein the fourth group, the fifth group, and the sixth group are arranged in this order along the flow direction after the first group, the second group, and the third group.
The method of the second aspect achieves the same advantages as the micropump device of the first aspect, and may be extended by respective implementations as described above for the micropump device of the first aspect.
Another aspect of this disclosure is related to a computer program comprising instructions which, when the program is executed by the controller of the micropump device of the first aspect, causes the control to perform the method of the second aspect.
In summary, this disclosure proposes a micropump device that employs piezoelectric transducers, which are actuated by respectively delayed control signals, in order to realize a net fluid flow in a fluid channel with a significant improvement of the pumping velocity over existing acoustic streaming micropump devices. As the piezoelectric transducers, PMUTs may specifically be used. The piezoelectric transducers can be controlled to generate a travelling wave - e.g. a SAW like unidirectional flexural plate wave - in the fluid channel, which causes a streaming of the fluid inside the fluid channel. The solution of this disclosure enables a high pumping flow rate of several hundreds of µL/min for the micropump device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above described aspects and implementations are explained in the following description of embodiments with respect to the enclosed drawings:

FIG. 1: shows schematically an acoustic micropump device according to this disclosure.
FIG. 2: shows a sectional view of a first example of an acoustic micropump device according to this disclosure
FIG. 3: shows a top view of a second example of an acoustic micropump device according to this disclosure.
FIG. 4: shows a sectional view of a third example of an acoustic micropump device according to this disclosure
FIG. 5: shows a sectional view of a fourth example of an acoustic micropump device according to this disclosure
FIG. 6: shows exemplary arrays of piezoelectric transducers for an acoustic micropump device according to this disclosure.
FIG. 7: shows exemplary arrays of piezoelectric transducers for an acoustic micropump device according to this disclosure.
FIG. 8: shows exemplary electrical control signals that can be used in an acoustic micropump device according to this disclosure.
FIG. 9: shows a method of operating an acoustic micropump device according to this disclosure.
FIG. 10: shows simulation results demonstrating the advantages of a micropump device according to this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows schematically an acoustic micropump device 10 according to this disclosure. The acoustic micropump device 10 is configured to create a directional net flow of a fluid, wherein the fluid is preferably a liquid. For example, the liquid may be water or an aqueous solution. In particular, FIG. 1 shows in (a) a sectional view of the acoustic micropump device 10, and in (b) a top view of the micropump device 10.
The acoustic micropump device 10 comprises a fluid channel 12. The fluid channel 12 is suitable to hold and transport the fluid, for example, liquid. The fluid channel 12 may be defined by channel walls, wherein the material of these channel walls may be suitable to be displaced to create plate waves, in particular, is elastic. The net flow of the fluid is created by the micropump device 10 in the fluid channel 12 and along a flow direction 11. The fluid channel 12 may, to this end, extend at least predominantly along the flow direction 11, or may even extend strictly along the flow direction 11, especially if this flow direction 11 is fixed. Notably, the flow direction 11 does not have to be as indicated in FIG. 1. The flow direction 11 may be defined from a first port of the fluid channel 12 to a second port of the fluid channel 12, for example, from an inlet port to an outlet port. The micropump device 10 may, however, be configured to create a net flow of fluid along different selectable flow directions, for instance, it may be configured to reverse the flow direction 11 to be from the second port to the first port.
The acoustic micropump device 10 further comprises a plurality of piezoelectric transducers 13, which are arranged spaced from each other and adjacent to the fluid channel 12. For instance, the piezoelectric transducers 13 maybe arranged on a wall or layer of the fluid channel 12 or vice versa. Each piezoelectric transducer 13 can be electrically activated, in particular, by a respective control signal 17 that is applied to one or more electrodes of the piezoelectric transducer 13. Each piezoelectric transducer 13 comprises a respective membrane 15, which is configured to vibrate, when the piezoelectric transducer 13 is electrically actuated.
The respective membrane 15 of the piezoelectric transducer 13 is further configured to acoustically couple to the fluid when present in the fluid channel 12, for example, to transfer its vibration to that fluid in the fluid channel 12. The elastic membrane 15 may thus be in contact with an elastic wall of the fluid channel 12. As an example, a lower channel wall that is close to or in contact with the membranes 15 of the piezoelectric transducers 13 may be made of silicon nitride (Si₃N₄). Other channel walls, for instance, top and side channel walls, may be made of an elastomer like Polydimethylsiloxane (PDMS).
The piezoelectric transducers may be implemented with different shapes (e.g., when viewed from the top). The shapes of the membranes 15 correspond to the shapes of the piezoelectric transducers 13, and the shapes of the cavities may be the same as the shapes of the membranes 15. For instance, as shown as example in FIG. 1(b), the piezoelectric transducers 13 may generally have round shapes (e.g., ellipsoidal). More specifically, the piezoelectric transducers 13 could have circular shapes. However, also other shapes, like rectangular shapes, square shapes, hexagonal shapes, octagonal shapes, or even more complex shapes are possible for the piezoelectric transducers 13.
The plurality of piezoelectric transducers 13 are organized into a set of at least three groups 14 of piezoelectric transducers 13. The set may be one of multiple sets as will be described later. In FIG. 1, as an example, three groups 14 are shown to be included in the shown set. Each group 14 comprises at least one piezoelectric transducer 13, that is, the plurality of piezoelectric transducers comprises at least three piezoelectric transducers 13. However, again as an example, in FIG. 1(b) two piezoelectric transducers 13 are shown per group 14. Of course, each group 14 could include more piezoelectric transducers 13 or only one piezoelectric transducer 13. The at least three groups 14 of the piezoelectric transducers 13 of the set are consecutively arranged along the flow direction 11, i.e., they are arranged one after the other and spaced from each other along an axis that runs parallel to the flow direction 11, which is indicated in the fluid channel 12 in FIG. 1.
The acoustic micropump device 10 further comprises a controller 16, for example, a computer or processor. The controller 16 may comprise processing circuitry, which is configured to perform, conduct or initiate the operations of the controller 16 described in this disclosure. The processing circuitry may comprise hardware and/or the processing circuitry may be controlled by software. The hardware may comprise analog circuitry or digital circuitry, or both analog and digital circuitry. The digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or multi-purpose processors. The controller 16 may further comprise memory circuitry, which stores one or more instruction(s) or executable program code that can be executed by the processing circuitry, in particular under control of the software. The execution causes the operations of the controller 16 described in this disclosure to be performed.
The controller 16 is configured to actuate the plurality of piezoelectric transducers 13. To do so, the controller 16 is configured to use at least three periodic electrical control signals 17, wherein each electrical control signal 17 is associated with one group 14 of the at least three groups 14 of the set, i.e., each control signal 17 is used by the controller 16 to actuate the piezoelectric transducers of said group 14 of the set. For example, in case of three groups 14 in the set, three shown electrical control signal 17a, 17b and 17c can be used, one for actuating the piezoelectric transducers 13 of each group 14 of the set. Notably, each reference element including a letter, like 14a or 17a, denotes a specific element of the element labelled with the general reference signs without letter, like 14 or 17 in this case.
The controller 16 is further configured to consecutively delay the at least three electrical control signals 17 to another, in accordance with the consecutively arranged groups 14 of the set, which are associated with the electrical control signals 17. The respective electrical control signals 17 and the respective delays between the control signals 17 can be configured such by the controller that the net flow of fluid along the flow direction 11 is caused.
FIG. 2 shows a sectional view of a first example of an acoustic micropump device 10 according to this disclosure. The micropump device 10 is based on the micropump device 10 that is schematically illustrated in FIG. 1. Same elements in FIG. 1 and FIG. 2 are labelled with the same reference signs and may be configured likewise.
FIG. 2 shows that the flow direction 11 can be between a first port 21 (port A) and a second port 22 (port B) of the fluid channel 12, wherein the first port 21 may act as an inlet for the fluid, while the second port 22 may act as an outlet for the fluid from the fluid channel 12. A net flow of the fluid may be created from the first port 21 to the second port 22 by the micropump device 10, particularly the controller 16, but this net flow could be reversed as well.
FIG. 2 further shows that the piezoelectric transducers 13 - as an example, the transducers 13 are PMUTs in FIG. 2 - are organized into more than one set 24, wherein the two or more sets 24 are consecutively arranged along the flow direction 11 next to the fluid channel 12. Like the set shown in FIG. 1, each set 24 in FIG. 2 comprises at least three groups 14 of piezoelectric transducers 13, wherein exactly three groups 14 are exemplarily shown in FIG. 2 per set 24. The groups 14 are consecutively arranged next to the fluid channel 12 along, i.e. parallel to, the flow direction 11. Notably, the piezoelectric transducers 13 may respectively work in the few MHz range, for instance, in a range of 1-10 MHz or in a range of 1-40 MHz.
The controller 16 (not shown in FIG. 2) is in this case configured to actuate the piezoelectric transducers 13 of each set 24 using the same at least three electrical control signals 17, i.e., each set 24 may be equally controlled as the set shown in FIG. 1. Each electrical control signal 17 is associated with one group 14 of each respective set 24. This allows creating and extending the net fluid flow over longer distances along the flow direction 11 in the fluid channel 12.
In the example shown in FIG. 2, the at least three groups 14 of the first set 24 comprise a first group 14a, a second group 14b, and a third group 14c arranged in this order and spaced from each other along the fluid channel 12 parallel to the flow direction 11. The at least three electrical control signals 17 accordingly comprise a first control signal 17a associated with the first group 14a, a second control signal 17b associated with the second group 14b, and a third control signal 17c associated with the third group 14c. That is, the controller 16 is configured to use the first control signal 17a to actuate the piezoelectric transducers 13 of the first group 14a, to use the second control signal 17b to actuate the piezoelectric transducers 13 of the second group 14b, and to use the third control signal 17c to actuate the piezoelectric transducers 13 of the third group 14c. Notably, the controller 16 may be further configured to, as illustrated in FIG. 2, use the first control signal 17a also to actuate the piezoelectric transducers 13 of a fourth group 14d of a further set 24, to, use the second control signal 17b also to actuate the piezoelectric transducers 13 of a fifth group 14e of the further set 24, and to use the third control signal 17c also to actuate the piezoelectric transducers 13 of a sixth group 14f of the further set. The fourth group 14d, the fifth group 14e, and the sixth group 14f are thereby arranged in the same order as the first group 14a, second group 14b, and third group 14c along the flow direction 11. The electrical control signals 17a, 17b, 17c can be reused to even control piezoelectric transducer groups 14 of more than two sets 24.
In the example of FIG. 2, the control signals 17, which are delayed with respect to another, are phase-shifted versions of a common periodic electrical control signal, for instance, an AC control signal. The electrical control signals 17 are phase-shifted to another in accordance with the consecutively arranged groups 14. In particular, the first periodic control signal 17a has a phase shift of about -120°, the second control signal 17b has a phase shift of about 0°, and the third control signal 17c has a phase shift of about 120° with reference to the common periodic electrical control signal, That is, the first and third control signal 17a, 17c are phase shifted by 120° with respect to the second control signal 17b (but in opposite phase directions). Notably, these phase shift numbers are examples only and may be set differently, as described above.
The respective membrane 15 of each piezoelectric transducer 13 may comprise or be formed by a piezoelectric layer 25. At least the piezoelectric transducers 13 of the same group 14 may have a common piezoelectric layer 25, but there may also be a common piezoelectric layer 25 for all the piezoelectric transducers 13. Each piezoelectric transducer 13 may comprise a cavity (as indicated in white at the locations in FIG. 2 where the abbreviations "PMUT" is written), wherein the respective membrane 15 of each piezoelectric transducer 13 is suspended in the cavity. The cavities may be formed in a common substrate 23 of the piezoelectric transducers 13 of the micropump device 10, and the membranes 15 maybe formed by the part of the piezoelectric layer 25 that is suspended in or over each cavity. The cavities may respectively have a width in a range of 10-150 µm, e.g. 50 µm, along the flow direction 11. A spacing between adjacent piezoelectric transducers 13 along the flow direction 11 may be at least or in a range of 50-200 µm, e.g. 120 µm. The spacing between adjacent piezoelectric transducers 13 may be defined as the spacing between their cavities. The spacing may be the same between each two adjacent piezoelectric transducers 13. In locations without cavity the common substrate 23 may be considered rigid and will thus not take part in the generation of the acoustic waves. That is, only the membranes 15 that are respectively suspended above the cavities may be vibrational (to implement a discontinuous membrane vibration). Due to the spacing between the transducers 13 the acoustic waves may only locally, i.e. above the cavity, generate an acoustic wave in the fluid present in the fluidic channel. For example, the free-standing membrane 15 per piezoelectric transducer 13 may formed by back-side etching of the substrate 23, which may be made of silicon in this case. As another example the cavities, over which the membranes 15 are suspended, may be formed by sacrificial release, and the substrate may be made of silicon oxide.
Each piezoelectric transducer 13 comprises a top electrode 27 and a bottom electrode 28, by which the piezoelectric transducer 13 is addressable, i.e., can be actuated. Thereby, at least each group 14 is individually addressable, but even each single piezoelectric transducer of the device 10 maybe individually addressable (by control signal 17). The membrane 15 of each piezoelectric transducer 13 is sandwiched between the two electrodes 27, 28 of said piezoelectric transducer 13.
The fluid channel 12 may be attached to the membranes 15 and/or the top electrodes 27 of the piezoelectric transducers 13, for example, by means of an elastic passivation layer 26. The passivation layer 26 may form a channel wall of the fluid channel 12, and may be made of silicon nitride.
By applying, with the controller 16, for example, AC control signals with a certain amplitude as the electrical control signals 17, a mechanical membrane vibration can be effected. For example, the top electrode 27 of each PMUT may be used to apply the respective control signal 17, and the bottom electrode(s) of the PMUTs may be commonly grounded, e.g., connected to a ground line 29. The different control signals 17a, 17b, 17c for the groups 14a, 14b, 14c may have a phase delay difference of 120° from group to group as described above. In this case, the membranes 15 of the PMUTs 13 generate a mechanical vibrational profile following the applied phase delay(s). This vibrational profile can subsequently generate a SAW-liked flexural plate wave with a uni-direction. Consequently, a straight fluid channel 12 may be plasma bonded on top of the PMUTs 13 to induce the acoustic streaming for directional continuous fluid flow in micropump applications.
FIG. 3 shows top view of a second example of an acoustic micropump device according to this disclosure. The second example may be the micropump device 10 of the first example of FIG. 2, but may also be a different example in some aspects.
FIG. 3 shows that the piezoelectric transducers 13 may be organized into an array 60 comprising a number of rows and a number of columns of piezoelectric transducers 13. The array 60 maybe bonded to the fluid channel 12. As exemplarily shown, the rows may align with the extension of the fluid channel 12 between its two ports 21, 22. As also exemplarily shown in FIG. 3, each column (vertical in FIG. 3) of the piezoelectric transducers 13 can be one unit cell of the array, e.g. a 1D array of transducers 13 (and corresponding to a group 14), and may thus be activated with one common control signal 17. Delayed electrical control signals 17 may be applied to the groups 14 (of two or more sets) to create a net fluid flow in the fluid channel 12, e.g. time delayed control signals 17 may be applied to the adjacent unit cells. Inside each unit cell, the transducers 13 maybe shorted, so that they are actuated by the identical electrical control signal 17.
FIG. 4 shows a sectional view of a third example of an acoustic micropump device 10 according to this disclosure. The micropump device 10 is based on the micropump device 10 schematically illustrated in FIG. 1 and is similar to that of FIG. 2. Same elements in FIGs. 1, 2 and 4 are labelled with the same reference signs and may be configured likewise.
In FIG. 4, the electrical control signals 17 are applied to the top electrodes 27 of the piezoelectric transducers 13. In contrast to FIG. 2, the micropump device 10 of FIG. 4 further comprises a grounded shielding layer 41, which is arranged between the top electrodes 27 and the fluid channel 12. By applying the electrical control signals 17 to the top electrodes 27, the electrical field may induce a dielectrophoresis force, which is beneficially eliminated by the use of the shielding layer 41. The shielding layer 41 has a ground connection and may be embedded in the elastic passivation layer 26, which may be made of silicon nitride, to generate electrical isolation.
FIG. 5 shows a sectional view of a fourth example of an acoustic micropump device according to this disclosure. The micropump device 10 is based on the micropump device 10 schematically illustrated in FIG. 1 and is similar to that of FIG. 2. Same elements in FIGs. 1, 2 and 5 are labelled with the same reference signs and can be configured likewise.
In contrast to FIG. 2 and 4, the electrical control signals 17 are applied to the bottom electrodes 28 of the piezoelectric transducers 13 in FIG. 5, and the top electrodes 27 of the piezoelectric transducers 13 are grounded, e.g., connected to a ground line 29. For instance, the top electrodes 27 may be a common grounded electrode for all transducers 13. The common top electrode 27 may be buried under the passivation layer 26. This implementation provides an alternative to eliminate the dielectrophoresis impact.
FIG. 6 shows exemplary array 60 of piezoelectric transducers 13 for an acoustic micropump device 10 according to this disclosure, for instance, any one of the previously shown. As described before, the piezoelectric transducers 13 are organized into the array 60 of rows and columns. In particular the array 60 has a size of M*N as it consists of N columns and M rows of transducers 13. A control signal 17 could be applied to each single piezoelectric transducer 13 by the controller 16, so that it can be controlled individually.
In the array 60 shown in (a) in FIG. 6, each row of piezoelectric transducers 13 comprises at least three groups 14 of piezoelectric transducers 13. The transducers 13 in each row can be actuated by the delayed electrical control signals, so as to move the fluid along the fluid direction 11 aligned with the rows. That is, in (a) the flow direction 11 is horizontal in FIG. 6. Alternatively, as shown in (b), the transducers 13 inside each column may be actuated by the delayed electrical control signals, so that the flow direction 11 is in vertical direction. In this case, each column comprises at least three groups 14 of piezoelectric transducers. Notably, in FIG. 6 each group 14 may have only one transducer 13, i.e., each transducer 13 may corresponds to a group 14, and each row (a) or column (b) may comprise more than three groups 14.
FIG. 7 shows exemplary array 60 of piezoelectric transducers 13 for an acoustic micropump device 10 according to this disclosure, for instance, any one of the previously shown. As described before, the piezoelectric transducers 13 are organized into the array 60 of rows and columns.
In FIG. 7, the at least one piezoelectric transducer 13 of each group 14 of the at least three groups 14 is arranged in a different row and in a different column than the at least one piezoelectric transducer 13 of the other groups 14 of the at least three groups 14. In (a) of FIG. 7, each group 14 is one transducer 13 and the transducers 13 share neither row nor column. That is a diagonal flow direction 11 through the array 11 can be achieved by applying the delayed electrical control signals 17 to these groups 14 (transducers 13). In (b), each group 14 comprises more than one, e.g. four, transducers 13, which may be shorted to be actuated by the identical electrical control signal 17. The groups 14 neither share row nor column, so that again a diagonal flow direction 11 is achievable.
FIG. 8 shows exemplary electrical control signals 17, which can be used in an acoustic micropump device 10 according to this disclosure to control different groups 14 of piezoelectric transducers 13.
For example, the piezoelectric transducers 13 may be PMUTs. A piezoelectric transducer 13 is typically driven under resonant frequency f_r (in the MHz range, e.g. 1-40 MHz). The actuating signal of any piezoelectric transducer 13 in the micropump device 10 may be one of the electrical control signals 17 that has the same frequency as its resonant frequency, i.e., f_signal = f_r . At least three different control signals 17 are needed to generate a directional travelling acoustic wave in the fluid channel 12. These at least three electrical control signals 17 (e.g., the first, second and third electrical control signals 17a, 17b, 17c) may only differ with a delay (i.e. they may otherwise have identical shape, amplitude and frequency). The electrical control signals 17 may be periodic that may be repeated over multiple periods of time.
A time period for the at least three electrical control signals 17 for a set 24 of piezoelectric transducers 13 may be $T_{signal} = \frac{1}{f_{r}}$ (and may be in the µs range). A time delay between adjacent periods of control signals may be $Δ t_{signal} = \frac{T_{signal}}{N}$ (where N is the number of piezoelectric transducers 13 included within one set 24). To generate a directional travelling acoustic wave, n ≥ 3. The generated travelling wave period may be T_wave = T_signal , and the travelling wave frequency may be f_wave = f_signal = f_r .
In the simplest case shown in (a), the electrical control signals 17 are continuous periodic sinusoidal signals. A pumping velocity in this case can be controlled by the amplitude of the applied voltage.
In (b), the electrical control signals 17 are pulsed signals, which are periodically applied at different time instances to the transducers 13. A time period for each repetition of the at least three control signals 17 is still determined by the resonant frequency of the PMUTs $T_{signal} = \frac{1}{f_{r}} (in μs range)$ .
In (c), pulse width modulation is used. A non-active time delay can be added in between each period of the control signals 17. A pumping velocity can be controlled by Δt_( non-active ).
FIG. 9 shows a flow-diagram of a method 90 for operating an acoustic micropump device according to this disclosure, for instance, any of the previously described devices 10. The method 90 is for creating the net flow of fluid along the flow direction 11. The method 90 comprises a step 91 of actuating the plurality of piezoelectric transducers 17 of the device 10 using the at least three periodic electrical control signals 17, wherein each electrical control signal 17 is used to actuate all piezoelectric transducers 13 of one group 14 of the at least three groups 14 of piezoelectric transducers 13 of a set. The method 90 also includes the step 92 of consecutively delaying the at least three electrical control signals 17 to another in accordance with the consecutively arranged groups 14 of the set associated with the electrical control signals 17. Notably, the steps 91 and 92 may be performed by the controller 16, and may be performed as one step.
FIG. 10 shows simulation results for a micropump device 10 where three single PMUTs 13 were included in one set 24, in order to demonstrate the generation of the directional flexural acoustic plate wave. According to the simulation results, under an AC excitation voltage amplitude of 10 V, the phase shifted PMUTs can generate a flexural plate wave displacement in the sub micrometer range, as shown in (a). The displacement is much larger - a maximum displacement of about 70 nm was achieved - than that of the SAW generated under the same excitation voltage amplitude by an IDT, which is in sub nanometer range. This leads to much higher acoustic pressure, as shown in (b) - a maximum acoustic pressure of almost 200 kPa was achieved - and this leads also a higher acoustic streaming velocity. A complete multiphysics simulation was further carried out between thermoviscous acoustic domain and laminar flow domain to extract the acoustic streaming flow velocity of the phase shifted PMUT micropump. A travelling flexural plate wave with three wavelength could be generated at the solid-fluid interface. According to the simulation results, by applying AC control signals 17 with the same amplitude and an equidistant phase delay of 120° between adjacent PMUTs, a directional acoustic streaming net flow can be successfully generated from the inlet to the outlet of the fluid channel 12The acoustic streaming flow velocity is found to be maximum under device resonance, and the flow at both inlet and outlet follows the Poiseuille flow profile. Compared to a conventional SAW micropump device, the micropump device 10 of this disclosure with the delay-controlled PMUTs 13 significantly increases the pumping flow velocity from a few mm/min to hundreds of mm/min, and can increase the pumping flow rate through the fluid channel 12 from a few µL/min to a few hundred µL/min. Further, for larger cavity sizes (cavity widths) of the PMUTs 13, the micropump device 10 may achieve a larger membrane displacement and an even higher pumping flow rate, as shown in (c).
In the claims as well as in the description of this disclosure, the word 'comprising' does not exclude other elements or steps and the indefinite article 'a' or 'an' does not exclude a plurality. A single element may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

An acoustic micropump device (10) for creating a net flow of fluid along a flow direction (11), the acoustic micropump device (10) comprising:
a fluid channel (12) for the fluid;

a plurality of piezoelectric transducers (13) arranged adjacent to the fluid channel (12);

wherein the piezoelectric transducers (13) are organized into a set (24) comprising at least three groups (14), the groups (14) being consecutively arranged along the flow direction (11), and each group (14) comprising at least one piezoelectric transducer (13);

wherein each piezoelectric transducer (13) comprises a respective membrane (15), which is configured to vibrate when the piezoelectric transducer (13) is electrically actuated and to acoustically couple to the fluid channel (12); and

a controller (16) configured to actuate the plurality of piezoelectric transducers (13) using at least three periodic electrical control signals (17), each electrical control signal (17) being associated with one group (14) of the at least three groups (14) of the set (24);

wherein the controller (16) is configured to consecutively delay the at least three electrical control signals (17) to another, in accordance with the consecutively arranged groups (14) of the set (24) associated with the electrical control signals (17), to create the net flow of fluid along the flow direction (11).
The acoustic micropump device (10) according to claim 1,
wherein the piezoelectric transducers (13) are organized into two or more sets (24), the sets (24) being consecutively arranged along the flow direction (11);

wherein each set (24) comprises at least three groups (14) of piezoelectric transducers (13), which are consecutively arranged along the flow direction (11); and

wherein the controller (16) is configured to actuate the piezoelectric transducers (13) of each set (24) using the same at least three electrical control signals (17), each electrical control signal (17) being associated with one group (14) of the set (24), to create the net flow of fluid along the flow direction (11).
The acoustic micropump device (10) according to claim 1 or 2,
wherein the at least three groups (14) of the set (24) comprise a first group (14a), a second group (14b), and a third group (14c), which are arranged in this order along the flow direction (11), and

wherein the at least three electrical control signals (17) are phase-shifted versions of a common periodic electrical control signal, wherein the at least three electrical control signals (17) are phase-shifted to another in accordance with the consecutively arranged groups (14) of the set (24) associated with the electrical control signals (17) and comprise:
a first periodic electrical control signal (17a) having a phase shift in a range of -120° to -70°, and being associated with the first group (14a);

a second periodic electrical control signal (17b) having a phase shift of 0°, and being associated with the second group (14b); and

a third periodic electrical control (17c) signal having a phase shift in a range of +70° to +120°, and being associated with the third group (14c).
The acoustic micropump device (10) according to one of the claims 1 to 3,
wherein the piezoelectric transducers (13) are organized into an array (60) of rows and columns;

wherein each row of piezoelectric transducers (13) is one group (14) of piezoelectric transducers (13); or

wherein each row of piezoelectric transducers (13) comprises at least three groups (14) of piezoelectric transducers (13).
The acoustic micropump device (10) according to one of the claims 1 to 3,
wherein the piezoelectric transducers (13) are organized into an array (60) of rows and columns;

wherein the at least one piezoelectric transducer (13) of each group (14) of the at least three groups (14) of the set (24) is arranged in a different row and in a different column than the at least one piezoelectric transducer (13) of the other groups (14) of the at least three groups (14) of the set (24).
The acoustic micropump device (10) according to claim 4 or 5,
wherein the fluid channel (12) is bonded to the array (60) of piezoelectric transducers (13).
The acoustic micropump device (10) according to one of the claims 1 to 6,
wherein the respective membrane (15) of each piezoelectric transducer (13) comprises a piezoelectric layer (25), which is sandwiched between a bottom electrode (28) and a top electrode (27) of the piezoelectric transducer (13); and

wherein the fluid channel (12) comprises an elastic layer (26) attached to the respective membrane (15) and/or to the top electrode (27).
The acoustic micropump device (10) according to claim 7,
wherein at least the piezoelectric transducers (13) of the same group (14) have a common piezoelectric layer (25).
The acoustic micropump device of claim 7 or 8,
wherein the electrical control signals (17) are applied to the top electrodes (27) of the piezoelectric transducers (17) and the bottom electrodes (28) of the piezoelectric transducers (13) are grounded; or

wherein the electrical control signals (17) are applied to the bottom electrodes (28) of the piezoelectric transducers (13) and the top electrodes (27) of the piezoelectric transducers (13) are grounded.
The acoustic micropump device (10) according to claim 9,
wherein the top electrodes (27) or the bottom electrodes (28) of two or more piezoelectric transducers (13) of the same group (14) are commonly connected to the controller (16) for receiving the same electrical control signal (17).
The acoustic micropump device (10) of claim 7 or 8, wherein the electrical control signals (17) are applied to the top electrodes (27) of the piezoelectric transducers (13), and the acoustic micropump device (10) further comprises
a grounded shielding layer (41) arranged between the top electrodes (27) and the fluid channel (12).
The acoustic micropump device (10) according to one of the claims 1 to 11,
wherein the respective membrane (15) of each piezoelectric transducer (13) is suspended in a cavity formed in a substrate (23) of the piezoelectric transducer (13);

wherein the cavity has a width in a range of 10-150 µm; and/or

wherein a spacing between any two adjacent piezoelectric transducers (13) is at least 50-200 µm.
A method (90) of operating an acoustic micropump device (10) according to one of the claims 1 to 12, so as to create the net flow of fluid along the flow direction (11), the method (90) comprising:
actuating (91) the plurality of piezoelectric transducers (17) using the at least three periodic electrical control signals (17), wherein each electrical control signal (17) is used to actuate all piezoelectric transducers (13) of one group (14) of the at least three groups (14) of the set (24); and

consecutively delaying (92) the at least three electrical control signals (17) to another in accordance with the consecutively arranged groups (14) of the set (24) associated with the electrical control signals (17).
The method (90) of claim 13, wherein
the at least three electrical control signals (17) are phase-shifted versions of a common periodic electrical control signal,

a first electrical control signal (17a) of the at least three electrical control signals (17) has a phase shift in a range of -120° to -70°, and is used to actuate all piezoelectric transducers (13) of a first group (14a) of the at least three groups (14) of piezoelectric transducers (13) of the set (24);

a second electrical control signal (17b) of the at least three electrical control signals (17) has a phase shift of 0°, and is used to actuate all piezoelectric transducers (13) of a second group (14b) of the at least three groups (14) of piezoelectric transducers (13) of the set (24); and

a third electrical control signal (17c) of the at least three electrical control signals (17) has a phase shift in a range of +70° to +120°, and is used actuate all piezoelectric transducers (13) of a third group (14c) of the at least three groups (14) of piezoelectric transducers (13) of the set (24); and

the first group (14a), the second group (14b), and the third group (14c) are arranged in this order along the flow direction (11).
The method (90) of claim 14, wherein
the first electrical control signal (17a) is further used to actuate all piezoelectric transducers (14) of a fourth group (14d) of piezoelectric transducers (13) of a further set (24);

the second electrical control signal (17b) is further used to actuate all piezoelectric transducers (13) of a fifth group (14e) of piezoelectric transducers (13) of the further set (24); and

the third electrical control signal (17c) is further used actuate all piezoelectric transducers (13) of a sixth group (14f) of piezoelectric transducers (13) of the further set (24);

wherein the fourth group (14d), the fifth group (14e), and the sixth group (14f) are arranged in this order along the flow direction (11) after the first group (14a), the second group (14b), and the third group (14c).