Surface Acoustic Wave Power Research Articles

О возможности создания эффективных устройств на поверхностных акустических волнах на частотах выше 6 ГГц

In the paper the interaction of surface acoustic waves (SAWs) with reflectors and interdigital transducers (IDTs) with high aspect ratio electrodes (HAR-electrodes) are investigated. The conditions for the SAW resonant reflection and propagation and the effective IDT conversion of the SAW power into electrical power at the load have been obtained. The real possibility of the effective SAW devices creation (filters, delay lines, resonators, RFID tags) at frequencies above 6 GHz (up to 12-15 GHz) using HAR-electrodes was demonstrated.

Controlling bacterial growth and inactivation using thin film-based surface acoustic waves.

Formation of bacterial films on structural surfaces often leads to severe contamination of medical devices, hospital equipment, implant materials, etc., and antimicrobial resistance of microorganisms has indeed become a global health issue. Therefore, effective therapies for controlling infectious and pathogenic bacteria are urgently needed. Being a promising active method for this purpose, surface acoustic waves (SAWs) have merits such as nanoscale earthquake-like vibration/agitation/radiation, acoustic streaming induced circulations, and localised acoustic heating effect in liquids. However, only a few studies have explored controlling bacterial growth and inactivation behaviour using SAWs. In this study, we proposed utilising piezoelectric thin film-based SAW devices on a silicon substrate for controlling bacterial growth and inactivation with and without using ZnO micro/nanostructures. Effects of SAW powers on bacterial growth for two types of bacteria, i.e., E. coli and S. aureus, were evaluated. Varied concentrations of ZnO tetrapods were also added into the bacterial culture to study their effects and the combined antimicrobial effects along with SAW agitation. Our results showed that when the SAW power was below a threshold (e.g., about 2.55 W in this study), the bacterial growth was apparently enhanced, whereas the further increase of SAW power to a high power caused inactivation of bacteria. Combination of thin film SAWs with ZnO tetrapods led to significantly decreased growth or inactivation for both E. coli and S. aureus, revealing their effectiveness for antimicrobial treatment. Mechanisms and effects of SAW interactions with bacterial solutions and ZnO tetrapods have been systematically discussed.

Manipulating exchange bias in Co/IrMn films by surface acoustic wave

Exchange bias (EB) has extremely important applications in spintronics, researchers have proposed various means to manipulate it. This work realizes the regulation of the EB field in Co/IrMn films sputtered on LiNbO3 substrate by the surface acoustic wave (SAW). The experimental results show that in the out-of-plane and in-plane EB of Co/IrMn films, both the coercivity and the EB field decrease with the increase of the SAW power. The dynamic strain field provided by the SAW transfer to the magnetic films changes the arrangement of the magnetic moments in the Co layer and IrMn layer, the rearrangement of magnetic moments leads to a reduction in the EB field. Our experiments provide an approach to manipulate the EB field, opening a potential avenue for manipulating antiferromagnetic moments in the future.

Acoustic charge transport in organic semiconductor films

We demonstrate the acoustic charge transport of optically induced excitons in two organic semiconductors, P3HT and MEH-PPV, up to a distance of 3 mm. The device consists of a surface acoustic wave (SAW) resonator transmitting SAW through a polymer layer where acoustic charge transport takes place and a polymer diode at the end to collect the charges. The voltage excitation is provided using an interdigital transducer (IDT) on a piezoelectric YZ lithium niobate substrate producing Rayleigh SAW at 42 MHz. Optical illumination up to 15 mW cm−2 intensity is applied to induce excitons in the polymer layer deposited on the lithium niobate substrate. The photogenerated excitons in the polymer are ionized by SAW field resulting in free carriers that are transported to the polymer diode by the travelling SAW. A surge in photovoltaic current in the diode is observed in the presence of SAW when the carriers are optically generated away from the diode. The maximum charge capacity and transfer efficiency of the acoustic transport are calculated for various SAW power and illumination intensities. A theoretical analysis of charge carrier dynamics in the presence of a moving SAW field is also performed using a semi-classical Hamiltonian of the system.

Coupling mechanism of kinetic and thermal impacts of Rayleigh surface acoustic waves on the microdroplet

An experimental study has been conducted to investigate the coupling mechanism between thermal and kinetic impacts of surface acoustic waves (SAW) using a water droplet (25 µl) on the zinc oxide (ZnO) thin-film piezoelectric substrate fabricated on an aluminium plate. The temperature is measured by an infrared (IR) thermal camera, and fluid streaming was detected by particles image velocimetry (PIV). The input power ranges from 0.096 W to 3.2 W resulting in a temperature rise and streaming velocity in the droplet up to 55 °C and 24.6 mm/s, respectively. It is found that the thermal impact is insignificant at lower input power (<0.50 W); however, this becomes dominant when the input power is>2.0 W. The study also found that heat inside the droplet is distributed via streaming from the heat source. The heat is distributed from the heat source where SAW power penetrates to the droplet. Another key finding of this investigation revealed that when the input power is>0.50 W, inverse heat transfer from the droplet to the substrate is observed due to the increase in fluid temperatures.

Surface Acoustic Wave Mitigation of Precipitate Deposition on a Solid Surface─An Active Self-Cleaning Strategy.

We demonstrate the application of a 20 MHz frequency surface acoustic wave (SAW) in a solid substrate to render its surface “self-cleaning”, redirecting the deposition of precipitating mass onto a nearby inert substrate. In our experiment, we confine a solution of poly(methyl methacrylate) polymer and a volatile toluene solvent between two substrates, lithium niobate and glass, at close proximity. We render the glass surface low energy by employing hydrophobic coating. In the absence of SAW excitation, we observe that the evaporation of the solvent yields polymer coating on the higher energy lithium niobate surface, while the glass surface is mostly devoid of polymer deposits. The application of a propagating SAW in the lithium niobate substrate mitigates the deposition of the polymer on its surface. As a response, we observe an increase in the deposition of the polymer precipitates on glass. Above a SAW power threshold, the polymer appears to deposit solely on glass, leaving the surface of the lithium niobate substrate devoid of polymer mass.

Reduction of Ice Adhesion Using Surface Acoustic Waves: Nanoscale Vibration and Interface Heating Effects.

Ice accumulation causes great risks to aircraft, electric power lines, and wind-turbine blades. For the ice accumulation on structural surfaces, ice adhesion force is a crucial factor, which generally has two main sources, for exampple, electrostatic force and mechanical interlocking. Herein, we present that surface acoustic waves (SAWs) can be applied to minimize ice adhesion by simultaneously reducing electrostatic force and mechanical interlocking, and generating interface heating effect. A theoretical model of ice adhesion considering the effect of SAWs is first established. Experimental studies proved that the combination of nanoscale vibration and interface heating effects lead to the reduction of ice adhesion on the substrate. With the increase of SAW power, the electrostatic force decreases due to the increase of dipole spacings, which is mainly attributed to the SAW induced nanoscale surface vibration. The interface heating effect leads to the transition of the locally interfacial contact phase from solid-solid to solid-liquid, hence reducing the mechanical interlocking of ice. This study presents a strategy of using SAWs device for ice adhesion reduction, and results show a considerable potential for application in deicing.

Surface acoustic wave-assisted spin–orbit torque switching of the Pt/Co/Ta heterostructure

Current-induced spin–orbit torque (SOT) allows for highly efficient control of magnetization for the magnetic memory and the logic application, but its high critical current density Jc limits its applications. Here, we report that the surface acoustic wave (SAW) presents a promising approach for reducing Jc in Pt/Co/Ta heterostructures with perpendicular anisotropy. By the second harmonic Hall measurements, we find that the damping-like SOT effective field is almost the same with or without SAW, implying that the reduction of Jc does not originate from the enhancement of the SOT. However, the current-induced domain wall velocity v under SAW is greatly enhanced. By fitting with the creep law under SAW, we find that lnv is linear with SAW power (P), which reveals that the effective pinning barrier is reduced linearly with increasing P. This feature can be well described by the time average of the periodically accumulated nucleation probability under SAW. The results shed light on the application of SAW-assisted SOT devices for low consumption storage.

Coalescence of Droplets in a Microwell Driven by Surface Acoustic Waves.

Microwell arrays are amongst the most commonly used platforms for biochemical assays. However, the coalescence of droplets that constitute the dispersed phase of suspensions housed within microwells has not received much attention to date. Herein, we study the coalescence of droplets in a two-phase system in a microwell driven by surface acoustic waves (SAWs). The microwell structure, together with symmetric exposure to SAW irradiation, coupled from beneath the microwell via a piezoelectric substrate, gives rise to the formation of a pair of counter-rotating vortices that enable droplet transport, trapping, and coalescence. We elucidate the physics of the coalescence phenomenon using a scaling analysis of the relevant forces, namely, the acoustic streaming-induced drag force, the capillary and viscous forces associated with the drainage of the thin continuous phase film between the droplets and the van der Waals attraction force. We confirm that droplet-droplet interface contact is established through the formation of a liquid bridge, whose neck radius grows linearly in time in the preceding viscous regime and proportionally with the square root of time in the subsequent inertial regime. Further, we investigate the influence of the input SAW power and droplet size on the film drainage time and demarcate the coalescence and non-coalescence regimes to derive a criterion for the onset of coalescence. The distinct deformation patterns observed for a pair of contacting droplets in both the regimes are elucidated and the possibility for driving concurrent coalescence of multiple droplets is demonstrated. We expect the study will find relevance in the demulsification of immiscible phases and the mixing of samples/reagents within microwells for a variety of biochemical applications.

Dynamic Behavior of Droplet Impact on Inclined Surfaces with Acoustic Waves.

Droplet impact on arbitrary inclined surfaces is of great interest for applications such as antifreezing, self-cleaning, and anti-infection. Research has been focused on texturing the surfaces to alter the contact time and rebouncing angle upon droplet impact. In this paper, using propagating surface acoustic waves (SAWs) along the inclined surfaces, we present a novel technique to modify and control key droplet impact parameters, such as impact regime, contact time, and rebouncing direction. A high-fidelity finite volume method was developed to explore the mechanisms of droplet impact on the inclined surfaces assisted by SAWs. Numerical results revealed that applying SAWs modifies the energy budget inside the liquid medium, leading to different impact behaviors. We then systematically investigated the effects of inclination angle, droplet impact velocity, SAW propagation direction, and applied SAW power on the impact dynamics and showed that by using SAWs, droplet impact on the nontextured hydrophobic and inclined surface is effectively changed from deposition to complete rebound. Moreover, the maximum contact time reduction up to ∼50% can be achieved, along with an alteration of droplet spreading and movement along the inclined surfaces. Finally, we showed that the rebouncing angle along the inclined surface could be adjusted within a wide range.

Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity

Surface acoustic waves (SAWs) in the GHz frequency range can inject spin currents dynamically into adjacent non-magnetic layers via the spin pumping effect associated with ferromagnetic resonance. Here, we demonstrate an enhancement of acoustic ferromagnetic resonance and spin current generation by a pair of SAW reflector gratings, which form an acoustic analog of the distributed Bragg reflector cavity. In the experiment, we confirmed 2.04 ± 0.02 times larger SAW power absorption in a device with cavity than in the case of no acoustic cavity. We confirmed up to 2.96 ± 0.02 times larger spin current generation by measuring electric voltages generated by the inverse Edelstein effect at the interface between Cu and Bi2O3. The results suggest that acoustic cavities would be useful to enhance the conversion efficiency in SAW driven coupled magnon–phonon dynamics.

Wide range of droplet jetting angles by thin-film based surface acoustic waves

Nozzleless jetting of droplets with different jetting angles is a crucial requirement for 2D and 3D printing/bioprinting applications, and Rayleigh mode surface acoustic waves (SAWs) could be a potential technique for achieving this purpose. Currently, it is critical to vary the jetting angles of liquid droplets induced by SAWs and control the liquid jet directions. Generally, the direction of the liquid jet induced by SAWs generated from a bulk piezoelectric substrate such as LiNbO3 is along the theoretical Rayleigh angle of ∼22°. In this study, we designed and manufactured thin-film SAW devices by depositing ZnO films on different substrates (including silicon and aluminium) to realize a wide range of jetting angles from ∼16° to 55° using propagating waves generated from one interdigital transducer. We then systematically investigated different factors affecting the jetting angles, including liquid properties, applied SAW power and SAW device resonant frequency. Finally, we proposed various methods using thin-film SAW devices together with different transducer designs for realizing a wide range of jetting angles within the 3D domain. A nozzleless jetting method is proposed using thin-film based surface acoustic wave devices to achieve a wide range of jetting angles for droplets.

Acoustothermal heating in surface acoustic wave driven microchannel flow

Surface acoustic wave (SAW) is a well-proven tweezer serving various applications such as particle manipulation, cell trapping and separations, fluid mixing, and biosensing. SAWs can cause significant heat generation due to acoustothermal heating as established experimentally. Theoretical understanding of acoustothermal heating is limited, primarily due to the difficulties presented by multiple time scales inherent in this phenomenon. We present a theoretical model based on a multiple scale perturbation approach to solve the fluid flow and heat transfer equations for SAW-driven acoustothermal heating of a Newtonian fluid in a microchannel. The first order fields are oscillatory with the same frequency as that of the SAW, whereas the second order components are time-averaged to account for the mean flow and temperature fields. We find that the temperature rise depends solely on the acoustic energy density and its conversion into internal energy via pressure work on the fluid and hydrodynamic transportation of heat. For a fixed aspect ratio, an increase in system size essentially increases the conversion of acoustic energy into internal energy, leading to an increase in temperature rise. On the other hand, an increase in SAW frequency for a given system size causes the acoustic energy density to increase and thereby increases the temperature rise. Temperature rise is found to increase linearly with SAW power, in agreement with experimental results reported in the literature. The quantitative model for the temperature field presented in this work will find applications in designing biosensors, microreactors, and in other SAW driven controllable digital microfluidic heating applications.

Computational and experimental analysis of droplet transportation/jetting behaviours driven by thin film surface acoustic waves

A Coupled Level Set Volume of Fluid (CLSVOF) approach has been applied to investigate severe deformation/transportation/jetting behaviours of sessile droplet driven by thin-film surface acoustic waves (SAW) devices. For validation of this computational method, a series of experimental studies of droplet transportation/jetting were performed using ZnO/Si thin film based SAW devices with resonant frequencies ranging from 64.49 MHz to 271.36 MHz. Good agreements between the computational and experimental results showed the capability of the developed CLSVOF method in modelling complex acoustofluidics phenomena such as significant internal streaming, pumping and jetting of the droplet driven by the propagating SAW.Results obtained from the computational model are used to clarify the fluidic mechanisms of droplet oscillation and wobbling behaviours during transportation. Numerical results reveal the liquid streaming patterns and airflow velocity field around the droplet at different stages of transportation/jetting process. Effects of droplet volume, the resonant frequency of SAW devices and applied SAW power on droplet transportation/jetting were investigated both theoretically and experimentally. In particular, comparisons between experimental and computational results showed that the model predicted well the minimum RF power to start droplet pumping and jetting at various resonant frequencies.

Unidirectional planar Hall voltages induced by surface acoustic waves in ferromagnetic thin films

The electromotive forces induced by surface acoustic waves (SAWs) are investigated in ferromagnetic thin films. CoFeB thin films deposited on LiNbO$_3$ substrates are patterned into Hall-bars to study the acoustoelectric transport properties of the device. The longitudinal and transverse dc voltages that develop in the Hall bars, which are parallel and orthogonal to the flow of the SAW, respectively, are measured under application of an in-plane magnetic field. The longitudinal voltage scales linearly with the SAW power and reverses its polarity upon changing the direction to which the SAW propagates, suggesting generation of a dc acoustic current via the SAW excitation. The magnetic field has little influence on the acoustic current. In contrast, the SAW induced transverse voltage shows significant dependence on the relative angle between the magnetic field and the SAW propagation direction. Such field angle dependent voltage resembles that of the planar Hall voltage induced by electric current. Interestingly, the angle dependent acoustic transverse voltage does not depend on the SAW propagation direction. Moreover, the magnitude of the equivalent angle dependent acoustic transverse resistance is more than one order of magnitude larger than that of the planar Hall resistance. These results show the unique acoustoelectric transport properties of ferromagnetic thin films.

Size tunable nanoparticle formation employing droplet fusion by acoustic streaming applied to polyplexes

Automated mixing of fluids with control over mixing parameters is of highest importance for reproducible production and chemical synthesis processes. We here introduce surface acoustic waves (SAW) induced mixing of µl droplets for tailorable nanoparticle (NP) formation. Nucleic acid therapeutics represent extremely potent and innovative approaches to a variety of medical challenges, such as the treatment of cancer and genetic diseases. In this study, we apply this method to produce nucleic acid polymer complexes. Fusing two droplets containing either pDNA or cationic polymers leads to the formation of well-defined polyplexes. We show that droplet size and incubation time do not influence the desired particle characteristics significantly. However, the resulting nanoparticle diameter strongly depends on the SAW power level and educt concentrations, which indicates a kinetically controlled assembly process, while the particle shape is largely unaffected. Applying our novel technique to the formation of three-component-NP, we find that the choice of the mixing order can be used to decrease NP size even further. To address the kinetic interplay between mixing and particle growth, we apply our technique to homogenous mixing at high salt concentrations followed by a subsequent dilution step. Finally, by comparing various hand and SAW mixed polyplexes, we demonstrate significant differences in size while the cytotoxicity and in vitro efficacy remain roughly the same.

Enhanced performance of 17.7 GHz SAW devices based on AlN/diamond/Si layered structure with embedded nanotransducer

Surface acoustic wave (SAW) devices using embedded interdigital transducers (IDTs) on an AlN/diamond/Si layered substrate are fabricated, and their performances are investigated. The Sezawa mode is the dominant resonance with the highest resonant frequency up to 17.7 GHz, a signal amplitude of 20 dB, and an electromechanical coupling coefficient of 0.92%. Comparing these SAW devices with those having the conventional IDTs on the same layered structure, the output SAW power and resonant frequency of devices are improved by 10.7% and 1.1%, respectively, for the embedded IDT devices. This is because the different field distribution leads to the different Bragg reflection and phase velocity for the two types of IDTs. The radiation frequency characteristics indicate that the advantages of the embedded IDTs would be useful for high frequency, high power applications such as monolithic integrated millimeter-wave integrated circuit and high speed communications.

Acousto-electric transport in MgO/ZnO-covered graphene on SiC

We investigate the acousto-electric transport induced by surface acoustic waves (SAWs) in epitaxial graphene (EG) coated by a MgO/ZnO film. The deposition of a thin MgO layer protects the EG during the sputtering of a piezoelectric ZnO film for the efficient generation of SAWs. We demonstrate by Raman and electric measurements that the coating does not harm the EG structural and electronic properties. We report the generation of two SAW modes with frequencies around 2 GHz. For both modes, we measure acousto-electric currents in EG devices placed in the SAW propagation path. The currents increase linearly with the SAW power, reaching values up to almost two orders of magnitude higher than in previous reports for acousto-electric transport in EG on SiC. Our results agree with the predictions from the classical relaxation model of the interaction between SAWs and a two dimensional electron gas.

Switching of magnetic moments of nanoparticles by surface acoustic waves

We report evidence of the magnetization reversal in nanoparticles by surface acoustic waves (SAWs). The experimental system consists of isolated magnetite nanoparticles dispersed on a piezoelectric substrate. Magnetic relaxation from a saturated state becomes significantly enhanced in the presence of the SAW at a constant temperature of the substrate. The dependence of the relaxation on SAW power and frequency has been investigated. The effect is explained by the effective ac magnetic field generated by the SAW in the nanoparticles.

Compact SAW aerosol generator

In this work, we discuss and demonstrate the principle features of surface acoustic wave (SAW) aerosol generation, based on the properties of the fluid supply, the acoustic wave field and the acoustowetting phenomena. Furthermore, we demonstrate a compact SAW-based aerosol generator amenable to mass production fabricated using simple techniques including photolithography, computerized numerical control (CNC) milling and printed circuit board (PCB) manufacturing. Using this device, we present comprehensive experimental results exploring the complexity of the acoustic atomization process and the influence of fluid supply position and geometry, SAW power and fluid flow rate on the device functionality. These factors in turn influence the droplet size distribution, measured here, that is important for applications including liquid chromatography, pulmonary therapies, thin film deposition and olfactory displays.