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Acoustic Streaming and Microparticle Enrichment within a Microliter Droplet Using a Lamb-Wave Resonator Array
Abstract
We report the nonlinear acoustic streaming effect and the fast manipulation of microparticles by microelectromechanical Lamb-wave resonators in a microliter droplet. The device, consisting of four Lamb-wave resonators on a silicon die, generates cylindrical traveling waves in a liquid and efficiently drives nine horizontal vortices within a 1−μl droplet; the performance of the device coincides with the numerical model prediction. Experimentally, the particles are enriched at the stagnation center of the main vortex on the free surface of the droplet in open space without microfluidic channels. In addition, the trajectories of the particles in the droplet can be controlled by the excitation power.
... For larger volumes, high streaming velocity is required. Based on higher frequency Lamb wave resonators (LWRs), rapid acoustic streaming energy conversion can be achieved, and it can be driven at a lower power [35]. ...
... In previous work, our research group has explored the acoustic streaming effects excited by a single LWR [36,37] and an array of four LWRs in a liquid environment, as well as using it for particle enrichment [35]. However, due to the presence of secondary vortices, it is challenging to form a single high-speed streaming tunnel for centrifuge applications. ...
... After solidification in the hydrogel, the assembly can be taken out. In previous studies, the vortices behind the LWR made the flow field distribution complex [35]. To achieve a more uniform fluid flow in the chamber and facilitate the quick enrichment of particles to the predetermined annular orbit, this paper designed a novel LWR with a new structure. ...
The µTAS/LOC, a highly integrated microsystem, consolidates multiple bioanalytical functions within a single chip, enhancing efficiency and precision in bioanalysis and biomedical operations. Microfluidic centrifugation, a key component of LOC devices, enables rapid capture and enrichment of tiny objects in samples, improving sensitivity and accuracy of detection and diagnosis. However, microfluidic systems face challenges due to viscosity dominance and difficulty in vortex formation. Acoustic-based centrifugation, particularly those using surface acoustic waves (SAWs), have shown promise in applications such as particle concentration, separation, and droplet mixing. However, challenges include accurate droplet placement, energy loss from off-axis positioning, and limited energy transfer from low-frequency SAW resonators, restricting centrifugal speed and sample volume. In this work, we introduce a novel ring array composed of eight Lamb wave resonators (LWRs), forming an Ultra-Fast Centrifuge Tunnel (UFCT) in a microfluidic system. The UFCT eliminates secondary vortices, concentrating energy in the main vortex and maximizing acoustic-to-streaming energy conversion. It enables ultra-fast centrifugation with a larger liquid capacity (50 μL), reduced power usage (50 mW) that is one order of magnitude smaller than existing devices, and greater linear speed (62 mm/s), surpassing the limitations of prior methods. We demonstrate successful high-fold enrichment of 2 μm and 10 μm particles and explore the UFCT’s potential in tissue engineering by encapsulating cells in a hydrogel-based micro-organ with a ring structure, which is of great significance for building more complex manipulation platforms for particles and cells in a bio-compatible and contactless manner.
... Its relatively low Young's modulus (as shown in Figure 6) also makes it susceptible to deformation under high-stress conditions, and it is prone to failure due to thermal stress in environments with large temperature fluctuations. Therefore, current research trends to use Lamb wave devices fabricated with Mo [86,87], with a high Young's modulus (as shown in Figure 6) and excellent corrosion resistance, or Au/Ti [16], with high conductivity and chemical stability, for liquid sensor applications. ...
A Lamb wave is a guided wave that propagates within plate-like structures, with its vibration mode resulting from the coupling of a longitudinal wave and a shear vertical wave, which can be applied in sensors, filters, and frequency control devices. The working principle of Lamb wave sensors relies on the excitation and propagation of this guided wave within piezoelectric material. Lamb wave sensors exhibit significant advantages in various sensing applications due to their unique wave characteristics and design flexibility. Compared to traditional surface acoustic wave (SAW) and bulk acoustic wave (BAW) sensors, Lamb wave sensors can not only achieve higher frequencies and quality factors in smaller dimensions but also exhibit superior integration and multifunctionality. In this paper, we briefly introduce Lamb wave sensors, summarizing methods for enhancing their sensitivity through optimizing electrode configurations and adjusting piezoelectric thin plate structures. Furthermore, this paper systematically explores the development of Lamb wave sensors in various sensing applications and provides new insights into their future development.
... In recent years, acoustic waves induced by the piezoelectric effect have been employed to drive liquids and manipulate particles in a wide range of miniaturized systems (especially microfluidic systems) [57][58][59][60][61][62][63][64][65][66][67]. These piezoelectric chips fabricated via advanced microelectromechanical system techniques [68,69] enable acoustofluidic micropumps (AFMPs) with great portability (simple and small structures, low power consumption, and low working voltage), high biocompatibility, a fast response time, and fair extensibility [23,[50][51][52][53][54], which theoretically meets the requirements of a portable system as an ideal micropump. ...
Miniaturization of health care, biomedical, and chemical systems is highly desirable for developing point-of-care testing (POCT) technologies. In system miniaturization, micropumps represent one of the major bottlenecks due to their undesirable pumping performance at such small sizes. Here, we developed a microelectromechanical system fabricated acoustic micropump based on an ultrahigh-frequency bulk acoustic wave resonator. The concept of an inner-boundary-confined acoustic jet was introduced to facilitate unidirectional flow. Benefitting from the high resonant frequency and confined acoustic streaming, the micropump reaches 32.620 kPa/cm³ (pressure/size) and 11.800 ml/min∙cm³ (flow rate/size), showing a 2-order-of-magnitude improvement in the energy transduction efficiency compared with the existing acoustic micropumps. As a proof of concept, the micropump was constructed as a wearable and wirelessly powered integrated drug delivery system with a size of only 9×9×9 mm³ and a weight of 1.16 g. It was demonstrated for ocular disease treatment through animal experimentation and a human pilot test. With superior pumping performance, miniaturized pump size, ultralow power consumption, and complementary metal–oxide–semiconductor compatibility, we expect it to be readily applied to various POCT applications including clinical diagnosis, prognosis, and drug delivery systems.
... Moreover, the simulated acoustofluidic patterns in Fig. 2 can be qualitatively compared with the existing literature, in which circumferential acoustic streaming vortices can be generated in local cavities surrounded by multiple radiation surfaces. 42,43 The calculated acoustofluidic fields and particle movement trajectories excited by four cross structures with three groups of clockwise-arranged radiation surfaces and one group of anticlockwise-arranged radiation surfaces are plotted in Fig. 3. In comparison to complete circumferential vortices existing in inner and outer regions that constituted of four cross structures in Fig. 2 due to the destruction of symmetrical characteristics, including the axial symmetry and the rotational symmetry, the acoustic streaming fields are no longer composed of single main vortices. ...
In this study, a novel strategy to generate sophisticated acoustic streaming vortices, which would be available for rotational manipulation of micro-/nano-scale objects, is proposed and simulated. All structural units in the microfluidic chamber are symmetric in design, and all radiation surfaces have the same settings of input frequency, oscillation amplitude, and initial phase. Different kinds of asymmetric acoustofluidic patterns can be generated in the originally static microfluidic chamber only because of the asymmetric arrangement of multiple radiation surfaces in space. The calculation results of kaleidoscopic acoustofluidic fields together with particle movement trajectories induced by cross structures with different radiation surface distributions further demonstrate the versatile particle manipulation capabilities of these functional microfluidic devices. In comparison to the existing oscillation modulation method, which requires multiple radiation surfaces with different initial phases, acoustofluidic devices with a same initial phase of all radiation surfaces can significantly reduce the required number of auxiliary signal generators and power amplifiers. The proposed generation method of acoustofluidic patterns is promising for microfluidic mixing without rotating machinery, driving operation of microrobots, and rotational manipulation of biological samples.
... Microfluidic chips and acoustic streaming can now be combined thanks to the rapid advancement of MEMS technology. Combining streaming with microfluidic chips can achieve advantages including low consumption, high efficiency, integrated platforms, good biocompatibility, easy manipulation, and contactless [3][4][5][6][7]. Thus, acoustic streaming has become an important tool for particle manipulation [8][9][10][11][12], cell capture [13], micromixing [14], micropump [15], material concentration [16], and chemical reactions [17]. ...
In microchannels, microstructure-induced acoustic streaming can be achieved at low frequencies, providing simple platforms for biomedicine and microfluidic manipulation. Nowadays, microstructures are generally fabricated by photolithography or soft photolithography. Existing studies mainly focused on the projection plane, while ignoring the side profile including microstructure’s sidewall and channel’s upper wall. Based on the perturbation theory, the article focuses on the effect of microstructure’s sidewall errors caused by machining and the viscous dissipation of upper wall on the streaming. We discovered that the side profile parameters, particularly the gap (gap g between the top of the structure and the upper wall of the channel), have a significant impact on the maximum velocity, mode, and effective area of the streaming.To broaden the applicability, we investigated boundary layer thickness parameters including frequency and viscosity. Under different thickness parameters, the effects of side profile parameters on the streaming are similar. But the maximum streaming velocity is proportional to the frequency squared and inversely proportional to the viscosity. Besides, the ratio factor θ of the maximum streaming velocity to the vibration velocity is affected by the side profile parameter gap g and sidewall profile angle α.
... Here, numerical simulations may play a crucial role, both in improving the understanding of the underlying physical acoustofluidic processes, and in easing the cumbersome development cycle consisting of an iterative series of creating, fabricating, and testing device designs. An increasing amount of numerical studies include piezoelectric dynamics in two-dimensional (2D) models [25][26][27][28], but in most cases the piezoelectric transducers are introduced in numeric models in the form of analytic approximations [29][30][31][32][33][34], and designs are often based on a priori knowledge of the piezoelectric effect in the unloaded substrates typically applied in telecommunications. In acoustofluidic devices, the acoustic impedance of the contacting fluid is much closer to that of the substrate, causing waves to behave very differently from those in telecommunications devices. ...
Surface-acoustic-wave (SAW) devices form an important class of acoustofluidic devices, in which acoustic waves are generated and propagate along the surface of a piezoelectric substrate. Despite their widespread use, only a few fully three-dimensional (3D) numerical simulations have been reported in the literature. In this paper, we present a 3D numerical simulation taking into account the electromechanical fields of the piezoelectric SAW device, the acoustic displacement field in the attached elastic material, in which a liquid-filled microchannel is embedded, the acoustic fields inside the microchannel, and the resulting acoustic radiation force and streaming-induced drag force acting on micro- and nanoparticles suspended in the microchannel. A specific device design is presented, for which numerical predictions of the acoustic resonances and the acoustophoretic response of suspended microparticles in three dimensions are successfully compared with experimental observations. The simulations provide a physical explanation of the observed qualitative difference between devices with acoustically soft and hard lids in terms of traveling and standing waves, respectively. The simulations also correctly predict the existence and position of the observed in-plane streaming-flow rolls. The simulation model presented may be useful in the development of SAW devices optimized for various acoustofluidic tasks.
... An increasing amount of numerical studies include piezoelectric dynamics in two-dimensional (2D) models [25][26][27][28], but mostly the piezoelectric transducers are introduced in numeric models in the form of analytic approximations [29][30][31][32][33][34], and designs are often based on a priori knowledge of the piezoelectric effect in the unloaded substrates typically applied in telecommunication. In acoustofluidic devices, the acoustic impedance of the contacting fluid is much closer to that of the substrate causing waves to behave much differently from those in telecommunications devices. ...
Surface acoustic wave (SAW) devices form an important class of acoustofluidic devices, in which the acoustic waves are generated and propagate along the surface of a piezoelectric substrate. Despite their wide-spread use, only a few fully three-dimensional (3D) numerical simulations have been presented in the literature. In this paper, we present a 3D numerical simulation taking into account the electromechanical fields of the piezoelectric SAW device, the acoustic displacement field in the attached elastic material, in which the liquid-filled microchannel is embedded, the acoustic fields inside the microchannel, as well as the resulting acoustic radiation force and streaming-induced drag force acting on micro- and nanoparticles suspended in the microchannel. A specific device design is presented, for which the numerical predictions of the acoustic resonances and the acoustophoretic repsonse of suspended microparticles in 3D are successfully compared with experimental observations. The simulation provides a physical explanation of the the observed qualitative difference between devices with an acoustically soft and hard lid in terms of traveling and standing waves, respectively. The simulations also correctly predict the existence and position of the observed in-plane streaming flow rolls. The presented simulation model may be useful in the development of SAW devices optimized for various acoustofluidic tasks.
Gigahertz acoustic streaming enables the synthesis of localized microjets reaching speeds of up to meters per second, offering tremendous potential for precision micromanipulation. However, theoretical and numerical investigations of acoustic streaming at these frequencies remain so far relatively scarce due to significant challenges including: (i) the inappropriateness of classical approaches, rooted in asymptotic development, for addressing high-speed streaming with flow velocities comparable to the acoustic velocity; and (ii) the numerical cost of direct numerical simulations generally considered as prohibitive. In this paper, we investigate high-frequency bulk streaming using high-order finite difference direct numerical simulations. First, we demonstrate that high-speed micrometric jets of several meters per second can only be obtained at high frequencies, due to diffraction limits. Second, we establish that the maximum jet streaming speed at a given actuation power scales with the frequency to the power of 3/2 in the low attenuation limit and linearly with the frequency for strongly attenuated waves. Last, our analysis of transient regimes reveals a dramatic reduction in the time required to reach the maximum velocity as the frequency increases (power law in –5/2), leading to characteristic time on the order of μs at gigahertz frequencies, and hence accelerations within the Mega-g range.
The generation and dispensing of microdroplets is a vital process in various fields such as biomedicine, medical diagnosis and chemistry. However, most methods still require the structures of nozzles or microchannels to assist droplet generation, which leads to limitations on system flexibility and restrictions on the size range of the generated droplets. In this paper, we propose a nozzle-free acoustic-based method for generating droplets using a gigahertz (GHz) bulk acoustic wave (BAW). Unlike most of the acoustofluidic approaches, the proposed method produces the droplet by pinching-off the liquid column generated by the acoustic body force at the oil-water interface. Benefitting from the focused acoustic energy and small footprint of the device, four orders of magnitude (ranging from 2 μm to 1800 μm) of droplet size could be produced by controlling the working time and power of the device. We also demonstrated cell encapsulation in the droplet and a high cell viability was achieved. The proposed acoustic-based droplet generation method exhibits capacity for generating droplets with a wide size range, versatility toward different viscosities, as well as biocompatibility for handling viable samples, which shows potential in miniaturization and scalability.
Diffusion limitations and nonspecific surface absorption are great challenges for developing micro-/nanoscale affinity biosensors. There are very limited approaches that can solve these issues at the same time. Here, an acoustic streaming approach enabled by a gigahertz (GHz) resonator is presented to promote mass transfer of analytes through the jet mode and biofouling removal through the shear mode, which can be switched by tuning the deviation angle, α, between the resonator and the sensor. Simulations show that the jet mode (α ≤ 0) drives the analytes in the fluid toward the sensing surface, overcomes the diffusion limitation, and enhances the binding; while the shear mode (0 < α < π/4) provides a scouring action to remove the biofouling from the sensor. Experimental studies were performed by integrating this GHz resonator with optoelectronic sensing systems, where a 34-fold enhancement for the initial binding rate was obtained. Featuring high efficiency, controllability, and versatility, we believe that this GHz acoustic streaming approach holds promise for many kinds of biosensing systems as well as lab-on-chip systems.
Over the past few decades, acoustofluidics, one of the branches of microfluidics, has rapidly developed as a multidisciplinary cutting edge research topic, covering many biomedical and bioanalytical applications. Acoustofluidics usually utilizes acoustic pressure and acoustic streaming effects to manipulate liquids and bioparticles. Acoustic manipulation using acoustic radiation force has been widely studied; however, with the recent development of new piezoelectric devices that enable faster acoustic streaming, particle manipulations using drag force induced by acoustic streaming have attracted more attention. Despite many review articles on acoustic radiation force-based acoustophoresis, acoustic streaming is less frequently covered. Here, we review the recent development of microscale acoustic streaming, especially high-frequency transducer-induced high-speed streaming, confinement and programed streaming, and acoustic streaming tweezers, which combine the acoustic radiation force and drag force to tackle the size limitations of conventional acoustic manipulations. A brief review of acoustic streaming theory and its generation is summarized. Recent progress in applying acoustic streaming for fluidic handling and bioparticle manipulations is reviewed. Representative applications of micro acoustic streaming are provided, and the key issues in these applications are analyzed. Finally, the future prospects of micro acoustic streaming in bioanalytical and biomedical applications are discussed.
Aggregation-dependent brightness (ADB) indirectly limits the in vitro performance of a pure aggregation-induced emission (AIE) probe in many ways; thus, controlling the aggregation state of the AIE probe is helpful for detecting an object of interest. Many studies are focused on the molecule design of the AIE probes, while less efforts have been made for the control of the aggregation of the AIEs. Here, an acoustic streaming tweezer (AST) generated using a gigahertz bulk acoustic wave resonator was applied to manipulate the aggregation status of the AIE probe and further enhance their performance for human serum albumin (HSA) detection. As the trapping size of the AST matches the working size of the AIE probe, the streaming can enrich and accumulate AIE nanoparticles, which then further trigger larger aggregates. Due to the ADB effect, the fluorescence intensity strongly increased, and thus, the detection limit of HSA was reduced to 0.5 μg/mL, which is low enough for kidney disease detection. Such an AST-assisted ADB strategy is potentially applicable to other AIE probes and can work as a portable choice for the biomedical detection.
Contactless acoustic manipulation of micro/nanoscale particles has attracted considerable attention owing to its near independence of the physical and chemical properties of the targets, making it universally applicable to almost all biological systems. Thin-film bulk acoustic wave (BAW) resonators operating at gigahertz (GHz) frequencies have been demonstrated to generate localized high-speed microvortices through acoustic streaming effects. Benefitting from the strong drag forces of the high-speed vortices, BAW-enabled GHz acoustic streaming tweezers (AST) have been applied to the trapping and enrichment of particles ranging in size from micrometers to less than 100 nm. However, the behavior of particles in such 3D microvortex systems is still largely unknown. In this work, the particle behavior (trapping, enrichment, and separation) in GHz AST is studied by theoretical analyses, 3D simulations, and microparticle tracking experiments. It is found that the particle motion in the vortices is determined mainly by the balance between the acoustic streaming drag force and the acoustic radiation force. This work can provide basic design principles for AST-based lab-on-a-chip systems for a variety of applications.
The contactless manipulation of microparticles and cells by using acoustic forces is important in many applications. However, multi-band acoustophoresis has been rarely investigated in the literature. In this Letter, we propose a microscale acoustofluidic system that has multiple orders of available Lamb modes for the acoustic trapping of microparticles at various frequencies. In our device, standing Lamb waves (SLWs) of specific orders can be selectively excited in a 300-μm-thick piezoelectric lithium-niobate (LiNbO3) crystal plate by a pair of interdigitated transducers (IDTs) at the corresponding frequency. We demonstrate the acoustophoretic trapping and patterning of 7-μm particles in a single acoustofluidic device with multiple available actuating frequencies. The approach to the proposed design and the working mechanisms are explained by using thin plate and a full-wave models that solve the dispersion relations and coupling fields of the piezoelectric SLW acoustofluidic system, respectively. Furthermore, we experimentally show that the stable and tight trapping of particles in the chamber can be achieved independently along two mutually orthogonal directions. This provides the essential ground for planar manipulations of microparticles and cells based on the proposed device. The results here can trigger more innovative designs and applications of acoustofluidic devices for microparticle manipulation and microfluidic mixing, with multi-frequency channels and a wide span of different actuating frequencies in one system.
This paper investigates the mechanism of a new acoustic micro-ejector using a Lamb wave transducer array, which can stably generate picoliter (pL) droplet jetting without nozzles. With eight transducers arranged as an octagon array, droplets are ejected based on the mechanism of combined acoustic pressure waves and acoustic streaming. The acoustic focusing area is designed as a line at the liquid center, which is the key factor for a large working range of liquid height. The experimental results show that the ejector can produce uniform water droplets of 22 μm diameter (5.6 pL in volume) continuously at a rate of 0.33 kHz with high ejection stability, owing to a large liquid height window and high acoustic wave frequency. By delivering precise ∼pL droplets without clogging issues, the acoustic ejector has great potential for demanding biochemical applications.
Acoustic tweezers are gaining increasing attention as a noncontact method that is capable of handling microparticles and nanoparticles in a controllable manner. By designing the acoustic field, objects, such as cells, bacteria, exosomes, and even worms, could be precisely and flexibly manipulated by the acoustic radiation force. With the advantages of non-invasiveness, label-free operation, and low power consumption, acoustic tweezers have been proven to be crucially important for a diverse range of applications, particularly in the biomedical domain. In this paper, we review the historical development and the current state of the theory of the acoustic radiation force. Furthermore, we introduce recent advancements in acoustic tweezers based on the standing wave, travelling wave, single beam, and arbitrary wave fields; its mechanism and potential applications are also presented. Finally, some perspectives referring to the future development of acoustic tweezers are discussed.
Profiling the kinetics of cell-matrix adhesion is of great importance to understand many physiological and pathological processes such as morphogenesis, tissue homeostasis, wound healing, and tumorigenesis. Here, we developed a novel digital acoustofluidic device for parallel profiling cell-matrix adhesion at the single-cell level. By introducing a localized and uniform acoustic streaming into an open chamber microfluidic device, the adherent cells within the open chamber can be detached by the streaming-induced Stokes drag force. By digitally regulating pulsed acoustic power from a low level to high levels, the hundreds of adherent cells can be ruptured from the fibronectin-coated substrate accordingly, and their adhesive forces (from several pN to several nN) and kinetics can be determined by the applied power and cell incubation time. As a proof-of-concept application for studying cancer metastasis, we applied this technique to measure the adhesion strength and kinetics of human breast cancer cells to extracellular matrix such as fibronectin and compared their metastatic potentials by measuring rupture force of cancer cells representing malignant (MCF-7 cells and MDA-MB-231 cells) and non-malignant (MCF-10A cells) states. Our acoustofluidic device is simple, easy for operation, and capable of parallelly measure hundreds of individual cells’ adhesion forces with a resolution at the pN level. Thus, we expect this device could be widely used for both fundamental cell biology research as well as the development of cancer diagnostics and tissue engineering technologies.
Performance of electroanalytical lab-on-a-chip devices is often limited by the mass transfer of electroactive species towards the electrode surface, due to the difficulty in applying external convection. This article describes the powerful signal enhancement attained with a 2.54 GHz miniature acoustic resonator integrated with an electrochemical device in a miniaturized cell. Acoustic resonator and an on-chip gold thin-film three-electrode electrochemical cell were arranged facing each other inside a structured polymethyl methacrylate chamber. Cyclic voltammetric and chronoamperometric responses of 1 mM ferrocene-methanol were recorded under resonator’s actuation at powers ranging from 0 to 1 W. Finite element analysis was carried out to study the sono-electroanalytical process. Acoustic resonator’s actuation greatly enhances the mass transport of electroactive species towards the electrode surface. The diffusion limited cyclic voltammetric and chronoamperometric currents increase around 10 and 20 times respectively with an input power of 1 W compared to those recorded under stagnant condition. The improvement in electroanalytical process is mainly associated with acoustic resonator’s vibration induced fluid streaming. The advantages of miniaturized acoustic resonator, including the submillimeter small size, amenability for mass fabrication, cost effectiveness, low energy consumption, as well as outstanding enhancement of coupled electrochemical processes, will enable the production of highly-sensitive compact electroanalytical devices.
Microcentrifugation constitutes an important part of the microfluidic toolkit in a similar way that centrifugation is crucial to many macroscopic procedures, given that micromixing, sample preconcentration, particle separation, component fractionation, and cell agglomeration are essential operations in small scale processes. Yet, the dominance of capillary and viscous effects, which typically tend to retard flow, over inertial and gravitational forces, which are often useful for actuating flows and hence centrifugation, at microscopic scales makes it difficult to generate rotational flows at these dimensions, let alone with sufficient vorticity to support efficient mixing, separation, concentration, or aggregation. Herein, the various technologies—both passive and active—that have been developed to date for vortex generation in microfluidic devices are reviewed. Various advantages or limitations associated with each are outlined, in addition to highlighting the challenges that need to be overcome for their incorporation into integrated microfluidic devices.
We demonstrate a type of boundary-driven streaming in a subwavelength slit (the slit width is smaller than the acoustic wavelength) between two phononic crystal plates (PCPs) by using the limiting velocity method. It is found that this streaming has the characteristics of four-quadrant streaming similar to classical Rayleigh streaming, and the smaller the slit width, the larger is the maximum velocity of streaming. The maximum velocity of the demonstrated streaming is approximately four orders of magnitude higher than that of classical Rayleigh streaming in two rigid plates with the same slit width of 15 μm. In addition, in the case of this streaming, the vortex sizes are related to the lattice constant of the PCPs, which can be much smaller than those realized in Rayleigh streaming. These effects originate from the strong antiphase coupling of the nonleaky zero-order antisymmetric A0 Lamb modes intrinsic to these thin plates. We denote this streaming as ultrafast Rayleigh-like streaming, which can find potential application in acoustofluidic devices for particle manipulation, rapid mixing and chemical reactions, etc.
In this paper, we propose and numerically simulate a novel optical trapping process based on the
enhancement and the confinement of both magnetic and electric near-fields by using gold Diabolo
Antenna (DA). The later was recently proposed to generate huge magnetic near-field when illuminated
by linearly polarized wave along its axis. Numerical 3D FDTD simulation results demonstrate the high
confinement of the electromagnetic field in the vicinity of the DA. This enhancement is then exploited
for the trapping of nano-particles (NP) as small as 30 nm radius. Results show that the trapping process
greatly depends on the particle dimensions and that three different regimes of, trapping at contact,
trapping without contact, or pushing can be achieved within the same DA. This doubly resonant structure opens the way to the design of a novel generation of efficient optical nano-tweezers that allow manipulation of nano-particles by simply changing the operation wavelength.
AlNLamb waveresonators enjoy advanced and attractive properties for enabling the next-generation single-chip radio frequency front-end, but their moderate effective electromechanical coupling coefficient (k²eff) poses a limit to their application in filters and multiplexers. Despite the fact that the reported k²eff enhancement techniques of doped AlN thin films which are expensive and trade off the quality factor (Q), the transducer topology itself extensively impacts the k²eff value. Although an AlN cross-sectional Lame mode resonator exhibiting a k²eff of 6.34% has been demonstrated without the need for changing the piezoelectric material, a detailed study of transducer design for AlNLamb waveresonators has not been conducted. In this work, we investigate the impact of (i) transducer configurations, (ii) electrode materials, (iii) electrode thicknesses, and (iv) interdigital transducer duty factors on the k²eff dispersive characteristics of one-port AlNLamb waveresonators by using the finite element analysis approach. By properly designing and optimizing the transducers, the k²eff of one-port AlNLamb waveresonators can be boosted to as high as 7.7%, showing great potential for applications of cellular frequency selection.
Periodic pattern generation using time-averaged acoustic forces conventionally requires the intersection of counterpropagating wave fields, where suspended micro-objects in a microfluidic system collect along force potential minimizing nodal or antinodal lines. Whereas this effect typically requires either multiple transducer elements or whole channel resonance, we report the generation of scalable periodic patterning positions without either of these conditions. A single propagating surface acoustic wave interacts with the proximal channel wall to produce a knife-edge effect according to the Huygens-Fresnel principle, where these cylindrically propagating waves interfere with classical wave fronts emanating from the substrate. We simulate these conditions and describe a model that accurately predicts the lateral spacing of these positions in a robust and novel approach to acoustic patterning.
Focusing and enriching submicrometer and nanometer scale objects is of great importance for many applications in biology, chemistry, engineering, and medicine. Here, we present an acoustofluidic chip that can generate single vortex acoustic streaming inside a glass capillary through using low-power acoustic waves (only 5 V is required). The single vortex acoustic streaming that is generated, in conjunction with the acoustic radiation force, is able to enrich submicrometer- and nanometer-sized particles in a small volume. Numerical simulations were used to elucidate the mechanism of the single vortex formation and were verified experimentally, demonstrating the focusing of silica and polystyrene particles ranging in diameter from 80 to 500 nm. Moreover, the acoustofluidic chip was used to conduct an immunoassay in which nanoparticles that captured fluorescently labeled biomarkers were concentrated to enhance the emitted signal. With its advantages in simplicity, functionality, and power consumption, the acoustofluidic chip we present here is promising for many point-of-care applications.
By means of ultrathin silicon membranes excited in the low ultrasound range, we show for the first time that it is possible to form two-dimensional Chladni patterns of microbeads in liquid. Unlike the well-known effect in a gaseous environment at the macroscale, where gravity effects are generally dominant, leading particles towards the nodal regions of displacement, we show that the combined effects of an ultrathin plate excited at low frequency (yielding to subsonic waves) together with reduced gravity (arising from buoyancy) will enhance the importance of microstreaming in the Chladni problem. Here, we report that for micrometric beads larger than the inner streaming layer, the microscale streaming in the vicinity of the plate tends to gather particles in antinodal regions of vibrations yielding to patterns in good agreement with the predicted modes for a liquid-loaded plate. Interestingly, a symmetry breaking phenomenon together with the streaming can trigger movements of beads departing from one cluster to another. We show that, for higher modes, this movement can appear as a collective rotation of the beads in the manner of a “farandole.”
Confinement of living microorganisms and self-propelled particles by an external trap provides a means of analysing the motion and behaviour of active systems. Developing a tweezer with a trapping radius large compared with the swimmers’ size and run length has been an experimental challenge, as standard optical traps are too weak. Here we report the novel use of an acoustic tweezer to confine self-propelled particles in two dimensions over distances large compared with the swimmers’ run length. We develop a near-harmonic trap to demonstrate the crossover from weak confinement, where the probability density is Boltzmann-like, to strong confinement, where the density is peaked along the perimeter. At high concentrations the swimmers crystallize into a close-packed structure, which subsequently ‘explodes’ as a travelling wave when the tweezer is turned off. The swimmers’ confined motion provides a measurement of the swim pressure, a unique mechanical pressure exerted by self-propelled bodies.
Significance
We present 3D acoustic tweezers, which can trap and manipulate single cells and particles along three mutually orthogonal axes of motion by recourse to surface acoustic waves. We use 3D acoustic tweezers to pick up single cells, or entire cell assemblies, and deliver them to desired locations to create 2D and 3D cell patterns, or print the cells into complex shapes. This technology is thus shown to offer better performance over prior cell manipulation techniques in terms of both accurate and precise motion in a noninvasive, label-free, and contactless manner. This method offers the potential to accurately print 3D multicellular architectures for applications in biomanufacturing, tissue engineering, regenerative medicine, neuroscience, and cancer metastasis research.
An acoustically actuated, bubble-based technique is developed to investigate the deformability of cells suspended in microfluidic devices. A microsized bubble is generated by an optothermal effect near the targeted cells, which are suspended in a microfluidic chamber. Subsequently, acoustic actuation is employed to create localized acoustic streaming. In turn, the streaming flow results in hydrodynamic forces that deform the cells in situ. The deformability of the cells is indicative of their mechanical properties. The method in this study measures mechanical biomarkers from multiple cells in a single experiment, and it can be conveniently integrated with other bioanalysis and drug-screening platforms. Using this technique, the mean deformability of tens of HeLa, HEK, and HUVEC cells is measured to distinguish their mechanical properties. HeLa cells are deformed upon treatment with Cytochalasin. The technique also reveals the deformability of each subpopulation in a mixed, heterogeneous cell sample by the use of both fluorescent markers and mechanical biomarkers. The technique in this study, apart from being relevant to cell biology, will also enable biophysical cellular diagnosis.
Standing surface acoustic waves (SSAW) are commonly used in microfluidics to manipulate cells and other micro/nano particles. However, except for a simple one-dimensional (1D) harmonic standing waves (HSW) model, a practical model that can predict particle behaviour in SSAW microfluidics is still lacking. Herein, we established a two-dimensional (2D) SSAW microfluidic model based on the basic theory in acoustophoresis and our previous modelling strategy to predict the acoustophoresis of microparticles in SSAW microfluidics. This 2D SSAW microfluidic model considers the effects of boundary vibrations, channel materials, and channel dimensions on the acoustic propagation; as an experimental validation, the acoustophoresis of microparticles under continuous flow through narrow channels made of PDMS and silicon was studied. The experimentally observed motion of the microparticles matched well with the numerical predictions, while the 1D HSW model failed to predict many of the experimental observations. Particularly, the 1D HSW model cannot account for particle aggregation on the sidewall in PDMS channels, which is well explained by our 2D SSAW microfluidic model. Our model can be used for device design and optimization in SSAW microfluidics.
In this letter, we demonstrate a technique to eliminate the spurious modes in aluminum nitride
Lamb wave resonators (LWRs). The transverse acoustic wave characteristics are examined, and a
resonance modulation theory on the regulation of mechanical boundary conditions is deducted. As
examples of embodiments, vertical and lateral protrusion structures are proposed for the
suppression. Finite element analysis verifies that the employment of these structures effectively
restrains the transverse modes, and the measured electrical performance of the LWR with
protrusions demonstrates an 11 dB reduction in the spurious response.
We present a theoretical analysis of the acoustic radiation force on a single small spherical particle, either a thermoviscous fluid droplet or a thermoelastic solid particle, suspended in a viscous and heat-conducting fluid medium. Within the perturbation assumptions, our analysis places no restrictions on the length scales of the viscous and thermal boundary-layer thicknesses δs and δt relative to the particle radius a, but it assumes the particle to be small in comparison to the acoustic wavelength λ. This is the limit relevant to scattering of ultrasound waves from nanometer- and micrometer-sized particles. For particles of size comparable to or smaller than the boundary layers, the thermoviscous theory leads to profound consequences for the acoustic radiation force. Not only do we predict forces orders of magnitude larger than expected from ideal-fluid theory, but for certain relevant choices of materials, we also find a sign change in the acoustic radiation force on different-sized but otherwise identical particles. These findings lead to the concept of a particle-size-dependent acoustophoretic contrast factor, highly relevant to acoustic separation of microparticles in gases, as well as to handling of nanoparticles in lab-on-a-chip systems.
This paper presents a transverse mode suppression theory and its experimental verification through aluminum nitride Lamb wave resonators (LWRs) operating at 142 MHz. An effective 2-D approximation model of the LWR is proposed, based on which the origin of transverse modes in LWR is investigated. The displacement distribution, resonant frequencies, and electromechanical coupling coefficients (k 2 t) of the main mode and its auxiliary transverse modes are obtained. A spurious mode suppression theory in terms of the expression of k 2 t in the 2-D model is proposed. Three kinds of electrodes are designed to suppress the transverse mode adjacent to the main mode, including a novel interdigital transducer gap technique that is reported for the first time. With the applicable geometries, these methods reduce the spurious response from 11.8 to <0.5 dB, without significantly affecting the figure of merit of the resonator. Index Terms— Aluminum nitride (AlN), Lamb wave resonator (LWR), micro electromechanical system (MEMS), spurious mode suppression.
We present a numerical study of the acoustophoretic motion of particles suspended in a liquid-filled PDMS microchannel on a lithium niobate substrate acoustically driven by surface acoustic waves. We employ a perturbation approach where the flow variables are divided into first- and second-order fields. We use impedance boundary conditions to model the PDMS microchannel walls and we model the acoustic actuation by a displacement function from the literature based on a numerical study of piezoelectric actuation. Consistent with the type of actuation, the obtained first-order field is a horizontal standing wave that travels vertically from the actuated wall towards the upper PDMS wall. This is in contrast to what is observed in bulk acoustic wave devices. The first-order fields drive the acoustic streaming, as well as the time-averaged acoustic radiation force acting on suspended particles. We analyze the motion of suspended particles driven by the acoustic streaming drag and the radiation force. We examine a range of particle diameters to demonstrate the transition from streaming-drag-dominated acoustophoresis to radiation-force-dominated acoustophoresis. Finally, as an application of our numerical model, we demonstrate the capability to tune the position of the vertical pressure node along the channel width by tuning the phase difference between two incoming surface acoustic waves.
We present a numerical study of the transient acoustophoretic motion of microparticles suspended in a liquid-filled microchannel and driven by the acoustic forces arising from an imposed standing ultrasound wave: the acoustic radiation force from the scattering of sound waves on the particles and the Stokes drag force from the induced acoustic streaming flow. These forces are calculated numerically in two steps. First, the thermoacoustic equations are solved to first order in the imposed ultrasound field taking into account the micrometer-thin but crucial thermoviscous boundary layer near the rigid walls. Second, the products of the resulting first-order fields are used as source terms in the time-averaged second-order equations, from which the net acoustic forces acting on the particles are determined. The resulting acoustophoretic particle velocities are quantified for experimentally relevant parameters using a numerical particle-tracking scheme. The model shows the transition in the acoustophoretic particle motion from being dominated by streaming-induced drag to being dominated by radiation forces as a function of particle size, channel geometry, and material properties.
This paper presents a systematic overview on the recent works on surface acoustic wave (SAW) driven microfluidics. SAW microfluidics is based on acoustic streaming induced by leaky SAW radiation into a liquid. The development of this field attracts attention from microfluidic research community due to its rapid actuation, programmable capability, simple and yet efficient operation. In our paper, SAW microfluidic applications are categorized into droplet-based applications and continuous-flow applications. Droplet is actuated into unique behaviours depending on the applied SAW power. A wide range of droplet based applications have been employed utilizing these behaviours. In continuous-flow system, applications are further categorized based on the interaction of travelling SAW and standing SAW with the bulk liquid. Finally, future perspectives of SAW driven microfluidics are discussed.
We've all seen images of astronauts in their spaceship cabins at zero gravity. Moving freely through the interior space, nothing inhibits them but the insubstantial friction of the air. Now imagine that they are repelled by the cabin walls. Instead of freely floating around, the hapless space travellers would be pushed to the place that is farthest away from all the walls. There they would be stuck, captured in a repulsive trap. Reporting in this issue (page 692), Krishnan et al.1 describe the realization of this scenario at the nanometre scale: an electrostatic trap for nanoparticles.
Despite its fundamental role in the dynamics of compressible fluids, bulk
viscosity has received little experimental attention and there remains a
paucity of measured data. Acoustic spectroscopy provides a robust and accurate
approach to measuring this parameter. Working from the Navier-Stokes model of a
compressible fluid one can show that the bulk viscosity makes a significant and
measurable contribution to the frequency-squared acoustic attenuation. Here we
employ this methodology to determine the bulk viscosity of Millipore water over
a temperature range of 7 to 50 degrees Celsius. The measured attenuation
spectra are consistent with the theoretical predictions, while the bulk
viscosity of water is found to be approximately three times larger than its
shear counterpart, reinforcing its significance in acoustic propagation.
Moreover, our results demonstrate that this technique can be readily and
generally applied to fluids to accurately determine their temperature dependent
bulk viscosities.
We present an experimental study of a droplet interacting with an ultrasonic
surface acoustic wave (SAW). Depending on the amplitude of the wave, the drop
can either experience an internal flow with its contact-line pinned, or (at
higher amplitude) move along the direction of the wave also with internal flow.
Both situations appear together with oscillations of the drop free-surface. The
physical origins of the internal mixing flow as well as the drop displacement
and surface waves are still not well understood. In order to give insights of
the underlying physics involved in these phenomena, we carried out an
experimental and numerical study. The results suggest that the surface
deformation of the drop can be related as a combination between acoustic
streaming effect and radiation pressure inside the drop.
A rapid particle concentration method in a sessile droplet has been developed using asymmetric surface acoustic wave (SAW) propagation on a substrate upon which the droplet is placed. Due to the asymmetry in the SAW propagation, azimuthal bulk liquid recirculation (acoustic streaming) is generated. Once the local particle concentration is sufficiently high within a particular streamline of the acoustic streaming convective flow, shear-induced migration gives rise to an inward radial force that concentrates the particles at the centre of the droplet. In this paper, a SAW device consists of a 0.75-mm thick, 127.68° Y–X-axis-rotated cut, X-propagating LiNbO3 for a substrate and an interdigital transducer electrode (IDT) with 25 straight finger pairs in a simple repeating pattern, 12 mm aperture, and a wavelength of λ = 440 μm was patterned on the substrate. The IDT was then driven with a sinusoidal signal at the resonance frequency f
0 of 8.611 MHz. To investigate the effect of particle type and size on the concentration process, three types of particles were used in this study, including fluorescent particles (1 μm), polystyrene microspheres (3, 6, 20, 45 μm), and living yeast cells (10–20 μm). Different RF powers were applied ranging from 120 to 510 mW. The concentration processes occurs within 2 to 20 s, depending on the particle size, type and input radio frequency (RF) power, much faster than currently available particle concentration mechanisms due to the large convective velocities achieved using the SAW device. Moreover, this concentration method is efficient, concentrating the particles into an aggregate one-tenth the size of the original droplet. Most importantly, bioparticles can also be concentrated by this method; we have verified that yeast cells are not lysed by the SAW radiation during concentration. By using the rapid concentration process described in this work, the breadth of applications and measurement sensitivity of SAW biosensor systems should be greatly enhanced.
Dielectrophoresis is a powerful technique for the handling of bioparticles ranging in size and structure from cells to proteins. The surge in experimental research on dielectrophoretic manipulation of biomacromolecules in recent years demands a renewed consideration of tools leveraging micro- and nanofabrication for dielectrophoresis. One aspect of such platforms that is less explored in literature is the volumetric electrokinetic flows generated in response to an alternating current (AC) potential at the electrode-electrolyte interface such as AC electroosmosis and AC electrothermal flows. Those electrokinetic flows may interfere with the dielectrophoretic force in certain experimental conditions. Hence, we provide a concise overview of the theory behind those phenomena and point out the key parameters for consideration and optimization. We further summarize experimental techniques for dielectrophoresis of the two most extensively studied biomacromolecules, DNA and protein molecules, and elaborate on their potential applications in separation, and mass transport for biosensing devices.
The use of ultrasonic fields to manipulate particles, cells and droplets has become widespread in lab on a chip (LOC) systems. There are two dominant actuation methods, the use of bulk acoustic waves (BAW) or surface acoustic waves (SAW). The development of BAW actuated systems have been underpinned by a robust understanding of the link between the ultrasonic field and forces which can be generated. In this work, we examine this link for standing surface acoustic waves (SSAW) comparing the relative strengths of streaming induced drag and acoustic radiation forces on suspended particles. To achieve this we have employed boundary conditions which accurately capture the travelling wave components of the pseudo-standing wave field, describe the key features of the acoustic radiation force fields and the acoustic streaming fields which can be generated, and finally we show that the relative importance of these two mechanisms is spatially dependant within a fluid chamber. The boundary condition used models the SSAW as two counter-propagating travelling waves, rather than assuming a standing wave directly. This allows the accurate inclusion of energy decay as the SAW couples into the fluid chamber and the resulting travelling wave component. This study shows that this previously neglected complexity of the SAW field is a critical factor in the nature of the resultant streaming field, as it gives rise to strong streaming rolls at the channel walls, which we validate experimentally. These rolls result in spatial variations of the dominant forces which in turn varies particle migration patterns spatially across the fluid domain.
We demonstrate an acoustofluidic device using Lamb waves (LWs) to manipulate polystyrene (PS) microparticles suspend-ed in a sessile droplet of water. The LW-based acoustofluidic platform used in this study is advantageous in that the device is actuated over a range of frequencies without changing the device structure or electrode pattern. In addition, the device is simple to operate and cheap to fabricate. The LWs, produced on a piezoelectric substrate, attenuate inside the fluid and create acoustic streaming flow (ASF) in the form of a poloidal flow with toroidal vortices. The PS particles experience direct acoustic radiation force (ARF) in addition to being influenced by the ASF, which drive the concentration of particles to form a ring. This phenomenon was previously attributed to the ASF alone, but the present experimental results confirm that the ARF plays an important role in forming the particle ring, which would not be possible in the presence of only the ASF. We used a range of actuation frequencies (45-280 MHz), PS particle diameters (1-10 μm), and droplet volumes (5, 7.5, and 10 μL) to experimentally demonstrate this phenomenon.
Electro-acoustic sensors proving an adequate operation in liquid media are appropriate candidates for biosensing or liquid monitoring applications. In this context, electro-acoustic devices based on Lamb waves have been widely used for such purpose during the last years. More particularly, S0 mode Lamb wave resonators (S0-LWRs) have shown promising in-liquid performance. However, a theoretical background describing their in-liquid sensing mechanisms has only been published recently. In this work we present the experimental verification of the previously developed theoretical background based on a finite element model. We discuss about similarities and discrepancies between model and experiments, stating a final model correctly describing the in-liquid sensitivity features of S0-LWRs. These devices show appreciable different sensitivities of the resonant frequency to liquid viscosity and density, being more sensitive to the latter. Additionally, when they are not electrically isolated, the influence of the liquid electrical properties is superimposed to the mechanical ones and can be correctly extracted.
In this study, we have investigated the motion of polystyrene microparticles inside a sessile droplet of water actuated by surface acoustic waves (SAWs), which produce an acoustic streaming flow (ASF) and impart an acoustic radiation force (ARF) on the particles. We have categorized four distinct regimes (R1-R4) of particle aggregation that depend on the particle diameter, the SAW frequency, the acoustic wave field (travelling or standing), the acoustic waves' attenuation length, and the droplet volume. The particles are concentrated at the centre of the droplet in the form of a bead (R1), around the periphery of the droplet in the form of a ring (R2), at the side of the droplet in the form of an isolated island (R3), and close to the centre of the droplet in the form of a smaller ring (R4). The ASF-based drag force, the travelling or standing SAW-based ARF, and the centrifugal force are utilized in various combinations to produce these distinct regimes. For simplicity, we fixed the fluid volume at 5 μL, varied the SAW actuation frequency (10, 20, 80, and 133 MHz), and tested several particle diameters in the range 1-30 μm to explicitly demonstrate the regimes R1-R4. We have further demonstrated the separation of particles (1 and 10 μm, 3 and 5 μm) using mixed regime configurations (R1 and R2, R2 and R4, respectively).
A longitudinal wave is radiated from a surface acoustic wave (SAW) when a liquid is loaded onto a SAW-propagating surface. Radiated longitudinal wave causes nonlinear phenomena, which is called SAW streaming. The SAW streaming phenomena depend on the applied electrical power. A droplet on the SAW-propagating surface vibrates, moves in the SAW-propagating direction, and ejects small droplets with increasing the applied power. The moving of a droplet is also named droplet manipulation. However, the mechanism of droplet manipulation using SAWs has not been clarified. It is important to understand the manipulation mechanism of droplets using a SAW to enable actual applications. To understand droplet manipulation, measurements of the radiation force in the droplet are necessary. In this study, we first observed streaming in a droplet during manipulation and found that the particles in the droplet accumulated at the front of the droplet. Then, we proposed the use of a flat spring with a strain gage to measure the radiation force in a water vessel. Fundamental characteristics of the force radiated from the SAW device were obtained experimentally.
Sensors based on thin film electroacoustic (TEA) devices have emerged as a promising alternative to quartz crystal microbalance and surface acoustic wave devices, in view of sensibility, miniaturization and easy integration. TEA devices include quasi-shear film bulk acoustic resonators (QS-FBAR) and S0 mode Lamb wave resonators (S0-LWR) based on AlN films. Despite the work done on the application of TEA devices as in-liquid biological and chemical sensors, a theoretical framework for S0-LWRs properly describing their sensing mechanisms is still needed. Here we validate a finite element analysis model on QS-FBARs and study the sensing mechanisms of S0-LWRs in liquid media. We show that S0-LWRs can sense changes in the dielectric permittivity of the liquid and demonstrate different sensitivities to viscosity and density. A complementary assessment of the S0-LWRs losses, dependent in a non-specific manner on the square root of the density viscosity product, provides the ability to discriminate density from viscosity changes on the entire device surface. Finally, with an S0-LWR optimization study we show that resolution improves with the decrease of the membrane thickness; however, a trade-off between sensitivity, quality factor and membrane fragility has to be considered.
Microfluidic devices have been used for handling and analyzing micro scale fluidic quantities with small concentrations, relying on smaller devices, with increased portability and availability. This paper proposes a solution to reduce the mixing time required in microfluidic devices analyses, without the need of pumps or moving parts. To speed up the required mixing, acoustic streaming is proposed. Two different piezoelectric materials, PZT and β-PVDF, are explored, concerning their actuation frequencies, for integration in microfluidic devices. Their layout, actuation frequency and behavior were compared for mixing two fluids inside a microfluidic reaction chamber. Numerical simulations and experimental tests showed flow vortices generation and a significant mixing time reduction for both materials: above 90% for PZT and above 80% for β-PVDF. It was also observed an increase of the temperature on the transducers surface, which is advantageous for applications in which heating is necessary. The results showed agreement between simulations and experimental tests which can be useful for predicting new materials behavior for improving microfluidic devices mixing.
Theories for calculating steady streaming associated with sound fields are reviewed, comparing the methods and approximations of various authors. Two illustrative problems are worked out, both for rectilinear flow due to irrotational sound fields. The first deals with a single attenuated plane wave traveling down a tube, as in Cady's quartz wind experiments. In the second, a pair of crossed plane waves is treated, giving rise to a quite different kind of streaming. In obtaining solutions, attention is given to boundary conditions; here, gradients of She excess static pressure, another second‐order quantity, come into consideration. Significantly, streaming speeds depend critically upon α, the attenuation constant, where α may be due to any common cause, such as heat conduction, scattering, thermal relaxation, etc. From these results it appears that streaming measurements cannot be used to distinguish between absorption mechanisms. Numerical values are given for a few cases; high flow speeds may be expected in a bubbly medium.
Microfluidic systems have faced challenges in handling real samples and the chip interconnection to other instruments. Here we present a simple interface, where surface acoustic waves (SAWs) from a piezoelectric device are coupled into a disposable acoustically responsive microfluidic chip. By manipulating droplets, SAW technologies have already shown their potential in microfluidics, but it has been limited by the need to rely upon mixed signal generation at multiple interdigitated electrode transducers (IDTs) and the problematic resulting reflections, to allow complex fluid operations. Here, a silicon chip was patterned with phononic structures, engineering the acoustic field by using a full band-gap. It was simply coupled to a piezoelectric LiNbO3 wafer, propagating the SAW, via a thin film of water. Contrary to the use of unstructured superstrates, phononic metamaterials allowed precise spatial control of the acoustic energy and hence its interaction with the liquids placed on the surface of the chip, as demonstrated by simulations. We further show that the acoustic frequency influences the interaction between the SAW and the phononic lattice, providing a route to programme complex fluidic manipulation onto the disposable chip. The centrifugation of cells from a blood sample is presented as a more practical demonstration of the potential of phononic crystals to realize diagnostic systems.
We report an experimental and numerical characterization of three-dimensional acoustic streaming behavior in small droplets of volumes (1-30 μl) induced by surface acoustic wave (SAW). We provide a quantitative evidence of the existence of strong nonlinear nature of the flow inertia in this SAW-driven flow over a range of the newly defined acoustic parameter F_{NA}=Fλ/(σ/R_{d})≥0.01, which is a measure of the strength of the acoustic force to surface tension, where F is the acoustic body force, λ is the SAW wavelength, σ is the surface tension, and R_{d} is the droplet radius. In contrast to the widely used Stokes model of acoustic streaming, which generally ignores such a nonlinearity, we identify that the full Navier-Stokes equation must be applied to avoid errors up to 93% between the computed streaming velocities and those from experiments as in the nonlinear case. We suggest that the Stokes model is valid only for very small acoustic power of ≤1 μW (F_{NA}<0.002). Furthermore, we demonstrate that the increase of F_{NA} above 0.45 induces not only internal streaming, but also the deformation of droplets.
We report the first use of ultrasonic acoustophoresis for the label-free separation of viable and non-viable mammalian cells within a microfluidic device. Cells that have undergone apoptosis are physically smaller than viable cells, and our device exploits this fact to achieve efficient sorting based on the strong size-dependence of acoustic radiation forces within a microchannel. As a model, we have selectively enriched viable MCF-7 breast tumor cells from heterogeneous mixtures of viable and non-viable cells. We found that this mode of separation is gentle and enables efficient, label-free isolation of viable cells from mixed samples containing 106 cells/mL at flow-rates of up to 12 mL/hr. We have extensively characterized the device and report the effects of piezoelectric voltage and sample flow-rate on device performance, and describe how these parameters can be tuned to optimize recovery, purity or throughput.
This work uses a finite volume method to investigate three-dimensional acoustic streaming patterns produced by surface acoustic wave (SAW) propagation within microdroplets. A SAW microfluidic interaction has been modelled using a body force acting on elements of the fluid volume within the interaction area between the SAW and fluid. This enables the flow motion to be obtained by solving the laminar incompressible Navier–Stokes equations driven by an effective body force. The velocity of polystyrene particles within droplets during acoustic streaming has been measured and then used to calibrate the amplitudes of the SAW at different RF powers. The numerical prediction of streaming velocities was compared with the experimental results as a function of RF power and a good agreement was observed. This confirmed that the numerical model provides a basic understanding of the nature of 3D SAW/liquid droplet interaction, including SAW mixing and the concentration of particles suspended in water droplets.
This article reviews acoustic microfluidics: the use of acoustic fields, principally ultrasonics, for application in microfluidics. Although acoustics is a classical field, its promising, and indeed perplexing, capabilities in powerfully manipulating both fluids and particles within those fluids on the microscale to nanoscale has revived interest in it. The bewildering state of the literature and ample jargon from decades of research is reorganized and presented in the context of models derived from first principles. This hopefully will make the area accessible for researchers with experience in materials science, fluid mechanics, or dynamics. The abundance of interesting phenomena arising from nonlinear interactions in ultrasound that easily appear at these small scales is considered, especially in surface acoustic wave devices that are simple to fabricate with planar lithography techniques common in microfluidics, along with the many applications in microfluidics and nanofluidics that appear through the literature.
Harnessing the ability to precisely and reproducibly actuate fluids and manipulate bioparticles such as DNA, cells, and molecules at the microscale, microfluidics is a powerful tool that is currently revolutionizing chemical and biological analysis by replicating laboratory bench-top technology on a miniature chip-scale device, thus allowing assays to be carried out at a fraction of the time and cost while affording portability and field-use capability. Emerging from a decade of research and development in microfluidic technology are a wide range of promising laboratory and consumer biotechnological applications from microscale genetic and proteomic analysis kits, cell culture and manipulation platforms, biosensors, and pathogen detection systems to point-of-care diagnostic devices, high-throughput combinatorial drug screening platforms, schemes for targeted drug delivery and advanced therapeutics, and novel biomaterials synthesis for tissue engineering. The developments associated with these technological advances along with their respective applications to date are reviewed from a broad perspective and possible future directions that could arise from the current state of the art are discussed.
Piezoelectric materials are widely used in the excitation of MHz frequency vibrations in devices for ultrasonic manipulation. An applied electrical voltage is transformed into mechanical stress, strain and displacement. Piezoelectric elements can be used in either a resonant or non-resonant manner. Depending on the desired motion the piezoelectric longitudinal, transverse or shear effects are exploited. Because of the coupling between electrical and mechanical quantities in the constitutive law the modelling of devices turns out to be quite complex. In this paper, the general equations that need to be used are delineated. For a one-dimensional actuator the underlying physics is described, including the consequences resulting for the characterization of devices. For a practical setup used in ultrasonic manipulation, finite element models are used to model the complete system, including piezoelectric excitation, solid motion and acoustic field. It is shown, how proper tailoring of transducer and electrodes allows selective excitation of desired modes.
Although optical tweezers based on far-fields have proven highly successful for manipulating objects larger than the wavelength of light, they face difficulties at the nanoscale because of the diffraction-limited focused spot size. This has motivated interest in trapping particles with plasmonic nanostructures, as they enable intense fields confined to sub-wavelength dimensions. A fundamental issue with plasmonics, however, is Ohmic loss, which results in the water, in which the trapping is performed, being heated and to thermal convection. Here we demonstrate the trapping and rotation of nanoparticles using a template-stripped plasmonic nanopillar incorporating a heat sink. Our simulations predict an ~100-fold reduction in heating compared with previous designs. We further demonstrate the stable trapping of polystyrene particles, as small as 110 nm in diameter, which can be rotated around the nanopillar actively, by manual rotation of the incident linear polarization, or passively, using circularly polarized illumination.