Article

Acoustic Streaming and Microparticle Enrichment within a Microliter Droplet Using a Lamb-Wave Resonator Array

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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.

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... 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. ...
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... 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]. ...
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... 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. ...
Preprint
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.
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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.
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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.
Article
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).
Article
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.
Article
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.
Article
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.
Article
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&apos;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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.