Figure 1 - uploaded by James Friend
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Surface acoustic waves are attenuated upon (a) contact with a fluid droplet; the energy is reradiated into the fluid at the Rayleigh angle θ R as a compressive wave. Without (b) asymmetric delivery or absorption of the acoustic radiation into the droplet there can be no (c) rotation and subsequent mixing of the droplet. Here, the droplet was placed to one side of the SAW causing rapid mixing of the water-glycerin (green) mixture.
Source publication
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. The SAW device consisted of a 0.75-mm thick, 127.68 YXaxis- rotated cut LiNbO3 as a substrate. An interdigital transducer electrode (IDT) with 25 straight finger pair...
Context in source publication
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... promising approach is the use of very high-frequency acoustic waves to perform all of the tasks common in droplet micro/nanofluidics-moving, splitting, combining, and pinning droplets-along with new processes [12,13,21] enabled via acoustic forces on objects within the droplets. Rotation of a droplet to perform mixing, for example, is shown in Fig. 1. Though one can use bulk Love and other such waves to perform manipulation and sensing [22], such waves are transmitted throughout the substrate holding the droplet, and so are subject to losses and diffraction from mount- ing. Surface acoustic waves are isolated to within 4-5 λ of the surface upon which they are generated, and so the ...
Citations
... The slow motion of the particles further convinced us that it was the acoustic pressure instead of the streaming that dominated the system when the capillary waves were initiated, considering how weak this effect is on the particles. 52 The capillary waves we observed are, therefore, not the result of acoustic streaming or other induced flow behaviors. The results of our simulation were confirmed with experimental particle migration measurements. ...
Remarkably, the interface of a fluid droplet will produce visible capillary waves when exposed to acoustic waves. For example, a small (∼1 μL) sessile droplet will oscillate at a low ∼102 Hz frequency when weakly driven by acoustic waves at ∼106 Hz frequency and beyond. We measured such a droplet's interfacial response to 6.6 MHz ultrasound to gain insight into the energy transfer mechanism that spans these vastly different time scales, using high-speed microscopic digital transmission holography, a unique method to capture three-dimensional surface dynamics at nanometer space and microsecond time resolutions. We show that low-frequency capillary waves are driven into existence via a feedback mechanism between the acoustic radiation pressure and the evolving shape of the fluid interface. The acoustic pressure is distributed in the standing wave cavity of the droplet, and as the shape of the fluid interface changes in response to the distributed pressure present on the interface, the standing wave field also changes shape, feeding back to produce changes in the acoustic radiation pressure distribution in the cavity. A physical model explicitly based upon this proposed mechanism is provided, and simulations using it were verified against direct observations of both the microscale droplet interface dynamics from holography and internal pressure distributions using microparticle image velocimetry. The pressure-interface feedback model accurately predicts the vibration amplitude threshold at which capillary waves appear, the subsequent amplitude and frequency of the capillary waves, and the distribution of the standing wave pressure field within the sessile droplet responsible for the capillary waves.
... It has been well documented that an acoustic radiation force can cause microparticles to migrate towards the pressure nodes, or antinodes, depending on their mechanical properties, and that particles can also be fractionized (according to their size and density), and thus concentrated and/or separated within the liquid50515253. A simple method to concentrate the particles within a droplet is to use an asymmetric distribution of SAW radiation along the width of the droplet as shown in figure 2(b), and this has been well documented in [45].Figure 10 shows the captured images of the starch particle concentration process within a 30 μl droplet at an input RF [45] reported that the flow phenomenon within liquid droplets due to the asymmetric positioning of SAW is similar to that obtained by the flow field between stationary and rotating disks, which is known as the Batchelor flow [54]. From the side view of the movies, the fluid was observed to be pushed upward just above the SAW propagation area which results in the primary azimuthal rotation within the droplet periphery, where the same flow feature has been reported in [45]. ...
... In this work, we make use of a Rayleigh wave, which is an axial-surface-normal polarized SAW [2]. SAW technology has been developed for the use of biotechnology, including mixing, bioparticle manipulation and pathogen detection in microfluidic devices [3][4][5]. When in contact with a liquid drop placed above the SAW substrate, bulk liquid recirculation is induced within the drop through a process known as acoustic streaming [6]. ...
In this paper, we investigate the ability to drive fluid streaming via a surface acoustic wave (SAW) into a porous bioscaffold structure, and to exploit this effect to deliver fluorescent particles/yeast cells into the scaffold as a potential rapid and efficient method for cell seeding in tissue engineering. The results demonstrate that the seeding process takes approximately 10 seconds, much shorter than that if the cell suspension were to perfuse through the scaffold under the effects of gravity alone (approximately 30 mins). By increasing the input power, both the velocity of the fluid flow and the particle seeding efficiency can be enhanced. At 560 mW, fluid velocities of the order 10 mm/s were achieved; in this case, the particle/yeast seeding efficiency is around 92%. In addition to rapid seeding, the SAW streaming induced perfusion is observed to significantly improve the uniformity of the scaffold cell distribution due to greater penetration into the scaffold. Finally, we verify using a methylene violet staining procedure that 80% of the yeast cells seeded by the SAW method within the scaffold remained viable.
... It has been well documented that an acoustic radiation force can cause microparticles to migrate towards the pressure nodes, or antinodes, depending on their mechanical properties, and that particles can also be fractionized (according to their size and density), and thus concentrated and/or separated within the liquid50515253. A simple method to concentrate the particles within a droplet is to use an asymmetric distribution of SAW radiation along the width of the droplet as shown in figure 2(b), and this has been well documented in [45].Figure 10 shows the captured images of the starch particle concentration process within a 30 μl droplet at an input RF [45] reported that the flow phenomenon within liquid droplets due to the asymmetric positioning of SAW is similar to that obtained by the flow field between stationary and rotating disks, which is known as the Batchelor flow [54]. From the side view of the movies, the fluid was observed to be pushed upward just above the SAW propagation area which results in the primary azimuthal rotation within the droplet periphery, where the same flow feature has been reported in [45]. ...
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.
... Better yet, both the requirement for large voltages and the problem of slow concentration were circumvented through the use of leaky surface acoustic waves (SAWs) to drive the microcentrifugation. As with other SAW-driven microfluidic actuation (Yeo and Friend 2009), including vibration-induced particle patterning and sorting (Li et al. 2008), drop translation (Wixforth 2003; Tan et al. 2007b), jetting (Tan et al. 2009) and atomization (Friend et al. 2008; Qi et al. 2008; Alvarez et al. 2008a, b, 2009), the microcentrifugation driven by SAW is extremely fast (Li et al. 2007), especially with the use of elliptical transducers to focus the SAWs (Tan et al. 2007a; Shilton et al. 2008). The perceived difficulties of using lithium niobate in SAW microfluidics is in some part countered by using fluid coupling to microfluidic devices in other materials (Hodgson et al. 2009). ...
... The perceived difficulties of using lithium niobate in SAW microfluidics is in some part countered by using fluid coupling to microfluidic devices in other materials (Hodgson et al. 2009). When subjected to asymmetrical SAW radiation, 5 lm particles seeded in a 30 ll drop are observed to concentrate rapidly in the center of the drop, as shown in Fig. 1. Li et al. (2007) attributed this phenomena to the competition between shear induced migration—arising due to azimuthal velocity gradients within the drop which result in the transport of the particles across azimuthal streamlines into the interior of the vortex (see Fig. 1)—and acoustically-driven inertial convection within the drop—acoustic streaming—which acts to disperse the particles. Below a critical threshold value in the applied power, approximately 300 mW, the shear diffusion process is dominant therefore leading to the concentration of the particles in the closed vortex. ...
... However, beyond this critical value, the acoustic streaming-driven convection dominates, giving rise to particle dispersion. In Li et al. (2007) and Shilton et al. (2008), the particle concentration dynamics were quantified using a relatively simple two-dimensional horizontal plane pixel intensity analysis of the temporal variation in gray-scale video frames, omitting the particle dynamics in a direction perpendicular to the plane. Whilst this provides a quantitative description of the dynamics sufficient to determine the underlying physical mechanisms, it ignores the effect of the three-dimensional flow field of the particle trajectory and hence also the concentration process. ...
Through confocal-like microparticle image velocimetry experiments, we reconstruct, for the first time, the three-dimensional
flow field structure of the azimuthal fluid recirculation in a sessile drop induced by asymmetric surface acoustic wave radiation,
which, in previous two-dimensional planar studies, has been shown to be a powerful mechanism for driving inertial microcentrifugation
for micromixing and particle concentration. Supported through finite element simulations, these insights into the three-dimensional
flow field provide valuable information on the mechanisms by which particles suspended in the flow collect in a stack at a
central position on the substrate at the bottom of the drop once they are convected by the fluid to the bottom region via
a helical spiral-like trajectory around the drop periphery. Once close to the substrate, the inward radial velocity then forces
the particles into this central stagnation point where they are trapped by sedimentary forces, provided the convective force
is insufficient to redisperse them along with the fluid up a central column and into the bulk of the drop.
... Recently, surface acoustic wave (SAW) technology has been developed for the use of biotechnology, including biofluidic mixing, particle trapping, and biosensors in microfluidic devices [15][16][17][18]. A SAW is like an earthquake wave propagating along the surface with an amplitude of a few tens of nanometers ( Fig. 1(a)). ...
... Though there are many forms of SAW, this study makes use of the Rayleigh wave, an axial-surface-normal polarized SAW [19]. There are many examples of the application of SAW technology for sensing [18]; the interaction of a fluid in contact with the propagating radiation allows the measurement of the fluid's viscosity, density and pH through the appropriate design of the SAW device. Wixforth et al. [20] has demonstrated that SAWs strongly interact with small amounts of liquid on the surface of a piezoelectric substrate by inducing bulk liquid recirculation through a process known as acoustic streaming ( Fig. 1(b)). ...
Fluid manipulations at the microscale and beyond are powerfully enabled through the use of 10-1,000-MHz acoustic waves. A superior alternative in many cases to other microfluidic actuation techniques, such high-frequency acoustics is almost universally produced by surface acoustic wave devices that employ electromechanical transduction in wafer-scale or thin-film piezoelectric media to generate the kinetic energy needed to transport and manipulate fluids placed in adjacent microfluidic structures. These waves are responsible for a diverse range of complex fluid transport phenomena - from interfacial fluid vibration and drop and confined fluid transport to jetting and atomization - underlying a flourishing research literature spanning fundamental fluid physics to chip-scale engineering applications. We highlight some of this literature to provide the reader with a historical basis, routes for more detailed study, and an impression of the field's future directions.
