(a) Surface acoustic waves are radiated into the drop at the Rayleigh angle , causing bulk recirculation within the drop. (b) The pressure wave at the Rayleigh angle that generates the fluid recirculation is sufficiently strong to generate a lift force on a 2 mm diameter steel ball placed on the drop, akin to the phenomena described by Sano et al.(Ref. 43). Consequently, the ball levitated above the substrate surface within the drop and transported along the direction of SAW propagation, which is from left to right in the image.
A 0.5 mm diameter steel ball placed within a water drop atop a Teflon-coated lithium niobate substrate spins at over 6000 rpm due to the secondary azimuthal flow induced within the drop as a consequence of the asymmetric SAW radiation passing into the drop.
The SPUDTs fabricated and used in this work, including a (a) 30 MHz straight SPUDT, a (b) 30 MHz focusing elliptical SPUDT (E1) with an approximate eccentricity of 0.616, a (c) 30 MHz focusing elliptical SPUDT (E2) with an approximate eccentricity of 0.831, and a (d) 30 MHz focusing circular SPUDT. The vibration displacement perpendicular to the substrate surface was measured using a scanning laser vibrometer across the area highlighted in each image; the results of the vibrometer scans are shown in Fig. 4.
The amplitude of surface displacement perpendicular to the lithium niobate substrate surface as SAW propagates from (a) a straight SPUDT, (b) a concentric elliptical SPUDT with eccentricity 0.831 (E1), (c) a concentric elliptical SPUDT with eccentricity 0.616 (E2), and (d) a concentric circular SPUDT. The scanned area for each transducer corresponds to the highlighted area shown in Fig. 3. Each cross indicates the position at which a drop will be placed to study SAW-induced mixing and particle concentration and dispersion.
(a) Linear rotation speeds for different input powers for the concentric circular, elliptical (E1) and straight SPUDTs. Images (b) and (c) show the particle trajectories as they recirculate within the drop, which were analyzed using the particle tracking software Diatrack to determine the rotational velocities. The images (b) and (c) are for low and high power excitation, respectively.
Concentration of particles in the drop via drop rotation induced by acoustic radiation from the focused elliptical SAW. Similar patterns were observed for the concentration driven by the circular and straight SPUDTs.
The normalized pixel intensity is a measure of the particle concentration; plotted here against time using the concentric circular SPUDT at different power levels it shows an increase in input power reduces the time required to concentrate the particles out of suspension.
Particle concentration times as a function of the applied power for the concentric circular, elliptical (E2), and straight SPUDTs.
Rapid mixing of water (dark) mixed with glycerine (light) using the (a) concentric circular, (b) elliptical (E1), and (c) straight SPUDT designs. For (a), the images at , 0.55, and 1.2 s are shown. For (b), the images at , 0.633, and 1.35 s are shown. For (c), the images at , 0.9, and 1.9 s are shown.
Normalized standard deviation in the pixel intensity as a function of time for the (a) concentric circular, (b) elliptical (E1), and (c) straight SPUDT designs.
Effect of the input power on the mixing enhancement, measured by the ratio of the effective diffusivity to the diffusivity due to pure diffusional mixing in the absence of SAW-driven convection when no power is applied.
(a) Micromixing and (b) particle concentration in a drop driven by SAW-driven bulk liquid recirculation. For micromixing (a), the mixing of the dye placed at the top of the drop is promoted by the primary internal streaming flow arising due to direct leakage of the SAW radiation from the substrate at the Rayleigh angle . The secondary azimuthal flow generated due to the asymmetric drop position on the substrate serves only to recirculate the dye. On the other hand, the concentration process is predominantly driven by particle shear-induced migration, which is promoted by the secondary azimuthal flow.
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