Continuous Particle Aggregation and Separation in Acoustofluidic Microchannels Driven by Standing Lamb Waves

Abstract

In this study, we realize acoustic aggregation and separation of microparticles in fluid channels driven by standing Lamb waves of a 300-μm-thick double-side polished lithium-niobate (LiNbO3) plate. We demonstrate that the counter-propagating lowest-order antisymmetric and symmetric Lamb modes can be excited by double interdigitated transducers on the LiNbO3 plate to produce interfacial coupling with the fluid in channels. Consequently, the solid–fluid coupling generates radiative acoustic pressure and streaming fields to actuate controlled acoustophoretic motion of particles by means of acoustic radiation and Stokes drag forces. We conducted finite-element simulations based on the acoustic perturbation theory with full-wave modeling to tailor the acoustic and streaming fields in the channels driven by the standing Lamb waves. As a result, the acoustic process and the mechanism of particle aggregation and separation were elucidated. Experiments on acoustic manipulation of particles in channels validate the capability of aggregation and separation by the designed devices. It is observed that strong streaming dominates the particle aggregation while the acoustic radiation force differentially expels particles with different sizes from pressure antinodes to achieve continuous particle separation. This study paves the way for Lamb-wave acoustofluidics and may trigger more innovative acoustofluidic systems driven by Lamb waves and other manipulating approaches incorporated on a thin-plate platform.

Particle manipulation with acoustic waves based on Hertz- Mindlin mechanics

“…The effect has been experimentally observed many times (e.g. in [1][2][3][4]). At the same time, theoretical description of the process is of interest since it can help optimize real systems once predicts the resulting behavior in a particular situation determined by a number of parameters such as wave characteristics, material properties, and geometric sizes and shapes.…”

“…Here Ay is the vertical amplitude, f is the wave frequency, kr is wavenumber, r(n) is the known ratio of the horizontal and vertical amplitudes that depend on n, Poisson's ratio. The equations of motion for bulk coordinates (x, y) of the small body of a mass m read (2) where T(b, a) and N(a) are friction and normal reaction forces, respectively, that depend on normal a and tangential b contact displacements (see Fig. 1).…”

“…It is well known that nonlinear contact frictional interaction can be used for transporting small solid objects on a substrate under the action of a surface acoustic wave. This principle is now applied to a number of practical cases: separation of granular mixtures, transport of grains in silos and conveyors, powder dosage in pipes and, in general, is of interest for food, chemical, pharmaceutical and coating industries [1][2][3]. Future applications are possible such as delivery of solid agents for defect healing, noninvasive excretion of undesirable particles, activation of micromachine components, and others.…”