Abstract
We present three-dimensional measurements of particle-size-dependent acoustophoretic motion of microparticles with diameters from 4.8 μm down to 0.5 μm suspended in either homogeneous or inhomogeneous fluids inside a glass-silicon microchannel and exposed to a standing ultrasound wave. To study the crossover from radiation force dominated to streaming dominated motion as the particle size is decreased, we extend previous studies to long timescales, where the particles smaller than the crossover size move over distances comparable to the channel width. We observe a particle-size-dependent particle depletion at late times for the particles smaller than the crossover size. The mechanisms behind this depletion in homogeneous fluids are rationalized by numerical simulations which take the Brownian motion into account. Experimentally, the particle trajectories in inhomogeneous fluids show focusing in the bulk of the microchannel at early times, even for the particles below the critical size, which clearly demonstrates the potential to manipulate submicrometer particles.
Highlights
Microscale acoustofluidics, relying on ultrasound-induced forces, has emerged as a tool in contemporary biotechnology to concentrate [1,2], trap [3,4], wash [5], align [6], synchronize [7], and separate suspended cells [8,9,10] in a flow-through format
To study the crossover from radiation force dominated to streaming dominated motion as the particle size is decreased, we extend previous studies to long timescales, where the particles smaller than the crossover size move over distances comparable to the channel width
We extend previous experimental studies of particle-size-dependent acoustophoretic motion of particles suspended in homogeneous and inhomogeneous fluids to long timescales, for which the particles below the crossover size move distances comparable to the size of the channel cross section, and we characterize the associated crossover from radiation force dominated to streaming dominated behavior
Summary
Microscale acoustofluidics, relying on ultrasound-induced forces, has emerged as a tool in contemporary biotechnology to concentrate [1,2], trap [3,4], wash [5], align [6], synchronize [7], and separate suspended cells [8,9,10] in a flow-through format. The acoustic streaming typically leads to mixing of particles inside the microchannel, and it has only been applied for particle manipulation in some special cases [11,12,13,14]. It is well established experimentally and theoretically that the particle velocity induced by the acoustic radiation force in a homogeneous fluid scales with the square of the particle radius [15,16,17].
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