Microfluidic Ultrasonic Shaker

Abstract

We report on a wave-driven microfluidic ultrasonic shaker that can stably trap and oscillate microparticles in water in a microchannel. The microfluidic ultrasonic shaking system mainly consists of a microfluidic chip and two orthogonally excited piezoelectric transducers: one for creating radiation force potential wells for three-dimensional trapping of microparticles and the other for generating sound destabilization to shake the trapped microparticles. It is shown that two standing waves of different but close frequencies provide a shaking force [approximately ${F}_{0}$cos(2\ensuremath{\pi}\ensuremath{\Delta}ft), with ${F}_{0}$ and \ensuremath{\Delta}f being the force amplitude and the frequency difference, respectively] that can repeatedly perturb a trapped microparticle and cause it to execute spontaneous oscillations. The dynamic behavior of microparticle oscillation driven by two sound waves is found to be equivalent to a forced harmonic oscillator (i.e., a mass on a spring) that has been extensively described in the literature. Specifically, experiments at various ultrasonic excitations show that the oscillating frequency of a stably trapped microparticle is determined by the frequency difference between the two sound waves, and the shaking amplitude scales inversely to the frequency difference and is affected by pressure amplitudes of both sound waves. We foresee that the illustrative microfluidic ultrasonic shaker presented here opens perspectives for dynamic manipulation of single microparticles or clusters in microscale acoustofluidic systems.