Particle separation and chemical gradient control via focused travelling surface acoustic waves

Abstract

A novel focused travelling surface acoustic waves (F-TSAW) based micro-chip is demonstrated here to continuously separate microparticles, generate chemical gradient, and uniformly mix fluids inside a PDMS mi-crofluidic channel. A pair of curved interdigitated electrodes deposited on a piezoelectric substrate (LiNbO3) produced unidirectional F-TSAW when high frequency AC signal is applied across the terminals of the trans-ducer. F-TSAW, when interacted with the fluid inside the microfluidic channel, imparted acoustic radiation force (ARF) to the suspended microparticles and induced acoustic streaming flow (ASF). ARF is used to sepa-rate the particles whereas ASF generated chemical gradient and uniformly mixed fluids. Previously reported acoustofluidic based particle separators use standing surface acoustic waves (SSAW) whereas chemical gradi-ent generator and micro-mixer depends on acoustic streaming induced by oscillating bubbles. F-TSAW based micro-chip did not require a trapped bubble and a cumbersome microchannel alignment step, which is essential for the working of SSAW particle separator, is also eliminated. All three functions - separation, gradient genera-tion and mixing - are performed on a single micro-chip with performance comparable with already reported devices. After selecting appropriate microchannel (w×h: 200um×40um) and flow rate (100uL/h-1200uL/h or 3.5mm/s-41.67mm/s), variable size polystyrene particles suspended in DI water are successfully separated with power input in excess of 235mW (23.7dBm) and separation efficiency of 100%. ARF separated micropar-ticles with diameter 10um from 3um by inducing an acoustophoretic separation distance whereas effect of ASF is negligible with the aforementioned experimental conditions. The trajectory of separated microparticles is slightly effected by the ASF but it does not affect the separation efficiency. F-TSAW micro-chip harnessed ASF to generate chemical gradient and mix fluids inside a microchannel. ...