Acoustothermal heating in surface acoustic wave driven microchannel flow

Abstract

Surface acoustic wave (SAW) is a well-proven tweezer serving various applications such as particle manipulation, cell trapping and separations, fluid mixing, and biosensing. SAWs can cause significant heat generation due to acoustothermal heating as established experimentally. Theoretical understanding of acoustothermal heating is limited, primarily due to the difficulties presented by multiple time scales inherent in this phenomenon. We present a theoretical model based on a multiple scale perturbation approach to solve the fluid flow and heat transfer equations for SAW-driven acoustothermal heating of a Newtonian fluid in a microchannel. The first order fields are oscillatory with the same frequency as that of the SAW, whereas the second order components are time-averaged to account for the mean flow and temperature fields. We find that the temperature rise depends solely on the acoustic energy density and its conversion into internal energy via pressure work on the fluid and hydrodynamic transportation of heat. For a fixed aspect ratio, an increase in system size essentially increases the conversion of acoustic energy into internal energy, leading to an increase in temperature rise. On the other hand, an increase in SAW frequency for a given system size causes the acoustic energy density to increase and thereby increases the temperature rise. Temperature rise is found to increase linearly with SAW power, in agreement with experimental results reported in the literature. The quantitative model for the temperature field presented in this work will find applications in designing biosensors, microreactors, and in other SAW driven controllable digital microfluidic heating applications.