The effects of microchannel aspect ratio on the relocation of inhomogeneous miscible fluids in acoustofluidics: numerical simulation

Abstract The development of acoustofluidics is addressing significant challenges across diverse fields, including biology, medicine, chemistry, engineering, and physics. However, current technology faces limitations in the precise manipulation of nanoscale objects, primarily due to acoustic streaming. Recent studies have shown that incorporating inhomogeneous fluids into microchannels can result in a relocation of mass concentration and a reduction in acoustic streaming. In this work, the effects of microchannel aspect ratio (AR) on the stability and relocation of inhomogeneous miscible fluids induced by bulk acoustic waves (BAW) were investigated. Numerical simulations revealed that in a stable configuration, the high-impedance fluid in the central region tends to sink due to gravity as the microchannel height increases. In contrast, fluid flow in an unstable configuration exhibits greater complexity. The relocation process, particularly the mass fraction of the high-impedance fluid, varies with the number of Rayleigh streaming rolls in the microchannel as its height increases. Furthermore, the role of boundary layer-driven streaming in fluid relocation was highlighted as the primary factor influencing the fluid profile changes at a slow time scale. These insights could potentially enhance the optimization of acoustofluidic microchannels and extend their potential applications in particle separation and fluid concentration.