Acoustic Forces Acting on Inhomogeneous Fluids Theory and Applications

Abstract

The primary focus is to provide a theoretical framework and clear understanding of the underlying principles and mechanisms governing the behaviour of inhomogeneous fluids under acoustic fields, enabling their effective utilization in heat transfer and droplet manipulation applications. Initially, a unified theory is developed to elucidate the phenomena of acoustic relocation and streaming suppression in inhomogeneous fluids, as well as streaming in a homogeneous fluid. The assumptions on mean Eulerian pressure and consequent formulations of acoustic body forces have resulted in a lack of clarity among the scientific community and our developed theory addresses these ambiguities in the literature. Remarkably, our theory predicts that the acoustic relocation/stabilisation of inhomogeneous fluids in a microchannel subjected to standing acoustic waves is possible only if there exists an impedance (Z = and#961;c) gradient, which agrees well with the recent experiments. Also, we successfully separate the streaming term and acoustic relocation term from the generalized acoustic body force thereby enhancing the understanding of the dynamics of the inhomogeneous fluids under acoustic fields. Followed by the theory, using linear stability analysis, we derive the characteristic equation that governs the stability of inhomogeneous fluids (with and without interfacial tension) under an acoustic body force. For fluids with interfacial tension, a non-dimensional number called the acoustic Bond number is obtained theoretically which characterizes the stable and unstable (relocation) regimes. The theoretical analysis of these studies provides valuable insights into the manipulation of particles (cells/droplets/beads) and in homogeneous fluids within micro channels under the influence of acoustic fields.