Viscoelastic control of acoustic particle migration and trapping in microchannels

TL;DR

This study explores how fluid viscoelasticity influences acoustic particle migration and trapping in microchannels, revealing new control mechanisms for particle manipulation in complex fluids.

Contribution

It introduces a perturbation framework for analyzing viscoelastic effects on acoustophoretic particle dynamics, highlighting the role of Deborah and viscous diffusion numbers.

Findings

Particle trapping locations depend on Deborah and viscous diffusion numbers.

High viscous diffusion number shifts trapping from bulk to near-wall regions.

Critical particle size for regime transition is smaller in viscoelastic fluids.

Abstract

Particle migration and trapping in ultrasonically actuated microscale flows arise from the competition between acoustic radiation forces and streaming-induced drag. While these mechanisms are well understood in Newtonian fluids, the role of fluid viscoelasticity in governing particle dynamics remains largely unexplored. Here, we investigate particle transport and trapping in a viscoelastic fluid within an ultrasonically excited microchannel under the combined action of acoustic streaming and radiation forces. Using a perturbation framework, we solve the continuity, momentum and constitutive equations for an Oldroyd-B fluid to obtain the oscillatory acoustic field and the resulting steady streaming flows in the bulk and near-wall boundary layers. Acoustic radiation forces, incorporated through a semi-analytical model, drives particle migration, while streaming-induced drag can oppose, alter or suppress trapping. We show that particle trajectories and equilibrium trapping locations are governed primarily by the Deborah number ($De$) and viscous diffusion number ($Dv$). At high $Dv$, increasing $De$ shifts the trapping location from the bulk region to the channel wall, pressure nodal line, channel centre or ultrasound symmetry line. We further determine the critical particle size governing the transition between radiation-dominated and streaming-dominated regimes as a function of $De$ and $Dv$. The critical particle size can become significantly smaller than that in a Newtonian fluid, enabling effective manipulation of submicron particles and overcoming a key limitation of conventional acoustofluidics. These results demonstrate how viscoelasticity fundamentally modifies acoustophoretic transport and establish new mechanisms for tunable particle migration and trapping in complex fluids.