Role of shear induced diffusion in acoustophoretic focusing of dense suspensions

We investigate the interplay between acoustic and shear induced diffusion (SID) forces in acoustophoretic focusing of dense suspensions in a microchannel. A theoretical model is presented which accurately predicts the width of the focused band in terms of shear rate, acoustic energy density, and particle concentration. The role of SID is clearly demonstrated by switching off the acoustic field, which leads to the instantaneous spreading of the focused band. At a given acoustic energy density and particle concentration, there exists a critical shear rate $Γcr$ above which the focusing of microparticles is prevented. For $Γ<Γcr$⁠, an equilibrium focused band is formed whose width remains constant downstream.