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Recent advances in the fabrication of microfluidic systems require handling of live cells and other micro particles as well as mixing. All this can, for example, be achieved using acoustic radiation forces and the viscous drag from the streaming flow.

Streaming: Due to the nonlinear terms in the Navier-Stokes equations, harmonic perturbation of the flow will lead to a net time-averaged flow called acoustic streaming. Acoustic streaming is a second order (nonlinear) acoustic effect. The effect can be simulated in two ways: either by direct simulation solving the nonlinear Navier-Stokes equations, or as shown here by separation of time scales.

Radiation Force: Due to nonlinear terms in the governing equations momentum can be transferred from an acoustic field to particles. This results in a net force acting on the particles – the acoustic radiation force.

The trajectory of particles in devices will be governed by the balance between the viscous drag force (from the streaming flow) and the acoustic radiation force. This model shows how to include and model both using COMSOL Multiphysics.

The model presented here is based on the papers:

1. P. B. Muller and H. Bruus, "Numerical study of thermoviscous effects in ultrasound-induced acoustic streaming in microchannels," Phys. Rev. E 90, 043016 (2014).

2. J. T. Karlsen and H. Bruus, “Forces acting on a small particle in an acoustical field in a thermoviscous fluid,” Phys. Rev. E 92, 043010 (2015).