Microparticle migration in Transport Flows: Oscillation-induced Rectification Forces*

The oscillation of interfaces in a fluid leads to rectified dynamics on time scales much longer than that of the oscillation, affecting the motion of both fluid elements (streaming) and particles in the medium (inertial migration). We have recently developed a scale separation 
formalism that efficiently describes forces of particle attraction to or repulsion from the interface in a radial flow situation where streaming is negligible. Extending this approach to microfluidic channel transport flows with two non-trivial components, we discuss additional forcing effects on the particles not present in radial flows as well as the role of Basset-Boussinesq corrections. Comparison is made with full simulations on the oscillatory time scale as well as with experiments using oscillating microbubbles, revealing a systematic dependence of the net particle displacement on the proximity of approach to the interface.  Depending on particle size and density contrast on the one hand, and experimental design and driving frequency on the other hand, particles can thus be efficiently deflected away from or towards the wall of the channel as desired.