Study on the mixing and migration behavior of micron-size particles in acoustofluidics

Slow mass transfer remains a limiting factor in microfluidic devices, as it is purely based on molecular diffusion. Hence, the completion of a targeted chemical reaction is extremely slow. Acoustic streaming is a great methodology to create lateral convection in otherwise laminar flow operated channels and offers the possibility to reduce mass transfer resistance dramatically. Moreover, the acoustic field can elegantly by used to handle micron-sized particles. Large particles ( > 2 mu m), experience a radiation force that is much larger than the drag force related to streaming and will rapidly focus at either the node or anti-node of the standing pressure wave in the channel. In the present contribution, the mass transfer rate in both a drag force dominated as well as in a radiation force dominated regime is studied by in situ tracking polystyrene particles of respectively 0.5 and 5 mu m. The 5 mu m particles focus on the pressure node experiencing a migration velocity that decreases as the node position is approached, whereas the 0.5 mu m particles remain in the vortex flow, characteristic for acoustic streaming. The experimentally obtained migration rates are in accordance with numerical simulations, suggesting that operational parameters as actuator displacement have been chosen appropriately. Mixing times of parallel flows were assessed and revealed an order of magnitude enhancement for small molecules, up to four order of magnitudes for 0.5 mu m diameter particles.