(22b) A Numerical Study on Steric Crowding Effect in Inertial Microfluidics

In square microchannels, flowing particles can laterally migrate and focus into streamlines due to hydrodynamic inertia. This inertial focusing has attracted intense attention for promising capability of passive particle manipulation without applying any external field. One aspect that remains unexplored is the effect of particle concentration on focusing behaviour. Although seemingly efficient for the manipulation, high particle concentration (1-3 v/v%) has been suggested to create steric crowding effect that might impede complete particle focusing. However, the concentration effect has not been studied because this steric crowding effect involves high-throughput flow on the micro scale of highly condensed suspension flow. This nature considerably defies experimental exploration. In this study, we numerically study the concentration effect on the inertial focusing behaviour for highly condensed suspension flowing in square microchannels. The numerical method is the discrete element method coupled with the direct numerical simulation for particle-fluid interaction [1]. This method is referred to as the DEM-DNS method. We find that complete focusing behaviour disappears above ~3 v/v%. It is also suggested that this steric crowding effect arises when flowing particles are within close distance in high concentration, and consequently, particle rotation is significantly reduced as a result of viscous effect of inter-particle hydrodynamics. This mechanism could also imply the essential role of particle rotation in formation of distinct streamlines found in square microchannels whose detailed mechanism has not been fully explained. Finally, the DEM-DNS method is shown to be a promising tool for inertial microfluidic analysis.

[1] H. Udono and M. Sakai, “A numerical study on dynamic inertial focusing of microparticles in a confined flow”, Granular Matter (accepted)