Directional locking and the role of irreversible interactions in deterministic hydrodynamics separations in microfluidic devices.

We performed macroscopic experiments on the motion of a sphere through an array of obstacles that highlight the deterministic nature of the lateral displacements that lead to particle separation in microfluidic systems. The motion of the spheres is irreversible and displays directional locking. The locking directions can be predicted with a single parameter that distinguishes between reversible and irreversible particle-obstacle collisions. These results stress the need to incorporate irreversible interactions to predict the movement of a non-Brownian sphere passing through a periodic array.