Separation of suspended particles by arrays of obstacles in microfluidic devices

The stochastic transport of suspended particles through a periodic pattern of obstacles in microfluidic devices is investigated by means of the Fokker-Planck equation and numerical simulations. Asymmetric arrays of obstacles have been shown to induce the continuous separation of DNA molecules, with particles of different size migrating in different directions within the microdevice (vector separation). We show that the separation of tracer particles only occurs in the presence of a permeating driving force with a nonzero normal component at the surface of the solid obstacles, and arises from differences in the local Peclet number of the particles. On the other hand, finite-size particles also exhibit nonzero, but small, migration angles in the case of nonpermeating fields. Monte Carlo simulations for different driving fields agree with the solutions to the Fokker-Planck equation.