ac separation of particles by biased Brownian motion in a two-dimensional sieve

We propose a technique for separation of particles based on a nonlinear coupling between an applied force and the induced transverse drift in a two-dimensional system with broken reflection symmetry transverse to the force. Further, if the system has reflection symmetry parallel to the force, the drift velocity is independent of the sign of the force. Thus, an ac force causes a net drift only in the transverse direction, allowing for efficient separation even in a very small system. In an appropriately constructed device, particles with diffusion coefficients below and above a threshold move in opposite directions, allowing for continuous separation of significant amounts of molecules.