Ratchets: the problems with boundary conditions in insulating fluids

We have constructed microfabricated devices, designed to have asymmetry in their physical structures, in order to fractionate objects under continuous-flow conditions. The fractionation of particles with the inclusion of diffusion and asymmetric structures requires a knowledge of not only statistical mechanics but also the external forces acting on the particles, since thermal Brownian fluctuations alone cannot serve to separate particles. We explicitly examine this problem in a device designed to fractionate biomolecules dissolved in water, and show that boundary conditions that influence the direction of the applied force are quite important in determining the efficiency of the device.