Entropic transport of finite size particles

Transport of spherical Brownian particles of finite size possessing radii R ≤ R(max) through narrow channels with varying cross-section width is considered. Applying the so-called Fick-Jacobs approximation, i.e. assuming fast equilibration in the direction orthogonal to the channel axis, the 2D problem can be described in terms of a 1D effective dynamics in which bottlenecks cause entropic barriers. Geometrical confinements result in entropic barriers which the particles have to overcome in order to proceed in the transport direction. The analytic findings for the nonlinear mobility for the transport are compared with precise numerical simulation results. The dependence of the nonlinear mobility on the particle size exhibits a striking resonance-like behavior as a function of the relative particle size ρ = R/R(max); this latter feature renders possible new effective particle separation scenarios.