  The rectilinear "drift" of particles in a hydrodynamic drift ratchet arises
from a combination of diffusive motion and particle-wall hydrodynamic
interactions, and is therefore dependent on particle diffusivity, particle
size, the amplitude and frequency of fluid oscillation and pore geometry. Using
numerical simulations, we demonstrate that the drift velocity relative to the
pore size is constant across different sized drift ratchet pores, if all the
relevant non-dimensional groups (Peclet number, Strouhal number and ratio of
particle to pore size) remain constant. These results clearly indicate for the
first time the scaling parameters under which the drift ratchet achieves
dynamic similarity, and so facilitates design, fabrication and testing of drift
ratchets for experiments and eventually as commercial micro/nano fluidic
separation devices.
