  We investigate analytically a microfluidic device consisting of a tube with
non-uniform but spatially periodic diameter, where a fluid driven back and
forth by a pump carries colloidal particles. Although the net flow of the fluid
is zero, the particles move preferentially in one direction due to ratchet
mechanism, which occurs by simultaneous effect of inertial hydrodynamics and
Brownian motion. We show that the average current is strongly sensitive to
particle size, thus facilitating colloidal particle sorting.
