Asymmetric pores in a silicon membrane acting as massively parallel brownian ratchets

The brownian motion of mesoscopic particles is ubiquitous and usually random. But in systems with periodic asymmetric barriers to movement, directed or 'rectified' motion can arise and may even modulate some biological processes(1). In man-made devices, brownian ratchets and variants based on optical or quantum effects have been exploited to induce directed motion(2-14), and the dependence of the amplitude of motion on particle size has led to the size-dependent separation of biomolecules(6,8,15). Here we demonstrate that the one-dimensional pores of a macroporous silicon membrane(16), etched to exhibit a periodic asymmetric variation in pore diameter, can act as massively parallel and multiply stacked brownian ratchets that are potentially suitable for large-scale particle separations. We show that applying a periodic pressure profile with a mean value of zero to a basin separated by such a membrane induces a periodic flow of water and suspended particles through the pores, resulting in a net motion of the particles from one side of the membrane to the other without moving the liquid itself. We find that the experimentally observed pressure dependence of the particle transport, including an inversion of the transport direction, agrees with calculations(17,18) of the transport properties in the type of ratchet devices used here.