The mysterious attraction between two flat, neutral surfaces was first predicted in 1948 by the Dutch physicist Henrik Casimir. It is a purely quantum effect arising from fluctuations of electromagnetic fields, which exert a radiation pressure on the surfaces that is, on average, stronger on the outer than the inner surfaces. The overall Casimir force is therefore weaker in the gap between the surfaces than elsewhere, drawing the surfaces together. Although the Casimir force decays quickly with distance, it becomes very important for surfaces separated by distances of several micrometres. As a result, the force is of great interest to those trying to built micrometre-sized machines.

More recently, physicists have discovered that if both surfaces are corrugated, rather than smooth, there will also be a lateral Casimir force acting on the plates. If the plates are held at a fixed distance, this lateral force tends to cause the plates to slide across one another until the corrugations are aligned such that the potential energy is a minimum. In Emig’s ratchet, one plate has a symmetric corrugation -- peaks with the same slope on either side -- while the other plate has an asymmetric corrugation -- peaks with a gradual slope on one side and a steep slope on the other (see figure “Corrugations”). According to Emig, this asymmetry allows the plates to slide easily over each other in one direction, but makes it more difficult for the plates to slip back in the opposite direction.

Emig’s ratchet could be actuated by causing one of the plates to vibrate relative to the other. This would cause the other plate to slide from one position of minimum energy to the next along the easy direction. Emig has calculated that the velocity at which the ratchet moves is proportional to the frequency of vibration – with oscillations in the kilohertz range causing a velocity of about 5 mm/s. Such a ratchet would have silicon plates a few micrometres thick with corrugation heights of 10 nm and a corrugation period of 1 µm.

Unlike electrostatic microratchets -- which have to be made from conducting materials -- the Casimir ratchets could be made from electrical insulators and do not require any electrical contacts or external electric fields. According to Emig, this means that the ratchet could operate in real-world environments such as in air or even with a liquid between the plates.

Chris Binns of the UK’s University of Leicester is familiar with Emig’s proposal and is currently building devices to explore how the lateral Casimir force can be exploited in micromachines. “We have already designed a machine that uses patterned surfaces to demonstrate the lateral force”, said Binns. “Making the pattern asymmetric should not be difficult.”