Colloidal systems, that is, micron size particles suspended in liquids, provide a magnificent testing ground for theories of out-of-equilibrium dynamics. Like atoms and molecules at moderate or high temperatures, the dynamics of colloidal particles is classical, dominated by the thermal agitation of the surrounding molecules in the liquid. Their constant bombardment induces a Brownian motion in the colloidal particle that, due to its large size, can be directly observed with a conventional microscope.

Current technology makes it easy to track the individual trajectories of colloidal particles in an experiment, whereas doing the same with individual atoms or molecules, while possible, is still challenging. Colloidal systems can thus be considered as large-scale manifestations of thermal fluctuations at the molecular level, which makes them easily observable. It should not come as a surprise, then, that the first experimental demonstrations of Brownian ratchets were carried out with colloidal systems.

Directed motion of colloidal particles in a flashing asymmetric potential

In the 1990s, Rousselet et al. (1994) successfully demonstrated a Brownian ratchet for colloidal particles. The experimental setup was a direct implementation of the flashing ratchet discussed in Chapter 2. Polystyrene latex spheres of diameters in the range 0.25 μm to 1 μm were set into directed motion following the flashing of the potential.

In that experiment, colloidal particles in solution were confined by two parallel glass slides. The asymmetric potential was generated by means of interdigitated electrodes, which had been previously deposited on one of the glass slides using photolithographic techniques. They were designed with a repeating pattern of a Christmas tree, as shown in Fig. 7.1, with a spatial period of 50 μm. Then, an applied a.c. voltage created an electric field E(x, y, z) that interacted with the polarizable latex particles, generating, between two neighboring electrodes, a dielectric potential -ΔαE2/2, with Δα, the particle's polarizability with respect to the solution, of asymmetric shape. To generate a current, this potential was switched on and off periodically. Figure 7.2 shows a series of images of the sample of colloidal particles during this on/off cycle.