Rectified motion of a mercury drop in an asymmetric structure

We describe a macroscopic experimental realization of force-free motion. Our system is a mercury drop, in a capillary of asymmetric roughness, subjected to an oscillatory force of zero average value. The force is of electrocapillary nature and the shape of the potential the drop is subjected to reflects the capillary roughness. In certain ranges of frequencies and intensities of the applied external force, the motion of the drop is rectified. On time scales much larger than the period of the applied force, this motion is uniform. The average velocity of the drop as a function of the frequency and intensity of the external field. These results compare qualitatively well with recent theoretical predictions. In particular, at a given frequency, there is an optimum value of the applied external force for which the average velocity is maximum.