A Brownian ratchet driven by the Coulomb friction

Granular media display amazing physics, but also attract interest because they allow to investigate at laboratory scale the effect of factors as general as, among others, linearity, dissipation and friction, and to induce what effect they can have on systems at different scale. A realization of a microscopic effect that exploits granular dynamics is the rectification of the unbiased fluctuations of a thermal bath, also known as ratchet effect. This requires statistical non-equilibrium conditions, that in granular systems are usually obtained from the inelastic collisions of the grains which also constitute the bath. We have recently devised and realized a granular ratchet based on a new source of non-equilibrium, the Coulomb friction. The ratchet consists of a suspended asymmetric wheel immersed in a granular gas. At high collisional rate dynamics are dominated by collisions and the ratchet exploits the usual collisional dissipation. At low collisional rate, however, a "friction-induced" torque appears that, for the specific wheel shape chosen, acts in the opposite direction with respect to the dissipation, resulting in the inversion of the wheel motion, and giving rise to a stochastic resonant peak in between the two regimes. We speculate that this new ratchet mechanism also works at the microscopic scale, where a thermal bath substitutes the granular gas. Our numerical simulations and analytical calculations demonstrate that in this case a net drift is indeed induced by the friction on an asymmetric wheel, even in the absence of inelasticity.