Andrea Gnoli - CNR ISC Roma #
Brownian ratchet driven by Coulomb friction #
Statistical non-equilibrium conditions, equivalent to a breakdown of
time-reversal symmetry, allow the rectification of unbiased fluctuations,  
which
is impossible in equilibrium systems.  Such a mechanism, also known as 
ratchet
effect, underlies the functioning of molecular motors which exploit
non-equilibrium chemical reactions to perform work in living organisms, 
e.g.
kinesin and myosin, or the complementary case of artificial nano-motors 
actuated
by non-equilibrium active fluids, e.g. bacteria. Recently, it has been
demonstrated that it is possible to rectify the fluctuations of 
macroscopic
mechanical devices: these are kept in out-of-equilibrium stationary states
through continuous energy dissipation balanced by random energy injection, 
and
are realized, for instance, by suspending an asymmetric probe in a 
fluidized
granular medium. Here, we demonstrate through a new experimental setup 
with a
rotating device subjected to granular collisions, the existence of a net 
ratchet
effect, originating entirely in the Coulomb friction acting on the contact
surface between the rotator and its bearing. Such a friction-induced 
torque acts
in the opposite direction with respect to the net torque provided by 
inelastic
collisions between the ratchet and the granular fluid: the interplay 
between
these two forces results in a resonant behavior and in a ratchet velocity
inversion point. Our experimental observations are reproduced by 
simulations and
explained by kinetic theory. This discovery paves the way to the 
realization of
Brownian motors in the realm of micro and sub-micrometer scales purely 
based
upon nano-friction.