OPTIMAL MODULATION OF A BROWNIAN RATCHET AND ENHANCED SENSITIVITY TO A WEAK EXTERNAL FORCE

We studied the dynamics of a Brownian particle moving in a spatially a nisotropic potential acted on by multiplicative temporal modulations s o that V(x,t) = g(t) U(x). Using the concept of the ''thermodynamic ac tion,'' we show that the class of modulation that maximizes the flow i s a square-wave in time. We also show that adding a weak, homogenous f orce F in synergy with the square-wave modulation can cause particles of slightly different size to move in opposite directions. The synerge tic change in velocity caused by F can be much greater than the drift velocity that would be caused by F alone.