Transport and Fractionation in Periodic Potential-Energy Landscapes
  Objects driven through periodically modulated potential-energy landscapes in two dimensions can become locked in to symmetry-selected directions that are independent of the driving force's orientation. We investigate this problem in the overdamped limit, and demonstrate that the crossover from free-flowing to locked-in transport can depend exponentially on an object's size, with this exceptional selectivity emerging from the periodicity of the environment. 
Kosta Ladavac, Matthew Pelton and David G. Grier