Nanoscale simulations of directional locking

When particles suspended in a fluid are driven through a regular lattice of cylindrical obstacles, their average motion is usually not in the direction of the force, and in the high Peacuteclet number limit, particles tend to lock into periodic trajectories along certain lattice directions. By means of molecular dynamics simulations we show that this effect persists for nanometer-sized particles and in the presence of molecular diffusion, provided the Peacuteclet number is not very small. The main effect of diffusion is to smooth the sharp transitions between locking directions found in the convective limit and to suppress the higher-order locking directions. We show that trajectory locking is independent of the driving mechanism and qualitatively insensitive to the particle and obstacle size and spacing. The absolute roughness of the solid surfaces is found to be the relevant quantity in locking. We observe trajectory locking in all cases, and in particular in semidilute suspensions of particles of different sizes. The degree of locking varies with particle size, and therefore these flows can have application as a nanoparticle separation technique. (C) 2010 American Institute of Physics. [doi:10.1063/1.3429297]