Slip at the liquid-solid interface reveals soliton propagation and solute partitioning *

The classical no-slip condition of fluid mechanics does not apply under certain circumstances. Rather, there is a finite liquid velocity at the solid surface that depends on the nature of the solid and liquid. It is poorly understood how this slip occurs, in particular, how liquid molecules move over the substrate. Of particular interest is propagation of nonlinear waves--solitons--conveying liquid molecules. Here we report that the slip velocity of a Lennard-Jones fluid over a cubic lattice solid substrate is due to soliton propagation. The angle between the direction of soliton propagation relative to that of the applied force depends on the type of liquid molecule. Consequently, different species of molecules can be spatially separated as they move over the substrate. Propagation of solitons is a fundamental process leading to slip at liquid-solid interfaces. The directional sensitivity of solitons yields insight into designing low friction surfaces and new technologies for chemical separations.