Directional Locking

Particles which are driven across periodic substrate potentials show a number of intriguing phenomena. Depending on the direction of the applied driving force F, the orientation of the particle's motion can substantially deviate from F but is locked-in to directions determined by the substrate's symmetry. Examples of such kinetically locked-in states range from atom migration on crystalline surfaces, driven charge density waves to flux flow in type-II superconductors. Also, it has been demonstrated that directional locking can be employed for sorting of colloidal particles according to their size, refractive index or chirality. When subjecting a monolayer of colloidal particles to quasiperiodic substrates potentials being created by interfering laser beams, we also observe dynamical ordering with a pronounced colloidal smectic phase. This suggests that dynamical ordering is not restricted to periodic potentials but also occurs under more general conditions.