Non-equilibrium simulation of optically trapped colloidal particles at liquid interfaces

A theoretical study of the non-equilibrium behaviour of colloidal particle monolayers under the influence of the displacement of a single, optically trapped colloidal particle is presented. Two different types of monolayer (i.e., one relatively rigid and the other relatively soft) confined to a liquid–liquid interface are investigated using the Stokesian dynamics simulation technique. An effective dipole–dipole interaction is assumed to act between the colloidal particles and viscous drag forces acting on the particles are taken into account. The calculations reveal periodic oscillations of the net force on the trapped particle for the soft monolayer and highly non-linear, non-monotonic variations for the rigid monolayer. The highly non-linear variations of the net force for the rigid monolayer are concomitant with rapid, cooperative particle rearrangements and large oscillations in the global orientational order parameter of the monolayer. These findings, combined with the results of optical-tweezer experiments, should be useful for the rheology of liquid interfaces and the investigation of Pickering emulsions.