Monte Carlo study of shear-induced alignment of cylindrical micelles in thin films

Abstract

The behavior of confined cylindrical micelle-forming surfactants under the influence of shear has been investigated using Monte Carlo simulations. The surfactants are modeled as coarse-grained lattice polymers, while the Monte Carlo shear flow is implemented with an externally imposed potential energy field which induces a linear drag velocity on the surfactants. It is shown that in the absence of shear, cylindrical micelles confined within a monolayer coarsen gradually with Monte Carlo "time" t, the persistence length of the micelles scaling as t^{0.24}, in agreement with the scaling obtained experimentally. Under the imposition of shear, the micelles within a monolayer align parallel to the direction of shear, as observed experimentally. Micelles confined within thicker films also align parallel to each other with a hexagonal packing under shear, but assume a finite tilt with respect to the velocity vector within the velocity-velocity gradient plane. We propose a novel mechanism for this shear-induced alignment of micelles based on breaking up of micelles aligned perpendicular to shear and their reformation and subsequent growth in the shear direction. It is observed that there exists a "window" of shear rates within which such alignment occurs. A phenomenological theory proposed to explain the above behavior is in good agreement with simulation results. A comparison of simulated and experimental self-diffusivities yields a physical timescale for Monte Carlo moves, which enables an assessment of the physical shear rates employed in our Monte Carlo simulations.