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

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 mechanism for this shear-induced alignment of micelles based on breaking up of micelles aligned perpendicular to the 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 simple 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 time scale for Monte Carlo moves, which enables an assessment of the physical shear rates employed in our Monte Carlo simulations.