Langevin simulations of rod-shaped object alignment by surface flow

Nano or micro-scale rod shaped objects suspended in a liquid flowing on a flat solid surface might be aligned or orientated by the nature of the liquid, type of flow and planar channel geometry containing the flow. Orientation might enhance or inhibit certain chemical reactions between the objects, the underlying surface, other chemicals or with the walls of the vessels holding the flowing suspension. The probability density function describing the orientations of the objects satisfies a Fokker-Planck equation whose solution is obtained with Langevin simulation for different surface flow parameters. The methods developed in the present work enable us to evaluate the orientation probability density function for a range of the Peclet number a covering several orders of magnitude, 10(-4) <= alpha <= 10(8). The experimental detection of orientation control is obtained optically from the measurement of dichroism and birefringence of the suspension. We describe different methods providing experimental proof of the onset of alignment control. (C) 2011 Elsevier B.V. All rights reserved.