Drag Forces on a Stationary Particle in Flowing Two-Dimensional Ordered Particle Monolayers:  Simulation and Measurement Using Optical Tweezers

Using a laser tweezer method, we have studied the drag force on a charged polystyrene latex particle adsorbed at the oil−water interface when an ordered 2D monolayer of identical particles is flowed around it. The drag force, measured as a function of the flow velocity and the lattice spacing in the particle monolayer, contains contributions from long-range electrostatic forces between the particles at the liquid interface. The interpretation of the experimental results is based on a model in which the repulsion arises primarily from the presence of a very small net electric charge at the particle−oil interface. This charge corresponds to a fractional ionization of the sulfate groups present at the particle surface exposed to the oil phase which is estimated from an independent experiment (see Aveyard, R.; Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Horozov, T. S.; Neumann, B.; Paunov, V. N.; Annesley, J.; Botchway, S. W.; Nees, D.; Parker, A. W.; Ward, A. D.; Burgess, A. N. Phys. Rev. Lett. 2002, 88, 246102-1) to be approximately 3.3 × 10-4 for the latex sample used. The experimentally determined drag forces are in reasonable agreement with results from computer simulations for a range of values of the velocity and the lattice spacing of the particle monolayer.