Capillary Assembly in Surfactant Monolayers

In this work we study capillary assembly in the presence of an insoluble surfactant monolayer by analyzing the rate of approach of particles attracted to each other by capillary interactions. The method simply requires video imaging of particles as they approach one another at an interface. In our case, cylindrically shaped particles in the zero Bond number limit are used so that gravity effects are neglected and the surface distortion depends solely on particle geometry and wetting properties. Far-field assembly is driven by the overlap of quadrupolar modes of the distortions created by the particles. For surfactants that form Newtonian surface layers on Newtonian subphases, the distance between the particles versus time to contact obeys a power law and yields a prefactor proportional to the surface viscosity. Surface viscosity measurements for pentadecanoic acid as a function of surface coverage are reported.