Dewetting behavior of aqueous cationic surfactant solutions on liquid films

Previous experimental work has shown that the spreading of a drop of aqueous anionic surfactant solution on a liquid film supported by a negatively charged solid substrate may give rise to a fingering instability (Afsar-Siddiqui, A. B.; Luckham P, F.; Matar, O. K. Langmuir 2003, 19, 703-708). However, upon deposition of a cationic surfactant on a similarly charged support, the surfactant will adsorb onto the solid-liquid interface rendering it hydrophobic. Water is then expelled from the hydrophobic regions, causing film rupture and dewetting. In this paper, experimental results are presented showing how the surfactant concentration and film thickness affect the dewetting behavior of aqueous dodecyltrimethylammonium bromide solutions. At low surfactant concentrations and large film thicknesses, the film ruptures at a point from which dewetting proceeds. At higher concentrations and smaller film thicknesses, the ruptured region is annular in shape and fluid moves away from this region. At still higher concentrations and smaller film thicknesses, the deposited surfactant forms a cap at the point of deposition that neither spreads nor retracts. This variation in dewetting mode is explained by considering the relative Marangoni and bulk diffusion time scales as well as the mode of assembly of the surfactant adsorbed on the solid surface.