Osmotically Driven Deformation of a Stable Water Film

An aspect of dynamic colloidal interactions that has received little attention is the osmotic stress associated with nonequilibrium distribution of solutes. Recent experiments on a mercury drop near a mica surface show a dimple forming on the mercury/water interface when there is a sudden change in the electric potential of the mercury drop coated with a self-assembled monolayer (SAM) of 11-mercapto-1-undecanoic acid thiol molecules. A reasonable hypothesis is that the dimple formation is due to the desorption of a fraction of the SAM from the mercury drop surface when the surface potential is changed. The osmotic pressure in the thin film region increases as a result of the presence of the thiol molecules in the region, giving rise to the observed dimple. A model including the effects of osmotic flow, disjoining pressure, interfacial tension and hydrodynamic pressure is developed to test the hypothesis. The simplest version of the model, in which desorption is uniform and instantaneous, can produce a dimple whose growth is significantly more rapid than its decay, in qualitative agreement with the data. However, quantitative agreement is lacking. Several refinements to the model, including effects such as the change in interfacial tension as thiols are desorbed, gradual thiol desorption, a change in disjoining pressure as charged thiols are desorbed and nonuniform desorption do not change the qualitative picture. The qualitative success of the model suggests the osmotic pressure mechanism is correct, but the detailed picture of the SAM desorption at positive mercury surface potentials is not sufficiently well understood. The model reveals that the osmotic dimple is not the time-reverse equivalent of the usual hydrodynamic dimple phenomenon. We suggest that transient deformation of thin films by osmotic flow is a new and little-studied mechanism influencing the structure of stable thin films and the interaction of deformable drops. This has implications for colloidal interactions in a broader range of systems where solute concentration may not be homogeneous, for example in solute transfer processes.