Osmotically driven deformation of a stable water film
by Sue Ann Chen
Abstract: Recent experiments on a mercury drop near a mica surface have revealed that a dimple forms 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. It is
suggested that the formation of the dimple is due to the desorption of a
fraction of the SAM from the surface of the mercury drop 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 and this in turn causes the observed dimple. The solute concentration c(r; t) is introduced as a new dependent variable in the system and the transport of the solute is described by a convection-diffusion equation. The thin film equation and the convection-diffusion equation combine to give a system of coupled partial differential equations. The effects of disjoining pressure, hydrodynamic pressure and total pressure are discussed. It appears that the simplest version of the model, in which desorption is assumed to be uniform, cannot explain key features of the experimental observations, indicating a more refined model or another mechanism is responsible.
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