Non-monotonic Pressure Field induced by Ionic Diffusion in Charged Thin Films

Dynamics of pressure field evolution inside thin films under the effect of ionic strength gradient is not well understood. Dynamics of the pressure field is important as it controls the film hydrodynamics and also change of contact angle due to the change of ionic strength. The major two potentials building the total pressure in thin films are osmotic and electrostatic potentials. In thin films, these two components are working against each other, while the reduction of ionic strength will decrease the osmotic pressure, it will increase the electrical double layer thickness. However, this interaction is controlled by transport of ions and the transport time-scale. Here, we present a model that couples Nernst–Planck and Poisson equations to simulate ionic transport and also Stokes equation augmented by the Maxwell stress tensor (MST) to simulate the pressure field. Results show a highly nonlinear behavior in the pressure field that is initiated by diffusion of the ions in a channel which is initially filled by a high ionic strength electrolyte and is exposed to a bulk solution with lower ionic strength. Results show that diffusion length (transport length) and the overlapping of the double layers affect the pressure field significantly. The results imply that in thin films where ionic diffusion is expected, interfaces can deform due to the nonlinear pressure field, which is triggered by the asymmetric and multidirectional transport of ions. This brings a new insight into thin film hydrodynamics that can contribute to understanding the dimple formation in thin films.