A Modified Disjoining Pressure Model for Thin Film Evaporation of Water

In the present work, a modified model to predict the disjoining pressure for thin film evaporation of water is presented. There has been some controversy about disjoining pressure modeling, especially for the case of polar liquids such as water. The conventional models for prediction of disjoining pressure, such as non-polar, logarithmic and exponential models, lead to different values of pressure and physically invalid thicknesses of the thin film near the non-evaporating region. In the present work, a modified disjoining pressure model is introduced based on multiplying a stretching function with the van der Waals component of disjoining pressure to consider the other intermolecular and surface effects such as structural forces and hydrogen bonds. Comparison of the present model with conventional ones shows that it can account for the effect of water molecules polarity over the entire extended meniscus. Also, the non-evaporating region thickness can be appropriately evaluated by this model, unlike the conventional ones. Moreover, the simple formulation of this model makes it suitable for analytical investigations of water thin film evaporation. Finally the effects of superheat, wall temperature and far field meniscus radius on the heat and mass transfer and fluid flow characteristic of thin film region are investigated and compared for the different disjoining pressure models.