Evaporation of sessile drops on polymer surfaces: Ellipsoidal cap geometry

The evaporation rate of sessile drops of water resting on poly(methyl methacrylate) polymer has been studied by using the recently published precise data of Rowan, Newton, and McHale. Mathematical expressions for an ellipsoidal cap drop resting on a solid surface having three parameters (base radius, height, and contact angle) are first derived. A vapor diffusion model of a drop based on a simplified two-parameter ellipsoidal cap geometry is also developed similar to Rowan et al. The expression for the rate of evaporation in the latter model reduces to that of Rowan et al.'s two-parameter model in the limit, as the drop shape becomes a spherical cap geometry. It is seen that the drop heights as calculated from the two-parameter ellipsoidal cap model fit the experimental data better than those evaluated from the spherical cap model. When the three-parameter ellipsoidal cap model is used, the surface area and volume of the drop show almost complete linear time dependence for most of the data.