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.

The classical spherical cap geometry which is used to define the shape of sessile drops resting on polymer surfaces has a deficiency, since it is a two-parameter model of the three measurable drop dimensions:  contact angle, base radius, and height. In this paper, mathematical expressions for a drop having all three measurable quantities as parameters are derived. The proposed new model is termed as the three-parameter spherical cap geometry. A vapor diffusion model for the drop depending on the two-parameter pseudospherical cap geometry is also developed that is similar to that of Rowan, Newton, and McHale. The evaporation rate of sessile drops of water resting on poly(methylmethacrylate) polymer has been studied by using their recently published precise data. When the present analysis is applied, the drop heights as calculated from the reduced two-parameter pseudospherical cap model fit the experimental data better than those calculated from the classical two-parameter spherical cap model. In addition, the drop volumes as calculated from the three-parameter spherical cap geometry give a linear decrease with time with better regression analysis results than those of the other models.

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