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There are two pure modes of evaporation of liquid drops on surfaces:  one at constant contact area and one at constant contact angle. Constant contact area mode is the dominating evaporation mode for water and many other drops on solids when the initial contact angle is less than 90°. However, the constant contact angle mode has been reported in a few instances, such as water drop evaporation on poly(tetrafluoroethylene) where the initial contact angle is greater than 90°. In this work, we report the evaporation of n-butanol, toluene, n-nonane, and n-octane drops on a poly(tetrafluoroethylene) surface, which occurs with constant contact angle mode and an initial angle of less than 90°. Video microscopy and digital image analysis techniques were applied to monitor the drop evaporation. The decrease of the square of contact radius of these drops was found to be linear with time for most of the cases. This paper discusses the theoretical background and compares the experimental data with results from the previous models.

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The evaporation process for small, 1-2-mm-diameter droplets of water from patterned polymer surfaces is followed and characterized. The surfaces consist of circular pillars (5-15 microm diameter) of SU-8 photoresist arranged in square lattice patterns such that the center-to-center separation between pillars is 20-30 microm. These types of surface provide superhydrophobic systems with theoretical initial Cassie-Baxter contact angles for water droplets of up to 140-167 degrees, which are significantly larger than can be achieved by smooth hydrophobic surfaces. Experiments show that on these SU-8 textured surfaces water droplets initially evaporate in a pinned contact line mode, before the contact line recedes in a stepwise fashion jumping from pillar to pillar. Provided the droplets of water are deposited without too much pressure from the needle, the initial state appears to correspond to a Cassie-Baxter one with the droplet sitting upon the tops of the pillars. In some cases, but not all, a collapse of the droplet into the pillar structure occurs abruptly. For these collapsed droplets, further evaporation occurs with a completely pinned contact area consistent with a Wenzel-type state. It is shown that a simple quantitative analysis based on the diffusion of water vapor into the surrounding atmosphere can be performed, and estimates of the product of the diffusion coefficient and the concentration difference (saturation minus ambient) are obtained.