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.

Roughening a hydrophobic surface enhances its non-wetting properties into superhydrophobicity. For liquids other than water, roughness can induce a complete roll-up of a droplet. However, topographic effects can also enhance partial wetting by a given liquid into complete wetting to create super-wetting. In this work, a model system of spreading droplets of a non-volatile liquid on surfaces having lithographically produced pillars is used to show that super-wetting also modifies the dynamics of spreading. The edge speed-dynamic contact angle relation is shown to obey a simple power-law and such power laws are shown to apply to naturally occurring surfaces.

In this work we present a reliable technique for the production of large areas of high aspect ratio patterns and their use as model super-hydrophobic systems. The high thickness and straight sidewalls possible with SU-8 were used to generate dense patterns of small pillars. The photoresist patterns could be used directly, without the need for micromoulding. A method is given allowing resist thickness to be varied over a wide range and a bottom antireflective layer was used to ease patterning on reflective substrates. This patterning technique allows rapid testing of wetting theories, as pattern size and depth can be varied simply and samples can be produced in sufficient numbers for laboratory use. We show how the static contact angle of water varies with pattern height for one sample-pattern and how static and dynamic contact angles vary with dimension using high aspect-ratio patterns.

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