Studies of the Dimensional Effects of SU-8 and PDMS Pillar Arrays on Hydrophobicity

The creation of hydrophobic and superhydrophobic surfaces has attracted tremendous attention in the past decade. Such surfaces provide unique and highly useful properties that are suitable for a wide range of applications. In this paper, we report our findings on the dimensional effects of SU-8 and polydimethylsiloxane (PDMS) pillar arrays on surface hydrophobicity. Pillar arrays with various dimensions are designed and fabricated on SU-8 and PDMS surfaces using optical lithography and soft lithography. The water droplet contact angles on these features are obtained with a goniometer to demonstrate how the related dimensions, including the diameter of each pillar and the distance between two pillars, affect the surface hydrophobicity. Theoretical analyses are also carried out to estimate the contact angles of water droplets based on Cassie and Wenzel models. The experimental and analytical results are presented and compared in this paper. The results demonstrate that the dimensional change of a pillar array has a direct impact on its surface hydrophobicity. The highest contact angle can only be achieved using optimum designs. Furthermore, there are clear differences in our measurement results between the SU-8 and PDMS pillar arrays. We believe that these differences come from the inherent hydrophobicity difference of the two polymers. Without further coatings or treatment steps, SU-8 pillar arrays can achieve contact angles up to 140° while the PDMS structures can achieve higher contact angles up to 170°. Last, the PDMS pillar arrays demonstrate a transition phase from the Cassie state to the Wenzel state during the wetting experiments.