Wetting transition and optimal design for microstructured surfaces with hydrophobic and hydrophilic materials

We present wetting transition of a water droplet on microstructured polymer surfaces using materials with different hydrophilicity or hydrophobicity: hydrophobic polydimethyl siloxane (PDMS) (theta(water) approximately 110 degrees) and hydrophilic Norland Optical Adhesive (NOA) (theta(water) approximately 70 degrees). The microstructures were fabricated by replica molding and self-replication with varying pillar geometry [diameter: 5 microm, spacing-to-diameter ratio (s/d): 1-10 (equal interval), height-to-diameter ratio (h/d): 1-5] over an area of 100 mm(2) (10 mm x 10 mm). Measurements of contact angle (CA) and contact angle hysteresis (CAH) demonstrated that wetting state was either in the homogeneous Cassie regime or in the mixed regime of Cassie and Wenzel states depending on the values of s/d and h/d. These two ratios need to be adjusted to maintain stable superhydrophobic properties in the Cassie regime; s/d should be smaller than approximately 7 (PDMS) and approximately 6 (NOA) with h/d being larger than approximately 2 to avoid wetting transition by collapse of a water droplet into the microstructure. Based on our observations, optimal design parameters were derived to achieve robust hydrophobicity of a microstructured surface with hydrophobic and hydrophilic materials.