Enhancement of Water Repellence by Hierarchical Surface Structures Integrating Micro-dome and Micro-pillar Arrays with Nanoporous Coatings

Abstract

Superhydrophobic surfaces with multi-scale topographies offer exceptionally high apparent water contact angles and low contact angle hysteresis by virtue of the small liquid{\textendash}solid contact fractions they enable. Natural water-repellent surfaces such as lotus leaves often feature dome-shaped micro-scale protrusions, whose lack of sharp edges also facilitates smooth droplet shedding without pinning. Engineered hydrophobic surfaces, however, have not yet fully exploited the merits of protrusions with controlled curvature. In this work, thermal re-flow of photoresist patterns followed by elastomeric casting was used to fabricate arrays of micro-domes with sizes 20{\textendash}50 {\mu}m. These microstructures were coated with a nanoporous zinc oxide film and fluorosilanized to produce hierarchical surface topographies with static water contact angles up to 169.7{\pm}0.4{\deg} and contact angle hysteresis as low as 14.7{\pm}1.3{\deg}. Performance of the micro-dome arrays significantly exceeded that of arrays of sharp-edged square pillars and flat surfaces coated with the same nanoporous film. The highest performance came from the smallest micro-domes (20 {\mu}m) and closest spacings (10 {\mu}m) investigated. For larger features, contact angles reduced and hysteresis increased {\textemdash} unexpected trends not explained by contact fraction alone. This simple fabrication technique could be adapted to manufacture large surfaces for droplet shedding, including in heat transfer applications.