Bio-Inspired Gas-Entrapping Microtextured Surfaces (GEMS): Fundamentals and Applications

Abstract

Omniphobic surfaces, which repel polar and non-polar liquids alike, have proven of value in a myriad of applications ranging from piping networks, textiles, food and electronics packaging, and underwater drag reduction. A limitation of currently employed omniphobic surfaces is their reliance on perfluorinated coatings/chemicals, increasing cost and environmental impact and preventing applications in harsh environments. Thus, there is a keen interest in rendering conventional materials, such as hydrocarbon-based plastics, omniphobic by micro/ nanotexturing rather than via chemical makeup, with notable success having been achieved for silica surfaces with doubly reentrant pillars (DRPs). We discovered a critical limitation of DRPs – they catastrophically lose superomniphobicity in the presence of localized physical damages/defects or on immersion in wetting liquids. In response, we pioneered bio-inspired gas-entrapping microtextured surfaces (GEMS) architecture composed of doubly reentrant cavities (DRCs). DRCs are capable of robustly entrapping air when brought into contact with liquid droplets or on immersion, which prevents catastrophic wetting transitions even in the presence of localized structural damage/defects. This dissertation presents our multifaceted research on DRCs via custom-built pressure cells, confocal laser scanning microscopy, environmental scanning electron microscopy, contact angle goniometry, high-speed imaging, and upright optical microscopy. Specific accomplishments detailed in this thesis include: (i) the microfabrication protocols for silica GEMS developed at KAUST; (ii) the characterization of GEMS’ omniphobicity via apparent contact angles and immersion; (iii) the demonstration of ~ 1000,000,000% delays in wetting transitions in DRCs compared to those in simple cavities (SCs) under hexadecane; (iv) a proposal for immersion of surfaces as a criterion for assessing their omniphobicity in addition to apparent contact angles; (v) effects of surface chemistry, hydrostatic pressure, and cavity dimensions on Cassie-to-Wenzel transitions in DRCs and SCs; (vi) the demonstration of “breathing” (liquid-vapor) interfaces in GEMS under fluctuating hydrostatic pressures; and (vii) the demonstration of directional wetting transitions in DRCs (or cavities in general) arranged in one- and two-dimensional lattices. The last chapter in the thesis presents future research directions such as breathing surfaces capable of preempting vapor condensation and gas replenishment.