Thin Film Condensation Supported on Ambiphilic Microstructures

Abstract

Ambiphilic surfaces have been used to support thin liquid films during condensation and imaged using environmental scanning electron microscopy (ESEM). Ambiphilic microstructures (a) are comprised of hydrophilic deep etched silicon micropillars with hydrophobic post tops made of PTFE deposited using iCVD. By restraining the growth of the liquid film using hydrophobic post tops (b), thermal resistance is reduced and heat transfer is increased. During condensation on ambiphilic microstructures the condensate initially fills the post array (b), but then bursts outward to accommodate continued production of liquid (c). This creates a low contact angle droplet on the surface (c), and could lead to complete flooding and decreased performance. With the addition of hydrophilic nanostructures to the micropost array (d), ambiphilic hierarchical structures have been fabricated with dedicated burst sites (e). During condensation the structures maintain a thin liquid film and excess liquid emerges from the burst sites as highly mobile spherical droplets (e). This maximizes the thin film area available for vapor-to-liquid phase change while minimizing thermal resistance across the condensate layer. Scale bars: (a) 5 µm, (b,c) 50 µm, (d) 2 µm, and (e) 25 µm.