Dewetting from Amphiphilic Minichannel Surfaces during Condensation.

Abstract

Condensation heat transfer can be altered significantly by changing the texture and material of a surface to promote droplet removal and therefore lower thermal resistance. These designs are often expensive and fragile, however, and are fabricated using micro- or nanoscale features that are not easily implemented in real-world systems. Here, we present a novel macromachined amphiphilic surface that promotes droplet removal and resists permanent flooding via a spontaneous dewetting transition. While much of the research in condensation involves condensing on surfaces that are fully or mostly hydrophobic, droplets on the surface presented here nucleate and grow inside the structure on a hydrophilic material. The absence of any coating between the liquid and the conductive surface has the benefits of both decreasing thermal resistance and enhancing nucleation density. When the liquid grows to a critical size inside the channel, its elongated shape becomes unstable and spontaneously dewets to form rounded droplets on the hydrophobic fin peaks. The removal of liquid from the channels promotes new growth on the bare hydrophilic material, while the emerged rounded droplets can more easily shed from the hydrophobic fins. The dewetting phenomenon is shown experimentally and characterized analytically such that a desired critical water slug length could be designed by changing geometric parameters of the surface structure. The macroscale machined surface is also more durable than typical nanofabricated surfaces and easier to manufacture, making the surface more applicable to use in real-world systems. Spontaneous condensate dewetting on the amphiphilic structure is expected to enhance the study of inhibiting flooding on condensing surfaces and provide new pathways for droplet shedding techniques without a requirement for nanothin hydrophobic coatings.