Tailoring wetting behavior at extremes

Abstract

In the classical understanding of liquid interactions with surfaces, liquid/surface chemistry dictates wetting behavior, requiring use of specific materials to achieve desired behavior. This restriction creates a number of challenges this thesis aims to address. First, high-thermal- conductance, hydrophobic coatings are used enhance condensation heat transfer, but have poor durability due to the extreme environment. We developed polymer infused porous surfaces, which 1. provided a large surface area to adhere and constrain the polymer to the condenser surface and 2. created a network of high-thermal-conductivity material through the otherwise low-thermal-conductivity polymer. These surfaces enhanced condensation heat transfer 8x and showed no degradation over 200 days.

Next, we demonstrated the use of reentrant microstructures and contact line pinning to shift the wetting paradigm, achieving any wetting behavior independent of the chemical nature of the surface and liquid, i.e., a surface with omniphobicity (repels all liquids), omniphilicity (wicks all liquids), switchability between repelling and wicking, and selectivity (repels or wicks only certain liquids). We then addressed robustness issues of reentrant microstructures during condensation on the surface by designing reentrant cavities with a pitch on the order of 100 nanometers. These dense, isolated cavities ensured nucleating droplets did not occur within all cavities and prevented liquid propagation within the structures, maintaining repellency to various liquids up to 10 'C below the dew point.

We explored alternative fabrication methods for omniphobic, doubly reentrant microstructures by using intrinsic stresses in thin films to induce bending, achieving omniphobicity with standard microfabrication processes. Finally, we enhanced wicking in pillar arrays by allowing pillar pitch and diameter to vary along the surface, optimizing each section of the surface for minimal pressure drop, increasing the wicking performance relative to uniform arrays. Each chapter of this thesis is dedicated to one of these challenging areas in tailoring wetting behavior at extremes.