Enabling Self-Propelled Condensate Flow During Phase-Change Heat Rejection Using Surface Texturing

This project is a collaborative effort between groups at University of California Davis and Auburn University. The central hypothesis of the proposed work is that asymmetry in surface microstructures can cause self-generated directional motion of the condensate. If such directional motion of the condensate can be achieved, a condenser with such microstructures can be combined with a similarly designed evaporator, developed under a previous NASA grant, to create a pumpless thermal management loop. The hypothesis will be tested on three specific asymmetric surface morphologies and compared against symmetric surface microstructures. The differences between the three morphologies lie in the surface wettability. Morphologies I and II will consist of hydrophilic and hydrophobic micro-structured asymmetric ratchets, respectively. The third morphology consisted of a combination of hydrophilic and hydrophobic faces on each ratchet. In accordance with the hypothesis, the overall objective was to characterize the effects of surface microstructures on droplet dynamics and/or film dynamics, and on heat transfer rate, by variation of the microstructure size or surface conditions. Both highly wetting fluids that are typically used in electronics thermal management as well as water were studied such that specific surfaces could be engineered for specific fluids.
Robust test surfaces with meso- and micro-scale surface texturing have been designed and demonstrated to effect droplet/film mobility, condensate removal, and consequently improved thermal performance of condenser surfaces in the horizontal orientation. This shows potential for use in microgravity systems because it allows surface drainage without the use of gravitational force. Specific achievements are as follows: 
•Demonstration and characterization of film mobility in Morphology I through experiments, numerical simulations, and analytical film drainage modeling. 
•Demonstration of enhanced heat transfer coefficient in Morphology I and II using asymmetric meso- and micro-scale structures compared with similar symmetric structures. 
•Demonstration of preferential adiabatic film/droplet motion on asymmetric structured surfaces with hydrophobic, hydrophilic and biphilic wettability (Morphologies I, II and III)