Liquid Propagation on Micro/Nanostructured Surfaces for Enhanced Heat Transfer

Abstract

Liquid propagation or wicking along micro/nanostructured surfaces attract numerous interest due to its importance in variety of engineering applications such as thermal management, desalination, power generation and microfluidics. The advantages of such surfaces are not only to drive the liquid by passive capillary pumping, but also to produce large thin-film evaporation area with low heat transfer resistance. In fact, a major bottleneck in thin-film evaporation is the limit of surfaces in supplying enough fluid through liquid propagation. Therefore, this thesis aims at probing physical insights into the liquid propagation phenomenon on micro/nano-engineered surfaces towards their practical applications. Fundamental understanding of microscale liquid propagation shows that capillary pressure during propagation is not constant and therefore. An average capillary pressure model is proposed based on maximum and minimum capillary pressure. Effective liquid height in a unit cell is further estimated by considering the liquid meniscus effect, which is crucial for permeability estimation. Average capillary pressure, average liquid height and contact angle models are validated through liquid rise rate experiments. The predictions of contact angle on micropillar sidewall are in agreement with interferometry measurements. The models were also used to simulate the droplet spreading and reasonably estimate spreading time and diameter when liquid droplets impact microstructured surfaces, which can facilitate the design of high heat flux cooling devices and inkjet printing. . In addition to fundamental understanding of liquid propagation along micropillar arrays, a group of scalable and low-cost nanostructured microporous surfaces, with micromeshes covering a flat substrate, are fabricated to enhance the capillary pumping. The liquid propagation along microporous surfaces are studied analytically and experimentally. An analytical capillary pressure model and a numerical permeability model are also developed to accurately model the liquid propagation phenomenon on various microporous surfaces. The model predictions agree with the experimental results within 10% error. The micromesh dimension was further optimized to achieve maximum mass flow rate, which shows outstanding wicking performance. This thesis presents insightful fundamental understanding of liquid propagation along microstructured surfaces and development of scalable and low-cost nanostructured surfaces for enhanced liquid propagation. The results from this thesis also provide valuable design guidelines of devices to achieve high-performance thermal energy transport.

Date of Award	Dec 2017
Original language	American English