Date of Award

5-14-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Shalabh Maroo

Keywords

Electronics Cooling, Liquid Infused Surfaces, Nanoscale Heat Transfer, Solar Vapor Generation, Superhydrophobicity, Thermal Management

Abstract

Surface texturing to create artificially engineered surfaces is a well-researched field in heat transfer. Depending on the surface energy, further texturing can drastically alter the wettability to obtain hydrophobic, superhydrophobic, or hydrophilic surfaces. These properties can then be used in a variety of practical applications such as water harvesting through evaporation and condensation, self-cleaning properties, and thermal management of electronic chips. In this work, wettability features of nanochannel wicks are experimentally tested and explored for applications ranging from creation of robust slippery surfaces to cooling of electronic chip.First, to advance dropwise condensation heat transfer, a process for creating a liquid infused surface (LIS) was developed; such surfaces have the ability to diminish the depletion of lubricant during condensation. The retention of silicon oil, used as a lubricant, was due to plasma treatment of the porous nanochannel sample prior to oil infusion, as well as strong capillarity because of the geometry of the nanochannels. While lower viscosity oil (5 cSt) provides high condensate drop mobility, it suffers faster depletion. So, 50 cSt was found to be a trade-off between drop mobility and oil depletion. Further, even the depleted LIS resulted in heat transfer coefficient (HTC) ~ 2.33 kWm-2K-1, which is ~ 162% improvement over flat silicon LIS. During condensation, rapid drop shedding on depleted nanochannel LIS with 50 cSt oil observed from little change in fraction of drops with diameter < 500 μm from ~ 98 % to only ~ 93 % after 4 hours of condensation. Durability of LIS was confirmed by total of 3 days of condensation on fresh LIS over an 18-day period. The sample still maintained the hydrophobic characteristic with a water contact angle ~104º after 18 days. In the next study, the porous nanochannel sample was treated and baked at high temperature with silicon oil, and subsequently depleted and coated with candle soot to obtain a superhydrophobic 1 surface (SHS). This SHS was then subjected to over 20 durability tests including extreme exposure to water (30 days submersion). Usually, a few days of water exposure or high humidity deteriorates the superhydrophobic surfaces, rendering them inappropriate for prolonged underwater applications. However, SHS fabricated in present study did not exhibit any significant degradation and remained superhydrophobic even after 5 months in open laboratory environment. Further, it demonstrated self-cleaning properties, organic compatibility (honey, soy sauce, chocolate syrup, all-purpose flour) and superoleophilicity, thus exhibiting potential real-world applications. Another feature of this SHS was its ability to restore superhydrophobicity from a forced degraded state. Next, hydrophilic properties of porous nanochannels were explored in designing a solar flux-based vapor generation system. An unconventional approach to separate heating side and vapor generating side yielded significantly higher vapor generation rate ~ 1.18 kgm-2h-1 even in the absence of heat supply. Testing under solar heat flux of 1.25 sun resulted in vapor generation rate of 4.87 kgm-2h-1 at surface temperature below 35ºC. The flexibility of the system, in regards to the choice of energy source apart from solar flux, was shown by using a resistive heater on the heating side. The maximum vapor generation rate of 17.12 kgm-2h-1 was achieved with resistive heat input of 5.08 W at low temperature of 62℃. These findings could guide the vapor generation systems towards achieving higher vapor generation rate at low power and temperature thresholds. In the later part of the work, performance of a device-scale nanochannel (122 nm depth and 10 μm width) based evaporator with FC72 as working fluid was demonstrated. FC72 is an ideal fluid for electronics cooling as it is nonpolar and dielectric with a low boiling point. The 1 mm thick evaporator provides a low form factor cooling solution desired in electronic cooling. The steady-state wicking distance of FC72 in the nanochannels varied from 21 mm to 8 mm depending on the evaporator’s working temperature. A new method to directly measure the evaporation of FC72 was developed to estimate the interfacial evaporative heat flux which eliminated the need for the contact angle measurement in nanochannels. The maximum evaporative heat flux was 0.93 kWcm-2 at ~ 65 ºC hot spot temperature. Numerical simulations were performed to quantify the heat losses from different components of the evaporator arrangement. This study provides a systematic approach to design thin film evaporators while delineating important parameters to help develop effective thermal management strategies for high-performance electronics.

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