Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 149734

149734 - Understanding Phase-Change in Nanostructures to Advance Thermal Transport Strategies 

Electronic devices are growing in power density, producing more heat in smaller packages and requiring innovative cooling strategies. Evaporation in nanostructured surfaces has the potential to provide high heat fluxes and has been studied extensively. Although evaporation in nanoporous structures occurs in nature and industrial processes, our understanding of evaporation kinetics of liquids, particularly water, confined in nanopores is still incomplete.

Quantifying phase change at a small scale is a significant challenge. Gravimetry-based techniques require exceptionally accurate scales or large devices, while typical imaging methods cannot visualize inside individual nanopores, requiring specialized equipment and limited environmental conditions. Calorimetric approaches often found in the literature also require specialized fabrication of testbeds for accuracy. Quartz crystal microbalances (QCMs) offer a precise, tunable, and inexpensive method for determining liquid evaporation out of nanopores. This poster presents our research on evaporation from single-ended nanopores, which was studied utilizing a QCM fabricated with highly-ordered anodic alumina nanopores.

QCMs, typically used to monitor thin film deposition processes, were modified in this study to quantify interfacial and wetting phenomena and phase-change kinetics. The resonant frequency of a QCM responds to stress acting on the surface, which for this application is the changing density (liquid to vapor) in the pores fabricated on the surface. Data acquisition using a quartz crystal with a 5 MHz nominal frequency can yield up to 10 samples/second with a resolution of 1 Hz on commercially available hardware. Theoretically, a 5 MHz QCM has a mass sensitivity of 56.6 Hz cm²/μg, hence, a mass resolution of 18 ng/cm². We fabricated nanopores on a QCM, achieving volumes of 0.3 μL or more, holding 0.3 mg or 233 μg/cm² of water. Pores filled with water, for example, would yield a response of 13000 Hz, far exceeding the device resolution, providing exceptional measurement capability for nanoporous evaporation kinetics. Our approach using the QCM allows high-precision measurements of both mass flux (~1.8 ng/cm²/s) and dynamic receding length of the evaporating liquid in the nanopores. Besides experiments, we used the kinetic theory of gases to model flow from the nanopores into the far-field. The model developed in this work builds on prior work and applies to single-ended and double-ended pores. It considers the conservation equations (mass, momentum, energy) and links experimental measurements in the far-field (temperature, pressure) and QCM-determined mass flux and receding length to phenomena occurring inside the pores during evaporation. In other words, the kinetics of water evaporation from long single-ended nanopores was quantified using a QCM as a high-precision mass sensor. The kinetic theory model determined the liquid phase pressure, the meniscus shape, and the dynamic accommodation coefficient (AC) throughout the evaporation process.

Our results indicate that confinement effects greatly influence mass accommodation at the liquid-vapor interface as the liquid meniscus recedes down the pore during evaporation. ACs were found to start high near the beginning of evaporation when the menisci are close to the top of the pores before dropping to values on the order of 1.e-04 after receding. The liquid side pressure at the meniscus reached a maximum magnitude of -26 MPa under these evaporation conditions and increased in magnitude as the humidity decreased. The strong tensile forces experienced by the water molecules may affect the ability of water molecules to transition from liquid to vapor, reducing accommodation at the liquid-vapor interface. Based on these observations, our future work would extend this model to include non-uniform liquid pressure in the pores for a more in-depth view of the meniscus shape and how the varying curvature of a meniscus influences the overall evaporation kinetics.

Presenting Author: Shankar Narayan Rensselaer Polytechnic Institute

Presenting Author Biography: An Academic and Thermal Scientist with over 20 years of experience leading research in liquid-vapor interfacial phenomena and nanoengineered materials for energy conversion and storage. Involved in research for advancing the understanding of evaporation and condensation processes, developing novel techniques for studying phase-change kinetics, and designing high-performance cooling systems. Expert in computational modeling, experimental characterization, and applying advanced materials to solve practical problems in thermal management, energy efficiency, and environmental sustainability. Adept at translating complex scientific concepts into actionable insights and leveraging academic expertise to drive innovative solutions and business growth.

Authors:

Shankar Narayan Rensselaer Polytechnic Institute
Brandon Murray Rensselaer Polytechnic Institute
Xue Li Rensselaer Polytechnic Institute