CONFIRMED: EAS PhD Dissertation Defense by Wen Jin (CSE Option/Mechanical Engineering)
EAS PhD Dissertation Defense by Wen Jin (CSE Option/Mechanical Engineering) DATE: January 10, 2024 TIME: 10:00 a.m. - 1:00 p.m. TOPIC: Computational investigation of impact of micron-sized water droplets onto freezing superhydrophobic surfaces and novel 3D numerical modeling of contact line pinning LOCATION: CSCDR - TXT 105 Zoom link: Please contact Dr. Raessi (mraessi@umassd.edu). ABSTRACT: This thesis presents a computational study on the impingement of micron-sized water droplets onto freezing superhydrophobic surfaces. In particular, the investigation is focused on the effect of surface wettability, temperature, droplet size and impact speed on the droplet dynamics and freezing behavior. A numerical approach based on the Volume-of-Fluid (VOF) method was employed to simulate the droplet-surface interaction and the droplet behavior during the freezing process. A theoretical model is presented for predicting the transition from bouncing to sticking after droplets impact freezing surfaces. The theoretical model, which relies on time scales of droplet spreading and droplet freezing, predicts the droplet behavior, i.e., bouncing off of or sticking to the freezing surface, as a function of substrate temperature and Weber number, which represents the ratio of inertia to surface tension. A constant in the theoretical model was determined based on a subset of simulation results obtained in this thesis. The theoretical model predictions were then verified using both experimental results of millimeter-sized drops and computational results of micron-sized droplets impacting freezing superhydrophobic surfaces, showing good agreements in both cases. In total, 720 simulations of droplet impact were performed to inform and validate the theoretical model. Moreover, a novel three-dimensional numerical scheme is developed for modeling discontinuous pinning along sharp straight edges. The proposed scheme is devised for multi-phase flow solvers that rely on the VOF method, although its fundamental concepts can be extended and applied to other methods. Following the Piecewise-Linear-Interface-Construction (PLIC) approach in VOF, the discontinuous pinning is modeled by adjusting the orientation of PLIC polygons located near a sharp edge according to the pinning stage. That is achieved by solving a root-finding problem and using a 3D geometrical toolbox, where the advancing contact angle determines critical volume fractions in numerical cells neighboring the sharp edge. Implementing the proposed scheme in our multi-phase flow solver, we assessed its performance using several test cases where contact line pinning effects dominate. To demonstrate the scheme's efficacy, we present quantitative comparisons of our results at various grid resolutions and with an experimental/theoretical study. Furthermore, we show quantitatively that without a numerical treatment of contact line pinning, the simulation results will be drastically different. Contact line pinning plays a critical role in several technologies including separation, lithography, lens fabrication, microfluidic flow control among numerous others. The proposed scheme will help to accurately capture the pinning effects in computational simulations of such applications. Acknowledgment: The research support from the National Science Foundation under CBET Grant No. 1336232 is gratefully acknowledged. ADVISOR: Dr. Mehdi Raessi (mraessi@umassd.edu, 508-999-8496), Dept of Mechanical Engineering COMMITTEE MEMBERS: Dr. Alfa Heryudono, Dept of Mathematics Dr. Hangjian Ling, Dept of Mechanical Engineering Dr. Jun Li, Dept of Mechanical Engineering Open to the public. All MNE and EAS students are encouraged to attend. For questions contact Dr. Mehdi Raessi (mraessi@umassd.edu, 508-999-8496)
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