We employ molecular dynamics simulations to investigate the evaporation process of nanosized droplets adsorbed on a substrate. Beads interacting with each other via Lennard-Jones (LJ) potentials are used to construct the simulation systems. The solid substrate contains 6 layers of beads forming a face-centered-cubic lattice. The bottom 3 layers are held rigid while the rest is kept at a constant temperature with a Langevin thermostat. A liquid droplet, consisting of LJ beads as well, is placed on top of the substrate. An appropriate amount of vapor beads are also supplied to the simulation box to help establish liquid-vapor equilibrium. To ensure adsorption, a stronger attraction is rendered between the droplet and a circular patch of 3 layers of beads at the center of the substrate surface while the rest of the substrate is made non-sticky for the fluid beads. During equilibration, the droplet and vapor are maintained at the same temperature as the thermalized substrate. After relaxation, the droplet adheres to the attractive patch as expected. Then a deletion zone is introduced into the top part of the vapor region. Fluid beads in this zone are removed at a given rate. To ensure that the evaporation dynamics and kinetics are properly captured, only the thermalized substrate is kept at the constant temperature during droplet evaporation. To carry out steady-state evaporation, the removed beads are reintroduced into a channel through the substrate and right below the droplet's center. These beads are then supplied to the droplet, compensating for the evaporation loss at the droplet surface. When the evaporation rate and the insertion rate are balanced, the system enters a steady state with the droplet undergoing continuous evaporation and its contact line pinned at the boundary of the attractive patch on the substrate. A one-to-one correspondence is found between the evaporation rate and the total number of fluid beads in the simulation box, as well as the contact angle of the droplet. Using this steady nonequilibrium system, we have mapped out the flow, temperature, and density fields inside and around the evaporating droplet as well as the local evaporation flux along the droplet surface with unprecedented resolutions. The results are used to test the existing theories on sessile droplet evaporation. / Master of Science / Droplet evaporation is a widespread natural phenomenon with numerous applications across various fields. While there has been extensive research on droplet evaporation, it remains a challenge to characterize the interior of the droplet and the local evaporation behavior on the droplet surface. Here we employ molecular dynamics (MD) simulation to model a nanosized droplet adsorbed on a substrate, which evaporates continuously while maintains a constant shape. This is realized by supplying the evaporated fluid back to the bottom of the droplet through an in-silico approach. Such a steady-evaporation system allows us to accurately map out the internal capillary flow of the evaporating droplet with a pinned contact line, where the droplet, vapor, and substrate meet. We find that local evaporation occurs faster near the contact line than at the apex of the droplet.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/117292 |
| Date | 02 January 2024 |
| Creators | Huang, Yisheng |
| Contributors | Physics, Cheng, Shengfeng, Kaplan, Cihan Nadir, Nguyen, Vinh |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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