Shajedul Hoque - B.Sc. Thesis

Atomistic Modeling of Functionally Gradient Wettability Surface and its Implication on Thin Film Liquid-Vapor Phase Change Phenomena

by

Mahmudul Islam,

Md Shajedul Hoque Thakur, and

Shahriar Alam

Under the Supervision of

Dr. Mohammad Nasim Hasan

Thesis Abstract

An atomistic model of functionally gradient wettability (FGW) surface has been developed to perform molecular dynamics simulations. The applicability of the developed model in maintaining its functionally determined wettability has been verified by comparing liquid contact angles. It is found that, by utilizing our present modeling method, a wide range of functionally gradient wettability surfaces can be generated for molecular dynamics (MD) studies. Thin-film phase change characteristics of liquid argon over FGW platinum surfaces governed by power-law function have been investigated through non-equilibrium molecular dynamics (NEMD) simulations. For proper comparison, hydrophilic as well as hydrophobic surfaces have been considered along with five different Power-law functionally gradient wettability (P-FGW) surfaces (0.2 ≤ p ≤ 5). The results show that increasing the function parameter (p) can greatly enhance the phase transition heat transfer due to the presence of a larger hydrophilic portion. The effects of enhanced heat transfer have been qualitatively characterized by monitoring the system snapshots of different time intervals, temperature, and pressure of liquid argon, and net evaporation rate. Also, the quantification of heat and mass transfer characteristics has been done by calculating the average heat flux and average evaporative mass flux. The heat transfer characteristics have been analyzed from both an atomistic point of view and a classical thermodynamics point of view. It has been observed that classical thermodynamics can be a predictive tool for nanoscale phase transition, especially in hydrophilic conditions. The averaged heat flux obtained from the rule of mixture (ROM) has been compared with the present results. The discrepancy between ROM and MD points towards an additional convective heat transfer mode that enhances the heat transfer characteristics of thin-film phase change over FGW surfaces. To further confirm and understand this additional heat transfer mode, we investigated thin-film evaporation of liquid argon over FGW surface governed by sigmoid function (S-FGW). Sigmoid function (1 ≤ s ≤ 10) ensures that the overall hydrophilic and hydrophobic proportions are the same for all the S-FGW surfaces so that a proper comparison between the evaporation characteristics can be done. Also, a patterned wettability surface is included in the investigation due to its relevance in contemporary literature. It is found that gradual change in wettability along a large portion of the surface leads to enhanced evaporation characteristics. S-FGW surface of s = 1 has better evaporation characteristics compared to the patterned wettability surface, which points out the numerous potential practical applications of FGW surfaces in thin-film phase change. We observed a large number of local argon clusters of different temperatures over S-FGW surfaces, which indicates local convection during evaporation. This local agitation is one of the key reasons behind the enhanced heat transfer characteristics of thin-film phase change of argon over S-FGW surfaces.

Publications from the thesis:

1. A Molecular Dynamics Approach to Thin Film Liquid Phase Change Phenomena on Functionally Gradient Wettability Surface (2020).

Authors: MSH Thakur, M Islam, S Alam, MN Hasan, Y Mitsutake, M Monde
Journal: Micro & Nano Letters (IET Digital Library)
DOI: 10.1049/mnl.2019.0657

Abstract:

An atomistic model of functionally gradient wettability (FGW) surface for molecular dynamics (MD) simulation has been proposed and developed. Using the present model, a non-equilibrium MD study has been conducted to investigate the effects of FGW on liquid thin film phase change characteristics over the FGW surface. A power function has been considered as the wettability governing function of the FGW surface and by varying its function parameter, various FGW surfaces have been studied. The simulation results show that the function parameter can be a significant modulation parameter for heat transfer characteristics associated with the phase transition. To gain insight into any additional heat transfer mode associated with the FGW surface, the wall heat fluxes have been compared with linear mixture rule predictions. It is found that, along with conduction heat transfer through the interface between solid FGW surface and liquid thin film, there exists convective heat transfer along the wettability gradient direction. This additional heat transfer mode, which is not present for uniformly wetted surfaces, causes significant enhancement of phase change characteristics. The results of the present MD simulation have been found consistent with macroscopic prediction based on classical thermodynamics theory.