Determining the surface tension of gas-liquid interfaces through metadynamics

  • Thomas Fyfe

Student thesis: Masters ThesisMaster of Philosophy

Abstract

The measurement of surface tension γ provides information on the behaviour of phases at an interface. γ can be determined both experimentally and through computer simulation. Here, a gas-liquid interface is constructed and simulated in order to determine the value of gas-liquid interfacial surface tension γGL. In terms of the simulation protocol, the methodology used is molecular dynamics (MD) with well-tempered metadynamics for a one component simulation of Lennard-Jones Ar. The system operates in the NPxxT ensemble with the simulation x-axis cell length Lx as the chosen collective variable (CV). This allows the system to transition from a pure liquid state to a coexistent gas-liquid state via cell expansion and vice versa via cell contraction multiple times over a typical simulation. The CV Lx is later converted to the total number of liquid atoms Nliq by way of a conservation relation. This simulation protocol allows for the construction of a free energy profile with respect to Nliq, from which the free energy change ΔG associated with interface formation can be found and consequently the thermodynamic definition for γGL can be calculated. This work assesses the viability of this new method by investigating scaling in pairwise cutoff size rc, interfacial surface area A and reduced temperature T*. In particular, with MD parameters of rc = 7 σ, A = 16 σ x 16 σ for T* = 0.9, 0.95, 1.0, the obtained values of γGL relative to experiment are in very good agreement and have average deviations of 4.1%, 1.6% and <0.1% respectively when compared to experimental γGL values. Further work in studying this method can investigate a wider range of reduced temperatures T*, different systems such as water, different interface geometries and multi component systems.
Date of AwardJul 2021
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsNorthern Ireland Department for the Economy
SupervisorJorge Kohanoff (Supervisor) & David Wilkins (Supervisor)

Keywords

  • Metadynamics
  • free energy
  • nucleation
  • gas-liquid
  • surface tension
  • Lennard-Jones

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