Determining the surface tension of gas-liquid interfaces through metadynamics

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 γ_GL. 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 NP_xxT ensemble with the simulation x-axis cell length L_x 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 L_x is later converted to the total number of liquid atoms N_liq by way of a conservation relation. This simulation protocol allows for the construction of a free energy profile with respect to N_liq, from which the free energy change ΔG associated with interface formation can be found and consequently the thermodynamic definition for γ_GL can be calculated. This work assesses the viability of this new method by investigating scaling in pairwise cutoff size r_c, interfacial surface area A and reduced temperature T^*. In particular, with MD parameters of r_c = 7 σ, A = 16 σ x 16 σ for T^* = 0.9, 0.95, 1.0, the obtained values of γ_GL 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 γ_GL 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 Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Jorge Kohanoff (Supervisor) & David Wilkins (Supervisor)