Determining the surface tension of gas-liquid interfaces through metadynamics

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.

Name dc:type.qualificationname

Master of Philosophy

Level dc:type.qualificationlevel

Masters Thesis

Grantor dc:publisher.institution

Queen's University Belfast

Year dc:date.issued

2021

Author dc:creator

• Fyfe, Thomas

Advisors dc:contributor.advisor

• Kohanoff, Jorge
• Wilkins, David

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

Language dc:language

eng

Identifier

oai:pure.qub.ac.uk/portal:studenttheses/971e6dce-6334-4f37-a62c-0da8cbdf8422

OAI identifier oai:identifier

oai:pure.qub.ac.uk/portal:studenttheses/971e6dce-6334-4f37-a62c-0da8cbdf8422