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Characterisation of the dominant liquid-liquid interface shape in confined systems during molecular simulations
This study investigates the equilibrium shapes of liquid-liquid interfaces confined in slit pores with rectangular geometries. We focussed on how the aspect ratio of the simulation box, described by a squeeze parameter k – the inverse simulation box base aspect ratio. This parameter significantly influences the interface shape. By analysing systems with complete non-wetting conditions, we derive analytical expressions for the surface areas of various interface shapes as functions of volume fraction, squeeze parameter and dimensionless lateral box size. Our findings reveal that as the simulation box becomes more squeezed, certain interface shapes like rolls become dominant among simulations results. Stability diagrams illustrate how the preferred interface type changes with system parameters. For highly squeezed boxes with the aspect ratio 1/𝑘>2, rolls account for over 50% of observed configurations, with probabilities exceeding 90% for specific volume fractions. This work provides a practical framework for predicting and controlling liquid-liquid interfaces in confined geometries, with applications in microfluidics and porous media. The study bridges a gap in understanding how geometric asymmetry affects interface stability, offering insights for initial simulation box design. The approach is validated through numerical calculations, demonstrating the utility for guiding simulations and engineering confined fluid systems.