Phase separation in fluids exposed to spatially periodic external fields

TL;DR

This paper investigates how spatially periodic external fields influence phase transitions in fluids, revealing the emergence of a modulated phase, its coexistence with vapor and liquid phases, and the conditions for its formation through simulations and mean-field theory.

Contribution

It introduces a new understanding of phase behavior under periodic fields, including a threshold wavelength for modulated phase formation and a novel simulation method for low interface tensions.

Findings

A modulated phase appears when the field wavelength exceeds a threshold.

The interface tension between phases is extremely low, around 10^{-4} kB T.

In two dimensions, the modulated phase does not form, altering the phase diagram.

Abstract

We consider the liquid-vapor type phase transition for fluids confined within spatially periodic external fields. For a fluid in d=3 dimensions, the periodic field induces an additional phase, characterized by large density modulations along the field direction. At the triple point, all three phases (modulated, vapor, and liquid) coexist. At temperatures slightly above the triple point and for low (high) values of the chemical potential, two-phase coexistence between the modulated phase and the vapor (liquid) is observed. We study this phenomenon using computer simulations and mean-field theory for the Ising model. The theory shows that, in order for the modulated phase to arise, the field wavelength must exceed a threshold value. We also find an extremely low tension of the interface between the modulated phase and the vapor/liquid phases. The tension is of the order 10^{-4} kB T per squared lattice spacing, where kB is the Boltzmann constant, and T the temperature. In order to detect such low tensions, a new simulation method is proposed. We also consider the case of d=2 dimensions. The modulated phase then does not survive, leading to a radically different phase diagram.