Simulating confined particles with a flat density profile

TL;DR

This paper introduces mirror-and-shift boundary conditions in particle simulations to produce a nearly monotonic density profile, aligning better with experimental observations for confined soft matter systems.

Contribution

The paper presents a novel boundary condition method that suppresses oscillatory density profiles in confined particle simulations, improving realism for soft matter applications.

Findings

Density profile becomes nearly monotonic with the new boundary condition.

Simulated profiles agree with neutron reflectometry data.

Method effectively models polymer brushes in solvent.

Abstract

Particle simulations confined by sharp walls usually develop an oscillatory density profile. For some applications, most notably soft matter liquids, this behavior is often unrealistic and one expects a monotonic density climb instead. To reconcile simulations with experiments, we propose mirror-and-shift boundary conditions where each interface is mapped to a distant part of itself. The main result is that the particle density increases almost monotonically from zero to bulk, over a short distance of about one particle diameter. The method is applied to simulate a polymer brush in explicit solvent, grafted on a flat silicon substrate. The simulated density profile agrees favorably with neutron reflectometry measurements and self-consistent field theory results.