Simulating confined particles with a flat density profile.

Area of Science:

• Soft matter physics
• Computational materials science
• Polymer physics

Background:

• Particle simulations with sharp walls often yield unrealistic oscillatory density profiles.
• This behavior is particularly problematic for modeling soft matter liquids and polymer systems.
• Existing simulation methods struggle to accurately represent density changes near interfaces.

Purpose of the Study:

• To develop a novel simulation method that produces monotonic density profiles near interfaces.
• To reconcile discrepancies between particle simulations and experimental observations in soft matter systems.
• To accurately model the density profile of a polymer brush grafted on a substrate.

Main Methods:

• Introduction of mirror-and-shift boundary conditions to particle simulations.
• Mapping simulation interfaces to distant parts of themselves to avoid artificial oscillations.
• Application of the method to simulate a polymer brush in explicit solvent on a silicon substrate.

Main Results:

• The proposed boundary conditions lead to an almost monotonic density increase from zero to bulk density.
• The density profile develops over a short distance, approximately one particle diameter.
• Simulations of a polymer brush show excellent agreement with neutron reflectometry and self-consistent field theory.

Conclusions:

• Mirror-and-shift boundary conditions are effective in generating realistic density profiles in particle simulations.
• This method provides a more accurate way to model soft matter systems, particularly polymer brushes.
• The approach bridges the gap between computational simulations and experimental measurements.