Anomalous Diffusion and Stress Relaxation in Surfactant Micelles

TL;DR

This study uses molecular dynamics simulations to explore anomalous diffusion and stress relaxation mechanisms in surfactant micelles, revealing how microstructure influences diffusion behavior and micellar dynamics.

Contribution

It provides the first detailed molecular-level mechanism of branch motion and stress relaxation in micellar fluids, linking microstructure to diffusion and rheological properties.

Findings

Branching structures lower viscosity due to stress relaxation.

Superdiffusive behavior linked to branch sliding and micelle breakage.

Microstructure determines normal, subdiffusive, or superdiffusive motion.

Abstract

We present the first molecular dynamics study to probe the mechanisms of anomalous diffusion in cationic surfactant micelles in the presence of explicit salt and solvent-mediated interactions. Simulations show that when the counter ion density increases, saddle-shaped interfaces manifest leading to the formation of branched structures. In experiments, branched structures exhibit lower viscosity as compared to linear and wormlike micelles, presumably due to stress relaxation arising from the sliding motion of branches along the main chain. Our simulations provide conclusive evidence and a mechanism of branch motion and stress relaxation in micellar fluids. Further, depending upon the surfactant and salt concentrations, which in turn determine the microstructure, we observe normal, subdiffusive and superdiffusive motion of surfactants. Specifically, superdiffusive behavior is associated with branch sliding, breakage and recombination of micelle fragments as well as constraint release in entangled systems.