Myelin figures: the buckling and flow of wet soap

TL;DR

This paper investigates the formation and growth of myelin figures in surfactant-water systems, proposing a fluid flow model driven by hydration gradients and identifying a mechanical instability as a key factor.

Contribution

It introduces a new model explaining myelin growth via surfactant flow and links mechanical instability to myelin formation, supported by experimental observations.

Findings

Myelin growth is driven by surfactant fluid flow due to hydration gradients.

A mechanical instability underlies the formation of myelin figures.

The model qualitatively reproduces various experimental growth behaviors.

Abstract

Myelin figures are interfacial structures formed when certain surfactants swell in excess water. Here, I present data and model calculations suggesting myelin formation and growth is due to the fluid flow of surfactant, driven by the hydration gradient at the dry surfactant/water interface; a simple model based on this idea qualitatively reproduces the various myelin growth behaviors observed in different experiments. From a detailed experimental observation of how myelins develop from a planar precursor structure, I identify a mechanical instability that may underlie myelin formation. These results indicate the mixed mechanical character of the surfactant lamellar structure, where fluid and elastic properties coexist, is what enables the formation and growth of myelins.