Surfactant Spreading on a Thin Liquid Film: Reconciling Models and Experiments

TL;DR

This study compares experimental observations of surfactant spreading on thin liquids with existing models, revealing significant discrepancies in monolayer regimes and suggesting the need for improved modeling approaches.

Contribution

It demonstrates that current models do not accurately predict surfactant spreading behavior in monolayer regimes, highlighting the necessity for revised theories.

Findings

Spreading exponent in monolayer regime is less than 1/10, much lower than predicted.

Significant differences in surfactant distribution between experiments and models.

Model inadequacy for describing insoluble surfactant spreading on thin films.

Abstract

The spreading dynamics of surfactant molecules on a thin fluid layer is of both fundamental and practical interest. A mathematical model formulated by Gaver and Grotberg 1990 describing the spreading of a single layer of insoluble surfactant has become widely accepted, and several experiments on axisymmetric spreading have confirmed its predictions for both the height profile of the free surface and the spreading exponent (the radius of the circular area covered by surfactant grows as t^{1/4}). However, these prior experiments have primarily utilized surfactant quantities exceeding (sometimes far exceeding) a monolayer. In this paper, we report that this regime is characterized by a mismatch between the timescales of the experiment and model, and additionally find that the spatial distribution of surfactant molecules differs substantially from the model prediction. For experiments performed in the monolayer regime for which the model was developed, the surfactant layer is observed to have a spreading exponent of less than 1/10, far below the predicted value, and the surfactant distribution is also in disagreement. These findings suggest that the model is inadequate for describing the spreading of insoluble surfactants on thin fluid layers.