Fluorescent visualization of a spreading surfactant

TL;DR

This study provides the first quantitative visualization of surfactant spreading on thin films, revealing novel features and confirming theoretical spreading exponents through fluorescence imaging.

Contribution

It introduces a fluorescence-based method for detailed spatiotemporal measurement of insoluble surfactant dynamics, uncovering features not predicted by existing models.

Findings

Observation of a Marangoni ridge and trailing trough during spreading.

Identification of a surfactant concentration peak trailing the leading edge.

Measured spreading exponents consistent with theoretical predictions.

Abstract

The spreading of surfactants on thin films is an industrially and medically important phenomenon, but the dynamics are highly nonlinear and visualization of the surfactant dynamics has been a long-standing experimental challenge. We perform the first quantitative, spatiotemporally-resolved measurements of the spreading of an insoluble surfactant on a thin fluid layer. During the spreading process, we directly observe both the radial height profile of the spreading droplet and the spatial distribution of the fluorescently-tagged surfactant. We find that the leading edge of spreading circular layer of surfactant forms a Marangoni ridge in the underlying fluid, with a trough trailing the ridge as expected. However, several novel features are observed using the fluorescence technique, including a peak in the surfactant concentration which trails the leading edge, and a flat, monolayer-scale spreading film which differs from concentration profiles predicted by current models. Both the Marangoni ridge and surfactant leading edge can be described to spread as $R \propto t^{\delta}$. We find spreading exponents, $\delta_H \approx 0.30$ and $\delta_\Gamma \approx 0.22$ for the ridge peak and surfactant leading edge, respectively, which are in good agreement with theoretical predictions of $\delta = 1/4$. In addition, we observe that the surfactant leading edge initially leads the peak of the Marangoni ridge, with the peak later catching up to the leading edge.