Spreading, Fingering Instability and Shrinking of a Hydrosoluble Surfactant on Water

TL;DR

This study experimentally explores the dynamics of surfactant droplets on water, revealing three regimes—spreading, shrinking, and resting—with a novel shrinking phenomenon linked to fingering instability.

Contribution

It introduces the observation of a shrinking regime in surfactant droplet dynamics, characterized by specific measurable parameters, expanding understanding of interfacial instability behaviors.

Findings

Identification of three distinct regimes: spreading, shrinking, resting.

Fingering instability appears at the droplet edge during spreading.

The shrinking regime is a newly observed phenomenon.

Abstract

We report an experimental investigation of spreading then shrinking of a surfactant droplet at the air-water interface with fingering instability appearing at the edge of the droplet stain. We find out that a droplet of a surfactant on the water shows three regimes of spreading, shrinking and resting at the appropriate parameters values. These regimes can be distinguished by measuring the mean square displacement of the droplet parts, fractal dimension of stain, or radius of fingerless part of the stain (to measure it, we define the angular box-counting dimension), versus time. The shrinking regime is a novel phenomenon.