Spreading, fingering instability and shrinking of a hydrosoluble surfactant on water

Abstract

An experimental investigation of spreading and shrinking of a surfactant droplet at the air-water interface with fingering instability appearing at the edge of the droplet stain was reported. It was found that a droplet of a surfactant on the water shows three regimes: spreading, shrinking and resting at the appropriate parameters values. We quantify these regimes by measuring the mean square displacement of the droplet parts, 〈r2〉, with respect to time. Behavior of the fractal dimension of stain in different time intervals also confirms this statement. By defining the angular box-counting dimension, the time variation of the fingerless part of the stain was studied. Three stages: spreading, shrinking and resting were observed again. In the first stage, the result is compatible with the previous works on spreading of the surfactant on a thin water film but the second stage is a novel phenomenon.