Unstable spreading of aqueous anionic surfactant solutions on liquid films. Part 1. Sparingly soluble surfactant

Experimental work has shown that the spreading of aqueous surfactant solutions on a thin liquid film may be accompanied by a fingering instability, the characteristics of which are dependent on the initial film thickness and surfactant concentration. Experimental results are presented showing how these parameters affect the fingering instability for a sparingly soluble anionic surfactant, sodium di-2-ethyl-hexyl sulfosuccinate. It is found that at surfactant concentrations well below the critical micelle concentration (cmc) stable spreading occurs but at concentrations in the vicinity of the cmc fingering develops behind the spreading front on water films as thick as 100 mum. At surfactant concentrations greater than the cmc, there is stable spreading with the formation of a central "disk" at lower thicknesses while fingering is observed at a higher thickness. Increasing the initial film thickness results in greater instability onset times and wider fingers, whereas increasing surfactant concentration gives rise to shorter onset times and narrower fingers.