Unstable spreading of aqueous anionic surfactant solutions on liquid films. 2. Highly soluble surfactant

The spreading of a surfactant solution across a thin water film may be accompanied by a fingering instability developing behind the spreading front. In this paper, the role of solubility on this instability is investigated by conducting spreading experiments using highly soluble surfactant solutions of sodium dodecyl sulfate over a range of concentrations on water films ranging from 25 to 100 mum in thickness. It is found that at surfactant concentrations up to and around the critical micelle concentration, spreading is largely accompanied by fingers upstream of the spreading front. In comparison with sparingly soluble ACT solutions (sodium di-2-ethylhexyl sulfosuccinate) studied in part 1 of this series, the instability becomes apparent sooner and the fingers are more pronounced and branched. Above the cmc, the instability takes a different form on thinner films, which was not noted in the sparingly soluble case, while fingering develops on thicker films.