Drops retracting while forming a rim.

Hypothesis: Surfactant laden droplets may spread faster on a substrate and, subsequently, retract (spontaneously reversing the drop's direction). We hypothesize that this Marmur-Lelah type retraction can be explained by a de Gennes-type triple line fluctuation expression that is modified to represent our anisotropic surfactant adsorption. This explanation requires that the retraction originates inner to the triple line (not at the triple line itself).

Experiments: Drops of oil with surfactant (mainly tetradecane with octadecylamine) were allowed to spread on freshly cleaved mica surfaces at various concentrations and recorded with a high-speed camera.

Findings: At low concentrations, the subsequent retraction left a rim of liquid at the triple line location of maximal spreading whose thickness was inversely related to the surfactant concentration. This indeed agrees with de Gennes' triple line fluctuation expression that is modified to fit the anisotropic adsorption of the surfactant during the spreading period, which induces a solid-liquid interfacial energy gradient. According to this explanation, the rim may exist also at high surfactant solution concentrations, however, there can be a limit at which the surfactant solution concentration is so high that the rim's thickness is practically zero.