The influence of surfactant on the bubble motion in Hele–Shaw cells

Abstract

The surfactant influence on the bubble motion in a Hele–Shaw cell was studied experimentally. In order to differentiate the cases with and without the surfactant influence, the motion of air bubbles and water drops driven by silicone oil was investigated. The water drops contained a predetermined amount of an organic surfactant (sodium dodecyl sulfate), so that the dependence of their motion on the surfactant concentration could be studied systematically. In case of air bubbles in silicone oil, surfactants were likely to have negligible influence, although they might be present as contaminants, and their translational velocities were observed to be close to the prediction of Taylor and Saffman [Q. J. Mech. Appl. Math. 12, 265 (1959)]. The bubble shapes were also in accordance with available theories for a surfactant-free system. The water drops, on the other hand, behaved very differently, in that the translational velocities were smaller by an order of magnitude and their shapes were very unusual. These observations are apparently consistent with those of Kopf-Sill and Homsy [Phys. Fluids 31, 18 (1988)], and the present study suggests that the perplexing observations by them may be due to the influence of surface-active contaminants.