Damping effect of surfactants on induced bubble shape deformations

Abstract

The addition of surfactants suppresses interfacial deformations, and surface elasticity is expected to play a critical role. High-speed imaging is used to study the influence of surfactants on the shape oscillations of air bubbles attached to a capillary and on the rebound of bubbles from a horizontal glass plate. The experiments are conducted in pure water and in aqueous solutions containing five different surfactants: terpineol, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, Triton X-100, and Triton X-45. The frequencies and damping times of the decaying oscillations and the restitution coefficients of the decaying rebounds are evaluated. The damping time and the restitution coefficient both decrease sharply with an increasing surfactant concentration and then reach a minimum before finally stabilizing. The oscillation frequency initially increases due to enhanced surface elasticity but then drops suddenly when the surfactant concentration corresponding to the maximum damping is reached. The suppression of bubble rebounds by surfactants bears a strong resemblance to the damping of quadrupole bubble oscillations. In both cases studied, a surface elasticity of about 4 mN/m is required to achieve the maximum damping of interfacial deformations, regardless of the surfactant used. Moreover, the bubble rebounds are found to be more susceptible to the presence of trace concentrations of surfactants.