Impact of surface dilation rate on dynamic surface tension

In past studies, estimating the dynamic surface tension in processes involving significant surface deformation relied heavily on empirical relations. Despite its importance, the detailed dynamics of surfactant adsorption onto a dilating surface have not been investigated extensively. In this study, we modified the pendant bubble tensiometer to provide direct observation on the dynamics of surfactant adsorption during rapid surface dilation. The relaxation of dynamic surface tension for bubbles rapidly expanding in the solution of four different bulk concentrations of Triton-X 100 was studied. It was found that surface tension rose with increasing surface area due to the decrease in surface concentration. Moreover, when surface dilational rate was increased, it was found that the relation between surface tension and surface area came to resemble the equation of state of two dimensional surfactant phase when dilational rates were larger than a certain threshold value, indicating that the adsorption of surfactant became negligible. This threshold value was found to increase with surfactant concentration. The results showed that the procedure established in this study could serve as a simple guideline to determine whether adsorption can be neglected in a process involving dilating surfaces. In addition, the detailed dynamical data reported in this work can serve as an experimental reference for multiphase flow simulations involving species that adsorb on interfaces.