Impingement of Triton X-100 solution drops on paraffin film–Relaxation of surface dilational rate

The impact dynamics of a solution drop of Triton X-100 impinging on a paraffin film was studied, specifically to develop a new method for evaluating the in situ surface dilational rate, λ. The drop images (side and top views) during the impingement process were sequentially digitized and used to calculate the interfacial areas of the liquid-air and liquid-solid interfaces. The data of interfacial area were then used to calculate the relaxation of λ. The surface dilational rate showed a strong oscillation at the beginning of the drop impact. It oscillated from 0 down to −390/s, up to 290/s, and then down to −200/s. This surface dilational rate is far above 2.4/s, which is the lower limit of λ for an expanding pendant bubble in a Triton X-100 solution with an isolated air-liquid surface (Lin et al., 2017). A simple but useful theoretical framework is given for the calculation of the dynamic surface dilational rate, surface concentration and surface tension of an impinging drop of surfactant solution.