Impacting Dynamics of Surfactant-Laden Droplets*

Surfactants are often used in liquid systems to modify surface tension and thus interfacial behaviour - for instance, to improve or adjust wetting in poorly wetting surfaces. In inkjet and spray technologies, surfactants are used to influence spreading, and therefore control the ink & paint coverage, and drop formation dynamics. In this work, we study the effect of surfactants on the behavior of droplets impacting flat dry surfaces. We combine high-speed imaging and quantitative image analysis to analyse the spreading dynamics on different surfaces through the dynamic contact angle. In our experiments, we use various surfactant solutions with the dynamic surface tension being characterised by high-speed tensiometry at time scales relevant to the droplet impact dynamics. By comparing the effects of different surfactants, surfaces, and dynamic conditions, we have gained insights into how the surfactant-induced dynamic surface tension influences the overall post-impact dynamics.