Surfactant-Induced Modification of Low Weber Number Droplet Impact Dynamics

The effect of surfactant molecular mass transport on the normal impact and spreading of a droplet of its aqueous solution on dry horizontal substrates is investigated experimentally for a range of Weber numbers (20−100). The postimpact dynamics of film spreading and its recoil behavior are captured using high-speed real-time digital imaging. Hydrophilic (glass) and hydrophobic (Teflon) substrates were used with water and aqueous solutions of three different surfactants of varying diffusion rates and ionic characteristics: SDS (anionic), CTAB (cationic), and Triton X-100 (nonionic). Their solutions facilitate larger spread and weaker surface oscillations compared to a pure water drop colliding at the same Weber number. On a hydrophobic surface, the drop rebound and column fracture are inhibited by the presence of the surface-active agent. Besides reagent bulk properties, dynamic surface tension, surface wettability, and droplet Weber number govern the transient impact−spreading−recoil phenomena. The role of dynamic surface tension is evident in comparisons of impact dynamics of droplets of different surfactant solutions with identical equilibrium surface tension and same Weber number. It was observed that higher diffusion and interfacial adsorption rate (low molecular weight) surfactants promote higher drop spreading factors and weaker oscillations compared to low diffusion/adsorption rate (high molecular weight) surfactants.