Molecular-Scale Reagent Transport-Induced Modification of Low Weber Number Droplet Impact Dynamics

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 for a range of Weber numbers (We = 20 – 100). The impact dynamics, spreading and recoil behavior are captured using a high-speed digital video camera at 4000 frames per second. Digital image-processing software was used to determine the drop spread and height of the liquid film on the surface from each frame. Experiments were performed with water and with two different aqueous solutions of Sodium Dodecyl Sulphate (SDS), one at its half critical micelle concentration (CMC) and another at twice CMC, on a hydrophilic surface (glass) and a hydrophobic surface (Teflon). It is seen that surfactant concentration, its dynamic surface tension behavior, and surface wettability govern the transient impact-spreading-recoil phenomena. A droplet of SDS solution exhibits higher maximum spread and weaker surface oscillations compared to a pure water drop colliding with the same velocity. On a hydrophobic (Teflon) surface, the drop rebound is inhibited by the presence of the surfactant. These effects are more pronounced at surfactant concentration of twice CMC compared to that at half CMC.