Abnormal wetting dynamics of Silwet-laden droplets on partially wetting substrates

Silwet L-77 surfactant has the unique ability to spread water droplets across surfaces to a greater extent than conventional surfactants within a second. By using Silwet, the N,N-dimethylformamide (DMF) droplets can exhibit self-propulsion on a glass substrate. The peculiar wetting dynamics of the Silwet-laden DMF droplets was explored on the partially wetting substrates, including the polytetrafluoroethylene slide and slippery liquid-infused porous surface with negligible contact angle hysteresis (CAH). The time evolution of the contact angle (CA) was monitored for both high and low Silwet concentrations. Moreover, the volume inflation/deflation method and inclined plane method were employed to investigate the CA relaxation. The sessile droplet exhibits long-time wetting dynamics and it spreads very slowly, depending on the Silwet concentration. The dynamic CA decreases for more than 20 min at low Silwet concentrations (e.g., 1 wt%), but the droplet reaches its equilibrium state within 4 min at high concentrations (e.g., 10 wt%). The relaxation behavior is also influenced significantly by CAH which is found to impede the initial CA relaxation. Finally, the abnormal wetting dynamics is explained by the time-varying interfacial tensions according to Young's equation.