Anomalous interfacial dynamics of pendant droplets of N,N-dimethylformamide containing Silwet

Background: Silwet L-77 has the unique ability to spread aqueous solutions across surfaces to a greater extent than conventional surfactants. Moreover, the Silwet-laden N,N-dimethylformamide (DMF) droplet can exhibit self-propulsion on substrates. Although the Marangoni effect is responsible, the origin of the surface tension gradient along the droplet remains unclear. Methods: The time evolution of a hanging Silwet-laden DMF droplet below a substrate is observed. Then, the dynamic surface tensions of pendant drops are determined for both high and low Silwet concentrations in ambient and saturated DMF vapor conditions. Significant Findings: For pendant drops with maximum volumes, the dynamic surface tensions are found to decrease slowly for more than 10 min at low Silwet concentrations (e.g., 1 wt%). However, at high Silwet concentrations (e.g., 10 wt%), the drop falls off around 1 min. As the initial drop volume is slightly less than its maximum value, surface tensions continue decaying at low Silwet concentrations but reach equilibrium values at high Silwet concentrations. Once the equilibrium surface tension is attained, the increment of the drop volume does not change the result. The peculiar interfacial dynamics is attributed to the weakly solvophobic nature of few siloxane groups of Silwet in DMF.