Electrically Controlled Spreading of a Surfactant-Laden Droplet on a Viscoelastic Interface

Electrically induced dynamic spreading of a droplet on a soft surface is characterized by intricate interactions between the moving contact line and the substrate deformation, which are explained by a complex interaction between elastic recovery and viscous dissipation that take place simultaneously. Here, we highlight the significance of an additional modulation in the interfacial energy brought about by the distribution of surfactant molecules surrounding the droplet, which causes an increase in the droplet's spreading rate, rather than the expected decrease in it due to energy dissipation at the viscoelastic interface. We attribute this to repartitioning of the surface energy that results in the dynamic reduction in the solid-liquid interfacial tension, overcoming the substrate viscosity-induced attenuation in the spreading rate. Using a scaling theory on the ensuing change in the contact angle as the droplet spreads dynamically, we further offer quantitative insights into the observed spreading dynamics. These findings allow for the rationalization of the sensitive reliance of droplet spreading on the initial contact angle, a phenomenon that has not yet been understood, in addition to providing a scientific basis for dynamic regulation of droplet spreading on soft biomimetic interfaces.