Surfactant-mediated control of wetting and mobility of droplets on polydimethylsiloxane (PDMS)-based slippery...

Controlling the wetting and mobility of droplets on a state-of-the-art lubricant-coated slippery surface has broad implications in microfluidics applications. However, a study on surfactant-mediated wetting on these surfaces is still missing. In this article, we report the effect of the oil-water interfacial tension change induced by the addition of the surfactant in an aqueous droplet on the static wetting, and the dynamics of droplet mobility on a polydimethylsiloxane (PDMS)-based slippery surface. The stability analysis employing the contact angle and Hamaker's constant measurements shows the presence of a stable lubricating film underneath the surfactant droplet on the slippery surface for all the concentrations up to 1.0 cmc. Interestingly, the surfactant in an aqueous droplet on a slippery surface enhances the wettability, following a modified Young's law. In addition, we found that the droplet shedding dynamics slow down due to the addition of surfactant. Furthermore, the dynamic frictional force measurements show that the dissipation force of the surfactant aqueous droplet depends on the viscosity and the shedding speed following the universal Landau-Levich-Derjaguin (LLD) law of friction force on lubricated surfaces as F_D ∼ 2πγRCa^2/3. Moreover, the influence of the surfactant on the dynamic dissipation force can be interpreted by the surface tension gradient at the oil-water interface near the ridge region, inducing a Marangoni stress. These results not only validate existing theoretical models of slippery surfaces but also provide insight into the mechanism to control the wetting and mobility of surfactant droplets on these surfaces for microfluidics applications.