Impact dynamics on SLIPS: Effects of liquid droplet's surface tension and viscosity

SLIPS (slippery liquid infused porous surface) is an emerging non-wetting surface with useful applications in various environmental areas. Because of its repellent properties to various liquids, the analysis of impact dynamics on SLIPS is very important for understanding liquid droplet's behaviors on SLIPS in dynamic conditions. However, most of the previous studies have mainly focused on the interaction of pure water with SLIPS. To study the dynamics of the impact of various liquid on SLIPS, we used a model system to investigate the effects of surface tension and viscosity. Surface tension and viscosity are regulated by changing the concentrations of ethanol and glycerol, respectively, in their water mixtures. When ethanol concentration increases from 0 to 20 wt%, surface tension drastically decreases from 72 to 38 mN/m. Similarly, viscosity varies from 1 to 6 cP by increasing the glycerol concentration from 0 to 50 wt%. In this study, several SLIPS samples were also prepared by employing various viscous lubricants. In pure water, the maximum spread factor (D-max/D-0) is measured under varying We conditions. A consistent dependence of D-max/D-0 similar to We(1/4) is demonstrated, according to a theoretical model. However, liquids with lower surface tension or higher viscosity than pure water exhibit noteworthy deviations from this model. A low surface tension enhances the wetting property, which results in a wider maximum spread diameter. On the other hand, an increased viscosity acts as friction during spreading, which results in a smaller maximum spread diameter. In the relationship between D-max/D-0 and We, the ethanol/water and glycerol/water mixtures both induce a reduced exponent dependence on We. In particular, the exponent value notably decreases to 0.209 for highly viscous liquid with a viscosity of 6 cP. This is mainly due to the amplifying effect in viscous dissipation originating from the increased viscosity and impact velocity. Our study clearly shows, for the first time, that liquid's surface tension and viscosity play a crucial role in the impact dynamics on SLIPS. This information is highly useful for understanding the impact dynamics of various liquids on SLIPS.