Force-based dynamic contact angles on lubricant-infused surfaces

Abstract: In this study, the macroscopic dynamic advancing and receding contact angles of an aqueous glycerin solution on lubricant-infused surfaces (LISs) were measured using the Wilhelmy plate technique. To construct LISs, polytetrafluoroethylene (PTFE) was roughened using 240-grit sandpaper to introduce micro-/nanostructures on the surface. Then, fluorinated synthetic oils were infused into the structures of the sanded PTFE surface to form the LISs. We found that the dynamic advancing contact angles increased with increasing capillary number, whereas the dynamic receding contact angles decreased with increasing capillary number. The viscosity ratio between the working fluid and lubricant oil was found to change the increasing/decreasing trends in the dynamic contact angles. As the viscosity ratio increased, the reduction in shear stress on the LISs shifted the change in the dynamic contact angles to higher capillary numbers. Unlike the contact angles measured by an optical method, the force-based contact angles are affected by the viscous force acting on the LISs. A sharp transition in the dynamic contact angle data at the critical capillary number was observed, and this was shifted toward higher capillary numbers as the viscosity ratio increased. The scaling behavior of the dynamic contact angles was found to follow the Cox-Voinov-Tanner scaling law. This study describes how LISs can be characterized based on dynamic contact angles. Graphical abstract: [Figure not available: see fulltext.]