Contact-Angle Hysteresis and Contact-Line Friction on Slippery Liquid-like Surfaces

Contact-line pinning (static) and dynamic friction forces arise from the fundamental interaction between droplets and solid surfaces. In this work, we explore a unique low-friction surface called Slippery Omniphobic Covalently Attached Liquid (SOCAL). SOCAL surfaces exhibit a remarkable combination of contact-angle hysteresis and contact-line friction properties, where they present low pinning but a high dynamic friction against the motion of a droplet. Volume variation experiments show that the droplets contact angle with the surface relaxes back to equilibrium values after inflow/outflow, which can be used to measure contact-angle hysteresis with significant improvement in accuracy. Additionally, we observe that the relaxation dynamics of the contact line are uniquely slow in this low pinning surface, a concept that defies the intuition of low friction interactions. Hence, we compare predictions made by the Cox-Voinov model and the contact-line model based on Molecular Kinetic theory to understand the dynamics at play in the motion of a water droplet on SOCAL surfaces. Our results feature a special relation between the low static friction and the unexpectedly high dynamic friction observed this type of surface, which is driven by the kinetics of the contact line. This work highlights the unique wettability of SOCAL surfaces, provides us with insight on the relation of static and dynamic friction, and calls attention to potential application for low-pinning, slow droplet shedding surfaces.