Unexpected increase in droplet nanofriction on a lubricant-impregnated smooth surface.

Friction working between two shearing solids has been studied for a long time. Once the applied shearing force overcomes the maximum static friction force, the friction transitions to plateau dynamic mode. However, friction is not limited to solids. A recent study revealed that the droplets on a hydrophobic surface exhibit a similar friction to solids, which revealed that the effective parameters for droplet friction are the roughness of the surface, droplet width, and shearing speed.

Recently, our group reported a new type of liquid-repellent surface, namely SPLASH(surface with π-interaction liquid adsorption, smoothness, and hydrophobicity). SPLASH is one of the lubricant-impregnated surfaces, whose base layer is smooth. Our motivation is to understand the droplet friction on the SPLASH; however, measuring the accurate droplet friction has been difficult for the classical method due to extremely low droplet friction. Here, we accurately measured the droplet friction on SPLASH using a sensitive capillary cantilever measurement that enables the detection of droplet friction with sub-µN accuracy. It seemed that droplet friction showed typical friction behavior. However, dynamical friction was increasing when droplet was reaching the edge of substrate. Investigations revealed that the friction increase is due to the long-range force from the edge of the substrate. This work experimentally study the origin of the increase in dynamic friction.