Decoupling Friction and Viscous Force at Solid-Liquid Interface via Gradient Measurement of AFM

The flow characteristics of fluids at the interface within the micronanoscale are significantly influenced by solid-liquid interactions, with friction force playing a crucial role. The challenge in accurately obtaining the friction force lies in determining the contact state between the fluid and the surface. Herein, we developed a method to decouple friction and viscous force at the solid-liquid interface via gradient measurement of atomic force microscopy (AFM) under elastohydrodynamic lubrication (EHL) conditions by the Stribeck curve, which ensured that the solid-solid direct contact could be avoided even under high pressure. It was found that the viscosity of two deep eutectic solvents (DESs) with different hydrogen bond donors on the mica surface differed by 30 times, but their hydrodynamic drag force remained similar, which was primarily attributed to the differing ratios of friction resistance to viscous resistance. This work introduces a novel method for quantifying friction between fluids and surfaces, which provides a foundation for understanding liquid flow behavior and designing advanced lubrication systems and surface materials.