Direct Measurement of the Elastohydrodynamic Lift Force at the Nanoscale

Friction in soft and wet contacts is crucial for industry and engineering, but it might also play a central role for the motion of various physiological and biological entities, as well as for the nanorheology of very soft materials. Here, we report on the first direct measurement of the elastohydrodynamic lift force acting on a sphere moving within a viscous liquid, near and along a soft substrate under nanometric confinement. Using atomic force microscopy, the lift force is probed as a function of the gap size, for various driving velocities, viscosities, and stiffnesses. The results are in excellent agreement with scaling arguments and a novel quantitative model developed from the soft lubrication theory, in linear elasticity, and for small compliances. For larger compliances, or equivalently for smaller confinement length scales, a saturation of the lift force is observed and its empirical scaling law is discussed.