Direct Measurement of the Elastohydrodynamic Lift Force at the Nanoscale

TL;DR

This study directly measures the elastohydrodynamic lift force on a sphere near a soft substrate at the nanoscale, revealing how the force varies with gap size, velocity, and material properties, and validating theoretical models.

Contribution

First direct measurement of nanoscale elastohydrodynamic lift force using atomic force microscopy, with validation of theoretical models and empirical scaling laws.

Findings

Lift force increases as gap decreases and saturates at small gaps.

Experimental results agree with soft lubrication theory and scaling arguments.

Empirical law describes lift force saturation for larger compliances.

Abstract

We present 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 force increases as the gap is reduced and shows a saturation at small gap. The results are in excellent agreement with scaling arguments and a 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, an empirical scaling law for the observed saturation of the lift force is given and discussed.