Dynamical force measurements for contacting soft surfaces upon steady sliding: Fixed-depth tribology

TL;DR

This study investigates the dynamic forces during steady sliding contact of soft surfaces, revealing non-monotonic friction behavior, force fluctuations with stick-slip patterns, and surface changes, linking tribology to granular flow.

Contribution

It introduces a fixed-depth experimental setup to measure force fluctuations and connects soft surface tribology to granular flow dynamics.

Findings

Friction force varies non-monotonically with sliding speed.

Force fluctuations exhibit strong stick-slip patterns.

Surface properties evolve during experiments.

Abstract

The tribology between surfaces can have a profound impact on the response of a mechanical system, such as how granular particles are driven to flow. In this work, we perform experiments that time-resolve the tangential and normal components of the force between two semi-cylindrical PDMS (polydimethylsiloxane) samples immersed in fluid, as they slide against each other in a range of controlled speeds. The time-averaged friction force shows a non-monotonic dependence on the sliding speed over four decades, which is consistent to the paradigmatic Stribeck diagram and three dynamical regimes associated with it. Our specially designed fixed-depth setup allows us to study the fluctuation of force that exhibits strong stick-slip patterns in one of the regimes. Data from repetitive experiments reveal that both the "onset speed" for the stick-slip patterns and its spatial location along the sample change gradually during the course of our experiments, indicating changes on the sample surfaces. In addition, we conduct counterpart experiments by using spherical samples rubbing against each other, to make a direct connection of the inter-particle tribology to the granular flow reported in our previous work [Phys. Rev. Lett. 126,128001 (2021)].