# Friction of viscoelastic elastomers with rough surfaces under torsional contact conditions

## TL;DR

This study investigates the frictional behavior of viscoelastic elastomers against rough and smooth surfaces under torsion, revealing how local shear stress depends on velocity and pressure, and highlighting nanoscale dissipation mechanisms.

## Contribution

It introduces a method to spatially resolve shear stress in torsional contacts and demonstrates the influence of surface roughness on frictional properties and contact area.

## Key findings

- Shear stress is velocity dependent for smooth contacts.
- Rough surfaces cause shear stress to depend on both velocity and pressure.
- Light transmission correlates with contact area and influences frictional behavior.

## Abstract

Frictional properties of contacts between a smooth viscoelastic rubber and rigid surfaces are investigated using a torsional contact configuration where a glass lens is continuously rotated on the rubber surface. From the inversion of the displacement field measured at the surface of the rubber, spatially resolved values of the steady state frictional shear stress are determined within the non homogeneous pressure and velocity fields of the contact. For contacts with a smooth lens, a velocity dependent but pressure independent local shear stress is retrieved from the inversion. On the other hand, the local shear stress is found to depend both on velocity and applied contact pressure when a randomly rough (sand blasted) glass lens is rubbed against the rubber surface. As a result of changes in the density of micro-asperity contacts, the amount of light transmitted by the transparent multi-contact interface is observed to vary locally as a function of both contact pressure and sliding velocity. Under the assumption that the intensity of light transmitted by the rough interface is proportional to the proportion of area into contact, it is found that the local frictional stress can be expressed experimentally as the product of a purely velocity dependent term, $k(v)$, by a term representing the pressure and velocity dependence of the actual contact area, $A/A_0$. A comparison between $k(v)$ and the frictional shear stress of smooth contacts suggests that nanometer scale dissipative processes occurring at the interface predominate over viscoelastic dissipation at micro-asperity scale.