Influences of microcontact shape on the state of a frictional interface

TL;DR

This paper demonstrates that the state of a frictional interface depends on contact geometry, not just contact area, revealing different aging behaviors and evolution of contact shapes under constant load.

Contribution

It introduces a new perspective on frictional interface states by highlighting the importance of contact shape, supported by direct visualization and a novel experimental protocol.

Findings

Contact shape influences interface state beyond total contact area.

Normal load and aging affect contact differently at the microscopic level.

Constant contact area conditions lead to ongoing interface evolution.

Abstract

The real area of contact of a frictional interface changes rapidly when the normal load is altered, and evolves slowly when normal load is held constant, aging over time. Traditionally, the total area of contact is considered a proxy for the frictional strength of the interface. Here we show that the state of a frictional interface is not entirely defined by the total real area of contact but depends on the geometrical nature of that contact as well. We directly visualize an interface between rough elastomers and smooth glass and identify that normal loading and frictional aging evolve the interface differently, even at a single contact level. We introduce a protocol wherein the real area of contact is held constant in time. Under these conditions, the interface is continually evolving; small contacts shrink and large contacts coarsen.