When are rough surfaces sticky?

TL;DR

This study uses extensive simulations and theory to understand how surface roughness influences adhesion, revealing a threshold strength for stickiness and challenging classical models that neglect adhesion range and asperity interactions.

Contribution

It provides a parameter-free analytic theory and extensive simulation data showing how roughness and material properties determine surface stickiness, contrasting with classical models.

Findings

Atomic contact area increases linearly with load.

A threshold adhesive strength determines stickiness.

Classical theories are inconsistent with simulation results.

Abstract

At the molecular scale there are strong attractive interactions between surfaces, yet few macroscopic surfaces are sticky. Extensive simulations of contact by adhesive surfaces with roughness on nanometer to micrometer scales are used to determine how roughness reduces the area where atoms contact and thus weakens adhesion. The material properties, adhesive strength and roughness parameters are varied by orders of magnitude. In all cases the area of atomic contact rises linearly with load, and the prefactor rises linearly with adhesive strength for weak interactions. Above a threshold adhesive strength, the prefactor changes sign, the surfaces become sticky and a finite force is required to separate them. A parameter-free analytic theory is presented that describes changes in these numerical results over up to five orders of magnitude in load. It relates the threshold strength to roughness and material properties, explaining why most macroscopic surfaces do not stick. The numerical results are qualitatively and quantitatively inconsistent with classical theories based on the Greenwood-Williamson approach that neglect the range of adhesion and do not include asperity interactions.