Linking energy loss in soft adhesion to surface roughness

TL;DR

This study quantitatively links adhesion hysteresis in soft materials to surface roughness by measuring contact behavior of PDMS on diamond substrates, revealing that energy loss correlates with true contact area rather than viscoelastic effects.

Contribution

It provides the first experimentally validated quantitative relationship between surface roughness and adhesion hysteresis in soft elastic materials.

Findings

Adhesion reduction equals energy needed for conformal contact.

Energy loss during contact is proportional to true contact area.

Roughness, not viscoelasticity, explains adhesion hysteresis.

Abstract

A mechanistic understanding of adhesion in soft materials is critical in the fields of transportation (tires, gaskets, seals), biomaterials, micro-contact printing, and soft robotics. Measurements have long demonstrated that the apparent work of adhesion coming into contact is consistently lower than the intrinsic work of adhesion for the materials, and that there is adhesion hysteresis during separation, commonly explained by viscoelastic dissipation. Still lacking is a quantitative experimentally validated link between adhesion and measured topography. Here, we used in situ measurements of contact size to investigate the adhesion behavior of soft elastic polydimethylsiloxane (PDMS) hemispheres (modulus ranging from 0.7 to 10 MPa) on four different polycrystalline diamond substrates with topography characterized across eight orders of magnitude, including down to the \r{A}ngstr\"om-scale. The results show that the reduction in apparent work of adhesion is equal to the energy required to achieve conformal contact. Further, the energy loss during contact and removal is equal to the product of intrinsic work of adhesion and the true contact area. These findings provide a simple mechanism to quantitatively link the widely-observed adhesion hysteresis to roughness rather than viscoelastic dissipation.