Abstract
Surface stresses have recently emerged as a key player in the mechanics of highly compliant solids. The classic theories of contact mechanics describe adhesion with a compliant substrate as a competition between surface energies driving deformation to establish contact and bulk elasticity resisting this. However, it has recently been shown that surface stresses provide an additional restoring force that can compete with and even dominate over elasticity in highly compliant materials, especially when length scales are small compared to the ratio of the surface stress to the elastic modulus, . Here, we investigate experimentally the contribution of surface stresses to the total force of adhesion. We find that the elastic and capillary contributions to the adhesive force are of similar magnitude and that both are required to account for measured adhesive forces between rigid silica spheres and compliant, silicone gels. Notably, the strain dependence of the solid surface stress contributes to the stiffness of soft solid contacts at leading order.
- Received 10 July 2017
DOI:https://doi.org/10.1103/PhysRevX.7.041031
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Sticky notes, adhesive bandages, product labels, and packing tape are all examples of everyday objects that rely on soft adhesives. Despite their ubiquitous application, much remains unknown about the physics of soft contact. Meanwhile, it is often assumed that classic theories developed for much stiffer materials can be used to interpret experiments with soft materials. We present experiments that demonstrate that soft solids stick differently than their stiffer counterparts.
Our experiment looks at the pull-off of small glass spheres from silicone gel substrates. We stick the spheres (ranging in diameter from about to ) to the silicone gel and then slowly pull them back off. By measuring force and imaging the shape of the contact zone at the same time, we find that the surface of the gel contributes significantly to its stiffness. The role of the surface also becomes more pronounced as we pull more.
These findings change the way we think about soft contacts. In the future, a complete theory of soft adhesion will need to account for both elastic mechanics and strain-dependent solid surface stresses.