Increased backing softness of planar smooth dry adhesives enhances contact area and frictional load capacity on a cylindrical substrate

Abstract

Dry adhesives can reattach to surfaces due to the reversible bond made by Van der Waals forces. These adhesives can therefore be used as gripping surface that has a high frictional load capacity, independent of the grasping force. In many grippers, the adhesive surface is often pressed into contact with another surface (’substrate’) in an open and closing motion. Since, generally, substrates have non-flat shapes, the adhesive has to be pressed into more directions at once to make full contact. An additional part to the adhesive system is needed here to transform the closing motion to a multi-directional preload on the adhesive surface. To realize this, a passive soft material behind the adhesive is added in this study.
Design objectives for such a material (’backing’) were formulated and different types of bio-inspired backing concepts (solid, sponge, and inflatable) were fabricated. To gain insight in the suitability of these backing concepts with regard to some of the objectives, minimal required preload and minimal residual stresses to avoid detaching forces, two things are measured. Firstly, backing softness was measured as the compression stress-strain characteristic of the backing. Secondly, the preload contact stress distribution of backings was qualitatively measured.
The adhesive was a thin planar adhesive material reinforced with a planar mesh. One sponge backing type and one inflatable backing type were selected as practical backings to fabricate an adhesive system with. An experiment to measure frictional performance was done with these systems whereby backing softness was varied. These cuboid adhesive systems were pressed onto a cylindrical substrate and, after removal of the preload, loaded in the direction of the reinforcement while measuring frictional load capacity and contact area.
For both these two adhesive systems types, experiments showed that an increased backing softness caused an even or greater contact area throughout the whole loading cycle. The linear correlation coefficient, between rest phase contact area and maximum load capacity was 0.96, and at the end of the load phase, between the ’slide’ contact area and ’slide’ load capacity was 0.99.
With an inflatable or sponge backing design it is possible to make a softer backing compared to a solid design made by the same material. Only the sponge backing type distributed the preload relatively even at low and high compression, owing it to its stress plateau in its compression stress-strain characteristic. Although the inflatable has an equal pressure internally and also shows such a plateau, it was found that its contact stress is not even, due to the effect of its outer hull.
Concluding, the addition of a soft backing to help make and keep contact with a general shaped substrate, and thereby increasing load capacity, promises a new design paradigm in synthetic dry adhesives. Furthermore, the results indicates functional relevance of the presence of a relatively large and soft volume between the bones and the adhesive surface of the fingers/toe pads of geckos, tree frogs and humans.