Adhesion Circle: A New Approach To Better Characterize Directional Gecko-Inspired Dry Adhesives.

The number of different designs of directional gecko-inspired adhesives has proliferated over the past 15 years, but some basic characterization tools are still nonstandardized, which can make direct comparisons of different adhesives in the literature difficult. By far the most common type of test for directional adhesives, the load-drag-pull (LDP) test is useful but can miss substantial information on the exact behavior of gecko-inspired adhesives in a variety of loading conditions. Other test techniques, including angled approaches and pull-offs, have been employed by a few groups but they are not as widely adopted; peel tests can be employed but require a larger amount of adhesive material to use in the test, which is not always practical given some current manufacturing constraints. Very few tests have looked at the effect of off-main axis loads on the performance of directional adhesives, however, and this quality of performance may be very important in applications where direct control over displacements or angle of pull-off in pitch and yaw of the peeling interface may not be practical or possible. To address this overlooked area of characterization, we introduce a new test concept for anisotropic adhesives, the adhesion circle, and also compare how the radial normal adhesion performance is altered depending on whether the pull-off comes after a displacement drag or when pulled at a constant angle from vertical after a preload. Testing directional adhesive designs made with different geometries shows that unexpected behaviors at pull-off angles not in the direction of the strong-weak axis can sometimes be seen. The complete adhesion circle tests should help better design directional adhesives for scaled up performance, and can be completed with relatively simple hardware that is typically used in most current directional adhesive tests.