Forcing the issue: testing gecko-inspired adhesives

Materials are traditionally tested either by imposing controlled displacements and measuring the corresponding forces, or by imposing controlled forces. The first of these approaches is more common because it is straightforward to control the displacements of a stiff apparatus and, if the material suddenly fails, little energy is released. However, when testing gecko-inspired adhesives, an applied force paradigm is closer to how the adhesives are loaded in practice. Moreover, we demonstrate that the controlled displacement paradigm can lead to artefacts in the assumed behaviour unless the imposed loading trajectory precisely matches the deflections that would occur in applications. We present the design of a controlled-force system and protocol for testing directional gecko-inspired adhesives and show that results obtained with it are in some cases substantially different from those with controlled-displacement testing. An advantage of the controlled-force testing approach is that it allows accurate generation of adhesive limit curves without prior knowledge of the expected behaviour of the material or the loading details associated with practical applications.

Automated summary

Traditional material testing methods involve controlling displacements or forces, with the former being more common. However, for gecko-inspired adhesives, a controlled force approach is more practical. Control displacement testing may lead to artifacts if the loading trajectory does not precisely match real-world deflections. Controlled force testing allows for accurate generation of adhesive limit curves without prior knowledge of material behavior or loading details associated with practical applications.