Capillary effect can be trivial in gecko adhesion

The present study is motivated by two classes of seemingly contradicting experiments on the effect of humidity or the capillary effect on adhesion. While one class of experiments suggests strong effect of humidity in gecko adhesion, those on micromachined surfaces indicate that the adhesion energy remains constant up to a relative humidity of 60–70% even for hydrophilic surfaces. To resolve this apparent paradox, we perform numerical simulations of the vertical peeling of a spatula pad adhered to a rough surface with periodic attachment sites. It is found that the reduction in material stiffness, which could be induced by moisture, leads to substantial increases in the pull-off force of the spatula pad, thereby providing a feasible explanation of relevant experimental observations. Our study suggests that the capillary effect can be trivial in gecko adhesion.

The paper can be accessed through http://dx.doi.org/10.1142/S1758825110000433