Grip on wet interfaces

Abstract

Drainage of fluid from wet adhesive-substrate interfaces is often mentioned to be an important mechanism in obtaining strong grip. It has been hypothesized that micropatterned adhesives – that contain features (e.g., pillars) separated by channels on their surface – have a better drainage capability compared to unpatterned adhesives. Nevertheless, the fluid flow behavior on micropatterned interfaces has not been analyzed before. In this work, the fluid flow on wet adhesive-substrate interfaces was captured on three focal planes and analyzed in terms of flow direction, flow trajectory and flow velocity. The aspect ratio and spacing ratio of the features on the micropatterned adhesives as well as the stiffness of the substrates were varied. Ultimately, the fluid flow velocity was related to the pull-off and friction forces of the adhesives on the substrates. From the results it appeared that the initiation of drainage through the channels of micropatterned adhesives is likely not the direct cause of changes in flow direction. A straight flow path and a semi-wave flow path are distinguished as the two main flow trajectories of the fluid flow. The flow velocity depends on the flow path width and the separation distance between the adhesive and the substrate: the flow velocity increases with a smaller flow path width, and is higher before than after adhesive-substrate contact was established. The flow velocity did not differ among the substrates, which could be the result of the effects of the stiffness and the wetting capability of the substrates cancelling each other out. Moreover, our results suggest that shear forces increase with a larger channel area and a smaller fluid volume, whereas this is not necessarily the case for pull-off forces. The outcomes of this study can be used to develop surfaces for the generation of strong grip under wet conditions.