Anatomic differences on the toe pad epithelial cells of torrent and tree frogs (elongated versus regular geometry) are believed to account for superior ability of torrent frogs to attach to surfaces in the presence of running water. Here, the friction properties of artificial hexagonal arrays of polydimethylsiloxane (PDMS) pillars (elongated and regular) in the presence of water are compared. Elongated pillar patterns show significantly higher friction in a direction perpendicular to the long axis. A low bending stiffness of the pillars and a high edge density of the pattern in the sliding direction are the key design criteria for the enhanced friction. The elongated patterns also favor orientation-dependent friction. These findings have important implications for the development of new reversible adhesives for wet conditions.

The authors thank the Deutsche Forschungs Gemeinschaft for financial support within the program SPP1420 “Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials” (Projects CA880/1, BU 1556/26); Dirk-Michael Drotlef (MPIP) for experimental help and Thomas Endlein (U Glasgow) for scientific discussions and graphics support

Peer reviewed