Sliding across veins becomes slipperier: Counterintuitive directional friction on nepenthes inner walls

Abstract

Abstract Directional friction on soft interfaces combining wetting heterogeneity with compliant contact remains unclear under ultralow normal loads, where lubrication and contact geometry may interact strongly. We probed the inner wall of Nepenthes pitchers, which is naturally divided into a hydrophobic waxy zone and a hydrophilic digestive zone, by sliding a compliant sphere parallel and perpendicular to the venation under dry contact, water lubrication, and oil lubrication. Both zones consistently exhibited a counterintuitive frictional anisotropy, with higher average friction during venation-parallel sliding than during venation-perpendicular sliding. The waxy and digestive zones encoded this anisotropy primarily through directional roughness and wettability, respectively, supporting a groove–wetting–hydrodynamic framework for direction-dependent liquid-film load sharing and interfacial shear.