The effect of surface water and wetting on gecko adhesion

Abstract

SUMMARYDespite profound interest in the mechanics and performance of the gecko adhesive system, relatively few studies have focused on performance under conditions that are ecologically relevant to the natural habitats of geckos. Because geckos are likely to encounter surfaces that are wet, we used shear force adhesion measurements to examine the effect of surface water and toe pad wetting on the whole-animal performance of a tropical-dwelling gecko (Gekko gecko). To test the effect of surface wetting, we measured the shear adhesive force of geckos on three substrate conditions: dry glass, glass misted with water droplets and glass fully submerged in water. We also investigated the effect of wetting on the adhesive toe pad by soaking the toe pads prior to testing. Finally, we tested for repeatability of the adhesive system in each wetting condition by measuring shear adhesion after each step a gecko made under treatment conditions. Wetted toe pads had significantly lower shear adhesive force in all treatments (0.86±0.09N) than the control (17.96±3.42N), as did full immersion in water (0.44±0.03N). Treatments with droplets of water distributed across the surface were more variable and did not differ from treatments where the surface was dry (4.72±1.59N misted glass; 9.76±2.81N dry glass), except after the gecko took multiple steps. These findings suggest that surface water and the wetting of a geckoʼs adhesive toe pads may have significant consequences for the ecology and behavior of geckos living in tropical environments.

Effects of surface wettability on gecko adhesion underwater

“…Unlike the capillary force of a liquid bridge [20,38], the nano-bubble capillary force decreases with the increase of the surface hydrophilicity under a determined nano-bubble volume, and even becomes repulsive in the case of a seriously hydrophilic substrate. All the results are well consistent with the experimental observations that geckos have strong adhesive force on hydrophobic substrates underwater, but cannot adhere well on hydrophilic ones underwater, for example, glass substrates [17,18]. The findings here also give possible explanations that why arboreal geckos like to inhabit on hydrophobic plant surfaces more than other substrates in wet environments [13].…”

“…Only few literatures have studied the wet adhesion mechanism of geckos so far [14][15][16][17][18][19][20]. Huber et al [14], Sun et al [16] and Pesika et al [19] have experimentally proved that capillary force also plays a significant role in gecko adhesion besides the van der Waals force and the adhesive force increases with the increase of relative humidity.…”

“…Huber et al [14], Sun et al [16] and Pesika et al [19] have experimentally proved that capillary force also plays a significant role in gecko adhesion besides the van der Waals force and the adhesive force increases with the increase of relative humidity. However, geckos cannot adhere on hydrophilic glass surfaces once sprayed with water [16,17]. Theoretical analysis of why the relative humidity and sprayed water yield two different effects on gecko adhesion has been investigated in one of our previous works [15].…”

“…Recently, systematic experiments of gecko adhesion on different substrates under different conditions have been carried out by Stark et al [17,18]. It is found that the dry adhesion strength of geckos is extremely strong on both hydrophilic and hydrophobic substrates.…”

Adhesive interactions of geckos with wet and dry fluoropolymer substrates

“…These thin substrates were fastened to acrylic plates to provide support, which were in turn mounted to a Rubbermaid container which was used to hold water during trials [27]. Geckos were fitted with two harnesses around their pelvis that were attached to a horizontally positioned force sensor on a motorized track, similar to the set-up used by Niewiarowski et al [26] and Stark et al [27] (electronic supplementary material, figure S1). Maximum shear adhesive force was defined as the point at which all four feet began to slip along the substrate (shear adhesion) (electronic supplementary material, figure S2).…”