Meeting Abstract
When tested on smooth, dry substrates in laboratory conditions the gecko can achieve upwards of 20 Newtons of force, which predictions suggest is only attaching about 3% of its adhesive hair-like setae. If all setae came into contact with a substrate at once the gecko would have a safety margin of almost 4000%. Clearly this margin is excessive, but why? When considering geckos in their natural environment we can begin to explore several explanations. For instance, the substrates a gecko moves along are likely not clean, often wet, rough and uneven. Previous work has shown that fouling the adhesive toe pads with dirt and water reduces adhesion and additional work suggests that rough surfaces and wet surfaces can also reduce adhesion. Thus taken together the gecko must have a high safety factor to negotiate the unpredictable substrates and conditions they likely encounter daily. To explore the additive effect of multiple challenges to the gecko adhesive system we tested adhesion on patterned substrates in air and in water. In addition, we tested adhesion on microscopically rough substrates that varied in surface wettability in both air and water. Our results show that first, patterned substrates reduce adhesion in water but not in air. Second, there is a critical size-scale related to the adhesive contact area of the setal tip. This minimum adhesion value is further reduced in water. Taken together our results suggest that when utilizing substrates that are not ideal (wet and rough) the gecko must rely on its impressive “over built” design to maintain adhesion, and in some circumstances even this is not enough.
