“Many insects cling to vertical and inverted surfaces with pads that
adhere by nanometre-thin films of liquid secretion. This fluid is an
emulsion, consisting of watery droplets in an oily continuous phase.
The detailed function of its two-phasic nature has remained unclear.
Here we show that the pad emulsion provides a mechanism that prevents
insects from slipping on smooth substrates. We discovered that it is
possible to manipulate the adhesive secretion in vivo
using smooth polyimide substrates that selectively absorb its watery
component. While thick layers of polyimide spin-coated onto glass
removed all visible hydrophilic droplets, thin coatings left the
emulsion in its typical form. Force measurements of stick insect pads
sliding on these substrates demonstrated that the reduction of the
watery phase resulted in a significant decrease in friction forces.
Artificial control pads made of polydimethylsiloxane showed no
difference when tested on the same substrates, confirming that the
effect is caused by the insects’ fluid-based adhesive system. Our
findings suggest that insect adhesive pads use emulsions with
non-Newtonian properties, which may have been optimized by natural
selection. Emulsions as adhesive secretions combine the benefits of
‘wet’ adhesion and resistance against shear forces.” (Dirks et al. 2009)