Contact Between Biological Attachment Devices and Rough Surfaces

Spatula is an individual adhesive element situated at the tip of an adhesive hair in geckos, spiders and some insects. Usually, the contact area and work of adhesion depends on the type of the substrate. It is interesting to note that the interaction of a spatula-like hair tip with the substrate is practically independent on the numerical approach by which the substrate is modeled. However, in the experiments on real animals, as well as in numerically-modeled adhesion of this type of contact systems, pull-off force drops at some particular substrate roughness. In this chapter, we present a numerical model, which is capable to explain experimentally found effect of the adhesion drop at the scale of spatula. Besides we apply our numerical approach to study dynamics of spatular tips during contact formation and show that the contact area of the tips increases under applied shear force, especially, when spatulae are misaligned prior to the contact formation. The shear force has an optimum, when maximal contact is formed, but no slip occurs. In such a state, maximal adhesion can be generated. Another factor influencing attachment is the pad secretion, which flow on rough substrates is studied numerically here. The obtained results demonstrate that an increase in the density of the substrate microstructures leads to an increase in fluid loss from the pad. Additional numerical study, discussed in this chapter, deals with adhesive properties of plant fruits used for dispersal. These fruits can readily stick using secretion provided by the set of glands arranged in a smart manner radially at the distal end of the cut-cone-shaped fruit.