Geometry-controlled adhesion: revisiting the contact splitting hypothesis

Following studies of biological attachment systems, the principle of contact splitting, according to which splitting up the contact into finer subcontacts increases adhesion, was introduced. However, numerous attempts at employing this principle in producing dry adhesives were unsuccessful, prompting us to test its validity. Here, we show that in addition to the increase in number of subcontacts, the contact splitting model also implies a built-in increase in contact area. Thus, based on this model, it is impossible to say which parameter leads to increase in adhesion, the increasing number of subcontacts, as accepted to think, or just an increase in contact area, which is a trivial result. To clarify this point, we show experimentally what happens if we keep the contact area constant, while increasing the number of subcontacts in the equal load sharing mode, which was never done before. In contrast to the contact splitting principle, our measurements clearly demonstrate that, in flat-punch-patterned conformal contact, the pull-off force remains the same even when the number of subcontacts increases by two orders of magnitude. Our finding suggests that the contact splitting idea can only work in thin-film-based contacts, which are indeed employed in most biological temporary attachment systems.