Preload-responsive adhesion: Effects of aspect ratio, tip shape and alignment

We tested the adhesive response of polymer surfaces structured with arrays of cylindrical fibrils having diameters of 10-20 mm and aspect ratios 1-2.4. Fibrils had two different tip shapes of end-flaps and round edges. A preloadinduced mechanical buckling instability of the fibrils was used to switch between the states of adhesion and non-adhesion. Non-adhesion in fibrils with round edges was reached at preloads that caused fibril buckling, whereas fibrils with end-flaps showed adhesion loss only at very high preloads. The round edge acted as a circumferential flaw prohibiting smooth tip contact recovery leading to an adhesion loss. In situ observations showed that, after reversal of buckling, the end-flaps unfold and re-form contact under prevailing compressive stress, retaining adhesion in spite of buckling. At very high preloads, however, end-flaps are unable to re-form contact resulting in adhesion loss. Additionally, the end-flaps showed varying contact adaptability as a function of the fibril-probe alignment, which further affects the preload for adhesion loss. The combined influence of preload,tip shape and alignment on adhesion can be used to switch adhesion in bioinspired fibrillar arrays.