Experimental analysis on the adhesion of living tokay geckos on nanorough surfaces

SUMMARY. With in vivo observations and time measurements, we have demonstrated that living geckos display adhesion times following Weibull statistics and the Weibull shape and scale parameters can be used to describe quantitatively the statistics of the adhesion times of different geckos (male or female), materials (glass or PMMA), and interfaces (virgin or machined PMMA surfaces). 1 INTRODUCTION The Tokay gecko's (Gekko gecko) ability to " run up and down a tree in anyway, even with the head downwards " was first observed by Aristotle, almost 25 centuries ago [1]. However, the pioneer study on gecko adhesion has been done by Hiller [2], who first provided SEM pictures of the setae, showing their hierarchical ultrastructure and high density of terminal spatulae; he first did a very careful experiment on living geckos, showing adhesion dependence on surface energy of the substrate. The structure of the digital setae of lizards was discussed [3]. In spite of this, only recently, the adhesive force of a single gecko foot-hair has been measured [4]. Like geckos, a comparable adhesive mechanism and adhesive ability, resulting in an extraordinary ability to move on vertical surfaces and ceilings, can be found in other creatures, such as beetles, flies and spiders. A comparison between the gecko and spider nanostructured feet is reported in Fig. 1 [5, 6]. Surface roughness strongly influences the animal adhesion strength and ability. Its role was shown in different measurements on flies and beetles, walking on surfaces with well defined roughness [11, 12, 13], on the chrysomelid beetle Gastrophysa viridula [14], on the fly Musca domestica [13] as well as on the Tokay geckos [15]. A minimum of the adhesive/frictional force, spanning surface roughness from 0.3 to 3 μm, was reported [13, 14]. The experiments on the reptile Tokay gecko [15] showed a minimum in the adhesive force of a single spatula at an intermediate root mean square (RMS) surface roughness around 100–300 nm, and a monotonic increase of adhesion times of living geckos by increasing the RMS, from 90 to 3000 nm. There are several observations and models in the literature, starting with the pioneer paper by Fuller and Tabor [16], in which roughness was seen to decrease adhesion monotonically. But there is also experimental evidence in the literature, starting with the pioneer paper by Briggs and Briscoe [17], which suggests that roughness need not always reduce adhesion. For example, in the framework of …