Role of contact electrification and electrostatic interactions in gecko adhesion

Abstract

Geckos, which are capable of walking on walls and hanging from ceilings with the help of micro-/nano-scale hierarchical fibrils (setae) on their toe pads, have become the main prototype in the design and fabrication of fibrillar dry adhesives. As the unique fibrillar feature of the toe pads of geckos allows them to develop an intimate contact with the substrate the animal is walking on or clinging to, it is expected that the toe setae exchange significant numbers of electric charges with the contacted substrate via the contact electrification (CE) phenomenon. Even so, the possibility of the occurrence of CE and the contribution of the resulting electrostatic interactions to the dry adhesion of geckos have been overlooked for several decades. In this study, by measuring the magnitude of the electric charges, together with the adhesion forces, that gecko foot pads develop in contact with different materials, we have clarified for the first time that CE does contribute effectively to gecko adhesion. More importantly, we have demonstrated that it is the CE-driven electrostatic interactions which dictate the strength of gecko adhesion, and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.

Keywords: contact electrification; electric double layer; electrostatic interactions; gecko adhesion.

Figure 1.

(a-1) An originally neutral gecko foot pad was brought close to a vertically aligned polymer thin film which was coated on an approximately 5 × 5 cm², mirror-finished, copper sheet. (a-2) The toe pad was placed on the polymer thin film and the surface charge density was measured. (a-3) The foot was pulled down and the toes were dragged over the polymer thin film for a 2–10 mm distance, depending on the type of the polymer. (a-4) Finally, the foot was pulled up—perpendicular from the thin film—until (a-5) the toes completely detached from the substrate. Characteristic changes in the shear strength (i.e. shear force per unit toe pad area) throughout all five steps of the in situ force/charge measurement tests on (b) Teflon AF and (c) PDMS. (d) Static shear strength values determined from force traces recorded during force/charge measurement tests on both Teflon AF and PDMS. (Online version in colour.)

Figure 2.

(a) Before contact of a nano-spatula—at the tip of a seta of a gecko toe pad—with the polymer thin film, both the spatula and the thin film were electrically neutral. (b) As the toe pad came into contact with the polymer thin film, electric charges separated between the nano-spatula and the thin film. The EDL, which was formed at the contact interface, induced certain electric charges in the backing copper sheet, which was grounded through an electrometer. (c) Electric charges that separated upon contact penetrated up to a depth of d_i and d_g into the polymer thin film and the contacted nano-spatula, respectively. D is the actual separation distance (approx. 0.3 nm) [7,21] between the nano-spatula and the thin film. (d) Surface charge densities measured right after contact of gecko toe pads with Teflon AF and PDMS. (Online version in colour.)

Figure 3.

F_t, the total lateral adhesion strength (i.e. shear strength), as well F_elc, the normal electrostatic adhesion strength (i.e. adhesion force per unit toe pad area), for contact of gecko toe pads with both Teflon AF and PDMS thin films. The schematic shows the contact of a nano-spatula at the tip of a seta of a gecko toe pad with the polymer thin film and the direction of the sliding of the toe pads as well as those of the generated shear and electrostatic forces. Both adhesion strength values were determined right before the initiation of the sliding of the toe pads over the polymer thin films. (Online version in colour.)