A Novel Bioinspired Switchable Adhesive with Three Distinct Adhesive States

Abstract

A novel switchable adhesive, inspired by the gecko's fibrillar dry attachment system, is introduced. It consists of a patterned surface with an array of mushroom-shaped pillars having two distinct heights. The different pillar heights allow control of the pull-off force in two steps by application of a low and a high preload. For low preload, only the long pillars form contact, resulting in a low pull-off force. At higher preload, all pillars form contact, resulting in high pull-off force. Even further loading leads to buckling induced detachment of the pillars which corresponds to extremely low pull-off force. To achieve the respective samples a new fabrication method called double inking is developed, to achieve multiple-height pillar structures. The adhesion performance of the two-step switchable adhesive is analysed at varying preload and for different pillar aspect ratios and height relations. Finally, the deformation behavior of the samples is investigated by in situ monitoring.

Keywords: bioinspired adhesives; patterned; pull-off; responsive; switchable.

Figure 1

PDMS sample before the dipping process a) and after the dipping process showing pillars with two different lengths b). Note that the artifacts in the pillar tips in (b) are due to reflections at the curved surface.

Figure 2

The graphs show force-displacement curves obtained from experiments with low, intermediate, and high preload. a) Stepwise detachment of LP for low preload, SP and LP for intermediate preload, and a hysteresis in the compressive part and low detachment forces at high preload are found. b) The tensile part of (a) is magnified. c) The adhesion curve at high preload and the corresponding in situ pictures.

Figure 3

Pull-off forces measured on samples with different AR at different velocities. The results stem from a) AR 4.0, b) AR 4.5, and c) AR 5.0 samples.

Figure 4

Schematic of mushroom tip preparation. A substrate is silanized and spin coated with liquid PDMS. A previously prepared pillar sample is dipped into the liquid PDMS layer forming droplets on the pillar tips. After pressing the pillars onto a heating stage, mushroom tips are formed. Subsequently, droplets of liquid pillars are applied to selected pillars and the curing process is repeated, resulting in mushroom shaped pillars of different length.

Figure 5

The schematic of a sample indicates SP and LP as well as their respective tip diameters. All geometric parameters were measured using an optical microscope.