Three-level hierarchal modeling of gecko toe adhesion
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1132-Z07-22
Three-level hierarchal modeling of gecko toe adhesion B. Chen1a, P. D. Wu1, H. Gao2, 1 Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, L8S 4L7, Canada. 2 Division of Engineering, Brown University, Providence, RI, 02912, USA.
ABSTRACT The underlying mechanisms of gecko adhesion have been investigated through modeling of the three-level hierarchical microstructures under gecko’s toe [1]. At the bottom of hierarchy, we show that the peeling strength of a spatula pad for attachment can be 10 times larger than that for detachment. At the intermediate level of hierarchy, we show that the 10 times difference in the peeling strength of a spatula pad for attachment and detachment leads to a 100 times difference in adhesion energy at the level of seta. At the top of hierarchy, the attachment of a gecko toe is modeled as a pad under displacement controlled pulling, which yields an adhesive strength more than sufficient for gecko’s body weight; On the other hand, the detachment is modeled as a pad under peeling, giving rise to a negligible peel-off force. The present study reveals that the hierarchical microstructures play critical roles in providing gecko with robust attachment and easy detachment. a
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INTRODUCTION Experiments have shown that gecko’s ability to climb on walls is mainly due to the van der Waals interaction between gecko’s toes and a surface [2,3]. The underlying adhesion mechanisms of how gecko can adhere robustly and, at the same time, elegantly release adhesion through a hierarchal microstructure under its toe has stirred a lot of interest. A detailed description of the hierarchal microstructure of a Gekko gecko can be found in, e.g. [4]: The toe of a Gekko gecko has a lamella structure containing numerous hairs called setae; each seta is about 120 µm long and has a cross-sectional diameter around 4.2 µm; the seta further branches into hundreds of spatulas through several shaft levels; at the very end of the branches, the spatula shaft holds a spatula pad, which is 0.3 µm in length, 0.2 µm in width and 5 nm in thickness. Various adhesion mechanisms for attachment and detachment of a gecko toe have been proposed [4,5]. However, the existing models are rather fragmented and incomplete, which have motivated us to set up a more coherent and self-consistent hierarchical modeling framework. This short paper summarizes our recent study [1] which is aimed to provide a more comprehensive description of the adhesion mechanisms of gecko from spatula pads at the bottom of hierarchy all the way up to the movements of gecko’s toes during attachment and detachment.
THEORY Adhesion at the bottom of hierarchy: a single spatula pad As a spatula pad approaches a surface, its tiny thickness results in a large portion of the spatula pad being firmly absorbed onto the surface via the van der Waals interaction. The relatively small adhesion process zone means that the apparent adhesion energy could
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