ADHESIVE FORCE OF A SINGLE GECKO SETA A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Lan Yu May, 2018 ADHESIVE FORCE OF A SINGLE GECKO SETA Lan Yu Thesis Approved: Accepted: Advisor Dean of College Dr. Ali Dhinojwala Dr. Eric J. Amis Committee Member Dean of the Graduate School Dr. Hunter King Dr. Chand K. Midha Department Chair Date Dr. Coleen Pugh ii ABSTRACT The gecko’s adhesion system uses arrays of setae to achieve strong and repeatable adhesion. Observation of the toe pad structure shows that shorter setae are generally proximal while longer ones distal [39]. We hypothesized that a seta of longer length would generate higher adhesive force, as long and slender setae have lower bending modulus, and therefore making it easier to contact spatulae with the surface. By following previous single seta experiments [11], we first measured the shear force generated by an isolated gecko seta using Nano Bionix. We also tested the length hypothesis by measuring the shear adhesion ability of single seta of varying lengths ranging from 80 to 140µm, which was taken from different parts of the gecko toe pad from distal to proximal. Measurements gave the result of shear forces of a single seta in the range of 80 to 250µN and an average of 136.81µN. This number is lower than the average value 194µN in previous studies, which is tested under a preload of 15µN [11]. In addition, results from 12 individual setae suggested that shear adhesion force of a single seta is independent of setal length and dependent on setal diameter. iii ACKNOWLEDGEMENTS I would like to profusely thank my advisor Dr. Ali Dhinojwala, for his guidance and encouragement at every step of graduate school. I would like to express my gratitude towards my committee member, Dr. Hunter King, for his valuable comments and suggestions. Besides, I want to thank Dr. Peter Niewiarowski and Dr. Todd Blackledge for allowing me to use their equipments. Also, I would like to show my gratitude to my coworkers Michael Wilson, Austin Garner, Angela Alicea and Jialu Li for giving me a lot of constructive advice and carrying out experiments together during the whole project. What is more, I would like to thank all my group members for helping me a lot during the daily experiments and also creating a harmonious atmosphere for research. At last, I feel grateful for the support of my parents, who help me a lot in my life. iv TABLE OF CONTENTS LIST OF FIGURES………………………………………………………………….vi LIST OF TABLES ………………………………………………………………….vii CHAPTER I. INTRODUCTION ………………………………………………………………….1 II. BACKGROUND ………………………………………………………………….6 III. EXPERIMENTAL ………………………………………………………………22 IV. RESULTS ……………………………………………………………………….26 V. CONCLUSIONS AND DISCUSSIONS ………………………………………...30 REFERENCES ……………………………………………………………………...31 v LIST OF FIGURES Figure Page 2.1 The adhesion system of gecko’ feet ................................................................... 8 2.2 The hierarchical structures on the adhesive toe pads of geckos......................... 9 2.3 Apparatus for single seta force measurement .................................................... 11 2.4 Adhesion force of a single seta .......................................................................... 12 2.5 Evidence for van der Waals forces as gecko adhesion mechanism ................... 17 2.6 SEM images of the proximal, intermediate and distal lamellae showing the difference in setal morphology .......................................................................... 19 2.7 Common trends in setal field configuration and dimensions across generalized proximal, intermediate and distal lamellae of Rhoptropus ................................ 19 3.1 Illustrations of shear force measurement of a single seta using Nano Bionix ... 23 3.2 Illustration of seta deformation over a small lateral displacement of the insect pin ...................................................................................................................... 23 3.3 Setal length and diameter measurement ............................................................. 25 4.1 Parallel shear force measured for one single seta as a function of time ............. 27 4.2 Maximal shear force measured for each seta as a function of (a) setal length (b) setal diameter ................................................................................................ 28 4.3 Maximal shear force measured for each seta as a function of setal aspect ratio 29 vi LIST OF TABLES Table Page 2.1 Surface characteristics of Tokay gecko feet 9 2.2. Mechanisms of adhesion rejected by studies 14 2.3 Adhesion force measured for spatula on different environments 18 vii CHAPTER I INTRODUCTION Geckos are lizards of the taxonomic family Gekkonidae that can be found in warm climates throughout the world. Geckos are well known for their dynamic attachment ability to climb on different smooth or rough surfaces and detach at will. This ability was referred to by Bhushan as reversible adhesion or smart adhesion [1]. Among over 1000 gecko species that involve an enormous range of morphological variation of the body sizes and toe pad morphology [2,3,4], Tokay geckos has been the one used for most researches because of its availability and large size [5][6][7]. Although the remarkable performance of gecko adhesion has been realized since the time of Aristotle, it was not until the late 19th century did people first recognize the microscopic hair-like structures covering their toe pads [8]. With the development of experimental equipment, scientists have discovered many of the secrets of gecko adhesion, exploring aspects that enable the gecko to adhere to and detach from surfaces, such as the mechanisms of adhesion, surface properties and adhesion strength [4, 9-23], yet the complex hierarchical structure on their toes is still generating puzzling new questions and valuable answers. The secret of the gecko’s adhesive properties lies in the surface morphology on their toe pads. Each gecko toe consists of several layers of hierarchical structure called lamellae [4]. Approximately half a million bristles called setae are coated on each gecko toe, and the ends of setae split into even-smaller hair-like structures called spatulae [23]. Measurements of adhesion have been accessible to all these length scales due to the advancement in experimental tools. 8 Although the structure of the gecko’ s adhesive system is well observed, the understanding of their function is full of trouble. The molecular mechanism underlying adhesion in setae has still remained unclear. Adhesion can be caused by at least 11 different types of intermolecular surface forces at the interface between solids and cannot always be distinguished from friction [28, 29]. Difficulties of these studies result from determining what material is interacting at the micro level. Three possible mechanisms, suction, electrostatics, and micro-interlocking were first proposed in the literature and later got ruled out by studies. Blackwall and Hepworth first brought up the idea that the setae act like miniature suction cups [30, 31]. However, no data can be used to support suction as an adhesive mechanism, and Dellit refuted this theory by carrying out the adhesion experiments in a vacuum [13]. In addition, Autumn’s measurements of single seta adhesion of greater than one atmosphere of adhesion pressure supported Dellit’s results and further proved the suction hypothesis wrong [11]. Dellit’s study in 1934 also eliminated another possible mechanism- electrostatic attraction, as a mechanism for setal adhesion, for the geckos could still adhere in ionized air. Nevertheless, it was found out that electrostatic effects can still be utilized to enhance adhesion even if other mechanisms are operating dominantly [20]. Dellit noted the fact that setae are recurved in a way that their tips point proximally, he then postulated a micro-interlocking hypothesis that these setae act like hooks, catching on surface irregularities [13]. Later in 1965, Ruibal and Ernst suggested that the spatulae lie flat against the surface when they are in contact with the substrate, and frictional force increases through the increasing number of spatulae making contact with the surface [4]. Ruibal and Ernst’s study was an important step in understanding the gecko’s adhesion mechanism, but it was still restricted by the micro-interlocking theory. It was Hiller that 9 first proposed that the material properties of the substrate but not its texture, determine the gecko adhesion abilities [5]. Hiller’s study rejected the micro-interlocking and friction hypotheses by proving that the gecko’s adhesion behavior is not mechanical but a molecular phenomenon. Furthermore, the fact that geckos can stick to polished glass up-side-down and that they are able to generate large adhesive forces on a molecularly smooth SiO2 surface indicated that surface irregularities are not necessary, but on the contrary, can impede adhesion [11]. In 1900, Haase first proposed that adhesion depends on the load, and it only occurs in the proximal direction along the axis of the toe [28]. Discovering that adhesive force was correlated with the water droplet contact angle of the surface, he also suggested that the gecko adhesion happens through intermolecular forces [5, 17]. Since then, following researches have been going in the direction of understanding the nature of these intermolecular forces. Thin film capillary adhesion was suggested