«UNIVERSITY OF CALIFORNIA Santa Barbara A Micro/Nano-Fabricated Gecko-Inspired Reversible Adhesive A Dissertation submitted in partial satisfaction of ...»
Figure V-1 Electron micrographs of silicon dioxide platforms supported by single slender pillars and coated with polymeric organorods. The structures are batch fabricated using standard bulk micro fabrication techniques requiring only a single lithography step. a, Array of four-fingered platform structures. Scale bar, 200 µm. b, Profile view of a organorod coated silicon dioxide platform supported by a single crystal silicon pillar. Scale bar, 20 µm. c, Magnified view of the multi-scale system. Scale bar, 20 µm. The integration of the ~200 nm diameter organorods with the 150 µm platform structures provides enhanced
organorod coated finger. Scale bar, 5 µm.
Figure V-2 Silicon dioxide platform supported by a single high-aspect-ratio pillar etched out of single crystal silicon using a modified micro fabrication technique. Scale bar 50 μm.
Figure V-3 Organic looking polymeric nanorods, “organorods”, fabricated utilizing the instability of a dielectric in a large electric field. The ~100 nm features were achieved using a bilayer to reduce the interfacial energy, thus reducing the instability wavelength and corresponding feature size. Scale bars, 10 and 2 μm, left and right respectively
Figure V-4 Comparison of hydrophilic and hydrophobic surfaces. a, (top) Water droplet partially wetting a hydrophilic organorod surface with a contact angle of
42.5° ± 2 (top). (bottom) Electron micrograph of ~120 nm diameter organic looking nanorods, “organorods”. Scale bar, 5 µm. b, (top) Water droplet resting on top of the highly-hydrophobic organorod surface with a contact angle of 145° ± 2. (bottom) Electron micrograph shows the increased diameter of the organorods to ~350 nm from the fluorocarbon hydrophobic coating applied through plasma deposition. Scale bar, 5 µm.
Figure V-5 Typical nanoindenter adhesion test results for a organorod covered structure indented with a semi-rough 3.175 diameter spherical aluminum tip.
The plot represents a typical load vs. displacement curve illustrating loading, unloading, and pull-off adhesion
hydrophilic polymeric surfaces. Test surfaces were indented with a semi-rough
3.175 diameter spherical aluminum tip. Increased normal loads created more surface contact between the spherical tip and surfaces increasing adhesion. The organorod morphology enhanced surface adhesion over the solid photoresist surface. Adhesion was further enhanced by integration of organorods with flexible silicon dioxide platforms, large data scatter was a bi-product of testing meso-scale adhesion with a rough surface. a, van der Waals and b, JohnsonKendal-Roberts (JKR) adhesion models predicting the collective adhesion of organorods over a given contact area, predicted by JKR contact mechanics for a sphere and flat surface. c and d, Modified van der Waals adhesion accounting for increased contact area attributed to conformation of platform “fingers” to the test sphere; c, two fingers. d, four fingers. The saturation point corresponds to complete finger attachment, in the case of four fingers (d) this value nicely bounds the experimental data.
Figure V-7 Nanoindenter adhesion testing results and theoretical models for hydrophobic polymeric surfaces. Surface adhesion between the sphere and test surfaces is enhanced by a hydrophobic coating on the organorods. The coating increases the size from ~120 nm to ~350nm and reduces the surface energy. a, van der Waals interaction energy model predicts an increase in adhesion with the larger organorods, but fails to capture the trend of the hydrophobic organorods. b, van der Waals and c, JKR models compensating for an increased radius of curvature- determined by scanning electron micrscopy- of
experimental data. d, Reduction of the interaction distance, do, from 0.165 nm to 0.09 nm in the van der Waals model. e, Combining the modified van der Waals models for hydrophobic organorods (do=0.09 nm) and the compliant platform model with 1 finger, excellent agreement with the meso-scale adhesion test is seen. Note the terminus of dotted line represents 1 complete finger adhesion.
Figure V-8 Scanning electron micrographs showing squashed organorods in the central portion of a structure. Scale bars, 20 and 10 μm, left and right respectively.
Figure VI-1 Scanning electron micrographs of the organorod morphology, scale bars 5 μm and 1 μm, left and right respectively
Figure VI-2 Schematic of the organorod growth mechanism. A bias is applied between oxygen plasma and the substrate, creating an electric field gradient, which acting on the dielectric polymer draws it in the direction of the gradient.94 Figure VI-3 Schematic of the nanoindenter test setup. The sphere represents the
3.175 mm aluminum sphere which is fixed to a shank fitted into the nanoindenter transducer. When the sphere is pressed into the platform structures a loading and unloading curve is produced. At the bottom of this curve there is a sudden jump from a negative force value to zero. This jump is indicative of pull-off associated with adhesion, and correspondingly taken to be the adhesive force
predictions. The circles represent a smooth polymer surface without modification. The squares represent a hydrophilic organorod surface on a solid substrate. The triangles represent a hydrophobic organorod surface on a solid substrate. Curve a represents a simple vdw model based on the number of organorods contacting the test surface, the radius of curvature of the organorod tips, and using a cut-off distance of 0.165 nm. The number of contacts is estimated by knowing the density of organorods and calculating the contact area of the indenter tip using JKR contact mechanics. Curve b is identical to curve a, but with a reduced cut-off distance in the vdw equation of 0.09 nm.... 96 Figure VII-1 Multiscale integrated compliant structures (MICS). Portion of a 2,500 array of MICS (top), scale bar 500 μm. Individual MICS (middle), scale bar 50 μm. Central portion of a platform showing organorod integration (bottom), scale bar 10 μm.
Figure VII-2 Scanning electron micrograph of organic looking polymeric nanorods (“organorods”) on the central portion of a platform, scale bars 5 μm and 2 μm, top and bottom respectively.
Figure VII-3 Adhesion strength versus applied normal load between a 5 mm diameter aluminum flat punch and a smooth photoresist surface (triangles); a surface of vertically aligned photoresist organorods (diamonds); and a surface of arrayed multiscale integrated compliant structures consisting of silicon dioxide platforms coated with organorods (squares).
solid substrate (diamonds) and a organorod surface on compliant structures (squares).
Figure VIII-1 – Electron micrographs of synthetic structures (left) and the analogous gecko structures (right), samples from a Tokay Gecko (Gekko Gecko). (A) Paddle surface coated with evenly spaced uncondensed aligned vertical polymer nanorods (left) and the branched terminus of a seta into spatulae (right), same magnification and scale bar 10 μm. (B) Freestanding nickel cantilevers and paddles coated with nanorods (left) and an array of setae (right), same magnification and scale bar 50 μm. (C) Low angle view of cantilevers showing upwards bending of the structures relative to the solid substrate (left) and a profile view of curving setal stalks (right), same magnification and scale bar 50 μm. (D) Lower magnification view of a portion of the synthetic array (left) and the setal array (right), scale bars 500 μm (left) and 200 μm (right).. 123 Figure VIII-2 - Stereomicrographs of the adhesive: (A) in the ‘ON’ state, no applied magnetic field, with the adhesive paddles facing vertically; and (B) in the ‘OFF’ state, with an applied magnetic field rotating the paddles sideways, concealing the adhesive faces. Scale bars, 100 μm
Figure VIII-3 – Schematic of the adhesion test apparatus. A laser interferometer monitors the deflection of a glass cantilever spring as a piezo actuator moves a 5 mm glass flat punch into and away from the test surface. The interaction forces are calculated by relating the stiffness and deflection of the cantilever upon contact with the surface
without and with an applied magnetic field, respectively. The insets represent actual adhesion data, where in the ‘ON’ state distinctive pull-off events were observed (top) and in the ‘OFF’ state no pull-off events were observed (bottom). Strength values were obtained by dividing the interaction force by the contact area of the paddles. In the ‘ON’ state, the devices showed an initial increase in adhesion with preload force, characteristic of increased surface contact with applied load (likely a result of slight misalignment between the 5 mm flat punch and test surface). Error bars represent 10 data sets at a specified displacement with no emission of outliers.
Figure IX-1 Electron micrographs of a double paddle configuration. The paddles are bending out of the base plane. By opposing each other the two paddles create a squeezing action, transforming a pure normal force into a combined normal and lateral force, enhancing a frictional component
Figure IX-2 Electron micrograph of a branched fingers bending out of plane. The branch fingers enhance the beams bending while providing surface area for adhesion
Figure IX-3 Electron micrographs of eight fingered branched platform structures coated with aligned vertical nanorods. The fingers are bending up out of the plane of the center support. Scale bars, in descending order, of 20, 100, 200 and 1000 μm
As a courtesy to the reader, a brief introduction of how to best navigate this thesis is given. For the general reader, the background information on the gecko, found in chapter II, may be quite interesting and important to understanding the research presented here. Also particular attention should be paid to the Chapters V and VIII, as these high impact journal publications highlight the significant findings in this research. As can be seen throughout this thesis, most work was focused on the development of micro/nanofabrication techniques for mimicking the hierarchical structure of the gecko adhesive.
Several new fabrication and characterization techniques were developed throughout the course of this work, leading to four journal publications and three conference proceedings. As a matter of completeness the journal publications and one conference proceeding has been included as chapters in this thesis. However, the reader may find the introductions, background and methods sections in each paper too brief, such is the nature of these publications. Therefore two important
chapters have been added to the beginning of the document:
Chapter II. A chapter giving a basic overview of the inspiration of the gecko adhesive system, the techniques used to create an analogue, and the methods employed to characterize the system.
Chapter III. A detailed chapter on the fabrication process, with extended commentary not found in the publications. While the background information on the gecko is important, this body of work was focused on developing new fabrication techniques for creating a hierarchical and reversible gecko inspired adhesive.
II. Background A. Biological AdhesionEquation Chapter 2 Section 1
1. Overview Figure II-1 Dude, a male Tokay Gecko (Gekko Gecko) sticking to a wall surface (Image by Jeff Clark, www.jeffclarkphotography.com).
The fine hair adhesive motif is found throughout nature in a variety of insects and lizards (1-8). While flies, beetles, crickets, spiders and anoles all make use of highly branched terminal structures to scale and stick to about any surface, the gecko is the largest animal to employ such a clinging motif. A Tokay Gecko (Gekko Gecko, Fig. II-1) weighs up to 300 grams and is able to run up, and down, vertical surfaces and across inverted surfaces (2). Not only does the gecko have the largest mass, it also has the most refined nanostructure, Fig. II-2. This is no coincidence as the structures with more branches produce more adhesion (discussed in detail in section 2 on adhesion mechanics).
Figure II-2 Gecko hierarchical adhesive system. Electron micrographs of:
A. the terminal spatular structure, scale bar 500 nm; B. the heavily branched spatular end of a setal stalk, scale bar 10 μm; C. setae extending from a lamellae surface, scale bar 20 μm. D. setal array showing the branched structures of the setae, scale bar 100 μm.
The gecko adhesive is a multi-scale hierarchical structure composed of β-keratin (3). While the nanostructures at the final termini of the system are of significant interest presently in the scientific community, it is the entire gecko system responsible for the amazing sticking ability of the gecko. The gecko has four feet, each containing five digits, Fig. II-3. The toes themselves are very flexible, able to roll up and away from a surface–important for the release mechanism. Within each toe are rows of imbricated lamellae supported by blood sinuses in the pad of the tarsus – which act as a sort of hydraulic suspension. The lamellae contain rows of thin slender fibers, called setae, approximately 130 μm in length and 20 μm in diameter (2), Fig. II-2. The terminus of each seta branches into thousands of smaller fibers, or spatular stalks. At the end of each of these stalks the structures flatten out into a flat 200 nm wide and 5 nm thick pad, or spatula (1, 2, 6, 9).
Figure II-3 Images of Dude, a Tokay Gecko, on a glass surfaces. Note the orientation of the toes relative to Dude’s leg and body orientation on the glass surface.