Date of Award

1-19-2012

Embargo Period

1-24-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor(s)

Metin Sitti

Abstract

Gecko feet stick to almost anything, in almost any condition (including underwater and in space), but do not stick unintentionally, do not stick to dirt, and enable the gecko to literally run up the walls. When climbing a smooth surface, geckos can attach and detach each foot very quickly (detaching a foot takes 15 milliseconds) and with almost no noticeable force, but if attached perfectly they could theoretically hold tens of times their body weight. In contrast to gecko adhesion, conventional adhesives, made of soft tacky materials, tend to leave residues, pick up dirt easily, stick to themselves strongly and are useless underwater. Gecko feet rely on completely different principles, utilizing arrays of tiny mechanical structures made of very stiff protein which react to pressing and dragging with some very smart behavior. This thesis work is primarily concerned with taking inspiration from the principles of gecko-adhesion in order to control the attachment of synthetic structured adhesives.

We present gecko-inspired angled elastomer micropillars with flat or round tip endings as compliant pick-and-place micromanipulators. The pillars are 35 μm in diameter, 90 μm tall, and angled at an inclination of 20°. By gently pressing the tip of a pillar to a part, the pillar adheres to it through intermolecular forces. Next, by retracting quickly, the part is picked from a given donor substrate. During transferring, the adhesion between the pillar and the part is high enough to withstand disturbances due to external forces or the weight of the part. During release of the part onto a receiver substrate, the contact area of the pillar to the part is drastically reduced by controlled vertical or shear displacement, which results in reduced adhesive forces. The maximum repeatable ratio of pick-to-release adhesive forces was measured as 39 to 1. We find that a flat tip shape and shear displacement control provide a higher pick-to-release adhesion ratio than a round tip and vertical displacement control, respectively. We present a model of forces to serve as a framework for the operation of this micromanipulator. Finally, demonstrations of pick-and-place manipulation of μm-scale silicon microplatelets and a cm-scale glass cover slip serve as proofs of concept. The compliant polymer micropillars are safe for use with fragile parts, and, due to exploiting intermolecular forces, could be effective on most materials and in air, vacuum, and liquid environments.

We present a study of the self-cleaning and contamination resistance phenomena of synthetic gecko-inspired adhesives made from elastomeric polyurethane. The phenomenon of self-cleaning makes the adhesive foot of the gecko robust against dirt, and makes it effectively sticky throughout the lifetime of the material (within the molting cycles). So far synthetic gecko adhesives fail to capture this behavior and self-cleaning remains the least studied characteristic in the field geckoinspired adhesives. In this work we use two distinct arrays of micropillars with mushroom-shaped tips made from polyurethane. The two geometries we use all have the same aspect ratios of pillar height to base diameter of about 2 to 1, and all have mushroom tips that are twice the diameter of base. The pillar tip diameters are 20 μm and 95 μm, and we will refer to them as the small and large pillars, respectively. We contaminate the adhesives with simulated dirt particles in the form of well-characterized soda lime glass spheres ranging in diameter from 1 to 250 μm. Both micropillar arrays recovered adhesive strength after contamination and cleaning through dry, shearing contact with glass. In a best case scenario, we found that large pillars contaminated with 150-250 μm diameter particles can rid the tips of contaminating particles completely and recover 90% of the initial adhesive strength. This finding is significant because it is the first demonstration of adhesion recovery through dry self-cleaning by contact to a non-sticky cleaning substrate. The degree to which adhesion is recovered is superior to any conventional adhesive and is nearly identical to the gecko itself.

This thesis presents a study of controlling adhesion in gecko-inspired adhesives. This control is achieved by maximizing or minimizing attachment strength on demand by simple mechanical loading, and enables robotic manipulation tasks and the recovery of adhesion after contamination. Looking forward, we can predict what is possible for gecko-inspired adhesives if the discoveries in this thesis are implemented, and if other shortcomings in the field are resolved. Looking at the applications already under development, it seems clear that medical adhesives have great potential, and climbing robots might achieve significant utility. In consumer products, gecko-adhesives might replace Velcro®and zippers in clothing, and might become a critical component in sports gear, e.g. soccer goal keeper and rock climber gloves. The reversible, controllable nature of the adhesion, as well as its incredible bonding strength, suggests more impressive possibilities for gecko-inspired adhesives: perhaps it might act as a fastener for temporary or emergency construction. We might yet see rolls of single-sided and double-sided gecko-tape sold in hardware stores, not as a replacement for duct tape, but as a replacement for nails, staples and screws.

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