The remarkable attachment system of geckos has inspired the development of dry microfiber adhesives through the last two decades. Some of the notable characteristics of gecko-inspired fibrillar adhesives include: strong, directional, and controllable adhesion to smooth and rough surfaces in air, vacuum, and under water; ability to maintain strong adhesion during repeated use; anti-fouling and self-cleaning after contamination. Given these outstanding qualities, fibrillar adhesives promise an extensive range of use in industrial, robotic, manufacturing, medical, and consumer products. Significant advancements have been made in the design of geckoinspired microfiber adhesives with the characteristic properties listed above, with the exception of the anti-fouling and self-cleaning features. The self-cleaning mechanism of the gecko’s adhesion system plays an important role to its ability to remain sticky in various environments. Similarly, enabling self-cleaning capability for synthetic microfiber adhesives will lead to robust performance in various areas of application. Presently, the practical use of fibrillar adhesives is restricted mainly to clean environments, where they are free from contaminants. The goal of this thesis is to conduct a detailed study of the mechanisms and mechanics of contact-based self-cleaning of gecko-inspired microfiber adhesives. This work focuses on contact self-cleaning mechanisms, as a more practical approach to cleaning. Previous studies on the cleaning of microfiber adhesives have mostly focused on mechanisms that involve complete removal of the contaminants from the adhesive. In this thesis, a second cleaning process is proposed whereby particles are removed from the tip of the microfibers and embedded between adjacent microfibers or in grooves patterned onto the adhesive, where they are no longer detrimental to the performance of the adhesive. In this work, a model of adhesion for microfiber adhesives that take the deformation of the backing layer under individual microfiber is developed. The dependence of adhesion of microfiber adhesives on the rate of unloading is also modeled and verified using experiments. The models of adhesion presented are later used to study the mechanics of contact self-cleaning of microfiber adhesives. Three major categories of self-cleaning are identified as wet self-cleaning, dynamic self-cleaning, and contact self-cleaning. A total of seven self-cleaning mechanisms that are associated with these categories are also presented and discussed. Results from the self-cleaning model and experiments show that shear loading plays an important role in self-cleaning. The underlying mechanism of contact self-cleaning due to normal and shear loading for spherical contaminants is found to be the particle rolling between the adhesive and a contacted substrate. Results from the model and experiments also show that small microfiber tips (much less than the size of the contaminants) are favorable for self-cleaning. On the other hand, large microfiber tips (much larger than the size of the contaminants) are favorable for anti-fouling of the microfiber adhesive. Results from this work suggests that the sub-micrometer size of the gecko’s adhesive fibers and the lamellae under the gecko toes contribute to its outstanding self-cleaning performance. The results presented in this thesis can be implemented in the design of microfiber adhesives with robust adhesion, self-cleaning and anti-fouling characteristic, for use in numerous applications and in various environments.

There has been significant interest in developing dry adhesives mimicking the gecko adhesive system, which offers several advantages compared to conventional pressure sensitive adhesives. Specifically, gecko adhesive pads have anisotropic adhesion properties: the adhesive pads (spatulae) stick strongly when sheared in one direction but are non-adherent when sheared in the opposite direction. This anisotropy property is attributed to the complex topography of the array of fine tilted and curved columnar structures (setae) that bear the spatulae. In this thesis, easy, scalable methods, relying on conventional and unconventional techniques are presented to incorporate tilt in the fabrication of synthetic polymer-based dry adhesives mimicking the gecko adhesive system, which provide anisotropic adhesion properties. In the first part of the study, the anisotropic adhesion and friction properties of samples with various tilt angles to test the validity of a nanoscale tape-peeling model of spatular function are measured. Consistent with the Peel Zone model, samples with lower tilt angles yielded larger adhesion forces. Contact mechanics of the synthetic array were highly anisotropic, consistent with the frictional adhesion model and gecko-like. Based on the original design, a new design of gecko-like dry adhesives was developed which showed superior tribological properties and furthermore showed anisotropic adhesive properties without the need for tilt in the structures. These adhesives can be used to reversibly suspend weights from vertical surfaces (e.g., walls) and, for the first time to our knowledge, horizontal surfaces (e.g., ceilings) by simultaneously and judiciously activating anisotropic friction and adhesion forces. Furthermore, adhesion properties between artificial gecko-inspired dry adhesives and rough substrates with varying roughness are studied. The results suggest that both adhesion and friction forces on a rough substrate depends significantly on the geometrical parameters of the substrate. The results in this study may be helpful for understanding how geckos overcome the influence of natural surface roughness. The novel designs of our dry adhesives open the way for new gecko-like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication. / acase@tulane.edu

Researches on biological adhesive systems in nature have changed a perspective view on adhesion that it is not only the area of surface chemistry, but also mechanics of interfacial geometry which can significantly effect on fracture strength and load distribution on the contact interface. Various synthetic fibrillar adhesives in previous works have shown enhanced interfacial bond strength with the capacity of adhesion control by exploiting mechanical deformation of the elastomeric fibrillar structures inspired by geckos. However, control of the interfacial load distribution has been focused on the size of micro-contact with single or a few of micro-/nano-fibers on planar surface, and not for a large contact area on complex three-dimensional (3D) surfaces. This thesis work aims at investigating principles of the interfacial load distribution control in multi-scale, ranging from micro-contact with single micro-fiber to a centimeter-scale contact with a membrane-backed micro-fiber array on non-planar 3D surfaces. The findings are also applied for developing a soft robotic gripper capable of grasping a wide range of complex objects in size, shape, and number, expanding the area of practical applications for bio-inspired adhesives in transfer printing, robotic manipulators, and mobile robots. This paper comprises three main works. First, we investigate the effect of tip-shapes on the interfacial load sharing of mushroom-shaped micro-fibrillar adhesives with precisely defined tipgeometries using high resolution 3D nano-fabrication technique. For a large area of non-planar contact interface, we fabricate fibrillar adhesives on a membrane (FAM) by integrating micro-fibers with a soft backing, which enables robust and controllable adhesion on 3D surfaces. Picking and releasing mechanism for the maximal controllability in adhesion are discussed. Finally, we propose a soft robotic architecture which can control the interfacial load distribution for the FAM on 3D surfaces, solving an inherit dilemma between conformability and high fracture strength with the equal load sharing on complex non-planar 3D surfaces.

No description available.

This study focuses on fabrication of dry adhesives mimicking gecko adhesion. We also look into the origin of the supreme adhesion of geckos, which have inspired the fabrication of fibrillar dry adhesives during the last decade or so. In principle, the superior material properties of ??-keratin (the main material comprising the fibrillar feature on gecko toe pads) along with the hierarchical high aspect-ratio fibrillar structure of geckos??? foot pad have enabled geckos to stick readily and rapidly to almost any surface in both dry and wet conditions. In this research, non-sticky fluoropolymer (Teflon AF) resembling ??-keratin rigidity and having an extremely low surface energy and dielectric constant was applied to fabricate a novel dry adhesive consisting of extremely high aspect-ratio nanopillars (200 nm in diameter) terminated with a fluffy top nanolayer. Both the nanopillars and the terminating layer were fabricated concurrently by replica-molding using a nanoporous anodic aluminum oxide membrane as the mold. In particular, upon infiltration of Teflon AF melt into the anodic aluminum oxide nanopores, the polymer melt fingered over the pore walls. The fingerlike structure formed during infiltration, subsequently collapsed after removal of the mold, developing a unique sheet-like nanostructure on top of the base nanopillars. Concurrent fabrication of the terminating nanostructure helps the fabrication of extremely high aspect-ratio (27.5???225) nanopillars which, up to an aspect-ratio of 185, neither collapse at the tip nor bundle. In order to fabricate nanopillars of different topographical properties, in our first approach, the height of the nanopillars as well as the size and density of the terminating nanostructure are carefully controlled by adjusting the processing temperature. Following that, a novel replica-molding technique for fabrication of bi-level Teflon AF nanopillars is reported. The developed technique relies on the concurrent heating and cooling of the Teflon AF melt which filled vertically-aligned alumina nanochannels. Unlike conventional polymer infiltration methods which consist of filling the mold by only heating the polymer above its glass transition temperature, in our novel method, the polymer melt is also simultaneously cooled down during the infiltration process. Concurrent cooling of the Teflon AF melt allows control over the interfacial instabilities of the polymer thin film, which forms ahead of the polymer melt upon its infiltration into the alumina nanochannels. Doing so, the geometrical properties of the subsequently developed peculiar fluffy nanostructure ??? after removal of the mold ??? on top of the extremely high aspect-ratio Teflon AF nanopillars (~25 ??m tall) are modified. In this project, we have also shown that the adhesion of the fabricated dry adhesives for the most part arises from electrostatic interactions of the applied polymer. In other words, Teflon AF, having an exceptional potential for developing electric charges at its surface upon contact with other materials via the so-called contact electrification phenomenon, can develop significant electrostatic interactions at its surface upon contact. In the current thesis, tribological results were discussed in detail to clarify the contribution of the structural properties of the fabricated dry adhesives toward their remarkable adhesion and friction forces generated via contact electrification. Nanopillars of specific geometrical properties have achieved remarkable adhesion and friction strengths, up to ~2.1 N/cm2 and 17 N/cm2, respectively (up to ~2.1 and 1.7 times larger than those of a gecko toe pad). It is commonly accepted that the adhesive performance of other synthetic bio-inspired dry adhesives is due to the formation of van der Waals interactions at the tip or side of the dry adhesives fibrils with the substrate they are brought into contact with. However, what has been usually neglected in this connection is that electrostatic interactions may also be developed at the contact between any two materials via the familiar contact electrification phenomenon. Although contact electrification is common and can have a large influence on interfacial interaction forces, its impact on adhesive properties of synthetic dry adhesives has been overlooked. Our results on adhesion of bi-level Teflon AF nanopillars, which can generate strong adhesion forces relying on electrostatic interactions arising from contact electrification, have brought to light again the idea that charging the surface of dry adhesives, specifically polymeric ones, can play a very crucial role in their adhesive behavior. From this perspective, the main reasons that have caused this lack of attention to this concept and the possible contributions of contact electrification to interfacial interactions of polymeric dry adhesives, other than bi-level Teflon AF nanopillars, are also thoroughly discussed in this thesis. Besides synthetic fibrillar dry adhesives, the possibility of the occurrence of contact electrification and its contribution to the supreme dry adhesion of geckos have also been overlooked for several decades. In this research, by the simultaneous measurement of electric charges and adhesion forces that gecko toe pads develop on two distinct substrates (a sticky and a non-sticky one), we have shown that the toe pads generate significantly large amounts of electric charge on both substrates. More importantly, we have found that there is a direct correlation between the contact electrification-driven electrostatic forces and the measured adhesion forces. Otherwise stated, we have shown that what makes the difference that geckos stick strongly to one surface and not to the other are the electrostatic interactions arising from contact electrification, and not van der Waals interactions, which have been considered as the prime source of adhesion of geckos for many years.

No description available.