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Author: Liehui Ge
Publisher:
ISBN:
Category : Adhesives
Languages : en
Pages : 176
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Book Description
Geckos' feet consist of an array of millions of keratin hairs that are hierarchically split at their ends into hundreds of contact elements called "spatula(e)". Spatulae make intimate contacts with surface and the attractive van der Waals (vdW) interactions are strong enough to support up to 100 times the animals' bodyweight. Tremendous efforts have been made to mimic this adhesion with polymeric materials and carbon nanotubes (CNT). However, most of these fall short of the performance of geckos. "Contact splitting principle", based on Johnson-Kendall-Roberts (JKR) theory, predicts that a vertically aligned carbon nanotube array (VA-CNT) will be at least 50 times stronger than gecko feet. Although 160 times higher adhesion was recorded in atomic force microscopy (AFM) measurements, macroscopic VA-CNT patches often show low or even no adhesion to substrates. The behavior of VA-CNT hairs near the contact interface has been explored using a combination of mechanical, electrical contact resistance, and scanning electron microscopic (SEM) measurements. Instead of making the expected end contacts, carbon nanotubes make significant side-wall contacts that increase with preload. Adhesion of side-wall contact CNTs is determined by the balance of adhesion in the contact region and the bending stiffness of the CNTs, thus a compliant VA-CNT array is required to make adhesive patches. Macroscopic patches of compliant VA-CNT array have been fabricated. Patches of uniform array have adhesive strength similar to that of geckos (10 N/cm2) on a variety of substrates and can be removed easily by peeling. When the array is patterned to mimic the hierarchical structures of gecko foot-hairs, strength increases up to four times. VA-CNT-based gecko adhesives are self-cleaning, non-viscoelasticity and give good strength in vacuum. These properties are desired in robotics, microelectronics, thermal management and outer space operations. Current theory still cannot completely explain adhesion of gecko feet. A series of experiments have been carried out to measure adhesion at different temperatures using a single protocol with two species of gecko that had been previously studied (G. gecko and P. dubia). Strong evidence of an effect of temperature was found but the trend was counterintuitive given the thermal biology of geckos and it violated the prediction by van der Waals interactions. Consequently, other factors (e.g., humidity) that could explain the variation in the observed clinging performance were examined. Evidence was found, unexpectedly, that humidity is likely an important determinant of clinging force in geckos. Both van der Waals and capillary forces fail to explain the shear adhesion data at the whole animal scale. Resolution of this paradox will require examination of the physical and chemical interaction at the interface and particular way in which water interacts with substrate and setae at the nanometer scale.
Author: Liehui Ge
Publisher:
ISBN:
Category : Adhesives
Languages : en
Pages : 176
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Book Description
Geckos' feet consist of an array of millions of keratin hairs that are hierarchically split at their ends into hundreds of contact elements called "spatula(e)". Spatulae make intimate contacts with surface and the attractive van der Waals (vdW) interactions are strong enough to support up to 100 times the animals' bodyweight. Tremendous efforts have been made to mimic this adhesion with polymeric materials and carbon nanotubes (CNT). However, most of these fall short of the performance of geckos. "Contact splitting principle", based on Johnson-Kendall-Roberts (JKR) theory, predicts that a vertically aligned carbon nanotube array (VA-CNT) will be at least 50 times stronger than gecko feet. Although 160 times higher adhesion was recorded in atomic force microscopy (AFM) measurements, macroscopic VA-CNT patches often show low or even no adhesion to substrates. The behavior of VA-CNT hairs near the contact interface has been explored using a combination of mechanical, electrical contact resistance, and scanning electron microscopic (SEM) measurements. Instead of making the expected end contacts, carbon nanotubes make significant side-wall contacts that increase with preload. Adhesion of side-wall contact CNTs is determined by the balance of adhesion in the contact region and the bending stiffness of the CNTs, thus a compliant VA-CNT array is required to make adhesive patches. Macroscopic patches of compliant VA-CNT array have been fabricated. Patches of uniform array have adhesive strength similar to that of geckos (10 N/cm2) on a variety of substrates and can be removed easily by peeling. When the array is patterned to mimic the hierarchical structures of gecko foot-hairs, strength increases up to four times. VA-CNT-based gecko adhesives are self-cleaning, non-viscoelasticity and give good strength in vacuum. These properties are desired in robotics, microelectronics, thermal management and outer space operations. Current theory still cannot completely explain adhesion of gecko feet. A series of experiments have been carried out to measure adhesion at different temperatures using a single protocol with two species of gecko that had been previously studied (G. gecko and P. dubia). Strong evidence of an effect of temperature was found but the trend was counterintuitive given the thermal biology of geckos and it violated the prediction by van der Waals interactions. Consequently, other factors (e.g., humidity) that could explain the variation in the observed clinging performance were examined. Evidence was found, unexpectedly, that humidity is likely an important determinant of clinging force in geckos. Both van der Waals and capillary forces fail to explain the shear adhesion data at the whole animal scale. Resolution of this paradox will require examination of the physical and chemical interaction at the interface and particular way in which water interacts with substrate and setae at the nanometer scale.
Author: Wil Mara
Publisher: Cherry Lake
ISBN: 1624317642
Category : Juvenile Nonfiction
Languages : en
Pages : 36
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Book Description
Learn about how nature has inspired technological innovations with this book on the similarities between gecko feet and a new adhesive tape. Integrating both historical and scientific perspectives, this book explains how gecko feet inspired the invention of an adhesive. Readers will make connections and examine the relationship between the two concepts. Sidebars, photographs, a glossary, and a concluding chapter on important people in the field add detail and depth to this informational text on biomimicry.
Author: Shihao Hu
Publisher:
ISBN:
Category : Adhesives
Languages : en
Pages : 151
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Book Description
Gecko feet integrate many intriguing functions such as strong adhesion, easy detachment and self-cleaning. Mimicking this biological system leads to the development of a new class of advanced fibrillar adhesives useful in various applications. In spite of many significant progresses that have been achieved in demonstrating the enhanced adhesion strength from divided non-continuous surfaces at micro- and nano- scales, directional dependent adhesion from anisotropic structures, and some tolerance of third body interferences at the contact interfaces, the self-cleaning capability and durability of the artificial fibrillar adhesives are still substantially lagging behind the natural version. These insufficiencies impede the final commercialization of any gecko inspired products. Hence here, we have focused our attentions on these critical issues in both (i) the gecko adhesive systems and (ii) the synthetic counterparts. (i) We tested the self-cleaning of geckos during locomotion and provided the first evidence that geckos clean their feet through a unique dynamic self-cleaning mechanism via digital hyperextension. When walking naturally with hyperextension, geckos shed dirt from their toes twice as fast as they would if walking without hyperextension, returning their feet to nearly 80% of their original stickiness in only 4 steps. Our dynamic model predicts that when setae suddenly release from the attached substrate, they generate enough inertial force to dislodge dirt particles from the attached spatulae. The predicted cleaning force on dirt particles significantly increases when the dynamic effect is included. The extraordinary design of gecko toe pads perfectly combines dynamic self-cleaning with repeated attachment and detachment, making gecko feet sticky yet clean. This work thus provides a new mechanism to be considered for biomimetic design of highly reusable and reliable dry adhesives and devices. (ii) A multiscale modeling approach has been developed to study the force anisotropy, structural deformation and failure mechanisms of a two-level hierarchical CNT structures mimicking the gecko foot hairs. At the nanoscale, fully atomistic molecular dynamics simulation was performed to explore the origin of adhesion enhancement considering the existence of laterally distributed CNT segments. Tube-tube interactions and the collective effect of interfacial adhesion and friction forces were investigated at an upper level. A fraction of the vertically aligned CNT arrays with laterally distributed segments on top was simulated by coarse grained molecular dynamics. The characteristic interfacial adhesive behaviors obtained were further adopted as the cohesive laws incorporated in the finite element models at the device level and fitted with experimental results. The multiscale modeling approach provides a bridge to connect the atomic/molecular configurations and the micro-/nano- structures of the CNT array with its macro-level adhesive behaviors, and the predictions from the modeling and simulation help to understand the interfacial behaviors, processes and mechanics of the gecko inspired fibrillar structures for dry adhesive applications.
Author: Andrew George Gillies
Publisher:
ISBN:
Category :
Languages : en
Pages : 125
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Book Description
The past decade has seen rapid advancement in gecko synthetic adhesives (GSAs) and their performance has also steadily increased. However, there still remains a gap between the capabilities of current GSAs, and the properties required for GSAs to perform as the gecko does: on natural undulating surfaces with several scales of roughness, in dirty environments where particle contamination is the norm, and for thousands or even tens of thousands of cycles. For continued progress to be made in GSAs, focus must shift from trying to attain high adhesive values under ideal conditions, to exploring the weaknesses in current GSAs and contrasting those with the principles that underpin the success of the natural gecko systems in real world challenging conditions. Here we show results from the testing and simulation of various GSA systems in rough environments, with contaminating particles of varying size and for repeated cycling. We report that with careful geometry and material consideration, large increases in 'real world' performance can be obtained, and in some cases active control can be utilized to increase controllability. To better understand adhesion on macroscopic rough surfaces, we studied the ability of live Tokay Geckos to adhere to an engineered substrate constructed with sinusoidal patterns of varying amplitudes and wavelengths in sizes similar to the dimensions of the toes and lamellae structures (0.5 to 6 mm). We found shear adhesion was significantly decreased on surfaces that had amplitudes and wavelengths approaching the lamella length and inter-lamella spacing, losing 95% of shear adhesion over the range tested. We discovered that the toes are capable of adhering to surfaces with amplitudes much larger than their dimensions even without engaging claws, maintaining 60% of shear adhesion on surfaces with amplitudes of 3 mm. As well, Gecko adhesion can be predicted by the ratio of the lamella dimensions to surface feature dimensions. In addition to setae, remarkable macroscopic-scale features of gecko toes and lamellae that include compliance and passive conformation are necessary to maintain contact, and consequently, generate shear adhesion on macroscopically rough surfaces. Similarly, we sought to understand the impact of surface roughness on the adhesion of two types of GSA arrays: those with hemispherical shaped tips and those with spatula shaped tips. Our simulations showed that the nanoscale geometry of the tip shape dramatically alters the macroscale adhesion of the array, and that on sinusoidal surfaces with roughness much larger than the nanoscale features, there is still a clear benefit to having spatula shaped features. Similar to experimental results found with the macroscale features of the gecko adhesive system, when roughness approaches the size of the fiber features, adhesion drops dramatically. We have also investigated the impact of two design parameters on the dry self-cleaning capability of GSAs by experimentally testing two GSAs after fouling with small (1 micron), medium (3-10 microns) and large (40-50 microns) particles. We found that a GSA made from a hard thermoplastic with nanoscopic fibers was able to recover 96-115% of its shear adhesion after fouling with small and large but not medium particles, while a GSA made from a soft polymer and microscopic fibers recovered 40-55% on medium and large particles. Further examination by scanning electron microscopy (SEM) revealed that the soft polymer structures were not shedding the smaller particles during recovery steps, but were instead being absorbed into the surface, and that, regardless of their size, particles did not release from the soft polymer surface. An analysis of the contact strength between fibers, particles and substrates of various dimensions and elasticity reveals that dry self-cleaning will be more effective for GSAs fabricated with smaller fiber diameters and for GSAs fabricated from materials with smaller loss functions, such as hard thermoplastics. This has important implications on the choice of materials and geometries used for GSAs when dry self-cleaning capability is a desired function in the material, and indicates that hard polymer GSAs with smaller fiber diameters are less prone to fouling. As indicated by results of dry self-cleaning on a passive soft polymer fibrillar adhesive, we set out to design a system with active control and release of particles. We have demonstrated controllable adhesion to glass spheres with a new magnetically actuated synthetic gecko adhesive made from a magnetoelastomer composite. Capable of controlling adhesion to glass spheres 500 microns to 1 mm, this represents an important step in realizing an adhesive with dry self-cleaning capabilities across a wide range of particle sizes. We also examined the behavior of high density polyethylene (HDPE) and polypropylene (PP) microfiber arrays during repeated cycles of engagement on a glass surface, with normal preload less than 40 kPa. We found that fiber arrays maintained 54% of the original shear stress of 300 kPa after 10,000 cycles, despite showing marked plastic deformation of fiber tips. This deformation was attributed to shear induced plastic creep of the fiber tips from high adhesion forces, adhesive wear or thermal effects. We hypothesize that a fundamental material limit has been reached for these fiber arrays, and that future gecko synthetic adhesive designs must take into account the high adhesive forces generated to avoid damage. Although the synthetic material and natural gecko arrays have a similar elastic modulus, the synthetic material does not show the same wear-free dynamic friction as the gecko. The discovery of this wear mechanism has uncovered a possible pathway to the fabrication of nanoscale spatula shaped tips. Spatula tips have been shown by the rough surface simulation to greatly improve adhesion strength. Several possible fabrication pathways are proposed and preliminary results on these fabrication techniques are presented.
Author: Bharat Bhushan
Publisher: Springer Science & Business Media
ISBN: 3540282483
Category : Technology & Engineering
Languages : en
Pages : 1157
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Book Description
The recent emergence and proliferation of proximal probes, e.g. SPM and AFM, and computational techniques for simulating tip-surface interactions has enabled the systematic investigation of interfacial problems on ever smaller scales, as well as created means for modifying and manipulating nanostructures. In short, they have led to the appearance of the new, interdisciplinary fields of micro/nanotribology and micro/nanomechanics. This volume serves as a timely, practical introduction to the principles of nanotribology and nanomechanics and applications to magnetic storage systems and MEMS/NEMS. Assuming some familiarity with macrotribology/mechanics, the book comprises chapters by internationally recognized experts, who integrate knowledge of the field from the mechanics and materials-science perspectives. They cover key measurement techniques, their applications, and theoretical modelling of interfaces, each beginning their contributions with macro- and progressing to microconcepts. After reviewing the fundamental experimental and theoretical aspects in the first part, Nanotribology and Nanomechanics then treats applications. Three groups of readers are likely to find this text valuable: graduate students, research workers, and practicing engineers. It can serve as the basis for a comprehensive, one- or two-semester course in scanning probe microscopy; applied scanning probe techniques; or nanotribology/nanomechanics/nanotechnology, in departments such as mechanical engineering, materials science, and applied physics. With a Foreword by Physics Nobel Laureate Gerd Binnig Dr. Bharat Bhushan is an Ohio Eminent Scholar and The Howard D. Winbigler Professor in the Department of Mechanical Engineering, Graduate Research Faculty Advisor in the Department of Materials Science and Engineering, and the Director of the Nanotribology Laboratory for Information Storage & MEMS/NEMS (NLIM) at the Ohio State University, Columbus, Ohio. He is an internationally recognized expert of tribology and mechanics on the macro- to nanoscales, and is one of the most prolific authors. He is considered by some a pioneer of the tribology and mechanics of magnetic storage devices and a leading researcher in the fields of nanotribology and nanomechanics using scanning probe microscopy and applications to micro/nanotechnology. He is the recipient of various international fellowships including the Alexander von Humboldt Research Prize for Senior Scientists, Max Planck Foundation Research Award for Outstanding Foreign Scientists, and the Fulbright Senior Scholar Award.
Author: Eric Verne Eason
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
The adhesive pads on gecko toes are complex systems containing structures at different size scales. Each toe is covered in flaps of skin called lamellae, which are in turn covered in arrays of microscopic hair-like structures known as setae. The tip of each seta splits into hundreds of even smaller nanoscale structures (spatulae) which produce adhesion through intermolecular van der Waals forces. Using this adhesive system, geckos can stick to a wide range of surfaces. One of the most interesting properties of gecko adhesive is controllable adhesion. An adhesive is called controllable if the stickiness can be switched on or off so it can be easily and repeatedly attached and detached. In gecko adhesive, the adhesion is controlled by the shear force: geckos can control their adhesive simply by applying a downwards shear force to their toes. In previous work, a controllable synthetic adhesive was developed that used shear force to control the adhesion similarly to gecko adhesive. The synthetic adhesive consisted of wedge-shaped microstructures made of polydimethylsiloxane (PDMS) silicone rubber, known as microwedges. This thesis presents a new micromachining manufacturing process for microwedge adhesives, which produces stronger adhesives with more varied geometries, enabling practical applications such as grasping and climbing devices for robots and humans. In addition, this thesis investigates the distribution of adhesive stress in natural gecko adhesive and synthetic microwedge adhesive through a combination of experimental measurements and theoretical modeling. In order for an adhesive system to produce the maximum possible adhesive force, the force must be uniformly distributed over the adhesive area. However, until now it was unknown how forces are distributed in gecko adhesive. To address this question and gain understanding of the gecko's adhesive system, the stress distribution over the toes of a live tokay gecko (Gekko gecko) was measured using a custom optical tactile sensor with 100 micrometer spatial resolution based on frustrated total internal reflection (FTIR). Additionally, the stress distribution in the synthetic microwedge adhesive is investigated with a theoretical model that describes the elastic deformation and adhesive interactions of adhesive microstructures. Adhesion is modeled using a cohesive zone model, where the normal and tangential forces generated along the side of the microwedge depend on the separation distance between the microwedge and the surface. Deformation is modeled using a geometrically exact beam model, where the microwedge is treated as a tapered beam undergoing bending, axial, and shear deformation. This modeling approach accurately reproduces the limit curve in force space of microwedge adhesive, describing the relationship between normal and shear force that gives rise to controllable adhesion. In both the tokay gecko toe and the synthetic adhesive, the stress distributions were found to be nonuniform. In the gecko, the normal stress varied significantly at the lamella scale, with compressive stresses observed in some areas even though the net stress over the toe was tensile. Likewise, the model predicts that the normal stress on an adhesive microwedge varies from tensile to compressive along the adhesive interface, with a net stress that is several times smaller than the maximum stress. If the stresses were distributed uniformly, both systems would be capable of supporting much larger loads (around 20 times larger for tokay gecko toes and 5 times larger for microwedges). The proposed model may be useful in evaluating new microwedge structures with modified geometry in order to design a structure that distributes stress more uniformly. Along with the capabilities of the new micromachining process, this could lead to the development of stronger controllable adhesives.
Author: Andrew M. Smith
Publisher: Springer Science & Business Media
ISBN: 3540310495
Category : Science
Languages : en
Pages : 296
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Book Description
Many creatures use adhesive polymers and structures to attach to inert substrates, to each other, or to other organisms. This is the first major review that brings together research on many of the well-known biological adhesives dealing with bacteria, fungi, algae, and marine and terrestrial animals. As we learn more about their molecular and mechanical properties we begin to understand why they adhere so well and with this comes broad applications in areas such as medicine, dentistry, and biotechnology.
Author: Alyssa Yeager Stark
Publisher:
ISBN:
Category : Adhesion
Languages : en
Pages : 143
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Book Description
The gecko adhesive system is a dry, reversible adhesive that is virtually surface-insensitive due to the utilization of intermolecular van der Waals forces. Remarkably, although detailed models of the adhesive mechanism exist and hundreds of gecko-inspired synthetics have been fabricated, our ability to fully replicate the system still falls short. One reason for this is our limited understanding of how the system performs in natural environments. To begin to resolve this I focused on one particular environmental parameter, water. Although thin layers of water can disrupt van der Waals forces, I hypothesized that geckos are able to retain or regain adhesive function on wet surfaces. I was motivated to investigate this hypothesis because many species of gecko are native to the tropics, a climate where we expect surface water to be prevalent, thus it is likely geckos have some mechanism to overcome the challenges associated with surface water and wetting. Despite the challenge water should pose to adhesion, I found that when tested on hydrophobic substrates geckos cling equally well in air and water. Conversely, on wet hydrophilic substrates geckos cannot support their body weight. Investigating these results further, I found that the superhydrophobic nature of the adhesive toe pads allows geckos to form an air bubble around their foot, which when pressed into contact with a hydrophobic substrate likely removes water from the adhesive interface. When the toe pads are no longer superhydrophobic however, geckos cannot support their body weight and fall from substrates. In order to regain adhesion geckos only need to take about ten steps on a dry substrate to self-dry their toe pads. Finally, when measuring a dynamic component of adhesion, running, we found that geckos are able to maintain speed on misted hydrophobic and hydrophilic substrates, contrary to what we would predict based on static shear adhesion measurements. In conclusion, my research provides a detailed investigation of how water affects the gecko adhesive system and has applications for synthetic design of adhesives which retain or regain function in water and further motivates the study of this remarkable system in a more environmentally relevant context.
Author: Yasong Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 156
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Book Description
Geckos are well known for being rapid climbers that have long existed in nature. The reversible and reusable adhesive on their feet intrigues scientists to explore a bio-mimetic adhesive, which inherits the adhesion properties of the gecko's adhesives. Recent advances in electron microscopy reveal the secret of gecko's climbing ability: there are hierarchical fibrillar structures branching from the skin of their climbing feet. Sizes of these hierarchical fibrils range from micrometer to nanometer. These fibrils are arranged to closely resemble a tree, and these tree like structures form a fibril forest on the skin of the climbing feet. Nano-fibrils in close proximity with the contacting surfaces interact with the substrate through intermolecular forces. Slender micro-fibrils extend the nano-fibrils, which are located at their open ends, to reach recesses of the contacting surfaces. The special arrangement of the fibrillar arrays enables quick attachment and detachment of the feet from surfaces of different materials and varying roughness. Inspired by the gecko's adhesive, artificial fibrillar adhesives have been sought developing for more than a decade. Early attempts were focused on making use of the intermolecular interaction by nano-fibrillar arrays. These artificial fibrillar adhesives have achieved great performance on flat surfaces but not as good when they were used on relatively rough surfaces. Recent attempts of preparing a hierarchical fibrillar structure, which contains fibrils in different length scales, have rare success on improving adhesion performance. Evidence of extra compliancy provided by the hierarchical structure is also not clear. This thesis provides evidence that there is a correlation between structure compliancy and adhesion performance of a hierarchical fibrillar adhesive. Improved compliancy and adhesion forces are observed on a hierarchical fibrillar structure with achievements of several milestones, which include developing methods for preparing and characterizing hierarchical fibrillar structures. Experimental results also reveal the interaction of fibrillar arrays with the contacting surfaces. Information obtained is valuable for future development and application of such artificial fibrillar adhesive.
Author: Bharat Bhushan
Publisher: Springer
ISBN: 3319282840
Category : Science
Languages : en
Pages : 607
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Book Description
This revised, updated and expanded new edition presents an overview of biomimetics and biologically inspired structured surfaces. It deals with various examples of biomimetics which include surfaces with roughness-induced superomniphobicity, self-cleaning, antifouling, and controlled adhesion. The focus in the book is on the Lotus Effect, Salvinia Effect, Rose Petal Effect, Oleophobic/philic Surfaces, Shark Skin Effect, and Gecko Adhesion. This new edition also contains new chapters on the butterfly wing effect, bio- and inorganic fouling and structure and Properties of Nacre and structural coloration.
