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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: 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: Melvin Alejandro Ramos
Publisher:
ISBN:
Category : Bio-inspired
Languages : en
Pages : 0
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Book Description
Geckos can scale verticals walls and even hang upside down on ceilings with ease. Their remarkable ability to adhere to a variety of surface conditions stems from their uniquely designed toepad structure. The anisotropic and hierarchical fibrillar design enables a high adhesion to preloading ratio, strong adhesion regardless of surface composition and roughness, self-cleaning, non-sticky at default sate, and anti-self-matting. These attributes and characteristics have stimulated the development of a new class of gecko-inspired synthetic adhesives (GSAs) over the past two decades. The objective of this project is to test the hypothesis that the ultimate contact shape and geometry of fibrillar adhesives play a crucial role to enhance the robustness and equal load sharing when the adhesive fibrils are engaged. Easy-detachment and self-cleaning processes may also be facilitated during the adhesive removal. Our approach to verify this hypothesis rest on a discretized cohesive zone finite element modeling and simulation of the adhesive interactions with imaginary target surfaces. This work is the first step to establish a holistic simulation scheme and parameterization, helping to bridge the gap between the microscopic structures and continuum level performances. Bi-linear traction-separation laws were employed in both normal and shear directions without implementing coupling or mixed mode criteria. Experimental results are obtained from our own research and the literatures. A regression analysis of the samples subjected a normal force has shown that the maximum stress correlates to increasing cohesive strength (Tn0) with an R2=0.9379. Other parameters such as the damage initiation (Îþn0), cohesive stiffness (Enn), fracture energy (Gn), and displacement at failure (Îþnf) all play minor roles, if not negligible, in the maximum stress as evident by their R20.4. However, a parametric study during shear loading has suggested an increase in Îþn0, Tn0, Enn, Îþnf, and fracture energy contributes to an increase in the maximum stress with an R20.95 for nearly every simulation setup. Depending on which parameters are increased can see anywhere from a 7% to 339% increase in the resulting maximum stress. Further analysis of the crack propagation is required to understand the stress distribution during shear and evaluate each parameter individually, but this work acts the foundational work for future researchers to fully address the hypothesis.
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: Lars Heepe
Publisher: Springer
ISBN: 3319591142
Category : Technology & Engineering
Languages : en
Pages : 357
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Book Description
This book deals with the adhesion, friction and contact mechanics of living organisms. Further, it presents the remarkable adhesive abilities of the living organisms which inspired the design of novel micro- and nanostructured adhesives that can be used in various applications, such as climbing robots, reusable tapes, and biomedical bandages. The technologies for both the synthesis and construction of bio-inspired adhesive micro- and nanostructures, as well as their performance, are discussed in detail. Representatives of several animal groups, such as insects, spiders, tree frogs, and lizards, are able to walk on (and therefore attach to) tilted, vertical surfaces, and even ceilings in different environments. Studies have demonstrated that their highly specialized micro- and nanostructures, in combination with particular surface chemistries, are responsible for this impressive and reversible adhesion. These structures can maximize the formation of large effective contact areas on surfaces of varying roughness and chemical composition under different environmental conditions.
Author: Cem Balda Dayan
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Bioinspired fibrillar structures have been promising for various disruptive adhesive applications. Especially micro/nanofibrillar structures on gecko toes can have strong and controllable adhesion and friction on a wide range of surfaces with residual-free, repeatable, self-cleaning, and other unique features. Also, in some environmental conditions (e.g., relative humidity, temperature), their adhesion performance increases according to literature. These findings can be integrated to design high-performance synthetic structural adhesives such as composite-based synthetic gecko-inspired adhesives. Additionally, there are some debates and theories about the reason for the increase of gecko adhesion in different environmental conditions. The related theories can be examined by studying them systematically. This investigation requires live geckos' and gecko-inspired synthetic adhesives' performance comparison in various environmental conditions. These findings can explore why adhesion increases and helps to design high-performance synthetic structural adhesives. Moreover, gecko-inspired synthetic adhesives' adhesion performance highly depends on their fabrication method. Due to fabrication limitations, the desired complex fibril designs sometimes cannot be fabricated. Advanced fabrication techniques can be integrated to minimize fabrication limitations and fabricate the desired designs almost freely. As a result, a two-photon-lithography-based three-dimensional printing technique can be used with an elastomeric material to manufacture more advanced free-body design fibrils. After all these findings, we can try to explore the outperformance of optimal designs for gecko-inspired synthetic adhesives. Previously, synthetic dry fibrillar adhesives inspired by such biological fibrils have been optimized in different approaches to increase their performance. Previous fibril designs for shear optimization are limited by pre-defined standard shapes in a narrow range primarily based on human intuition, which restricts their maximum performance. In this aspect, we can combine the Bayesian optimization and finite-element-method-based shear mechanics simulations to find shear-optimized fibril designs automatically. In addition, fabrication limitations can be integrated into the simulations to have more experimentally relevant results. The computationally discovered shear-optimized structures are fabricated, experimentally validated, and compared with the simulations. Both experimental and simulation results show that the shear-optimized fibrils perform better than the pre-defined standard fibril designs. This design optimization method can be used in future real-world shear-based gripping or non-slip surface applications, such as robotic pick-and-place grippers, climbing robots, gloves, electronic devices, and medical and wearable devices.
Author: Zhilong Peng
Publisher: Springer
ISBN: 3662469553
Category : Technology & Engineering
Languages : en
Pages : 107
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Book Description
The thesis systematically investigates the factors which influence many animals’ robust adhesion abilities and micro-reversible adhesion mechanisms, including the geometric principles of their adhesion, relative humidity, surface roughness and pre-tension. Studies exploring biological adhesion mechanisms are not only of great significance for the design of advanced adhesive materials and adhesion systems for micro-climbing robots, but also very helpful for resolving the problem of adhesion failure in MEMS/NEMS.
Author: Yongkwan Kim
Publisher:
ISBN:
Category :
Languages : en
Pages : 118
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Book Description
Over the last decade, geckos' remarkable ability to stick to and climb surfaces found in nature has motivated a wide range of scientific interest in engineering gecko-mimetic surface for various adhesive and high friction applications. The high adhesion and friction of its pads have been attributed to a complex array of hairy structures, which maximize surface area for van der Waals interaction between the toes and the counter-surface. While advances in micro- and nanolithography technique have allowed fabrication of increasingly sophisticated gecko mimetic surfaces, it remains a challenge to produce an adhesive as robust as that of the natural gecko pads. In order to rationally design gecko adhesives, understanding the contact behavior of fibrillar interface is critical. The first chapter of the dissertation introduces gecko adhesion and its potential applications, followed by a brief survey of gecko-inspired adhesives. Challenges that limit the performance of the current adhesives are presented. In particular, it is pointed out that almost all testing of gecko adhesives have been on clean, smooth glass, which is ideal for adhesion due to high surface energy and low roughness. Surfaces in application are more difficult to stick to, so the understanding of failure modes in low energy and rough surfaces is important. The second chapter presents a fabrication method for thermoplastic gecko adhesive to be used for a detailed study of fibrillar interfaces. Low-density polyethylene nanofibers are replicated from a silicon nanowire array fabricated by colloidal lithography and metal-catalyzed chemical etching. This process yields a highly ordered array of nanofibers over a large area with control over fiber diameter, length, and number density. The high yield and consistency of the process make it ideal for a systematic study on factors that affect adhesion and friction of gecko adhesives. The following three chapters examine parameters that affect macroscale friction of fibrillar adhesives. Basic geometric factors, namely fiber length and diameter, are optimized on smooth glass for high friction. The test surfaces are then processed to intentionally introduce roughness or lower the surface energy in a systematic and quantifiable manner, so that the failure mechanisms of the adhesive can be investigated in detail. In these studies, observed macroscale friction is related to the nano-scale contact behavior with simple mechanical models to establish criteria to ensure high performance of fibrillar adhesives. Chapter 6 presents various methods to produce more complex fiber structures. The metal-assisted chemical etching of silicon nanowires is studied in detail, where the chemical composition of the etching bath can be varied to produce clumped, tapered, tilted, and curved nanowires, which provide interesting templates for molding and are potentially useful for applications in various silicon nanowire devices. Hierarchical fiber structures are fabricated by a few different methods, as well as composite structures where the fibers are embedded in another material. A way to precisely control tapering of microfibers is demonstrated, and the effect of tapering on macroscale friction is studied in detail. The final chapter summarizes the dissertation and suggests possible future works for both further investigating fibrillar interfaces and improving the current gecko adhesive.
Author: Noe Esparza
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Research into how the gecko lizard is able to climb a wide variety of surfaces has re- vealed an adhesive system that takes a fundamentally different approach than is found in conventional pressure-sensitive adhesives such as sticky tape. The gecko's adhesive system is composed of setal stalks, each thinner than a human hair and terminating in spatulae only 250 nm across. The entire hierarchical system is composed of beta- keratin, a tough, hydrophobic material, somewhat harder than the alpha-keratin of human fingernails. The geometry of the setae and spatulae allow them to conform to surfaces in a manner similar to very soft materials, but without the tendency of tacky materials to become fouled with dirt. Using the gecko adhesive system as inspiration, Biomimetics and Dexterous Ma- nipulation Laboratory developed an adhesive that is suitable for robotic climbing ap- plications. The smallest features of this adhesive are arrays of sharp wedges molded from silicone rubber. A tapered feature was pursued because it is capable of repro- ducing the "frictional adhesion" property of the gecko's adhesive system. Frictional- adhesion defines a behavior for which increasing the shear stress imposed at a contact increases the available adhesive stress perpendicular to the surface. A consequence of frictional adhesion is that one can control the amount of adhesion by controlling the applied shear load. In the present case, the behavior arises from the fact that sharp wedge-shaped features initially present very little area as they are brought into contact with a surface. However, they bend over when the array is loaded in shear, so that the contact area and the adhesion grow in proportion. This thesis seeks to understand how the details of the tapered wedge geometry, including the wedge profile and angle of inclination, influence the frictional adhesive behavior. The analysis includes a combination of numerical finite element modeling and empirical pull-off tests. The constraints on material stiffness, wedge geometry and spacing are also studied, as affected by possible failure modes such as self-sticking of adjacent wedges (leading to "clumping"). The desire to test wedges at various angles of inclination lead to the development of a new micro-machining process for creating molds for the wedge arrays. This process affords much greater freedom to control the wedge size and geometry than a previous lithographic process. However, a byproduct of the machining process is that the wedges have a non-negligible surface roughness on their contacting faces, which compromises their performance. Consequently, a new process was developed to improve the surface finish by "inking" the molded wedges, depositing a thin film of liquid silicone rubber onto their faces and providing a smoother surface. The resulting microwedges achieve more than double the maximum adhesion and several times the adhesion at low levels of shear than previous microwedges from molds created using the lithographic process. Although the microwedges stick well to smooth, flat surfaces such as glass, they cannot conform to surfaces with undulations higher than a couple of micrometers. In addition, the array of microwedges must be precisely aligned with surfaces so that all wedges are uniformly loaded. To mitigate these limitations, some approximation to the gecko's compliant hierarchy of lamellae, setae and spatulae is needed. The solution presented in this thesis is a two-layer hierarchical system in which the arrays of wedges are supported by a larger array of angled pillars. In between the pillars and wedges is a film of solid silicone rubber, which bridges the gaps between pillars and helps to create a relatively uniform loading of the wedges. A combination of numerical analysis and empirical pull-off tests is used to understand the relationships among pillar dimensions, pillar spacing and film thickness that govern the performance of this structure. At one extreme, the loading can become sufficiently non-uniform that some wedges lose contact with the surface, resulting in a loss of adhesion. At the other extreme, the structure is too stiff to accommodate surface undulations and misalignment. The thesis concludes with a summary of the results on wedges and hierarchical adhesive structures, and discusses the implications for future work.
Author: Robert A. Sayer
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Abstract: Several creatures including insects, spiders, and lizards, have developed a unique clinging ability that utilizes dry adhesion. Geckos, in particular, have developed the most complex adhesive structures capable of smart adhesion--the ability to cling on different smooth and rough surfaces and detach at will. These animals make use of on the order of a million microscale hairs (setae) (about 14000/mm2) that branch off into hundreds of nanoscale spatulae. This hierarchical surface construction gives the gecko the adaptability to create a large real area of contact with surfaces. van der Waals forces are the primary mechanism utilized to adhere to surfaces and capillary forces are a secondary effect that can further increase adhesive force. Although a gecko is capable of producing on the order of 20 N of adhesive force, it retains the ability to remove its feet from an attachment surface at will. A man-made fibrillar structure capable of replicating gecko adhesion has the potential for use in dry, superadhesive tapes that would be of use in a wide range of applications. These adhesives could be created using micro/nanofabrication techniques or self-assembly. A fibrillar polyvinylsiloxane (PVS) sample consisting of an array pillars (about 230/mm2) approximately 50 um in diameter, 70 um in height and 60 um center-to-center was compared to an unstructured sample. Structured roughness was found to be more important than random roughness in adhesion. The added roughness of the structured sample increased the hydrophobicity of the surface.
Author: Srinivasan Arul Suresh
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
The last 20 years have seen considerable interest in bioinspired dry adhesives, based on discoveries regarding the adhesive system of the gecko and some arthropods. Such adhesives typically have the advantage of being reusable, leaving no residue, and allowing control of the adhesion through loading states. However, the number of practical applications of these adhesives remains small. One possible reason is that unlike in mechanical design, where design, simulation, and testing methodologies are all well established, there are significant gaps in all of these phases of engineering as applied to gecko-inspired adhesives. There are a variety of methods and metrics used for evaluating adhesives, often giving differing results, and even in some cases results that do not accurately reflect those observed in practical applications. Even with an accurate evaluation of an adhesive material, refining the design is challenging, as the design and manufacturing methods are typically time-consuming, highly constraining, or both. At the same time, there continues to be growing interest in the use of these adhesives in wide-ranging applications including reusable tapes and bandages; improved and more gentle industrial grippers; and grasping objects in space, where the combination of large objects, low contact forces, and lack of atmosphere make adhesives of particular interest. To address this growing need for improved ability to design and manufacture adhesives tailored to these applications, a three-pronged approach is taken. An improved method for testing gecko-inspired adhesives is presented. Unlike the common testing paradigms published in the literature, which impose a fixed displacement between the adhesive material and a test surface, the proposed testing method uses a series elastic configuration to apply forces to the adhesive. This shift in test control from displacement-space to force-space allows the testing conditions to be aligned to those seen in applications; whether for climbing, grasping, or adhesive tapes, nearly all applications of gecko-inspired adhesives fundamentally involve force-space constraints in normal conditions. It is shown that by testing the adhesives in similar conditions to those observed in use, the measured limit curves better reflect those seen in practice. Further, in cases where the adhesive structures are more complicated, or more integral to the performance of the adhesive--such as the directional, controllable adhesives at the core of this work--force-space testing enables measuring the full capabilities of the adhesive, which in many cases are impossible to measure in displacement-space. With the ability to accurately measure more complex limit curves, spatial variation is investigated as a means to improve the ability to create adhesives with novel parameters. In this case, the property of interest is a high friction ratio, the ratio of friction in a preferred direction to friction in the opposite direction, a property of the natural gecko adhesive system. Taking inspiration from the spatial variation found on the gecko's feet, an adhesive structure with wedges of varying length is developed, modeled, and analyzed. The friction ratio of this adhesive is measured, indicating an improvement of orders of magnitude over the current state of the art. Further, this adhesive structure also demonstrates the possibility of simplifying the adhesive design problem. Rather than developing a single complex feature to provide all of the desired properties, spatial variation permits the development of multiple features that are individually simpler but interact to provide more complex behavior. A discussion of the manufacturing process and associated fabrication constraints for these designed adhesive geometries follows. The process is an extension of a previous manufacturing process developed for making uniform adhesives. This is coupled with methods for directly incorporating adhesives into larger assemblies to create tightly coupled adhesive and sensing systems. Finally, a simplified design framework is presented, synthesizing many of the concepts from the prior sections. The current state of the art in adhesive simulation and modeling, while useful for understanding and explaining various specific aspects of adhesive design, is not adequate for directly analyzing the adhesion of complex adhesive geometries. The framework is intended to be a heuristic that synthesizes concepts from the various models of adhesion to provide useful guidance for thinking about adhesive designs for particular applications.
