Dry Adhesives Research Articles

Tribological properties of vertically aligned carbon nanotube arrays

Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction μ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of μ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect μ after a preconditioning by a few sliding cycles. However, a strong decrease of μ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.

Adhesive interactions of geckos with wet and dry fluoropolymer substrates.

Fluorinated substrates like Teflon® (poly(tetrafluoroethylene); PTFE) are well known for their role in creating non-stick surfaces. We showed previously that even geckos, which can stick to most surfaces under a wide variety of conditions, slip on PTFE. Surprisingly, however, geckos can stick reasonably well to PTFE if it is wet. In an effort to explain this effect, we have turned our attention to the role of substrate surface energy and roughness when shear adhesion occurs in media other than air. In this study, we removed the roughness component inherent to commercially available PTFE and tested geckos on relatively smooth wet and dry fluoropolymer substrates. We found that roughness had very little effect on shear adhesion in air or in water and that the level of fluorination was most important for shear adhesion, particularly in air. Surface energy calculations of the two fluorinated substrates and one control substrate using the Tabor-Winterton approximation and the Young-Dupré equation were used to determine the interfacial energy of the substrates. Using these interfacial energies we estimated the ratio of wet and dry normal adhesion for geckos clinging to the three substrates. Consistent with the results for rough PTFE, our predictions show a qualitative trend in shear adhesion based on fluorination, and the quantitative experimental differences highlight the unusually low shear adhesion of geckos on dry smooth fluorinated substrates, which is not captured by surface energy calculations. Our work has implications for bioinspired design of synthetics that can preferentially stick in water but not in air.

Three-dimensional dynamic surface grasping with dry adhesion

Most robotic grasping research focuses on objects that are either not large in comparison to the gripper or have small graspable features; however, there are important applications that involve large flat or gently curved surfaces. Examples include robots that grasp the solar panels of space craft, handle large panels in manufacturing, or climb or perch on surfaces. We present a solution for grasping such surfaces consisting of groups of tiles coated with a controllable gecko-inspired adhesive. The tiles are loaded with two sets of tendons: one for distributing the forces evenly while grasping and the other for release. The gripper is passive and can attach and detach with little effort so that it does not disturb either the robot or the object to be grasped. The maximum gripping force in the normal direction can be over 1000 times greater than the required detaching force. The gripper is also fast, allowing a flying quadrotor to attach to a surface milliseconds after the tiles make contact. We present a model of the gripping mechanism and use the model to design the layout of the tiles to best support anticipated normal and tangential loads.

Electrokinetics of scalable, electric-field-assisted fabrication of vertically aligned carbon-nanotube/polymer composites

Composite thin films incorporating vertically aligned carbon nanotubes (VACNTs) offer promise for a variety of applications where the vertical alignment of the CNTs is critical to meet performance requirements, e.g., highly permeable membranes, thermal interfaces, dry adhesives, and films with anisotropic electrical conductivity. However, current VACNT fabrication techniques are complex and difficult to scale up. Here, we describe a solution-based, electric-field-assisted approach as a cost-effective and scalable method to produce large-area VACNT composites. Multiwall-carbon nanotubes are dispersed in a polymeric matrix, aligned with an alternating-current (AC) electric field, and electrophoretically concentrated to one side of the thin film with a direct-current (DC) component to the electric field. This approach enables the fabrication of highly concentrated, individually aligned nanotube composites from suspensions of very dilute (ϕ=4×10−4) volume fraction. We experimentally investigate the basic electrokinetics of nanotube alignment under AC electric fields, and show that simple models can adequately predict the rate and degree of nanotube alignment using classical expressions for the induced dipole moment, hydrodynamic drag, and the effects of Brownian motion. The composite AC + DC field also introduces complex fluid motion associated with AC electro-osmosis and the electrochemistry of the fluid/electrode interface. We experimentally probe the electric-field parameters behind these electrokinetic phenomena, and demonstrate, with suitable choices of processing parameters, the ability to scalably produce large-area composites containing VACNTs at number densities up to 1010 nanotubes/cm2. This VACNT number density exceeds that of previous electric-field-fabricated composites by an order of magnitude, and the surface-area coverage of the 40 nm VACNTs is comparable to that of chemical-vapor-deposition-grown arrays of smaller-diameter nanotubes.

Cephalopod-Inspired Miniaturized Suction Cups for Smart Medical Skin.

Biomimetic miniaturized suction cups (mSCs) are designed for the patient friendly, dry adhesives of smart medical skin. Both strong van der Waals force and induced negative pressure by the ultrasoft mSCs facilitate tight skin coupling without discomfort or irritations, improve sensitivities of the embedded stretchable electronics for continuous vital sign monitoring, and enable multiple drug reloading without loss of the adhesion. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Contributions of adhesion and hysteresis to coefficient of friction between shoe and floor surfaces: effects of floor roughness and sliding speed

Improving shoe–floor friction in order to reduce slip and fall accidents requires thorough understanding of the factors that contribute to friction. The friction between a sliding viscoelastic material (shoe) and a hard surface (floor) has two major components: adhesion and hysteresis. This study aimed to quantify the effects of floor roughness and sliding speed on adhesion and hysteresis to determine how each component contributes to the coefficient of friction. Experiments were conducted on a pin on disc tribometer using ceramic tiles with three levels of roughness, six sliding speeds, two common shoe materials and four liquid lubricants. Hysteresis was measured using a lubricant that minimised adhesion. Dry and lubricated adhesion was measured by subtracting hysteresis from the coefficient of friction. Analysis of variance regression models were used to determine the contributions of hysteresis, dry adhesion, sliding speed and fluid to lubricated coefficient of friction. Increased floor roughness led to increased hysteresis, while increased sliding speed reduced both adhesion and hysteresis. These findings are consistent with theory that states that larger asperities increase hysteretic deformation and that sliding speed affects deformation and real area of contact between a viscoelastic material and a hard surface. The model correctly predicted 83% of variation in coefficient of friction based on dry adhesion, hysteresis and fluid dependent constants. The sensitivity of hysteresis friction to shoe material and floor roughness indicates that optimising these parameters may be effective at reducing slip accidents on oily floor surfaces.

Rectangle-capped and tilted micropillar array for enhanced anisotropic anti-shearing in biomimetic adhesion.

Dry adhesion observed in the feet of various small creatures has attracted considerable attention owing to the unique advantages such as self-cleaning, adaptability to rough surfaces along with repeatable and reversible adhesiveness. Among these advantages, for practical applications, proper detachability is critical for dry adhesives with artificial microstructures. In this study, we present a microstructured array consisting of both asymmetric rectangle-capped tip and tilted shafts, which produce an orthogonal anisotropy of the shearing strength along the long and short dimensions of the tip, with a maximum anti-shearing in the two directions along the longer dimension. Meanwhile, the tilt feature can enhance anisotropic shearing adhesion by increasing shearing strength in the forward shearing direction and decreasing strength in the reverse shearing direction along the short dimension of the tip, leading to a minimum anti-shearing in only one of the two directions along the shorter dimension of the rectangular tip. Such a microstructured adhesive with only one weak shearing direction, leading to well-controlled attachment and detachment of the adhesive, is created in our experiment by conventional double-sided exposure of a photoresist followed by a moulding process.

Wet adhesion of graphene

Interfacial adhesion between graphene and various substrate materials is essential for practical applications of graphene. To date, most of the studies on adhesion of graphene have assumed dry adhesion of van der Waals type. In this paper, we conduct molecular dynamics simulations to study the traction–separation behaviors for wet adhesion of graphene on amorphous silicon oxide covered by a thin layer of water. Three stages of the traction–separation relations are identified and they are analyzed by simple, approximate continuum models. The work of separation is found to be close to the theoretical value dictated by the interaction potential between graphene and water. The maximum traction is found to be set by the critical stress for cavitation at the water/graphene interface. With morphological evolution of water from cavitation to capillary bridging, the range of interaction extends to about 3 nm before complete separation of graphene. Compared to van der Waals interactions for dry adhesion of graphene, the work of separation for wet adhesion is smaller, the maximum traction is lower, but the interaction range is longer. It is noted that the properties of wet adhesion depend sensitively on the graphene–water interactions, which may vary considerably from hydrophobic to hydrophilic interactions.

Temperature-Induced Switchable Adhesion using Nickel-Titanium-Polydimethylsiloxane Hybrid Surfaces.

A switchable dry adhesive based on a nickel–titanium (NiTi) shape-memory alloy with an adhesive silicone rubber surface has been developed. Although several studies investigate micropatterned, bioinspired adhesive surfaces, very few focus on reversible adhesion. The system here is based on the indentation-induced two-way shape-memory effect in NiTi alloys. NiTi is trained by mechanical deformation through indentation and grinding to elicit a temperature-induced switchable topography with protrusions at high temperature and a flat surface at low temperature. The trained surfaces are coated with either a smooth or a patterned adhesive polydimethylsiloxane (PDMS) layer, resulting in a temperature-induced switchable surface, used for dry adhesion. Adhesion tests show that the temperature-induced topographical change of the NiTi influences the adhesive performance of the hybrid system. For samples with a smooth PDMS layer the transition from flat to structured state reduces adhesion by 56%, and for samples with a micropatterned PDMS layer adhesion is switchable by nearly 100%. Both hybrid systems reveal strong reversibility related to the NiTi martensitic phase transformation, allowing repeated switching between an adhesive and a nonadhesive state. These effects have been discussed in terms of reversible changes in contact area and varying tilt angles of the pillars with respect to the substrate surface.

Nanofibrous Adhesion: The Twin of Gecko Adhesion

Inspired by dusty spider dragline silk, we studied the adhesive interaction between artificial nanofibers and their aerosol surroundings. The nanofibers are found to be able to actively capture particulate matters from the environment, exactly as the spider dragline silk does. Examinations prove that such nanofibrous adhesion is insensitive to the chemical nature of the fibers and the physical states of the particulate matter and depends only on the fiber diameters. Such facts indicate that nanofibrous adhesion is a case of dry adhesion, mainly governed by van der Waals force, sharing the same mechanism to gecko adhesion. Nanofibrous adhesion is of great importance and has promising potential. For instance, in this work, nanofibers are fabricated into a thin and translucent filter, which has a filtration performance, as high as 95%, that easily outperformed ordinary ones. We believe that this adhesive property of nanofibers will open up broader applications in both scientific and industrial fields.

干黏附碳纳米管垂直阵列的转移及其黏附性能评价

Vertically aligned carbon nanotube arrays(VACNTs) exhibit high aspect-ratios and excellent mechanical strength, and have attracted attention in the field of gecko-inspired dry adhesives. However, the poor interfacial bonding between the silicon substrate and the VACNTs, the hard and brittle silicon substrate which is suitable for the growth of VACNTs in chemical vapor deposition(CVD) technique limit the development and application of VACNTs as the gecko-inspired dry adhesive. To overcome these limitations we present a simple and cost-effective heating transfer method without other media-assisted. We combine this with thermal oxidation detachment to transfer the grown VACNTs onto flexible polycarbonate(PC) substrates. The transfer process provides two different VACNTs morphologies; a curly entangled segment in the bottom and an aligning end segment in the top. The bottom transfer maintains the top morphology of the pristine VACNTs, which differs from the flip-over process known as top transfer. This transfer process results in a double transferred VACNTs structure which is a new type of dry adhesive. By optimizing the transfer process, the interfacial bonding force between the VACNTs and PC substrates is nearly three times stronger than that of grown VACNTs on silicon substrates at 250°C. The shear adhesion strengths of the bottom- and top-transferred VACNTs were tested and analyzed. Then, the shear adhesion strength of the transferred VACNTs were tested and analyzed according to the difference between the morphology and the degree of graphitization on the top of VACNTs after the top/bottom transfer methods. The shear adhesion strength of the transferred VACNTs increased with the normal preload. The adhesion strength of the bottom-transferred VACNTs was approximately 5 N/cm2, whereas that of the top-transferred VACNTs was 2.8 N/cm2 under a 16 N/cm2 normal preload force. The bottom-transferred VACNTs showed better adhesion because of the increasing contact area and higher degree of graphitization on the top of the structure according to scanning electron microscope and Raman spectra. Our transfer method is nondestructive, fast and suitable for VACNTs with a height greater than 200 um. Moreover, this method is not restricted by the area of the VACNTs, and thus can improve their integrity and flexibility, and increase their application as gecko-inspired dry adhesives.

Adhesion and friction of an isolated gecko setal array: The effects of substrates and relative humidity

Abstract Geckos can climb steadily and quickly on different surfaces under various environmental conditions. Extensive experiments have been conducted using gecko toe, setal array, single seta and single spatula to explore the physical principles of gecko adhesion and friction. This work focuses on the effect of environment (i.e. substrate materials and relative humidity [RH]) and experimental conditions (i.e. preload, sliding velocity, and sliding direction) on gecko adhesion and friction through an isolated gecko setal array. Experimental results show that when sliding a setal array in gripping direction, adhesion and friction can be enhanced by more than 200% by increasing surface energy, 60% by increasing relative humidity, and only slightly by increasing sliding velocity; an appropriate preload increase can also cause an enhancement. When RH >40%, adhesion and friction become saturated, and higher sliding velocity and preload can realize the saturation at a lower RH. When a preload is oversized in gripping direction or when sliding a setal array against substrates in releasing direction, the setal array deforms severely and always exhibits a preload-dominated repulsion. A strong anisotropic property is revealed when sliding a setal array in gripping and releasing direction. These results help understand the remarkable abilities of gecko swiftly controlling strong attachment and easy detachment, which inspire the design and fabrication of gecko-inspired reversible dry adhesives.

Nanofibers: Clumping Criteria of Vertical Nanofibers on Surfaces (Adv. Mater. Interfaces 5/2015)

There are numerous nanofiber based structures and applications, such as in soft lithographic stamps, bio-inspired dry adhesives, ultraviolet nanolasers, solar cells, and micro-/nano-sensors. The clumping behavior of nanofibers is important for those designs. In article 1400466, Y. Tian and co-workers develop a universal model that can predict the sequences of side-side, side-tip, and tip-tip clumping of nanofibers by considering the adhesion and the bending restoring force.

Adhesion properties of camelina protein fractions isolated with different methods

The objective of this research was to investigate the effects of protein extraction methods on the adhesion performance of different camelina protein fractions. Physicochemical properties were also studied, including the electrophoresis profile; rheological, thermal, and morphological properties; and crystallization. Two camelina protein fractions, globulin and glutelins, were isolated from defatted camelina meal using three different methods resulting in total of six protein fractions including globulin 0–2 and glutelin 0–2. Dry adhesion strength of camelina protein adhesives exhibited nearly 100% wood cohesive failure at the curing temperatures of 150–190°C, except glutelin 2 and globulin 0. The overall adhesion performance of globulin fraction behaved better than glutelin fraction. The greatest wet shear strength of globulin 1 and 2 was around 3.3MPa, curing at 190°C. The wet shear strength of glutelin 2 was inferior to glutelin 1 due to the negative effects of NaCl. Glutelin had higher protein aggregation than globulin, as indicated by higher crystallinity, higher thermal stability, and dense protein aggregation. The compact structure of glutelins may partially contribute to their lower adhesion strength.

Adhesion of a carbon nanotube array and its polydimethylsiloxane composite in vacuum and thermal vacuum

Developing dry adhesion technologies for space environments will greatly enhance the manned spaceflight, on-orbit servicing, and operational capability of non-cooperative targets. The complexity of space conditions requires strict standards for the performance of adhesives in aerospace devices. In this study, vacuum and thermal vacuum environments are simulated for testing the frictional attachment-detachment properties and adhesion strength of a carbon nanotube (CNT) array and polydimethylsiloxane (PDMS)/CNT array composite. In vacuum, the frictional attachment-detachment behavior of both CNT array and PDMS/CNT array composite shows no obvious differences from that in atmospheric environment. The frictional adhesion strength of the composite is improved in vacuum, whereas that of the CNT array remains unchanged compared with those in atmospheric environment. Both materials exhibit good adhesion stability and reusability in vacuum and atmospheric environment. In thermal vacuum, the normal and shear adhesion strength are maintained for the CNT array and the composite. In vacuum and thermal vacuum, the CNT array and its composite retain effective adhesion properties, and could form the basis of a CNT dry adhesive for space application.

Human climbing with efficiently scaled gecko-inspired dry adhesives.

Since the discovery of the mechanism of adhesion in geckos, many synthetic dry adhesives have been developed with desirable gecko-like properties such as reusability, directionality, self-cleaning ability, rough surface adhesion and high adhesive stress. However, fully exploiting these adhesives in practical applications at different length scales requires efficient scaling (i.e. with little loss in adhesion as area grows). Just as natural gecko adhesives have been used as a benchmark for synthetic materials, so can gecko adhesion systems provide a baseline for scaling efficiency. In the tokay gecko (Gekko gecko), a scaling power law has been reported relating the maximum shear stress σmax to the area A: σmax ∝ A(-1/4). We present a mechanical concept which improves upon the gecko's non-uniform load-sharing and results in a nearly even load distribution over multiple patches of gecko-inspired adhesive. We created a synthetic adhesion system incorporating this concept which shows efficient scaling across four orders of magnitude of area, yielding an improved scaling power law: σmax ∝ A(-1/50). Furthermore, we found that the synthetic adhesion system does not fail catastrophically when a simulated failure is induced on a portion of the adhesive. In a practical demonstration, the synthetic adhesion system enabled a 70 kg human to climb vertical glass with 140 cm(2) of adhesive per hand.

Effect of Wood Preservatives on Surface Properties of Coated Wood

Effect of wood preservatives (waterborne and organicborne) on the performance of surface finishing properties is investigated. Sapwood of scots pine, (Pinus sylvestrisL.), oriental beech (Fagus orientalisLipsky), and chestnut (Castanea sativaMill.) specimens (300 × 100 × 15 mm along the grain) were impregnated with aqueous solution of 2% CCA, 2% Tanalith E, 1% boric acid, and Immersol aqua. Surface roughness, dry film thickness, adhesion strength, gloss measurement, scratch, and abrasion resistance were determined according to related standards for treated and untreated samples. The results indicated that surface roughness and adhesion strength depended on wood species and the chemical composition of preservatives. Generally, waterborne wood preservatives increased the surface roughness of wood while the organic-based wood preservatives decreased it. The organic-based wood preservatives decreased adhesion but they increased gloss value. Wood preservatives did not affect the scratch resistance which was found to depend on properties of the coating. All the wood preservatives increased abrasion resistance.

Gecko gaskets for self-sealing and high-strength reversible bonding of microfluidics.

We report in this work a novel reversible bonding technique for elastomeric microfluidic devices by integrating gecko-inspired dry adhesives with microfluidic channels which greatly enhances the bonding strength of reversibly sealed channels. The concept is applicable to nearly any elastomer and can be used to bond against any smooth surface which allows for van der Waals interactions. It does not require any solvents or glues or sources for plasma activation or thermal-compressive loading to aid the bonding process and is achievable at zero extra cost. We also demonstrate a quick fabrication technique involving soft master thermo-compressive molding of these microfluidic devices with thermoplastic elastomers. The resultant devices can be used for both pressure driven and non-pressure driven flows. We report the maximum contained pressure of these devices manufactured from two grades of styrene ethylene butylene styrene (SEBS) by conducting a burst pressure test with various substrates.

Enhanced adhesion of bioinspired nanopatterned elastomers via colloidal surface assembly.

We describe a scalable method to fabricate nanopatterned bioinspired dry adhesives using colloidal lithography. Close-packed monolayers of polystyrene particles were formed at the air/water interface, on which polydimethylsiloxane (PDMS) was applied. The order of the colloidal monolayer and the immersion depth of the particles were tuned by altering the pH and ionic strength of the water. Initially, PDMS completely wetted the air/water interface outside the monolayer, thereby compressing the monolayer as in a Langmuir trough; further application of PDMS subsequently covered the colloidal monolayers. PDMS curing and particle extraction resulted in elastomers patterned with nanodimples. Adhesion and friction of these nanopatterned surfaces with varying dimple depth were studied using a spherical probe as a counter-surface. Compared with smooth surfaces, adhesion of nanopatterned surfaces was enhanced, which is attributed to an energy-dissipating mechanism during pull-off. All nanopatterned surfaces showed a significant decrease in friction compared with smooth surfaces.

A state-of-the-art review and analysis on the design of dry adhesion materials for applications such as climbing micro-robots

This article highlights the design considerations for the development of robust and durable bio-inspired synthetic adhesives.