Dry Adhesives Research Articles

INVESTIGATION OF THE EFFECT OF MULTI-LAYER COATING ON CORROSION PREVENTION PERFORMANCE FORMED BY THE APPLICATION OF SOLVENT-BASED ZINC LAMEL COATING ON ALKALI ZINC AND ALLOY ZINC COATING

Bu çalışmada, alkali çinko, çinko-nikel ve çinko-demir alaşım kaplama prosesleri ile çözücü bazlı çinko lamelli kaplama prosesi yeniden düzenlenerek bir araya getirilmiş ve iki farklı kaplama prosesinin tekil üstün özellikleri birleştirilerek üstün korozyon önleme performansına sahip ürünlerin üretilmesine olanak sağlayan yeni bir proses geliştirilmiştir. Yürütülen deneysel çalışmalar sonucunda elde edilen ve görsel uygunluk kriterini sağlayan kaplamalar kuru yapışma, su direnci, nem direnci, tuz sisi, çevrimsel korozyon ve kimyasal direnç testlerine tabi tutulmuştur. Geliştirilen yeni proses, yüksek korozyon direncine (>1000 saat tuz sis testi ve >63 döngü döngüsel korozyon testi), yüksek sıcaklık direncine (300 °C'de bir saat), kimyasal dirence ve elektriksel yalıtkanlığa sahip parçaların üretilmesini olanak sağlamıştır. Testlerden elde edilen sonuçlar otomotiv endüstrisinin beklentileriyle kıyaslanmış ve beklentilerden daha üstün özellikler elde edilmiştir.

High Barrier Sustainable Paper Coating Based on Engineered Polysaccharides and Natural Rubber

The demand for paper-based packaging solutions as an alternative to incumbent single-use petroleum-based polymers for food packaging applications is ever-increasing. Paper-based packaging enables direct options for recycling and, in the case of inadvertent leakage, avoids the formation of microplastics through natural biodegradation. However, typical paper-based formats do not offer sufficient barrier performance which meets these sustainability objectives. Replacement of the plastic film with paper formats will reduce product shelf-stability and increase food wastage from farm to table. This work employed enzymatically polymerized microcrystalline glucan (MCG) as a functional additive in natural rubber (NR)-based coating formulations. Typically, NR coating formulations containing 0–50 wt % MCG were fabricated at a constant coating thickness with a constant solid content. The influence of the MCG on the wet and dry strength, rheology, adhesion strength, and barrier properties such as moisture, oxygen, and grease barrier of the formulated coatings was investigated. Also, further study on the effect of solid content and light crosslinking on the barrier properties was conducted. The optimized NR/MCG coating displayed promising mechanical properties as well as oxygen and oil barrier properties. Furthermore, light crosslinking of the NR/MCG coating enhances the barrier properties of the paper coating. Overall, the formulated NR/MCG paper coatings exhibited excellent wet strength in conjuncture with outstanding barrier properties, indicating their potential as a sustainable alternative in the food packaging industry.

Versatile Adhesion-Based Gripping via an Unstructured Variable Stiffness Membrane.

Reversible and variable dry adhesion is a promising approach for versatile robotic grasping. Variable stiffness materials with a modulus that can be tuned using an external stimulus offer a unique approach to realize dynamic control of adhesion. In this study, an unstructured shape memory polymer (SMP) membrane with variable stiffness is used to pick-and-place three-dimensional objects. The variable stiffness of the SMP allows the membrane to conform to and make good contact with objects of various shapes in its soft state and then achieve high adhesive load capacity by switching to the stiff state. Release of objects is realized by switching to the soft state. The ratio between the high-adhesion and low-adhesion state is demonstrated to be >2000 on a curved substrate and ∼115 on a flat substrate. This gripper exhibits no adhesion in the unactivated state and maintains adhesion passively once actuation is complete.

Biomimetic textiles: An innovative approach towards conserving the future

Superhydrophobicity, self-repair, self-cleaning, energy conservation, dry adhesion, adaptive development, drag reduction, and other natural phenomena have evolved over billions of years to produce more efficient textile solutions than equivalent man-made solutions. Few innovative biomimetic technologies viz., functional surfaces, structural colours, fibre structures, self-healing, thermal insulation, and other characteristics that can be used for prospective textile goods. Biomimetic research is a fast-growing discipline, and exploiting the issue's full potential in the manufacture of unique and sustainable textiles requires a multidisciplinary approach based on a comprehensive knowledge of nature. Biomimicry has the potential to enhance man-made materials and pave the way for the next generation of technological, high-performance materials, including novel materials and characteristics, creative structures and designs, and product and process sustainability. In this study, the potential of a bio-inspired textile structure is explored to the best extent conceivable. The potential use of different biomimetic fabrics was also considered. In essence, this technique can serve as a source of motivation for further material advancement.

Moisture damage mechanism and material selection of HMA with amine antistripping agent

This study investigates the moisture damage mechanism and material selection of hot-mix asphalt (HMA) with amine-based antistripping agent (ASA). The surface free energy (SFE) of virgin and ASA modified asphalts and five aggregates with different acidities were measured, yielding the compatibility of each asphalt-aggregate combination. The corresponding compacted mixtures were evaluated by tensile strength ratio (TSR) considering the freeze–thaw and MIST conditioning. The use of amine ASA changed the thermodynamic properties of asphalt by decreasing the nonpolar components and increasing the polar components, which enhanced the work of adhesion between asphalt and aggregate and reduced the free energy released at the presence of water. The wettability of asphalt over aggregate was also increased due to the ASA, suggesting a better coating quality of asphalt mixtures was achieved. All those changes led to the increased energy ratio (ER), indicating the improved moisture resistance and compatibility of materials. At the molecular level, it might be because the amino groups from ASA adsorbed the silanol groups at the aggregate surface, leading to a stronger bond between asphalt and aggregate. The asphalt mixtures with acidic aggregate exhibited more serious moisture damage, which could be attributed to the lower dry adhesion energy and the larger free energy released with moisture. To enhance the moisture resistance, the selection of compatible asphalt-aggregate combinations seemed more effective than the use of amine ASA. The ERs of all the asphalt-aggregate combinations correlated well with the TSR results, and the SFE-based criteria for material selection of HMA were tentatively proposed.

Gecko-inspired dry adhesives for heritage conservation – tackling the surface roughness with empirical testing and finite element modelling

Gecko-inspired dry adhesives (GDAs) have been developed in an attempt to replicate in polymer material the natural ability of some gecko lizards to attach to nearly any surface. Geckos achieve this with nano-sized structures on their feet that facilitate van der Waals's interactions with the surfaces. The conservation of cultural heritage is an area that could benefit greatly from the introduction of a versatile and easily reversible adhesive. However, the multitude of surface types and various surface textures encountered in this field make the adaptation of GDAs difficult. In this research two types of GDAs, with flat tips and with mushroom-shaped tips have been assessed using pull-off tests on three substrate materials. These are based on real heritage objects’ surfaces (copper and ceramic) with different levels of surface roughness from The Hunterian collection. Adhesive strength varied between different GDAs and as expected adhesive strength reduced with increased substrate roughness. The finite Element Modelling (FEM) of the pull-off tests closely matched empirical results and showed how different behaviours on the microlevel can affect the GDA behaviour on rough surfaces. It helped to understand the microscale behaviour of two different types of GDAs tested. The research has shown the necessary direction for experimental and theoretical research on GDAs which will enable them to be adopted more widely in heritage conservation.

Scalable Fabrication of Bioinspired Controllable Dry Adhesive by Roll‐to‐Roll Slitting

Controllable dry adhesives have attracted significant interest; however, their development and application remain limited owing to their high cost, complexity, and machine precision required for their fabrication. Herein, a scalable roll‐to‐roll technique for fabricating wedge‐shaped controllable dry adhesives for use in robotic grasping tasks is developed. Wedge‐shaped structures with various geometrical dimensions are manufactured, and the structure with optimal wedge dimension (50 μm width and 30° tip angle) demonstrates peak and stable normal adhesion values of 14 and 6.8 kPa, respectively. Moreover, parallel grippers are designed to grasp everyday objects that have flat surfaces and a variety of geometries and surface textures. This novel scalable, low‐cost, and simple method can be used to manufacture a straight and continuous wedge structure in large scale for controllable dry adhesives that can be effectively applied in robotic manipulation tasks.

Improvement of the dual barrier/active corrosion inhibition function of the epoxy composite filled with zinc doped-phytic acid-modified graphene oxide nanosheets

In this research, a smart epoxy coating was designed using GO sheets functionalized (non-covalently) with phytic acid (Ph) molecules and zinc (II) cations. Ultraviolet-visible spectroscopy (UV–Vis), Raman spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and inductively coupled plasma (ICP) were conducted to prove the adsorption of inhibiting species on the GO surface. Alongside the electrochemical impedance spectroscopy (EIS) and polarization methods, grazing incidence XRD (GI-XRD) and FE-SEM analyses were carried out on the plates exposed to the modified GO-containing solutions. The coated samples were subjected to the EIS, salt spray test (SST), pull-off, and cathodic delamination (CD) methods. The characterization analyses confirmed the adsorption of the Zn2+ cations and Ph molecules. The adequate inhibition efficiency of the GOZn400Ph nanosheets in the saline solution was verified by the electrochemical techniques. The GOZnPh/EP coating exhibited the highest low-frequency impedance value after 49 days (~109 Ω cm2). In addition to the superior barrier protection properties, the GOZnPh nanosheets promoted active protection ability. The inclusion of the GOZnPh increased the dry adhesion strength and reduced the adhesion loss.

Ultrastrong underwater adhesion on diverse substrates using non-canonical phenolic groups

Robust underwater adhesion is challenging because a hydration layer impedes the interaction between substrates and adhesives. Phenolic adhesives inspired by marine creatures such as mussels were extensively studied, but these adhesives have not reached the adhesion strength and substrate diversity of Man-made dry adhesives. Here, we report a class of ultrastrong underwater adhesives with molecular phenolic designs extending beyond what nature has produced. These non-canonical phenolic polymers show versatile adhesion on various materials, with adhesion strengths exceeding 10 MPa on metal. Incorporating even just a small amount (<10%) of non-canonical phenolic groups into a polymer is sufficient for dramatically enhancing underwater adhesion, suggesting that this new class of phenolic materials will be incorporated into various industrial polymer systems in the future.

An instant, repeatable and universal supramolecular adhesive based on natural small molecules for dry/wet environments

Development of supramolecular adhesion systems from small molecules is a promising strategy to construct adhesives integrating synthetic simplification and sophisticated properties. Here, we report a facile and green one-pot strategy for fabricating a solvent-free and dynamic supramolecular adhesive via co-heating three readily available feedstocks, a natural small biological molecule (α-lipoic acid), a natural plant polyphenol (tannic acid), and ferric chloride. The adhesive is injectable and self-healing, which can work instantly and repeatedly on various materials (e.g., polytetrafluoroethylene (PTFE)) in diverse conditions (dry, underwater, pH < 7, various ionic environments, and even in vacuum) at room temperature, showing promising applicability in complex environments. The self-strengthening of the supramolecular network with time further improves the adhesion strength of the adhesive for the glass (from about 22 kPa to exceeded 300 kPa after 28 d). More importantly, the adhesive exhibits great potentials to be applied in biomedical fields with wet environments. This ready-to-use and high-performance supramolecular adhesive, constructed by natural small molecules via an extremely facile preparation method, represents a promising universal strategy for both dry and wet adhesion.

Attachment of bioinspired microfibrils in fluids: transition from a hydrodynamic to hydrostatic mechanism.

Reversible and switchable adhesion of elastomeric microstructures has attracted significant interest in the development of grippers for object manipulation. Their applications, however, have often been limited to dry conditions and adhesion of such deformable microfibrils in the fluid environment is less understood. In the present study, we performed adhesion tests in silicone oil using single cylindrical microfibrils of a flat-punch shape with a radius of 80 µm. Stiff fibrils were created using three-dimensional printing of an elastomeric resin with an elastic modulus of 500 MPa, and soft fibrils, with a modulus of 3.3 MPa, were moulded in polyurethane. Our results suggest that adhesion is dominated by hydrodynamic forces, which can be maximized by stiff materials and high retraction velocities, in line with theoretical predictions. The maximum pull-off stress of stiff cylindrical fibrils is 0.6 MPa, limited by cavitation and viscous fingering, occurring at retraction velocities greater than 2 µm s-1. Next, we add a mushroom cap to the microfibrils, which, in the case of the softer material, deforms upon retraction and leads to a transition to a hydrostatic suction regime with higher pull-off stresses ranging from 0.7 to 0.9 MPa. The effects of elastic modulus, fibril size and viscosity for underwater applications are illustrated in a mechanism map to provide guidance for design optimization.

Machine learning-based optimization of the design of composite pillars for dry adhesives

A machine learning-based optimization strategy was used to find optimal designs for adhesive composite pillars with either high adhesion strength or high adhesion tunability. Neural networks were trained with data generated by finite element analysis to predict the adhesion strength of composite pillars with different designs; an average prediction error of less than 1% was achieved. Through a sensitivity study with the trained neural networks, it is found that the geometry of the stiff core above a critical cut off height has no effect on the interfacial stress distribution and the adhesion of the pillar. A randomly initialized constrained optimization algorithm was then applied to the trained neural networks to find the optimal composite pillar design. A composite pillar design with a stiff core that has an enlarged tapered flat end is optimal for realizing robust and high adhesion, since it can achieve high adhesion under different loading and contact conditions. The optimized pillar has a critical normal detachment force that is nearly 11 times that of a homogenous pillar and 1.7 times that of a composite pillar with a simple wide rectangular core under normal loading. A composite pillar with a thin flat stiff core shows the highest effective adhesion difference between being loaded with a normal force and being loaded with a moment.

Correlation-function-based microstructure design of alloy-polymer composites for dynamic dry adhesion tuning in soft gripping

Tunable dry adhesion is a crucial mechanism in compliant manipulation. The gripping force can be controlled by reversibly varying the physical properties (e.g., stiffness) of the composite via external stimuli. The maximal gripping force Fmax and its tunability depend on, among other factors, the stress distribution on the gripping interface and its fracture dynamics (during detaching), which in turn are determined by the composite microstructure. Here, we present a computational framework for the modeling and design of a class of binary smart composites containing a porous low-melting-point alloy (LMPA) phase and a polymer phase, in order to achieve desirable dynamically tunable dry adhesion. We employ spatial correlation functions to quantify, model, and represent the complex bi-continuous microstructure of the composites, from which a wide spectrum of realistic virtual 3D composite microstructures can be generated using stochastic optimization. A recently developed volume-compensated lattice-particle method is then employed to model the dynamic interfacial fracture process, where the gripper is detached from the object, to compute Fmax for different composite microstructures. We focus on the interface defect tuning mechanism for dry adhesion tuning enabled by the composite, and find that for an optimal microstructure among the ones studied here, a tenfold dynamic tuning of Fmax before and after the thermal expansion of the LMPA phase can be achieved. Our computational results can provide valuable guidance for experimental fabrication of the LMPA–polymer composites.

Height-Tunable Replica Molding Using Viscous Polymeric Resins.

Replica molding is one of the most common and low-cost methods for constructing microstructures for various applications, including dry adhesives, optics, tissue engineering, and strain sensors. However, replica molding provides only a single-height microstructure from a mold and master molds produced by an expensive photolithography process are required to prepare microstructures with different heights. Herein, we present a strategy to control the height of micropillars from the same mold by varying the cavity size of the micromold and the viscosity of the photocurable polyimide resin. The height of the constructed micropillar decreases in the case of small microcavities or high viscosity resin. In addition, the height of the micropillar arrays could be arbitrarily patterned by applying a masking technique. We believe that this cost-effective technique can be applied to metasurfaces for manipulation of electromagnetic signal or in biomedical applications including cell-culture and stem-cell differentiation.

Novel high-performance and long-life anti-corrosion coating with rust conversion and self-healing characteristics based on waste biomass derived carbon nanosheets

Fabrication of coating system with high-performance and long-life anti-corrosion performance is an important way of prolonging service life and reduces the waste of metal equipment. Herein, one smart coating with rust conversion and self-healing characteristics was prepared though the addition of tannic acid (TA)-grafted cornstalks derived carbon nanosheets (CNS) and 8-hydroxyquinoline (8-HQ)-loaded mesoporous silica (MS) into the coating. Thermogravimetric analysis results indicated that the grafting/loading rates for TA and 8-HQ were 17.5% and 27.3%, respectively. Pull-out test results reflected that the value of dry adhesion of the smart coating at the steel/coating interface was two times higher than that of the pure coating. Electrochemical impedance spectroscopy results reflected that the anti-corrosion performance and service life of the coating were evidently improved, and the coating exhibited self-repairing property. /Z/f=0.01Hz of smart coating was still 1.8 × 107 Ω cm2 after 80 days immersion. The high-performance and long-life of the coating were attributed to the sheet structure of CNS, formation of TA-Fe layer at steel/coating interface as well as chelate products between 8-HQ and corrosion products. It is expected that the prepared smart coating could find wide application due to the simplification the operations and improvement the service life of traditional coating.

Lignin-Glyoxal: A Fully Biobased Formaldehyde-Free Wood Adhesive for Interior Engineered Wood Products

In this study, a biobased phenolic adhesive was successfully developed by entirely substituting both petroleum-based phenol and formaldehyde with an unmodified corn stover biorefinery lignin and glyoxal (a biobased dialdehyde), respectively. Lignin-glyoxal resins were synthesized using an alkaline catalyst with a molar ratio of lignin to glyoxal of 1:2. Chemical, thermal, and mechanical properties of the lignin, lignin-based resins, and final adhesives were assessed following appropriate standard test methods. The analysis of lignins and lignin-based resin molar mass was performed using gel permeation chromatography. The lignin-glyoxal resin was found to have a 3-fold higher average molecular weight than the starting lignin, demonstrating the successful integration of lignin into the polymeric resin network. The curing of the formulated adhesives was studied using differential scanning calorimetry and dynamic mechanical analysis. Although the lignin-glyoxal resin had a higher curing temperature (167 °C) than a conventional phenol-formaldehyde resin (142 °C) and the formulated lignin-formaldehyde resin (146 °C), the rate and degree of cure were similar or better than the other two resins. The adhesion strengths of the formulated adhesives were determined using single-lap-joint veneer samples cured according to recommended press parameters for commercial adhesives. The lignin-glyoxal adhesive had a relatively high dry adhesion strength (3.9 MPa), with over 90% wood failure, but failed the wet adhesion test (boiling water test). Although the formulated lignin-glyoxal adhesive failed the boiling water test, it had excellent stability at room temperature water, remaining intact after 1 week during the water immersion test. The high dry adhesion strength makes this class of lignin-based formaldehyde-free adhesives a unique biobased glue for the production of interior grade plywood and oriented strand boards.

Gecko inspired reversible adhesion via quantum dots enabled photo-detachment

• High efficiency photo driven reversible adhesion smart surface inspired by gecko. • The smart surface could increase up to 88.5 °C in 60 s trigger by MoO 3 dots. • The mechanism of the reversible process were discussed. Inspired by the detachment behavior of gecko toe valgus, a reversible dry adhesive surface was developed, which is composed of polydimethylsiloxane (PDMS) micropillar arrays and MoO 3-x quantum dots (MoO 3-x QDs). The reversibility of adhesion is caused by the thermal expansion of the overall structure of the PDMS micropillar array due to the thermal response of MoO 3-x QDs to external stimuli. Under infrared laser irradiation, the interface temperature of MoO 3-x -PDMS surface increased from 20 °C to 88.5 °C in 60 s. The MoO 3-x QDs-induced photothermal conversion facilitate the reduction of the real contact fraction between the bio-inspired micropillar arrays and the opposing/target surface, resulting in a significant decrease in adhesion. This work demonstrates a new strategy for realizing remote control of smart adhesives or surfaces. The mechanism elucidated in this work may find a broad application, such as in climbing robots, micro-manipulation/micro-selection in advanced manufacturing, active self-cleaning surface for solar panels, fibrillar dry adhesives and touch sensitive grippers in outer space explorations, and more.

Surface Adaptable and Adhesion Controllable Dry Adhesive with Shape Memory Polymer.

Gecko feet consist of numerous micro/nano hierarchical hairs and exhibit a high adhesion onto various surfaces by van der Waals forces. The gecko, despite its mighty adhesion, can travel efficiently with a rapid adhesion switching due to the end of the hairs on the gecko feet are slanted in one direction. Herein, a shape memory polymer (SMP)-based switchable dry adhesive (SSA), inspired by gecko feet, having tremendous surface adaptability and adhesion switching capability, is reported. The SSA shows not only high adhesion to the various surfaces (≈332.8kPa) but also easy detachment (nearly 3.73kPa) due to the characteristic of SMP, which can reversibly recover from a deformed shape to its initial shape. On the basis of the novel adhesion switching property, it is suggested the SSA-applied advanced glass transfer system can lead to feasible application. This experiment confirms that an ultrathin and light glass film is transferred easily and sustainably, and it is believed that the SSA may be a breakthrough and a powerful alternative for not only conventional dry adhesives but also the next-level transfer systems.

Predicting the adhesion strength of micropatterned surfaces using supervised machine learning

Fibrillar dry adhesives have shown great potential in many applications thanks to their tunable adhesion, notably for pick-and-place handling of fragile objects. However, controlling and monitoring alignment with the target objects is mandatory to enable reliable handling. In this paper, we present an in-line monitoring system that allows optical analysis of an array of individual fibrils (with a contact radius of 350 µm) in contact with a smooth glass substrate, followed by the prediction of their adhesion performance. Images recorded at maximum compressive preload represent characteristic contact signatures that were used to extract visual features. These features, in turn, were used to create a linear model and to train different linear and non-linear regression models for predicting adhesion force depending on the misalignment angle. Support vector regression and boosted tree models exhibited highest accuracies and outperformed an analytical model reported in literature. Overall, this new approach enables predictions in gripping objects by contact observations in near real-time, which likely improves the reliability of handling operations.

Laser-induced morphology-switchable slanted shape memory microcones for maneuvering liquid droplets and dry adhesion

Slanted micro-/nano-structures play pivotal roles in a diversity of fields, including water-proof engineering and fogdrop collection. In light of recent advances in fabricating slanted microstructures by using photolithography or reactive ion etching techniques, however, a complex, environmentally unfavorable, and tedious fabrication process makes them far from practical in application. Herein, we present a viable strategy to prepare a slanted shape memory microcone array (SSMMA) by combining the femtosecond laser oblique microfabrication and replica-mold method. Thanks to its fast temperature-responsive feature, SSMMA enables the transition of adhesion forces to effectively control the sliding of droplet on the surface. The underlying principle of the adjustable migration behavior of droplet is that SSMMA switches between the slanted and collapsed states. Moreover, we systematically studied the influence of the microcone spacing/height together with the microcone bending angle on the wetting performance of water droplet. More significantly, the resulted SSMMA analogous to a “machine hand” is experimentally demonstrated to be competent for the grab and transfer of fragile and smooth objects (e.g., coverslip) with a maximum adhesion force of ∼19.404 mN. The current study opens up an avenue for rapidly fabricating functional slanted microstructures for practical usage.