Gel Friction Research Articles

Low-Hysteresis and Tough Ionogels via Low-Energy-Dissipating Cross-Linking.

Low-hysteresis merits can help polymeric gel materials survive from consecutive loading cycles and promote life span in many burgeoning areas. However, it is a big challenge to design low-hysteresis and tough polymeric gel materials, especially for ionogels. This can be attributed to the fact that higher viscosities of ionic liquids (ILs) would increase chain friction of polymeric gels and eventually dissipate large amounts of energy under deformation. Herein, a chemical design of ionogels is proposed to achieve low-hysteresis characteristics in both mechanical and electric aspects via hierarchical aggregates formed by supramolecular self-assembly of quadruple H-bonds in a soft IL-rich polymeric matrix. These self-assembled nanoaggregates not only can greatly reinforce the polymeric matrix and enhance resilience, but also exhibit low-energy-dissipating features under stress conditions, simultaneously benefiting for low-hysteresis properties. These aggregates can also promote toughness and subsequent anti-fatigue properties in response to external cyclic mechanical stimuli. More importantly, these ionogels are presented as a model system to elucidate the underlying mechanism of the low hysteresis and fatigue resistance. Based on these findings, it is further demonstrated that the supramolecular low-hysteresis strategy is universal.

Fast to Slow Slip Transition in Stick-Slip Friction of Polymer Gels with Artificial Roughness

Fast to Slow Slip Transition in Stick-Slip Friction of Polymer Gels with Artificial Roughness

Control of Surface Morphology, Adhesion and Friction of Colloidal Gels with Lamellar Surface Interactions

AbstractDespite recent advances in polyelectrolyte systems, designing responsive hydrogel interfaces to meet application requirements still proves challenging. Here, semicrystalline colloidal gels composed of poly(methacrylamide‐co‐methacrylic acid) are investigated in water with storage moduli in the MPa range. A combination of SEM, X‐ray scattering, and NMR reveals the evolution of the colloidal microstructure, crystallinity, and hydrogen bonding with varying monomer ratio. The gels with the finest colloidal microstructure exhibit the most dissipative rheological behavior and are selected for the study of their interfacial characteristics and underlying interactions. Microstructure stabilization and dynamics results from short‐range (attractive) hydrogen bonding and hydrophobic forces, and long‐range (repulsive) electrostatic interactions—the “SALR” pair potential. Further, the gel's surface exhibits a submicron colloidal topography that greatly determines (colloidal‐like) friction as a result of the viscoelastic deformation of the colloidal network, while electrostatic near‐surface interactions propagate in lamellar adhesion. The dynamic and reversible nature of the involved interactions introduces a stimulus responsive behavior that enables the electrotunability of adhesion and friction. This study advances the knowledge necessary to design complex hydrogel interfaces that enable spatial and dynamic control of surface properties, which is of relevance for applications in biomedical devices, soft tissue design, soft robotics, and other engineered tribosystems.

Ultralow friction of basil-based gel in the presence of ethanol as a green lubricant for biomedical applications

Superlubricity is an ideal approach to save the service life of medical devices from the tribology point of view. Recently, basil seed gel (BSG) is proved to be a potential candidate with ultralow friction. In this paper, the friction coefficient of BSG is significantly reduced by introducing ethanol. The results indicate that the ethanol molecules enter the BSG network, contributing to transforming the loose structure of polysaccharide nanosheets to the integrated structure. Thus, the mechanical strength of the adsorption layer is improved, which prevents the lubrication film from being destroyed during the sliding process and thus reduces the friction loss. Such macroscopic superlubricity of BSG-ethanol lubricant demonstrates promising potential in biomedical applications such as catheters and gastric tubes.

Controlling the Friction of Gels by Regulating Interfacial Oxygen During Polymerization

Hydrogel surfaces are of great interest in applications ranging from cell scaffolds and transdermal drug-delivery patches to catheter coatings and contact lenses. In this work, we propose a method to control the surface structure of hydrogels, thereby tailoring their frictional properties. The method is based on oxygen inhibition of the free-radical polymerization reaction during synthesis and enables (i) control of friction over more than an order in magnitude and (ii) spatial control of friction as either a continuous gradient or a distinct pattern. The presented method has successfully been applied to acrylamide-, diacrylate- and methacrylate-based gels, illustrating the universality of the presented method, and its potential use in the above-mentioned applications.Graphical

Semi-convertible Hydrogel Enabled Photoresponsive Lubrication

Summary A semi-convertible hydrogel with reversible photoresponsive supramolecular lubricating ability is demonstrated by integrating a responsive supramolecular system of α-cyclodextrins/polyethylene glycol (α-CD/PEG) and competitive guest 1-[p-(phenylazo)benzyl]pyridinium bromide (AzoPB) within a poly(vinyl alcohol) (PVA) and polyacrylamide (PAAm) framework. The competitive host-guest interaction between α-CD/PEG supramolecular network and AzoPB after UV and visible-light irradiation enables the appearance and disappearance of the sol-state layer on the hydrogel surface while the PVA and PAAm consistently keeps the framework. This semi-convertible process realizes the reversible photoresponsive lubricating ability and variable toughness. We envision that this finding will offer a possible way for fabricating novel smart devices far beyond lubrication, leading to an emerging branch of interdisciplinary materials science that integrates supramolecular chemistry, hydrogel, and surface science.

Friction between a plane strain circular indenter and a thick poroelastic substrate

This paper presents a computational study on the role of poroelasticity in gel friction. Motivated by recent experimental studies in the literature, we develop a plane strain finite element model to elucidate the contact mechanics between a circular indenter and a thick poroelastic substrate under both normal and shear loadings. Two cases are considered: i) steady state sliding under fixed normal displacements, and ii) relaxation under fixed normal and shear displacements. In steady state sliding, we find that a net friction force can arise even if no intrinsic adhesive or frictional interaction is implemented at the indenter/substrate interface. Such friction force exhibits a non-monotonic dependence on the sliding velocity and peaks at an intermediate velocity. Our model reveals that this friction force is induced by poroelastic diffusion in the gel substrate which can lead to considerable asymmetry in both the contact profile and contact pressure. In terms of relaxation, if the indenter/substrate interface is set to be frictionless, we find that the friction force induced by poroelasticity relaxes to zero with a characteristic time much faster than that of the normal force. When a finite friction coefficient is introduced at the interface, the normalized relaxation curve for the friction force approaches that for the normal force as the friction coefficient increases. These modeling results suggest that poroelasticity can be an important contributing mechanism for gel friction.

Sliding Walk With Friction Control of Double-Network Gel on Feet of Inchworm Robot

Soft-matter robotics, which is named for robots made from soft materials, will possibly conquer the limit of conventional rigid body dynamics of hard robots made from hard materials. For example, a snake-like robot, of which body is made from soft materials, will walk in various environments including narrow space like drain pipes with complicated structure. Here we developed a walking robot with the friction control of double-network hydrogel (DN gel) on its feet, as the first model based on the concept of the soft-matter robotics. It is found that with appropriate control of the friction of DN gel, the inchworm walking robot has realized its high enough velocity for pipe inspection. We discussed the physical principle of the walking motion of the inchworm search robot and its actual mechanism to provide thrust for its walking motion.

Low-velocity super-lubrication of sodium-alginate/polyacrylamide ionic-covalent hybrid double-network hydrogels.

Structural and frictional behaviours of sodium alginate (SA)/polyacrylamide (PAAm) ionic-covalent hybrid, sequential double-network (DN) hydrogels against glass have been investigated in water, NaCl and CaCl2 aqueous solutions using a rotational rheometer. Dilution of adsorptive elastohydrodynamic friction for the PAAm covalent network with repulsive hydrodynamic lubrication for the minor SA ionic network was found to control the frictional stresses of the SA/PAAm gels within between those of the SA and PAAm single-network gels. A tentative qualitative model was proposed to describe the impact of ionic environmental solution on the frictional behaviour of the hybrid gel by selectively affecting the SA-network structure and friction. It was revealed that strong Debye shielding in the NaCl solution significantly reduced the thickness of the electric double layer for hydrodynamic lubrication of the SA network, which made the SA/PAAm gel's friction the highest among the three solutions. Dramatically increased ionic cross-linking of the SA network in the CaCl2 solution, although effectively mediated by the PAAm-network flexible skeleton, still functioned partially to conserve a portion of the SA fractional boundary-friction at the interface, making the friction of the hybrid gel intermediate among the three solutions. In contrast, extreme hydration of the SA network in water sharply increased the volume fraction of its unshielded hydrodynamic lubrication at the interface, which greatly reduced the SA/PAAm's friction to the lowest among the three solutions. We have thus incorporated for the first time both super-lubrication (frictional coefficients of below 10(-2) over low sliding-velocities of 3 × 10(-5) to 2 × 10(-3) m s(-1)) and previously reported high fracture energy (over 9000 J m(-2)) into a single ionic-covalent hybrid DN hydrogel, which is the SA/PAAm (∼1/8.5 w/w) gel in water. Effects of inversion of DN-formation sequence further indicated that frictional behaviours (i.e. frictional stress-sliding velocity profiles) of the hybrid sequential DN hydrogels (SA/PAAm and PAAm/SA), respectively, were primarily determined by those of the second networks (PAAm and SA), presumably due to the formation of first-second network "core-shell" structures at the blob scale. Frictional stress of the SA/PAAm gel was increased monotonically with external normal pressure at all of the sliding velocities investigated in the three solutions, which was in agreement with the predictions from the repulsion-adsorption model proposed by Gong et al.

316 Interaction between Artificial Surface Asperities in Stick-Slip Friction of Gels

316 Interaction between Artificial Surface Asperities in Stick-Slip Friction of Gels

Impact of formulation and saliva on acid milk gel friction behavior.

Rheological analysis is commonly used to evaluate mechanical properties in studies of food behavior. However, rheological analysis is often insufficient to describe food texture as evaluated by descriptive sensory analysis. Additionally, traditional rheometry does not account for changes in food behavior as a function of saliva incorporation into the food during mastication. The objectives of this study were to evaluate friction behavior of acid milk gels with and without the addition of saliva, and to determine relationships between frictional behaviors and mechanical and sensory behaviors. Acid milk gels were prepared with 12.5% total solids comprising nonfat dry milk, whey protein isolate, waxy maize starch, and gelatin in different ratios. The addition of starch was found to have significant impact on acid milk gel frictional behavior. Addition of saliva resulted in a change in frictional behavior over the entire sliding speed range measured. Correlations were found between rheological, tribological, and sensory behavior, suggesting that an underlying mechanism may impact both viscosity and friction behavior. Additional study is needed to further explore the links between food structure, rheology, tribology, and sensory texture. Application of tribology in food science allows measurement of friction behavior of foods. Matching both rheological and tribological behavior is important to creating reduced-fat or reduced-sugar products with similar mouthfeel to the original product.

J044031 ゲル機械材料の表面摩擦測定方法の確立([J044-03]ソフトマター・イノベーション(3))

In the last decade, several innovative ways to enhance the mechanical properties of polymer gels have been proposed. In 2003, the most effective but simple way was proposed to synthesize double network gels, whose compression fracture stress reached about 30MPa, while that of common gels is several tens kPa. Our group has focused on PAMPS-PDMAAm DN gel, because it possibly has both biocompatibility and permeability, which are good for developing artificial articular cartilage and artificial blood vessel. Also it is possibly used for rapid additive manufacturing with 3D gel printer. Here we develop a novel appratus to observe the surface friction of the DN gels. We hope to apply this appartus for various studies about the tribological behavior of the gels, especially about the effect of external electric field on the gel friction.

Visualization of Interface in Sliding Friction of Polymer Gels

Visualization of Interface in Sliding Friction of Polymer Gels

509 PAMPS-PDMAAm DNゲルのIn vitro摩擦摩耗特性(計算力学・材料力学II)

509 PAMPS-PDMAAm DNゲルのIn vitro摩擦摩耗特性(計算力学・材料力学II)

Friction and Lubrication of Gels: Toward the Low Friction of Bio-tissue

Friction and Lubrication of Gels: Toward the Low Friction of Bio-tissue

Friction of Soft Gel in Dilute Polymer Solution

We study the friction behaviors of poly(vinyl alcohol) (PVA) gel sliding against glass surface in dilute poly(ethylene oxide) (PEO) aqueous solution with various molecular weights, Mw, and concentrations. At low sliding velocity (10-5, 10-4 m/s), distinct PEO polymer effects are observed: The frictional stress in PEO 2E4 (Mw = 2 × 104 g/mol) solutions is lower than that in pure water, decreasing with the increase in PEO concentration and reaching a minimum at the crossover concentration, c*. However, in higher molecular weight solution, PEO 4E6 (Mw = 4 × 106 g/mol), this friction reduction effect is only observed for very dilute concentration (0.01c* solution), and the friction stress in higher concentration (0.1c*, 0.3c*, and c* solution) is higher than that in pure water, accompanied by the appearance of “plateau”. At fast sliding velocity (10-2, 10-1 m/s), all the friction curves in dilute PEO solution superpose with the curve in pure water, independent of Mw and concentration of PEO. These results in...

Surface sliding friction of negatively charged polyelectrolyte gels

The friction between two polyelectrolyte gels carrying the same or opposite sign of charges has been investigated using a rheometer. It is found that the friction was strongly dependent on the interfacial interaction between two gel surfaces. In the repulsive interaction case, especially, the friction was extremely low. The friction behavior is attempted to be described in terms of the hydrodynamic lubrication of the solvent layer between two like-charged gel surfaces, which is formed due to the electrostatic repulsion of the two gel surfaces. From the theoretical analysis (hydrodynamic mechanism), the friction behaviors were explained qualitatively, all of the experimental results, nevertheless, could not be understood well. The viscoelastic feature of the gel and the non-Newtonian behavior of water at the friction interface are considered to be important to elucidate the gel friction.

Friction and Lubrication of Hydrogels: Its Richness and Complexity

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Friction and lubrication of hydrogels—its richness and complexity

Biological connective tissues, such as cartilage and corneal stroma, are essentially hydrogels consisting of fibrous collagen and proteoglycans. Little is known of the surface properties of the hydrogel, although we observe fascinating tribological behavior in biological soft tissues, such as extremely low friction between animal cartilages. We consider that the role of the solvated polymer network existing in the extracellular matrix as a gel state is critically important in the specific frictional behavior of cartilages. In order to elucidate the general tribological features of a solvated polymer matrix, the friction of various kinds of hydrogels has been investigated, and very rich and complex frictional behaviors are observed. The friction force and its dependence on the load differ with the chemical structure of the gels, surface properties of the opposing substrates, and the measurement conditions, which are totally different from those of solids. Most importantly, the coefficient of friction of gels, , varies over a wide range and exhibits very low values (≈ 10-10), which cannot be obtained from the friction between two solid materials. A repulsion-adsorption model has been proposed to explain the gel friction, which says that the friction is due to lubrication of a hydrated layer of polymer chains when the polymer chain of the gel is non-adhesive (repulsive) to the substrate, and the friction is due to elastic deformation of the adsorbed polymer chain when it is adhesive to the substrate.

Static Friction of Agar Gels: Formation of Contact Junctions at Frictional Interface

We measure the static friction F c of agar gels on glass substrates immersed in water. F c is independent of the nominal contact area, and increases with the normal load and the duration t w of contact prior to sliding. Using a confocal laser-scanning microscope, many fine dark spots are clearly visible in the optical reflection distribution images on the glass interface in contact with the gel. The total area of the dark spots increases with t w , corresponding qualitatively to that of F c . These observations indicate that F c originates from the formation of the contacting spots at the interface, which is not due to asperities of the gel surface alone but related to the drainage of water trapped by the gel polymer network at the frictional interface.