Search for: All records

Abstract The viscosity of fluids and their dependence on shear rate, known as shear thinning, plays a critical role in applications ranging from lubricants and coatings to biomedical and food-processing industries. Traditional models such as the Carreau and Eyring theories offer competing explanations for shear-thinning behavior. The Carreau model attributes viscosity reduction to molecular distortions, while the Eyring model describes shear thinning as a stress-induced transition over an activation energy barrier. This work proposes an extended-Eyring model that incorporates stress-dependent activation volumes, bridging key aspects of both theories. In modifying transition-state theory by using an Evans-Polanyi perturbation analysis, we derive a generalized viscosity equation that accounts for the molecular-scale rearrangements governing fluid flow. The model is validated against computational and experimental data, including shear-thinning behavior of pure squalane and polyethylene oxide (PEO) aqueous solutions. Comparative analysis with Carreau-Yasuda and conventional Eyring models demonstrates excellent accuracy in predicting viscosity trends over a wide range of shear rates. The introduction of stress-dependent activation volumes provides a description of molecular exchange kinetics accounting for structural reorganization under shear. These findings offer a unified framework for modeling shear thinning and have broad implications for designing advanced lubricants, polymer solutions, and complex fluids with tailored flow properties. Graphical Abstract 

Interactions between molecules in the synovial fluid and the cartilage surface may play a vital role in the formation of adsorbed films that contribute to the low friction of cartilage boundary lubrication. Osteoarthritis (OA) is the most common degenerative joint disease. Previous studies have shown that in OA-diseased joints, hyaluronan (HA) not only breaks down resulting in a much lower molecular weight (MW), but also its concentration is reduced ten times. Here, we have investigated the structural changes of lipid-HA complexes as a function of HA concentration and MW to simulate the physiologically relevant conditions that exist in healthy and diseased joints. Small angle neutron scattering and dynamic light scattering were used to determine the structure of HA-lipid vesicles in bulk solution, while a combination of atomic force microscopy and quartz crystal microbalance was applied to study their assembly on a gold surface. We infer a significant influence of both MW and HA concentrations on the structure of HA-lipid complexes in bulk and assembled on a gold surface. Our results suggest that low MW HA cannot form an amorphous layer on the gold surface, which is expected to negatively impact the mechanical integrity and longevity of the boundary layer and could contribute to the increased wear of the cartilage that has been reported in joints diseased with OA. 

Abstract Charged double network (DN) hydrogels are widely studied for their desirable mechanical strength and tunable properties. In this work, the influence of polymer concentration on microstructure and properties of agarose/polyacrylic acid DN hydrogels is studied. Agarose, the first network, is a brittle biopolymer, while polyacrylic acid (PAAc) is a weak polyelectrolyte. The microstructure, visualized in liquid environment, displays an agarose scaffold coated and interconnected by PAAc, deviating from the common assumption of an entangled double network. Importantly, the charging of PAAc in the hydrogel is regulated not only by the pH and weak polyelectrolyte effects, but also by the restricted swelling of the double network, and hence, it is an inherent regulation mechanism of charged hydrogels. The interactions between the hydrogel and the ionic environment induce microstructural changes and charging of the double network, impacting surface properties such as topography, stiffness, and adhesion, which are spatially resolved by liquid‐environment atomic force microscopy. The responsiveness of the DN hydrogels significantly depends on both polymer concentrations and ion concentrations. These findings provide insights into the responsive behavior of double network hydrogels and reveal universal mechanisms for charged hydrogels, which can guide the future development of functional soft materials. 

Abstract Despite 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. 

Abstract The ability to modulate polyacrylamide hydrogel surface morphology, rheological properties, adhesion and frictional response is demonstrated by combining acrylic acid copolymerization and network confinement via grafting to a surface. Specifically, atomic force microscopy imaging reveals both micellar and lamellar microphase separations in grafted copolymer hydrogels. Bulk characterization is conducted to reveal the mechanisms underlying microstructural changes and ordering of the polymer network, supporting that they stem from the balance between hydrogen bonding in the substrate‐grafted hydrogels, electrostatic interactions, and a decrease in osmotically active charges. The morphological modulation has direct impacts on the spatial distribution of surface stiffness and adhesion. Furthermore, lateral force measurements show that the microphase separations lead to speed and load‐dependent lubrication regimes as well as spatial variation of friction. A proof of concept via salt screening demonstrates the dynamic control of surface morphology and adhesion. This work advances the knowledge necessary to design complex hydrogel interfaces that enable spatial and dynamic control of surface morphology and thereby of friction and adhesion through modulation of hydrogel composition and surface confinement, which is of significance for applications in biomedical devices, soft tissue design, soft robotics, and other engineered tribosystems. 

Abstract While many mechanistic studies have focused on the lubricious properties of ionic liquids (ILs) on ideally smooth surfaces, little is known about the mechanisms by which ILs lubricate contacts with nanoscale roughness. Here, substrates with controlled density of nanoparticles are prepared to examine the influence of nanoscale roughness on the lubrication by 1‐hexyl‐3‐methyl imidazolium bis(trifluoromethylsulfonyl)imide. Atomic force microscopy is employed to investigate adhesion, hydrodynamic slip, and friction at the lubricated contact as a function of surface topography for the first time. This study reveals that nanoscale roughness has a significant influence on the slip along the surface and leads to a maximum slip length on the substrates with intermediate nanoparticle density. This coincides with the minimum friction coefficient at sufficiently small contact stresses, likely due to the lower resistance of the IL film to shear. However, at the higher pressures applied with a sharp tip, friction increases with nanoparticle density, indicating that the IL is not able to alleviate the increased dissipation due to roughness. The results of this work point toward a complex influence of the surface topology on friction. This study can help design ILs and nanopatterned substrates for tribological applications and nano‐ and microfluidics. 

Abstract Ionic liquids (ILs) are proposed as potentially ideal electrolytes for use in electrical double layer capacitors. However, recent discoveries of long‐range electrostatic screening in ILs have revealed that this understanding of the electrical double layer in highly concentrated solutions is still incomplete. Through precise time‐dependent measurements of wide‐angle X‐ray scattering and surface forces, novel molecular insight into their electrical double layer is provided. An ultraslow evolution of the nanostructure of three imidazolium ILs is observed, which reflects the reorganization of the ions in confined and unconfined (bulk) states. The observed phase transformation in the bulk consists of the ILs ordering over at least 20 h, reflected in an expansion or contraction of the spacing between the ions organized in domains of ≈10 nm. This transformation justifies the evolution of the electrical double layer measured in force measurements. Subtle differences between the ILs arise from the intricate balance between electrostatic and non‐electrostatic interactions. This work reveals a new time scale of the evolution of the IL structure and offers a new perspective for understanding the electrical double layer in ILs, with implications on diverse areas of inquiry, such as energy storage, lubrication, as well as micro‐ and nanoelectronics devices.