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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 

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. 

Understanding fluid viscosity is crucial for applications including lubrication and chemical kinetics. A commonality of molecular models is that they describe fluid flow based on the availability of vacant space. The proposed analysis builds on Goldstein’s idea that viscous transport must involve the concerted motion of a molecular ensemble, referred to as cooperatively rearranging regions (CRRs) by Adam and Gibbs in their entropy-based viscosity model for liquids close to their glass transition. The viscosity data for propylene carbonate reveal a non-monotonic trend of the activation volume with pressure, suggesting the existence of two types of CRR with different compressibility behaviors. This is proposed to result from a change in CRR free volume (<0.2 GPa) and a growth in its size (>0.2 GPa). We use Evans–Polanyi perturbation theory to develop an analytical model for the structural changes of the CRR in function of pressure and temperature and their effect on Eyring viscosity. This analysis shows that the activation energies and volumes scale with the CRR size. Using the compressibility data of propylene carbonate, we show that the activation volume of the CRR at low pressures depends on the compressibility of an ensemble comprised of the first coordination shell around a molecule. At higher pressures, we apply an Adam–Gibbs-type analysis to model the increase in CRR size and its effect on viscosity, where the increase in size is estimated from propylene carbonate’s heat capacity. However, this analysis also reveals deviations from the Adam and Gibbs model that will guide future improvements. 

Ionic liquids (ILs) are an extremely exciting class of electrolytes for energy storage applications because of their unique combination of properties. Upon dissolving alkali metal salts, such as Li or...