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Surface textures decrease friction in lubricated sliding with Newtonian fluids. Viscoelastic non-Newtonian lubricants can enhance frictional performance, but the optimal rheological material properties and their coupling to the texture design are non-obvious. In this study, we present a simultaneous design of both surface texture shape and non-Newtonian properties, which can be achieved by fluid additives that introduce viscoelasticity, shear thinning, and normal stress differences. Two models with different fidelity and computational cost are used to model laminar non-Newtonian fluid flow between a rotating flat plate and a textured disk. At lower fidelity, we use the Criminale-Ericksen-Filbey (CEF) constitutive model and a thin-film approximation for conservation of momentum (Reynolds equation). At higher fidelity, we use a fully nonlinear constitutive model typically applicable to polymer solutions (multimode Giesekus model) and the full 3-D momentum equations. Fluid additive design is parameterized by two relaxation modes each having a timescale, added viscosity, and a nonlinear anisotropic drag parameter. To manage the computational complexity and constraints between design variables, we use our previously developed multiobjective adaptive surrogate modeling-based optimization (MO-ASMO) method. A new data-driven extension of MO-ASMO is introduced that constructs general boundaries to prevent attempts to evaluate designs that would lead to simulation failure. We demonstrate the efficiency of our MO-ASMO method and provide insights into co-designing the lubricant and textured surface. The Pareto-optimal solutions include fluid designs with both high and low viscoelastic additive loading. We rationalize this trade-off and discuss how the optimal design targets can be physically realized. [ABSTRACT FROM AUTHOR]

Surface texturing can decrease friction in lubricated sliding contact. The majority of existing experimental work has focused on symmetric-depth-profile surface textures. This experimental work examines asymmetric-depth-profile surface textures using gap-controlled experiments with Newtonian fluids on a custom tribo-rheometer setup. Measurements of normal force and shear load are reported as a function of texture geometry, gap height, and bi-directional sliding velocity. This work shows that, in the absence of cavitation, surface texture depth symmetry must be broken to produce normal forces (through viscous effects) for gap-based Reynolds Number up to Re h = ρ V h η = 1.21 . Asymmetric surface textures reduce shear stress and generate normal load, and therefore decrease the effective friction coefficient, which we observe to be smallest for the shallowest texture angle tested, β =5.3°. [ABSTRACT FROM AUTHOR]

We demonstrate that field-responsive magnetorheological fluids can be used for variable-strength controllable adhesion. The adhesive performance is measured experimentally in tensile tests (a.k.a. probe-tack experiments) in which the magnetic field is provided by a cylindrical permanent magnet. Increasing the magnetic field strength induces higher peak adhesive forces. We hypothesize that the adhesion mechanism arises from the shear resistance of a yield stress fluid in a thin gap. This hypothesis is supported by comparing the experimentally measured adhesive performance to the response predicted by a lubrication model for a non-Newtonian fluid with a field-dependent yield stress. The model predictions are in agreement with experimental data up to moderate field strengths. Above a critical magnetic field strength the model over-predicts the experimentally measured values indicating non-ideal conditions such as local fluid dewetting from the surface. [ABSTRACT FROM AUTHOR]

• We observe synergistic coupling between asymmetric textures and a polymer solution. • Normal thrust is positive in both sliding directions, contrasting Newtonian results. • Laminar hydrodynamic friction reduction is reported. • An optimal angle of texture asymmetry decreases the coefficient of friction. • Friction reduction is caused by Weissenberg rather than Deborah number effects. We present an experimental investigation with surface textures and a Non-Newtonian polymer solution (polyisobutylene in oil) to determine synergistic effects on laminar hydrodynamic friction reduction. Gap controlled experiments were performed on a custom parallel disk tribo-rheometer to systematically vary the Reynolds number (0.1–16), Weissenberg number (0.1–30), and Deborah number (0.005–0.5) in bi-directional sliding motion with five different texture geometries: cylindrical holes with varying asymmetric angle β. Cavitation effects are not present, thus normal force is produced solely by the textures and the lubricant rheology. Contrary to Newtonian fluids without cavitation, the normal force is positive, independent of the direction of motion when shear normal forces dominate purely viscous effects. We observe that the normal thrust can be larger than a simple superposition of forces due to viscous hydrodynamics and shear normal stresses. This synergistic coupling between the asymmetric textures and the polymer solution (at high Wi) produces an optimal angle of texture asymmetry β for dramatically decreasing the effective coefficient of friction compared to the Newtonian oil without polymer. [ABSTRACT FROM AUTHOR]

• Framework for Uncertainty Quantification (UQ) applied to Generalized Newtonian Fluid. • Isolated parametric uncertainties reveal model-form uncertainty. • For sedimenting spheres, predictions with UQ compared to experiments. • Model is adequate for a weakly shear-thinning fluid, within uncertainty bounds. • Model inadequacy is quantified for a strongly shear-thinning Carbopol suspension. We elucidate how uncertainties from rheological measurements manifest in a Generalized Newtonian Fluid (GNF) flow prediction. As specific examples, the resistance and velocity of a settling sphere under gravity in Carbopol and Polyvinylpyrrolidone (PVP) solution was predicted using a Herschel–Bulkley and a Carreau model. Uncertainties in model predictions are quantified, accounting for parametric uncertainties in geometry and rheological models. While the prediction for a settling sphere in PVP falls within the quantified uncertainty bounds, the self-consistent predictions for Carbopol do not. This quantifies the inadequacy of a GNF model for Carbopol in this configuration. Generalizing these results, a validation procedure is presented as a framework for including model-parameter uncertainties in model predictions and identifying model-form error. This information provides quantitative assessments to inform decisions. [ABSTRACT FROM AUTHOR]