The purely viscoplastic paradigm has forged the understanding of yield stress materials, leading to misconceptions associated to the set of quantities that are needed to characterize yielding transition and also to the significance of the measured quantities (Thompson et al., 2018). The following assertions, usually taken for granted, are actually simplistic constitutive assumptions: (i) there exists a scalar that represents the yielding condition, (ii) there is a universal rule that maps the yield stress tensor onto that scalar, (iii) the von Mises yielding criterion holds (Thompson et al., 2018). That is, these statements are not necessarily true for a general yield stress material. In the present study we investigate the yielding of seven materials in two extensional loading conditions: traction and compression. We show that the von Mises criterion does not perform well for the yielding of these materials. A new criterion that includes the third invariant of the (deviatoric) yield stress tensor is proposed to handle the differences between traction and compression values of the stress at the yielding point. Even this criterion is not able to accommodate the yielding obtained in shear loading for these materials. In this regard, the most likely scenario is that no universal criterion exists for all kinds of yield stress materials.

The enhanced thermal conductivity of nanofluids justifies their application to traditional niches of heat transfer fluids. For Newtonian materials, however, viscosity, pressure drop, and ultimately, pumping power, also increase with nanoparticle concentration. A different behavior, though, may be observed with non‐Newtonian nanofluids. This possibility motivated the present work, in which aqueous solutions of refrigeration secondary fluid and prime‐mover coolant were enriched by graphene nanoparticles (thickness: 0.55‐3.74 nm; average length: 5‐10 μm). Viscosity variation with shear stress was experimentally investigated for temperatures within (−10°C < T < 25°C). Shear thinning and shear thickening were observed and described in detail by means of flow curves for each fluid sample (base fluid and nanofluids). The patterns for the curve fitting parameters of a modified version of the Herschel‐Bulkley equation were analyzed, tabulated, and modeled. Finally, the relative apparent viscosity data were compared with four classical models. HIGHLIGHTS Synthesis of graphene non‐Newtonian heat transfer nanofluids. The spectrum of temperatures ranged from –10°C to 25°C. Shear thinning and shear thickening were observed and described in detail. Herschel‐Bulkley‐like equation was observed, parameters patterns were modeled. Relative apparent viscosity data were compared with four classical models.

During cold start of internal combustion engines, coolant temperature, and thermal conductivity are key parameters in the heat transfer processes that ultimately affect pollutant emissions and engine performance. Hereupon the use of coolants with suspended nanoparticles, to enhance thermal conductivity, emerged as a promising technology. However, for Newtonian materials, viscosity also increases with nanoparticle concentration. To overcome increased pumping power, the use of non-Newtonian nanofluids makes such application potentially feasible, specifically for shear-thinning materials, in which a higher shear rate leads to reducing shear viscosity due to higher shear stress. Accordingly, a nanofluid, suitable for engine cooling (0.2 wt.% MWCNT-engine coolant/distilled water 30/70 v/v%), was here fabricated and mapped. Shear rate and temperature were varied, with focus on cold start investigation. Shear thinning and shear thickening regions were mapped according to the shear rate levels, for each temperature considered. The nanofluid behaved as shear-thinning material for the entire range of temperatures (−10°C–25°C). Above shear rates of 500 s⁻¹ and flow curves with temperatures below −5°C, a prominent shear thickening behavior was observed. Additionally, the relative apparent viscosity data were compared with four classical models. Regarding the curve fitting parameters of a modified Herschel-Bulkley equation, above 0°C, the apparent yield stress, [ ], was invariant with temperature. Besides, for the temperature range from 0°C to 20°C, the flow index remained approximately constant. For temperatures above −5°C, infinite-shear-rate viscosity and consistency index presented a linear decrease and a third-degree polynomial-like behavior, respectively.