Evidence of viscoplastic behavior of exfoliated graphite nanofluids

Abstract

The rheological behavior of ethylene glycol-based nanofluids containing exfoliated graphite nanoplatelets has been carried out using a cone-plate Physica MCR rheometer. Initial experiments based on flow curves were carried out, the flow curves were based on the controlled shear stress model, these tests show that the studied nanofluids present non-Newtonian shear thinning behavior with yield stress. Furthermore, linear viscoelastic experiments were conducted in order to determine the viscoelastic behavior: using strain sweep and frequency sweep tests the storage and loss modulus were determined. The fractal dimension (Df) was estimated from the suspension static yield-stress and volume fraction (ϕ) dependence, and was determined to be Df = 2.36, a value consistent with a process of aggregation of RLCA type (reaction limited cluster aggregation). This value is unusual if compared with other nanofluids, and can be regarded as a result of the bidimensionality of the suspended nanoplatelets. Finally, creep-recovery tests and mechanical models confirm the viscoplastic nature of our nanofluids, a feature never shown so far for this type of systems, increasing the solid-like character in the range of concentrations studied if compared with other nanofluids reported in the literature. This is a result of the combination of a remarkable internal structure and strong interactions, which evidence an unexpected behaviour sharing many solid-like features.

Two-Dimensional Tungsten Disulfide-Based Ethylene Glycol Nanofluids: Stability, Thermal Conductivity, and Rheological Properties

“…Beyond the flow point, the viscous nature became dominant, which indicated the liquid-like behavior of the nanofluids [ 56 ]. The similar phenomenon of structured nanofluids has also been observed in previous studies for exfoliated graphite nanoplatelets and titanium nitride-based EG nanofluids [ 54 , 55 ].…”

“…This region is referred to as the linear viscoelastic range (LVER). In this region, the nanofluid behaves like a gel, with a pronounced elastic modulus, and the stress value that corresponds to the storage (G’) modulus in this limiting region is referred to as the true static yield stress [ 55 ]. It can also be observed from Figure 9 that, after a certain value of deformation, the structure breakdown started, followed by the nanofluid flow.…”

Thermal and Physical Characterization of PEG Phase Change Materials Enhanced by Carbon-Based Nanoparticles

“…It must be pointed out that, even though nanofluids continue having a liquid-like appearance (in all cases tang δ > 45°) at temperatures at which the base fluid is supposed to be liquid, the addition of nanoparticles led to an increase in the storage modulus. This increase in the elastic behavior is usually observed when rigid elements are suspended in their network [ 102 , 103 ].…”

Heat Transfer Capability of (Ethylene Glycol + Water)-Based Nanofluids Containing Graphene Nanoplatelets: Design and Thermophysical Profile

“…However, other thermophysical properties, such as dynamic viscosity ( η ), density ( ρ ), or even specific heat capacity ( c p ), should also be taken into account in order to assess whether the replacement of the conventional fluid with the new nanofluid would be really beneficial, as well as to make technical calculations of thermal facilities. Dynamic viscosity or the rheological characteristics in general have a critical effect on the type of flow and consequently on the heat transfer and the necessary pumping power, for example [36]. Although the complex behavior of nanofluids precludes a generalization of their thermophysical properties, the addition of nanoparticles usually leads to higher thermal conductivities, viscosities, and densities as well as to lower specific heat capacities.…”