Rheological non-Newtonian behaviour of ethylene glycol-based Fe2O3 nanofluids
© Pastoriza-Gallego et al; licensee Springer. 2011
Received: 4 July 2011
Accepted: 25 October 2011
Published: 25 October 2011
The rheological behaviour of ethylene glycol-based nanofluids containing hexagonal scalenohedral-shaped α-Fe2O3 (hematite) nanoparticles at 303.15 K and particle weight concentrations up to 25% has been carried out using a cone-plate Physica MCR rheometer. The tests performed show that the studied nanofluids present non-Newtonian shear-thinning behaviour. In addition, the viscosity at a given shear rate is time dependent, i.e. the fluid is thixotropic. Finally, using strain sweep and frequency sweep tests, the storage modulus G', loss modulus G″ and damping factor were determined as a function of the frequency showing viscoelastic behaviour for all samples.
Research on nanofluids characterization has progressed remarkably in the last decade [1–8]. The first studies were performed at the US Argonne National Laboratories reporting anomalous thermal conductivity enhancements, beyond the prediction of classic models. Nowadays, it is well known that this effect depends on particle size, concentration, nature of base fluids, pH, temperature and nanoparticles clustering [5, 7]. Moreover, it has been shown that other transport properties exhibit also unusual behaviour, including viscosity and rheological properties [9–12]. This means that the thermophysical profile of a nanofluid may be tuned to meet the requirements for a given industrial application. From a practical point of view, this offers numerous benefits [8, 13, 14] as improved heat transfer and stability, microchannel cooling without clogging, or reduction in required pumping power. Thus, nanofluids have emerged as suitable tailored working fluids in industrial, engineering and medical applications [5, 13–15], but this requires a rigorous analysis of heat transfer and rheological properties. The effective viscosity of a nanofluid constitutes a key property as it governs the ease of flow, pressure drop and thus the pumping power involved during flow applications . Concerning rheological behaviour of nanofluids, only a reduced number of studies can be found (see, e.g. Prasher et al. , Kwak and Kim , Chen et al. [2, 9, 19], Rao , Namburu et al. , Chevalier et al. ), evidencing a gap where further studies concerning rigorous characterization of their Newtonian behaviour limits and their viscoelastic trend are necessary. Moreover, recent studies have identified nanoparticle structuring/aggregation as a dominant mechanism for the thermal conductivity enhancement of nanofluids, and rheological analysis can provide a useful insight on their structure .
Following our previous research on nanofluids [22–25], we present experimental evidence of non-Newtonian behaviour of nanofluids obtained by dispersing hematite (Fe2O3) nanoparticles in ethylene glycol (EG). These ferrofluids are termed as smart functional fluids, due to some of its unique features, manifesting simultaneously fluid and magnetic properties, and have found applications in mechanical engineering, aerospace and bioengineering [26, 27]. The selected base fluid, EG, constitutes an excellent benchmark to compare viscosity results with literature. As an example of rheological analysis of ferrofluids, Hong et al.  studied water-based Fe3O4 nanofluids, reporting shear-thinning behaviour.
In this work, homogeneous and stable suspensions of commercial hexagonal scalenohedral-shaped α-Fe2O3 (hematite) nanoparticles in EG were prepared at concentrations up to 25% in mass fraction (6.6% in volume fraction). The average nanoparticle diameter value determined was 29 ± 18 nm. More details about nanofluid preparation, stability and characterization have been recently reported . These nanofluids were subjected to rheological analyses using a Physica MCR 101 rheometer (Anton Paar, Graz, Austria). The equipment allows to control torques between 0.5 μN·m and 125 mN·m and normal force from 0.1 to 30 N. The cone-plate geometry with a cone diameter of 25 mm and a cone angle of 1° was used. All experiments are conducted at a constant gap of 0.048 mm, and an initial stabilization period of 100 s is given for achieving constant temperature (303.15 K) using a Peltier system. Three replicates at each experimental condition were carried out.
Results and discussion
With the aim to check the operation of this rheometer using a cone-plate geometry and at shear rates up to 1,000 s-1 in the flow curves, initial experiments based on flow curves at controlled shear stress were carried out for pure EG, diisodecyl phthalate (DiDP) and polyalpha olefin (PAO-40). DiDP and PAO-40 represent Newtonian reference materials  in the moderate- to high-viscosity region. If compared with literature [10, 25, 29–32], excellent agreement is obtained for viscosities, with average deviations of 1.5%, 1.1% and 0.8% for EG [10, 25, 32], DiDP [30, 31] and PAO-40 , respectively.
The rheological studies were performed under two types of flow [33, 34]. The first is a non-linear viscoelastic experiment, the flow curve, or measurement of shear viscosity (η) as a function of shear rate (). The second is the linear viscoelastic oscillatory experiment, leading to the determination of frequency-dependent energy storage modulus G' (elastic) and loss modulus G″ (viscous), which reveal the mechanical properties of the material under small amplitude oscillatory shear. Oscillatory shear measurements within the linear viscoelastic domain, intended to measure G' and G″, represent a useful way of characterizing complex fluids.
Shear thinning of well-dispersed suspensions can be linked to the modifications in the structure and arrangement of interacting particles . Shearing may cause the particles to orient in the direction of flow and its gradient. This can break agglomerates and hence reduce the amount of solvent immobilized by the particles. The interaction forces may then decrease and cause a lowering in the flow resistance and the apparent viscosity of the system.
The inset in Figure 1 shows the time evolution of shear viscosity for the 25 wt.% EG/Fe2O3 sample, and its decrease evidences thixotropic behaviour or a structure loss under shear. For this reason, all flow curves were measured after the preliminary application of a constant stress during 500 s. This time evolution of viscosity had not been reported for nanofluids so far, but it must be considered when performing viscosity measurements of nanofluids because it may also produce spurious trends for the measured data.
This work evidences the non-Newtonian nature of EG/Fe2O3 nanofluids, showing shear thinning and thixotropy. All samples show viscoelastic nature, suggesting that a combination of particle aggregation and shape effects is the mechanism for its high-shear rheological behaviour, which is also supported by the thermal conductivity measurements [7, 24, 39]. G' decreases after a certain critical strain, and G″ presents an overshoot phenomenon. Finally, the results of the frequency sweep show that the damping factor presents a maximum against frequency, corresponding to a continuous evolution with concentration from viscous to elastic nature. This is an evidence of important aggregation and structural changes in the samples, a subject still poorly studied that deserves further attention.
The authors acknowledge CACTI (Univ. de Vigo) for technical assistance, and Univ. de Vigo, Xunta de Galicia (grant PGIDIT07PXIB314181PR), Min. de Educación y Ciencia (grant CTQ2006-15537-C02/PPQ) and Min. de Ciencia e Innovación (Ramón y Cajal Program), all in Spain, for financial support.
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