Yielding and flow of highly concentrated, few-layer graphene suspensions (original) (raw)
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Rheology and microstructure of dilute graphene oxide suspension
Graphene and graphene oxide are potential candidates as nanofluids for thermal management applications. Here, we investigate the rheological properties and intrinsic viscosity of aqueous suspension of graphene and use the measured intrinsic viscosity to determine the aspect ratio of graphene oxide. Dilute suspension of graphene oxide (0.05 to 0.5 mg/mL) exhibits a shear thinning behavior at low shear rates followed by a shear-independent region that starts at shear rate between 5 and 100/s depending on the concentration. This shear thinning behavior becomes more pronounced with the increase of particle loading. Moreover, AFM imaging of the dried graphene oxide indicates the evolution of irregular and thin low fractal aggregates of 0.3–1.8 nm thickness at lower concentrations to oblate compact structures of 1–18 nm thickness of nanosheets at higher concentration. These observations elucidate the microstructure growth mechanisms of graphene oxide in multiphase systems, which are important for nanofluids applications and for dispersing graphene and graphene oxide in composite materials. The suspension has a very high intrinsic viscosity of 1661 due to the high graphene oxide aspect ratio. Based on this intrinsic viscosity, we predict graphene oxide aspect ratio of 2445. While the classical Einstein and Batchelor models underestimate the relative viscosity of graphene oxide suspension, Krieger–Dougherty prediction is in a good agreement with the experimental measurement.
Viscosity and Rheological Properties of Graphene Nanopowders Nanofluids
Entropy, 2021
The dynamic viscosity and rheological properties of two different non-aqueous graphene nano-plates-based nanofluids are experimentally investigated in this paper, focusing on the effects of solid volume fraction and shear rate. For each nanofluid, four solid volume fractions have been considered ranging from 0.1% to 1%. The rheological characterization of the suspensions was performed at 20 ∘C, with shear rates ranging from 10−1s−1 to 103s−1, using a cone-plate rheometer. The Carreau–Yasuda model has been successfully applied to fit most of the rheological measurements. Although it is very common to observe an increase of the viscosity with the solid volume fraction, we still found here that the addition of nanoparticles produces lubrication effects in some cases. Such a result could be very helpful in the domain of heat extraction applications. The dependence of dynamic viscosity with graphene volume fraction was analyzed using the model of Vallejo et al.
Journal of Nanoparticle Research, 2013
The viscosity of Graphene nano-sheet suspensions (GNS)and its behavior with temperature and concentration have been experimentally determined. A physical mechanism for the enhanced viscosity over the base fluids has been proposed for the poly-dispersed GNSs. Experimental data reveals that enhancement of viscosity for GNSs lie in between that of Carbon Nanotube Suspensions (CNTSs) and nano-Alumina suspensions (nAS) , indicating the hybrid mechanism of percolation (like CNTs) and Brownian motion assisted sheet dynamics (like Alumina particles). Sheet dynamics and percolation, along with a proposed percolation Network Dynamicity Factor; have been used to determine a dimensionally consistent analytical model to accurately determine and explain the viscosity of poly-dispersed GNSs. It has been hypothesized that the dynamic sheets behave qualitatively analogous to gas molecules. The model alsoprovides insight into the mechanisms of viscous behavior of different dilute nanoparticle suspensions. The modelhas been found to be in agreement with the GNS experimental data, and even for CNT and nano-Alumina suspensions.
International Communications in Heat and Mass Transfer, 2018
Energy efficiency of systems stands out as a critical parameter to achieve sustainability. Heat transfer processes spend an important part of overall energy consumption, so the enhancement of its efficiency is an important research field. Nanofluids have received increasing attention to reach this goal because of its initial conception as thermal-improved working fluids. Nevertheless, the increase in thermal conductivity produced by the dispersion of nanoparticles should not be the only focus. In forced convection processes, viscosity plays an important role in achieving an improvement of the exchange of heat. In addition, the energy consumption needed to pump the new fluid is directly related to its viscosity. In this work, the rheological behaviour of different loaded functionalized graphene nanoplatelet dispersions (nanoparticle mass concentrations, wt%, of 0.25, 0.50, 0.75 and 1.0) in three based fluids (water, propylene glycol:water mixture at 30:70 wt% and propylene glycol:water mixture at 50:50 wt%) has been investigated by using a rotational rheometer. A double cone geometry was employed together with a cover specially appropriated to help thermostatization and avoid evaporation at high temperatures. The viscosity curves in the shear rate range from 10 to 1000 s −1 , with 10 points per decade, and in the temperature range from 283.15 to 353.15 K, with 10 K step, were determined for the three base fluids and the twelve different nanofluids. The dynamic viscosities values of the base fluids were compared with those available in the literature, absolute average deviations being lower than the experimental uncertainty. The dynamic viscosities of the nanofluids were obtained for the Newtonian ranges and their dependences with temperature and nanoadditive concentration were analysed, as well as the influence of the base fluid on these trends. Furthermore, the fitting parameters of an equation that models temperature and nanoparticle concentration dependences are provided, which allow to describe viscosity data for each of the three sets of nanofluids with deviations lower than 1.8%. Finally, linear viscoelastic oscillatory experiments were performed for those samples that showed evidences of Non-Newtonian behaviour in the non-linear tests. These oscillatory tests were carried out in the deformation range from 0.1 to 1000% at constant frequency, 1 Hz, and at 283.15 and 293.15 K. Unlike the base fluids and other nanofluids sets, the nanofluids of the analysed propylene glycol:water 50:50 nanofluid set at 283.15 K and 293.15 K present a clear shear thinning (pseudoplastic) non-Newtonian behaviour, the Newtonian plateaus being easily identified as the concentration rises in the lowest deformations range.
Structural and mechanical characterization of platelet graphite nanofibers
Carbon, 2007
Platelet graphite nanofibers have been characterized by scanning electron microscopy, transmission electron microscopy, electron diffraction, X-ray photoemission spectroscopy, and atomic force microscopy. The results show that the graphene sheets are stacked parallel to each other and are perpendicular to the fiber axis; the interlayer spacing is 0.34 nm. A small fraction of carbon atoms are bonded to oxygen. Solid-state nuclear magnetic resonance shows that hydrogenated carbons are under the detection limit (<5%) and that the nanofibers are dominated by sp 2-bonded carbons. Mechanical measurements were made on individual nanofibers by nanoindentation.
Fluids
This work studies the influence of the concentration and oxidation degree on the rheological behavior of graphene oxide (GO) nanosheets dispersed on polyethylene glycol (PEG). The rheological characterization was fulfilled in shear flow through rotational rheometry measurements, in steady, transient and oscillatory regimes. Graphene oxide was prepared by chemical exfoliation of graphite using the modified Hummers method. The morphological and structural characteristics originating from the synthesis were analyzed by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, and atomic force microscopy. It is shown that higher oxidation times increase the functional groups, which leads to a higher dispersion and exfoliation of GO sheets in the PEG. Moreover, the addition of GO in a PEG solution results in significant growth of the suspension viscosity, and a change of the fluid behavior from Newtonian to pseudoplastic. This effect is r...
Macroscopic Graphene Membranes and Their Extraordinary Stiffness
Nano Letters, 2008
The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal and other characteristics of this one-atom-thick material. Here we describe a new and highly-reliable approach for making graphene membranes of a macroscopic size (currently up to 100 µm in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by one side only do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.
Rheological Properties of Graphene Oxide Liquid Crystal, Carbon 2014
Carbon, 2014
We report the rheological properties of liquid crystalline graphene oxide (GO) aqueous dispersion. GO dispersions exhibit typical shear thinning behaviors of liquid crystals, which is described by power law or simple Curreau model. Irrespective of the shear rate, shear viscosity exhibits sudden decrease with the increase of GO composition around a critical volume fraction, / c = 0.33%, demonstrating typical colloidal isotropic-nematic phase transition. Dynamic measurements reveal the liquid-like (isotropic phase, G 0 > G 00 ) behavior at a low GO composition (/ $ 0.08%) and solid-like (liquid crystalline) behavior at higher compositions (/ $ 0.45%), where G 0 exceeds over G 00 . Nematic gel-like phase is confirmed at a higher GO composition over / > 0.83%, where both G 0 and G 00 moduli are nearly independent of frequency (x). Simple power law scaling arguments are introduced to model the dependence of yield stress and viscoelastic moduli on the GO composition. We also observed the yield stress and rigidity percolation transition above phase transition composition / c > 0.33% with a percolation exponent of 1.3 ± 0.1. These rheological insights provide valuable information for the liquid crystalline processing of GO based materials including fibers, sheets and other complex structures for electronic/optoelectronic and energy storage/conversion applications.
Formulation and micro-extrusion of high concentration graphene slurries
Ceramics International, 2016
In this study we have investigated the feasibility of preparing high concentration graphene slurries and their rheological properties as a function of various parameters including the sonicating time, solid loading, surfactant concentration, different types of graphene, aqueous and non-aqueous solvents. The feasibility of micro-extruding slurries with high graphene loading has also been studied. We demonstrate that slurries with graphene loading at 13 wt.% (equivalent to 6.3 vol.%) can be prepared to have pseudoplastic (shear thinning) properties. The 6.3 vol.% graphene is the highest graphene loading ever reported in the open literature. An unusual phenomenon of increasing graphene loading leading to reduced viscosity after graphene loading has reached a critical value has been observed for the first time. This unusual phenomenon is attributed to the forced alignment of graphene flakes during shearing when graphene loading is high enough and has been utilized to prepare slurries with high graphene loading and pseudoplastic properties. Micro-extrusion of these high concentration graphene slurries reveals the capability of the extrudate to maintain its shape right after extrusion of the slurries. Thus, it is expected that these high concentration graphene slurries can be used to fabricate three-dimensional objects through layer-by-layer fabrication methods with high fabrication rates in the near future.