Dynamic viscosity measurement in non-Newtonian graphite nanofluids
© Duan et al.; licensee BioMed Central Ltd. 2012
Received: 10 May 2012
Accepted: 2 July 2012
Published: 2 July 2012
The effective dynamic viscosity was measured in the graphite water-based nanofluids. The shear thinning non-Newtonian behavior is observed in the measurement. On the basis of the best fitting of the experimental data, the viscosity at zero shear rate or at infinite shear rate is determined for each of the fluids. It is found that increases of the particle volume concentration and the holding time period of the nanofluids result in an enhancement of the effective dynamic viscosity. The maximum enhancement of the effective dynamic viscosity at infinite rate of shear is more than 24 times in the nanofluids held for 3 days with the volume concentration of 4% in comparison with the base fluid. A transmission electron microscope is applied to reveal the morphology of aggregated nanoparticles qualitatively. The large and irregular aggregation of the particles is found in the 3-day fluids in the drying samples. The Raman spectra are extended to characterize the D and G peaks of the graphite structure in the nanofluids. The increasing intensity of the D peak indicates the nanoparticle aggregation growing with the higher concentration and the longer holding time of the nanofluids. The experimental results suggest that the increase on effective dynamic viscosity of nanofluids is related to the graphite nanoparticle aggregation in the fluids.
KeywordsDynamic viscosity Graphite nanofluids Nanoparticle aggregation Non-Newtonian flow
Nanofluids, consisting of suspended nanoscale solid particles, can improve thermal conductivity and heat transfer coefficient from the base fluids [1–10]. The effectiveness of thermal property enhancement of nanofluids depends on nanoparticle amount, particle size, particle materials, particle shape, base fluids, etc. However, since nanofluids are suspensions with nanoparticles in their base fluids, achieving a stable dispersion in nanofluids would benefit industrial applications. Nanoparticles are expected to stabilize the fluids more effectively than the microparticles, the fluid properties including the dynamic viscosity would change accordingly. The viscosity of nanofluids is important for nanofluid transport related to flow dynamics and heat transfer. The spherical shaped Al2O3, TiO2, and the other nanoparticles have been widely studied in the nanofluids [2–5, 8]. The results showed an enhancement on the effective dynamic viscosity as an increase of concentrations. A strong correlation was indicated between the rheological behavior and the structure of nanoparticles in the nanofluids. The Al2O3-water nanofluid exhibited as a Newtonian flow after freshly prepared, but a shear thinning non-Newtonian flow after the aggregation was formed in the nanofluids. The dynamic viscosity had a significant increase as a result. However, the properties can be resumed after the re-ultrasonication process, in which the aggregates were dispersed again . The main nonspherical nanoparticles in suspensions under the study are carbon nanotubes (CNTs) and graphite on thermal conductivity [1, 9, 10]. However, there are limited reports on the nanofluid dynamic viscosity with the nanoparticles at different heights to width aspect ratios, especially for the graphite nanoparticles. Ding et al. measured the dynamic viscosity of CNTs-water nanofluids as a function of shear rate, showing the fluids with a non-Newtonian property . The viscosity of the nanofluids was found to increase with the increasing concentrations of CNTs and the decreasing temperature. Yang et al. investigated the rheological behavior of poly(α-olefin) solutions dispersed by the rodlike CNTs with an aspect ratio of about 30, or the disklike graphite nanoparticles an aspect ratio of about 0.025 . The nanofluids acted as a shear thinning fluids. The above studies suggest that the nanoparticles may aggregate in the base fluids, and the aggregation would affect the rheological properties. To explicate the phenomena, the investigation of the dynamic viscosity is carried out in the graphite-water nanofluids for the potential application. The morphology of the nanoparticle aggregates and the structure in molecular vibration are demonstrated in this paper by using a transmission electron microscope and a Raman spectroscope, respectively.
In the experiments, a two-step method was used to prepare the graphite water-based nanofluids. The graphite nanoparticles were supplied by SkySpring Nanomaterials, Inc. (Houston, TX, USA) with a reported average size of 3 to 4 nm. We dispersed the nanoparticles in the 40 mL deionized water to prepare the nanofluids with the volume concentrations at 1 %, 2 %, 3 %, and 4 % without adding any surfactant in order to study the effect of nanoparticle aggregation. The next step for the nanofluids was to undergo a mechanical stirring process with a magnetic stirrer at a rotation speed 540 rpm for about 7 h. Then, the nanofluids were performed under ultrasonication by using the ultrasonic bath (Elmasonic E 15H, Singen, Germany) continuously for about 1.5 h to prevent the well-dispersed fluids from aggregation initially.
The effective dynamic viscosity of the graphite-water nanofluids was measured directly with a standard controlled shear rate rheometer (Contraves LS 40, Mettler-Toledo, Greifensee, Switzerland) which has a cup and bob geometry. This instrument requires only a volume of liquid of approximately 5 mL. The instrument was calibrated by measuring the dynamic viscosity of the deionized water. The calibration results showed the measurement error within ±1% from the viscosity value of 0.000891 Pa·s. All measurements in this study were performed at 1 atm and 298.15 K. The effective dynamic viscosity of the nanofluids was measured instantly after the ultrasonication agitation. Thereafter, the same nanofluids were measured again after 3 days, which is determined by the experimental observation with the stratified fluids. Before the measurement, the fluids were shaken to prevent the possible particle sediment in the measurement. The relative effective dynamic viscosity is calculated with a reference value of the base fluid (pure water).
A transmission electron microscope (TEM, JEOL, JEM-2010, Tokyo, Japan) is applied to reveal the microstructural morphology of the graphite particles in the dried samples from the nanofluids. In preparation, the nanofluids in the volume fraction of 1% were diluted so as to reduce the possibility of the particle agglomeration in preparing the TEM samples in the drying process . Then, a little drop of the nanofluid samples was dried naturally by placing on the copper grid coated with carbon film. The TEM instrument was used at an operating voltage of 200 kV in the graphite-water nanofluids instantly after preparation (fresh) and 3 days after, respectively. The Raman spectra were to disclose the molecular structure of materials. A Renishaw inVia Raman spectroscope (Wotton-under-Edge, UK) was applied by using the 514 nm He-Ne laser source with a laser power setting at 10 mW to determine the structure of nanoparticle aggregation on the basis of the molecular vibration. The fresh nanofluids and the fluids held for 3 days were sampled and then measured at room temperature for the Raman spectra.
Results and discussion
Fitting parameters of the steady shear measurement for the nanofluids held 3 days (nff)
Fitting parameters of the steady shear measurement for the nanofluids held 3 days (nfo)
The fitting errors of the effective dynamic viscosity of fresh nanofluids and 3-day fluids
As listed in Table 3, the average of absolute fitting errors is 0.49% for the 1 vol% fresh graphite-water nanofluids compared with the value at 0.98% in the 1% nanofluids held for 3 days. For a higher volume concentration at 4%, the mean of absolute fitting errors is 1.28% for the fresh nanofluids, but 1.87% for the 3-day fluids. The absolute average fitting error has a smaller value compared with the mean of the absolute fitting errors in the same nanofluids. The maximum absolute average fitting error is 0.11% for the fresh nanofluids while it is 0.24% for the 3-day fluids. It can be seen that the fitting curve is very close to the experimental data.
Similarly, Kim et al. reported that the CNT-based nanofluids had such the phenomenon by pointing out that the high surface effect and the strong van der Waals force drive the nanoparticles to form the aggregation in the suspensions . Since most of aggregates might be destroyed under high shear rates, the nanofluids are shown in shear thinning non-Newtonian behaviors [9, 11, 15]. We have to mention that the graphite-water nanofluids have a different flow property from the Al2O3-water nanofluids, which are Newtonian flows if the nanofluids are freshly prepared, but non-Newtonian flows if the fluids have large aggregates . The different results might be from the various nanoparticle sizes, species, and configuration. The graphite-water nanofluids might have the particle aggregation just after the preparation, and show the non-Newtonian flow properties. The size of aggregation in nanofluids would increase with the increase of the particle volume concentration and the holding time. Thus, it would take a higher force to break the ligand structure among particles in the aggregated fluids [9, 11], as a result, a high effective dynamic viscosity ratio can be observed in Figure 3.
The TEM images of the graphite particles dried from the nanofluids are shown in Figure 4. It is found that the average diameter of graphite particles dispersed in the nanofluids just after the ultrasonic agitation is up to 50 nm shown in Figure 4a. The nanoparticles are still larger than those specified by the supplier in the powder form. It suggests that the graphite nanoparticles have aggregated into a certain size even in the fresh nanofluids, resulted from the high surface effect of nanoparticles and the inter-particle attraction [11, 18]. However, the graphite particles were significantly aggregated if the nanofluids were held for 3 days, as shown in Figure 4b. The size of aggregated graphite particles is larger than 150 nm at least. The highly fragmented aggregation clusters were found in the microstructure analysis. The aggregation of nanoparticles in the graphite-water nanofluids increased with a longer holding time. It supports the aforementioned discussion that the effective dynamic viscosity increases at the 3-day fluids. Note that the nanoparticle boundary is detectable in the aggregates shown in Figure 4a. We can estimate that the largest dimension of the particle is at about 18 nm. Thus, if the 3 to 4 nm is treated as the thickness of the graphite nanoparticle, the height to width aspect ratio could be up to 0.17, which is much larger than those used by Yang et al. .
The effective dynamic viscosity of the graphite-water nanofluids is experimentally found to decrease with an increase of shear rate in a given particle volume fraction (Figures 1 and 2). The nanofluids act as the shear thinning non-Newtonian flows. The data of the effective dynamic viscosity in nanofluids are fitted numerically, the relative effective dynamic viscosity at infinite rate of shear increases to 2.92 in the fresh nanofluids at 4 vol% in comparison of the base fluid, but 24.86 for the nanofluids held for 3 days (Figure 3). The microstructure of the diluted nanofluids indicates that the aggregation of nanoparticles is significantly higher in the 3-day fluids than that in the fresh nanofluids, as shown in Figure 4. The Raman spectra are used for showing the formation of larger graphite nanoparticle aggregation with an increase of the volume concentration or the holding time of the nanofluids in Figure 5. This study suggests that the aggregation would happen in the nanofluids which have not been treated specially by adding the surfactant, controlling the pH value, etc. The aggregation would dramatically change the nanofluid properties including viscosity consequently.
FD is an assistant professor on thermofluids at Nanyang Technological University. TFW was an undergraduate student for his final year project under FD. AC is a Ph.D. student on nanofluids.
The authors acknowledge the support of MOE AcRT Tier 1 and the help from the staff of Materials Lab in Nanyang Technological University.
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