An emerging field of nanofluids has the potential to transform multiple industries in terms of their thermal management techniques and could bring evolution in stable and higher-performing devices.
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The term “Nanofluids” was first proposed by Choi of Argonne National Laboratory USA in 1995, which meant nanoparticle fluid suspensions. After which, this interdisciplinary field has developed extensively.
Nanofluids are fluids containing nanomaterials having a size in the range of one to 100 nm. These nanomaterials could be metallic nanoparticles like aluminum, copper, and nickel. Alternatively, they could be metal oxides like aluminum oxide, titanium oxide, cuprous oxide, silicon dioxide, and magnetite, or other materials like aluminum nitride, silicon carbide, and graphene.
Nanofluids are mainly utilized in heat transfer systems as they show thermal properties superior to those of their host fluids. This technology shows that the thermal properties of conventional fluids like water, engine oil, or ethylene glycol, could be enhanced by uniform dispersion of nanomaterials forming a stable suspension in the host fluid.
Nano-size and high surface-to-volume ratio of nanomaterials cause high thermal conductivity, reduced clogging in the direction of flow, high heat transfer rate, and long-term stability of the nanofluids.
How are Nanofluids Formulated?
There are mainly two techniques through which nanofluids are formulated: the one-step method and the two-step method.
In the one-step method, nanoparticles are synthesized and dispersed into a fluid simultaneously following physical or chemical processes. This method has the advantage of less aggregation and high stability but has small-scale production and high costs.
In the two-step method, nanoparticle synthesis and dispersion take place in two steps. This is the most popular method of nanofluid formulation.
Viscosity is one of the properties of nanofluids that determines their properties. In general, the viscosity of fluids could directly influence the pumping power, pressure drop in laminar flow, and convective heat transfer in fluids. Hence, to determine the thermal properties of fluids used in heat transfer systems, analysis of the viscosity of fluids is essential.
Parameters Determining the Viscosity of Nanofluids:
The viscosity of nanofluids is measured through rheometers or viscometers; in most cases, a rotational rheometer is used. These instruments include the concentric cylinder/spindle type rheometer or the cone and plate type rheometer and, in some instances, rheometers containing both types. Few studies had reported the use of oscillating piston viscometer, capillary viscometer, and vibro viscometer to measure the viscosity of nanofluids.
Various parameters influence the viscosity of nanofluids, such as particle shape, particle size, the volume fraction of nanomaterials in the fluid, surfactants used, shear rate, particle aggregation, and pH value of the liquid.
Viscosity of nanofluids is different for different sizes of the same nanoparticles. In general, an increase in nanofluids' viscosity has been reported with an increase in particle sizes. However, few studies reported the decrease in viscosity with the increase in particle size.
The reason for the decrease in viscosity was ascribed to the increased resistance at the nanoparticle fluid interface due to the increased surface-to-volume ratio in smaller nanoparticles.
The shape of the dispersed nanomaterial has shown a strong influence on the viscosity of nanofluids. Timofeeva et al. (2009) reported that nanofluids with elongated particles have increased viscosity compared to nanofluids with spherical nanoparticles.
The effect of volume concentration of nanomaterials in the viscosity of nanofluids has been mostly found to increase with the increase in even a small volume fraction of nanomaterial in the nanofluid.
Temperature is reported as the most influential factor in the viscosity of nanofluids. As described by Thomas et al. (2011), the attractive force within the particles and with the fluid decreases with the increase in temperature; hence the viscosity decreases.
A study based on nanofluids of zirconia nanoparticles in ethylene glycol and another investigating alumina nanoparticles in ethylene glycol has shown the same trend of decrease in viscosity with an increase in temperature.
The pH of the nanofluid is another factor over which the viscosity has shown dependency. Research has reported that there is an optimal value of pH at which the nanofluid dispersion is found stable.
In a study reported by Wang Xian-Ju et al.,(2013) nanofluids with the smallest viscocity containing alumina and copper nanoparticles were found at pH values of 7.7 - 8.9 for alumina and >7.6 for copper nanoparticles based nanofluids, respectively.
Shear rate is another parameter that strongly influences the viscosity of nanofluids; it helps distinguish between Newtonian and non-Newtonian nanofluids.
There are many more parameters over which the viscosity of nanofluids depends; however, there is no theoretical formula to accurately measure the nanofluid’s viscosity. Hence, further work has to be done to determine new models that can accurately determine viscosity in nanofluids.
Applications of Nanofluids:
Studies have shown that the nanofluids acquire extremely desirable thermal properties such as higher thermal conductivity, convection heat transfer coefficient, and critical heat flux compared to the base fluid. These thermal features of nanofluids open up a wide door for many applications. Hence, nanofluid technology has emerged as a new field in science where researchers could discover innovative applications.
Heat management in many devices is an essential need in many industries. Heat rejection requirements are frequently increasing in the industry of micro and nano-electromechanical systems (MEMs and NEMS), power electronics, integrated circuits, semiconductor lasers, and many more. As for the stable performance of devices, their thermal management is a critical requirement.
Nanofluids have been identified as next-generation coolants such as smart coolants in computers or as safe coolants in nuclear reactors. A study conducted at Nuclear science and engineering department of Massachusetts Institute of Technology has studied the feasibility of using nanofluids as coolants in nuclear applications.
However, many technical issues are faced in the commercialization of nanofluids, such as volume production, low-cost production, highly stable dispersion of nanofluids, and environmental issues. Further research has to be done to develop the synthesis and applications of nanofluids to progress in the device/instrument perspectives.
Continue reading: Why is Understanding the Rheology of Nanofluids Important?
References and Further Reading
Choi, S.U. (2009) Nanofluids: from vision to reality through research. Journal of Heat transfer, 131(3). Available at: https://doi.org/10.1115/1.3056479.
Wang, L., Chen, H. and Witharana, S. (2013) Rheology of nanofluids: a review. Recent patents on nanotechnology, 7(3), pp.232-246. Available at: https://pubmed.ncbi.nlm.nih.gov/24330046/
Thomas, S. and Sobhan, C.B.P. (2011) A review of experimental investigations on thermal phenomena in nanofluids. Nanoscale research letters, 6(1), pp.1-21. Available at: https://doi.org/10.1186/1556-276X-6-377.
Timofeeva, E.V., Routbort, J.L. and Singh, D. (2009) Particle shape effects on thermophysical properties of alumina nanofluids. Journal of applied physics, 106(1), p.014304. Available at: https://doi.org/10.1063/1.3155999.
Goharshadi, E.K. and Hadadian, M. (2012) Effect of calcination temperature on structural, vibrational, optical, and rheological properties of zirconia nanoparticles. Ceramics International, 38(3), pp.1771-1777. Available at: https://doi.org/10.1016/j.ceramint.2011.09.063.
Xian-Ju, W. and Xin-Fang, L. (2009) Influence of pH on nanofluids' viscosity and thermal conductivity. Chinese Physics Letters, 26(5), p.056601. Available at: https://doi.org/10.1088/0256-307X/26/5/056601.
Wong, K.V. and De Leon, O. (2010) Applications of nanofluids: current and future. Advances in mechanical engineering, 2, p.519659. Available at: https://doi.org/10.1155%2F2010%2F519659.
Mishra, P.C., Mukherjee, S., Nayak, S.K. and Panda, A. (2014) A brief review on viscosity of nanofluids. International nano letters, 4(4), pp.109-120. Available at: https://doi.org/10.1007/s40089-014-0126-3.
Zhang, S. and Han, X. (2018) Effect of different surface modified nanoparticles on viscosity of nanofluids. Advances in Mechanical Engineering, 10(2), p.1687814018762011. Available at: https://doi.org/10.1177%2F1687814018762011.