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Experimental investigation on the stability and density of TiO2, Al2O3, SiO2 and TiSiO4

At present, literature data on density of nanofluids are still scarce and controversial. In this paper, density data for nanofluids formed by water, as a base fluid, and solid nanoparticles of two different materials are presented. Density was measured by using a Density Meter DA-130N from Kyoto Electronics. The obtain result is compared with previous models and found incompatible for temperature variations.


Introduction
Researchers have always been thoroughly looking for ways to optimize the energy use in various devices. This is achievable through different tools. Nanofluids, defined as the mixture of a base fluid and nano-sized (1-100 nm) solid particles, have been recently introduced as one of the novel solutions in this issue. Because of their enhanced capability in terms of heat transfer, these fluids could be implemented as cooling media in different applications. However, existence of solid particles leads to interesting characteristics in the fundamental thermo-physical properties of nanofluids. Thermal conductivity, viscosity, density and stability have been investigated throughout the recent years by many researchers. . For the first time, Masuda et al. [1] reported a nearly 30% increase in the thermal conductivity of Al 2 O 3 /water nanofluids. In other early studies different increment in the thermal conductivity were reported like 40% increase [2] for copper and 15% increment [3] for alumina nanoparticles. The mechanism of thermal conductivity enhancement has been discussed by some studies like [4][5][6]. Brownian motion and thermophoresis have been stated to be the main factors behind the heat transfer behavior of nanofluids.
Viscosity of nanofluids has become of importance since it can majorly affect the performance of the system. Rheological behavior of nanofluid flow has been studied recently by many researchers. Reports indicate that viscosity generally increases due to the existence of nanoparticles [7,8]. Effective parameters on the viscosity like temperature, particle size and volumetric concentration have been investigated by some researchers [9][10][11]. Nevertheless, study on the density of nanofluids still seems to be confined to few researches. Sommers and Yerkes [12] investigated the density of alumina propanol nanofluid. Density characterization in this study was done by two different methods.
Nanofluids with oxide nanoparticles are interesting because they can be produced more easily than metallic or carbon nanotubes nanoparticles, and they are relatively cheap In this study, four different types of water based nanofluids such as, TiO 2 , SiO 2 , Al 2 O 3 and TiSiO 4 has been characterized.   Figure 2 given below shows the images of nanoparticles mixed in deionized water, which was obtained using Libra 120 transmission electron microscope at University Malaya. The images obtained show that the particles are indeed very small, whereas some of the particles match the manufacturer's specified size, while some exceeds the specified size (shown in the title boxes).

Nanofluid preparation
For this investigation, Nanofluids were prepared by dispersing nanoparticles in distilled water using two step method. Nanoparticles were dispersed into the base fluid using ultra sonic agitation. We used 30 mins of sonication time for preparing the samples. Different volume concentrations of nanoparticles (0.05%, 0.5%, 1%, 3% and 5%) were used to prepare distilled water based nanofluids mechanically. The density and stability of the above mentioned nanofluids will be further investigated with different surfactants such as, PEG, HTAB, SDS and PEG.

Conclusion
Thermo-physical property of nanofluids was evaluated in this work. The physical properties at varying temperatures are significant for engineering calculations. The present model of density doesn't satisfy the experimental results. The model was claimed valid for Al 2 O 3 for up to 4% vol. concentration but it failed when temperature variation is considered. However, the Pak and Chow model shows an excellent agreement with our experimental value at 25⁰C. Density variations with