In this paper, the effects of alignment, pH, surfactant and solvent on heat transfer nanofluids containing Fe2O3 and CuO nanoparticles are studied and analyzed. The microscope images show that Fe2O3 could form some kind of alignment spontaneously in water even without external magnetic field. With the addition of external magnetic field, the alignment is strengthened. In water, the magnetic particle agglomeration to larger size occurs easily, which makes the directional alignment much faster and easier. Ethylene glycol solvent and chemical surfactant sodium dodecyl benzene sulfonate, NaDDBS could separate the Fe2O3 and CuO nanoparticles well in the fluids and avoid possible aggregation. Therefore, magnetic alignments are hard to observe. The measured thermal conductivities of each individual sample coincide with the microscope images and assumptions. In addition, pH values of Fe2O3 and CuO nanoparticles are measured and it has been determined that at those pH values, thermal conductivities of those nanoparticles would not be influenced according to the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The highlight of this paper is that our microscope images could well explain most of the literature data and conclusions and may open new door to better understanding fundamental nature of nanofluids.

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It has been shown that the alignment of Iron (III) oxide (Fe2O3) nanoparticles in water (H2O) can enhance the thermal conductivity of nanofluids. To better understand solvent effects such as hydrogen bonding, viscosity, and polarity, nanofluids were prepared by mixing Fe2O3 nanoparticles and various solvents (water, ethanol, 1-propanol, isopropanol, 2-propanone, hexane, cyclohexane, ethylene glycol, glycerol, etc.), and the dispersions and alignments of the Fe2O3 nanoparticles in these solvents with and without an applied magnetic field were investigated using an optical microscope. The microscope images indicated that inter-molecule hydrogen bonding of the solvents with one OH group (water, ethanol, 1-propanol, and isopropanol) could help to disperse and align the Fe2O3 nanoparticles. The intra-molecular hydrogen bonding causes a dramatic increase in viscosity for fluids with multiple OH groups, such as ethylene glycol (C2H6O2) and glycerol (C3H8O3), and makes the Fe2O3 nanoparticles dispersion and alignment difficult. Adding water to those fluids could lead to significantly reduced viscosity and make the particles disperse and align well. Polarity studies indicated that higher polarity yields better dispersion and alignment of the Fe2O3 nanoparticles. Thermal studies showed that thermal conductivity of nanofluids containing metal oxide particles with hydrogen bonding in solvents is enhanced compared to the theoretically calculated data. Intermolecular hydrogen bonding between water and ethylene glycol increases the thermal conductivity of nanofluids while decreasing the fluid viscosity. The results also well explain why 50 wt. % water/50 wt. % ethylene glycol is an excellent commercial coolant. Since high thermal conductivity enhancement with minimal viscosity increase is the primary goal of heat transfer nanofluids, this current research may open new doors to better understanding of the fundamental nature of nanofluids.

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