Effect of Nanoparticles Aggregation on Thermal and Electrical Conductivities of Nanofluids

We study the thermal conductivity (k), electrical conductivity (sigma), viscosity (eta) and refractive index (n) of two non-aggregating (stable) nanofluids of gamma-Al2O3 and SiO2 and two aggregating (unstable) nanofluids of TiO2 and alpha-Al2O3. The k, sigma, eta and n are found to increase with increase in volume fraction of nanoparticles in all the four systems. For stable nanofluids, the experimental k data falls within the effective medium theory (EMT) limit at lower concentration but deviates at higher concentration. For aggregating nanofluids, the experimental k values are found to be more than the EMT predictions. The k is found to be time independent and dependant, respectively in stable and unstable nanofluids. Our studies show that the volume fraction of nanoparticle has a strong influence on the sigma of nanofluids. The phase contrast microscopic study shows the presence of micron scale aggregates in TiO2 and alpha-Al2O3 nanofluids and the absence of micron scale aggregates in gamma-Al2O3 and SiO2 nanofluids. The effect of aggregation in unstable nanofluids leads to a strong modification of their viscosity and refractive index.