We study the effect of nanoparticle aggregation on thermal conductivity (k), viscosity (η) and size distribution of a stable and unstable nanofluid of iron oxide (Fe3O4) and copper oxide (CuO), respectively. The k, η, and size are found to be time independent in Fe3O4 while they are time dependant in CuO nanofluid. The microscopic study shows a progressive increase in the size of aggregate from nano to micron in CuO nanofluid. Our results show the prominent role of agglomeration on thermal properties and the importance of surface functionalization for improved stability and performance of nanofluids.

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