Role of Adsorbing Moieties on Thermal Conductivity and Associated Properties of Nanofluids

We probe the contributions of adsorbing moieties (surface. active species) on thermal conductivity (k), rheology, density, contact angle, and refractive index of nanofluids. The role of surface morphology, initial thermal conductivity of solid particles and their number density on thermal property enhancement of nanofluids is also studied. Our studies on a model soft sphere system, consisting of Micelles of an average size of 2.5-7 nm with different headgroup charges, show that the thermal conductivity of ionic surfactant micelles follows the effective medium theory of poor thermal conductors, i.e., 1 - 3 phi/2 with the interfacial resistance tending to infinity, where phi is the volume fraction. The results suggest that the long alkyl chain group of nonionic surfactant micelles are poor thermal conductors at very low concentrations with large interfacial tension compared to their anionic counterparts. The k measurement in aqueous alumina, silica, and nonaqueous iron oxide nanofluids with particle size in the range of 12-15 nm shows that the presence of excess surfactant does not lower the thermal conductivity enhancement in nanofluids, though the latter has a much lower thermal conductivity than the base fluid. The adsorbed moieties indeed enhance the stability of nanoparticles in the base fluids. Further, our studies show that the k enhancement in nanofluids is independent of the initial thermal properties of suspended particles. For both "soft" and "hard" sphere particles, the k follows the effective medium theory for poor and good conductors, respectively. Further, our results show that the orientation of cylinders and their aspect ratio are key to the thermal conductivity enhancement in nanofluids. While cylinders with higher aspect ratio and random ordering give a lower k, orientated cylinders with higher aspect ratio provide dramatic enhancement in the k. These finding are very useful for engineering efficient nanofluids for thermal managements.