Thermorheological properties of nanostructured dispersions

Abstract

Nanostructured dispersions, which consist of nanometer-sized particles, tubes, sheets, or droplets that are dispersed in liquids, have exhibited substantially higher thermal conductivities over those of the liquids alone. While it is desirable to synthesize a fluid that has improved heat transfer characteristics, it is necessary that the viscosity remain low, so as not to appreciably increase the pumping power needed to employ these fluids in "real world" applications. To this end, the theological and thermal properties of twenty-six different nanostructured dispersions were examined. In terms of rheometry, both steady flow and creep tests were employed, while the transient hot wire technique was utilized to perform measurements of the thermal conductivity of each fluid. Characterization of the dispersed phase was completed using dynamic light scattering and transmission electron microscopy. In particular, the dispersion properties examined were nanostructure material, nanoparticle size, base fluid material, nanostructure concentration, and presence of a surfactant. It was observed that several of the fluids or nanopowders obtained from commercial manufacturers either contained no particles, had the presence of a relatively large proportion of water in ethylene glycol-based fluids, or were composed of particles with sizes far in excess of those claimed by the manufacturer.

(cont.) Ultimately, it was determined that while most of the fluids studied demonstrated Newtonian or slightly shear thinning behavior, several of the fluids exhibited undesirable yield stresses that could be attributed to the formation of a network structure of aggregated nanoparticles. However, it was observed that the addition of a surfactant helped to keep the nanoparticles from clustering to the same degree, thereby eliminating the presence of a yield stress, and reducing the viscosity of the fluid over the entire range of shear rates. The surfactant also contributed to an increase in thermal conductivity enhancement, thereby producing a highly desirable behavior.