Suppression of Thermal Conductivity Enhancement in Carbon Nanofluids Caused by Surfactant High Concentration

Abstract

In this study, structural and thermal properties of dispersed carbon nanomaterials such as single-wall carbon nanotubes and graphene in water are investigated. Carbon nanostructures are dispersed in the fluid through surfactant and ultrasonication treatment. Structural studies of the samples are carried out by Raman spectroscopy which show that the dispersion process leads to the formation of two kind of nanofluids: nanostructured graphene based and single-wall carbon nanotubes ones. The first in the form of graphene dots. The thermal diffusivities of the samples are measured by thermal lens technique. Contrary to what is normally expected our nanofluids show a considerable reduction in thermal conductivity. The reduction is stronger for graphene dots than for the carbon nanotubes. This peculiar behavior of the thermal conductivity is attributed to the high concentration ratio of surfactant to dispersed particles, as well as, the formation of air layer or bubbles attached to the nanoparticles due to sonication process. To evaluate this hypothesis, simulation of the effective thermal conductivity based on Murshed model is conducted for spherical and cylindrical particles surrounded by air layer. The simulation shows an effective decrease in thermal conductivity, which corroborates with the experimental trend.