Particle Aspect-Ratio and Agglomeration-State Effects on the Effective Thermal Conductivity of Aqueous Suspensions of Multiwalled Carbon Nanotubes

The effective thermal conductivities of aqueous nanofluids containing surfactant-stabilized multiwalled carbon nanotubes were measured and compared with the predictions of effective medium theory (Nan, C.-W., , 1997, "Effective Thermal Conductivity of Particulate Composites With Interfacial Thermal Resistance," J. Appl. Phys., 81(10), pp. 6692-6699). Detailed characterization of nanotube morphology was carried out through electron microscopy, while the nanotube agglomeration state was monitored through optical microscopy and absorption measurements. An optimum surfactant-to-nanotube mass ratio was found for the particular surfactant, sodium dodecylbenzene sulfonate, which resulted in the greatest increase in thermal conductivity. Taking into consideration the volume-weighted aspect ratio of the nanotubes, the measured thermal conductivities of the suspensions were shown to be in good agreement with calculations for a reasonable choice of interfacial resistance on the particle/liquid interface. The effect of particle aspect ratio on the suspension's thermal conductivity was further demonstrated and compared with theory by reducing the nanotube length through intense ultrasonication. The effect of particle aggregation on the thermal conductivity was also investigated by destabilizing previously stable suspensions with ethanol addition, which causes surfactant desorption and bundling of nanotubes. The measured thermal conductivities were correlated with absorption measurements and microscopic visualizations to show that particle aggregation decreases the thermal conductivity of the nanofluid by reducing the effective particle aspect ratio. [DOI: 10.1115/1.4001364]