## 2 Nanofluid Thermal Conductivity
### Effects of Surfactant on Nanofluid Thermal Conductivity
It was observed that over a time period of 8 days, the nanofluid containing surfactant remained stable, while greater sedimentation was observed in the nanofluid containing no surfactant. It was suggested that the surfactant molecules on the surfaces of the nanoparticles created bonds between the nanoparticles and the base liquid, and limited aggregation, leading to more stable suspensions. Using a KD2 Pro Thermal Property Analyzer, [PERSON] et al. [39] measured the thermal conductivity of FeC-water nanofluid as a function of temperature and nanoparticle volume concentration. Due to the carbon-rich nature of FeC particles, low viscosity carboxymethyl cellulose sodium salt was used as a surfactant. The surfactant was used in a concentration of 3 g/L. It was observed that the surfactant itself did not have a significant effect on the thermal conductivity of the base fluid. However, it was suggested that the surfactant may have indirectly influenced the thermo-physical properties of the nanofluid by improving the stability. [PERSON] et al. [33] tested the stability of TiO\({}_{2}\)-water nanofluid using CTAB, acetic acid (AA), OA, and SDS as surfactants. It was observed that only AA and CTAB surfactants produced stable nanofluids. In some cases, researchers have observed negative effects of surfactants on the thermal conductivity of nanofluids. Using a KD2 Pro Thermal Property Analyzer, [PERSON] et al. [22] measured the thermal conductivity of Al\({}_{2}\)O\({}_{3}\)-water nanofluid as a function of temperature and nanoparticle volume concentration. The stability of the nanofluid was tested using CTAB, SDBS, and SDS as surfactants. It was observed that SDS and CTAB surfactants did not increase stability of the nanofluid. SDBS surfactant showed the highest stability, yielding a more homogeneous distribution of nanoparticles within the base liquid. It also aided in the reduction of particle clustering. However, it was observed that the effective nanoparticle sizes were larger, due to surfactant layers on the surfaces of the nanoparticles. At a given temperature and volume fraction, the nanofluid containing SDBS surfactant had slightly lower thermal conductivity than the nanofluid containing no surfactant. Using the transient hot-wire method, [PERSON] et al. [23] measured the thermal conductivities of Ag-Ethylene Glycol, Ag-hexane, and Ag-water nanofluids. OA, oleylamine, and Gum Arabic were used as surfactants to aid in stability. It was observed that surfactant had no effect on the thermal conductivities of Hexane and Ethylene Glycol. The thermal conductivity of water decreased with the addition of Gum Arabic surfactant, due to the lower thermal conductivity of the surfactant in comparison to water. From the experimental observations discussed above, it can be suggested that surfactant, when used at an optimum concentration, can create uniform, stable suspensions in a nanofluid, which in turn leads to increased thermal conductivity. However, further experimentation should be completed in order to determine the optimum surfactant concentration. The effects of various surfactants on the thermal conductivities of nanofluids composed of CuO, Cu, and Al\({}_{2}\)O\({}_{3}\) nanoparticles dispersed in water at room temperature are shown in Figure 17. It is observed that the thermal conductivity of nanofluids increases with surfactant concentration up to a maximum value, and then decreases. The decrease in thermal conductivity at higher concentrations of surfactant may be a result of the greater amount of surfactant present on the surfaces of the nanoparticles allowing a narrower area for heat transfer. Brief discussion of effects of surfactant on thermal conductivity of nanofluids is given in Table 3.  
Figure 17: Effect of surfactant on the thermal conductivity of CuO–water and Al\({}_{2}\)O\({}_{3}\)–water nanofluids at room temperature, as a function of surfactant weight concentration.