## 2 Nanofluid Thermal Conductivity
### Effects of Surfactant on Nanofluid Thermal Conductivity  
Scientists have studied the effects of various surfactants on the thermal conductivity of nanofluids [16; 17; 18; 19; 20; 21; 22; 23]. Surfactant was mixed with the base liquid of various nanofluids in order to produce a uniform distribution of nanoparticles, and to prevent the agglomeration of nanoparticles. The high surface energy of nanoparticles leads to agglomeration and settling, thus resulting in a lack of stability in nanofluids. Low stability in nanofluids leads to low thermal conductivity. Thus, surfactants are added with the goal of creating more stable nanofluids, and in turn increasing the thermal conductivity [18]. Several researchers have observed that surfactants aid in preventing the agglomeration of nanoparticles but can lead to decrease thermal conductivity at higher concentrations. Using the transient hot-wire method, [PERSON] et al. [16] measured the thermal conductivity of TiO\({}_{2}\)-water nanofluid using oleic acid (OA) and CTAB as surfactants. The surfactants were used at very low concentrations, between 0.01 and 0.02 volume percent, in order to avoid altering the heat transfer characteristics of the nanofluid. CTAB was found to be a more effective surfactant in producing a stable nanofluid, by preventing nanoparticle clustering and in turn creating a homogeneous distribution of nanoparticles throughout the base liquid. Using a KD2 Pro Thermal Property Analyzer, [PERSON] et al. [17] measured the thermal conductivities of CuO-water and Al\({}_{2}\)O\({}_{3}\)-water nanofluids containing sodium dodecyl benzene sulfonate (SDBS) surfactant as a function of surfactant weight concentration. The addition of the negatively charged surfactant altered the surface charges of the positively charged CuO and Al\({}_{2}\)O\({}_{3}\) nanoparticles, creating repulsion forces between the nanoparticles. The repulsion forces prevented clustering of the nanoparticles, and in turn led to a more stable nanofluid with higher thermal conductivity. It was suggested that ideal thermo-physical properties can be obtained by creating optimal surface charges on the nanoparticles. It was observed that the thermal conductivity of the nanofluid increased up to 0.1 W% for Al\({}_{2}\)O\({}_{3}\)-water nanofluid, and up to 0.15 W% for CuO-water nanofluid. Above these concentrations, the thermal conductivity of the nanofluid decreased significantly. It was suggested that this decrease was caused by the greater \begin{table}
\begin{tabular}{c c c c} \hline
**Reference** & **Base Liquid** & **Temperature (\({}^{\circ}\)C)** & **Thermal Conductivity (W/\(m\).K)** \\ \hline [PERSON] et al. [11] (1999) & Pump Fluid & 24 & 0.1287 \\ \hline [PERSON] et al. [42] (2017) & Glycerol & 20 & 0.2837 \\ \hline [PERSON] et al. [28] (2010) & Transformer Oil & 20 & 0.111 \\  & & 30 & 0.11 \\  & & 40 & 0.109 \\  & & 50 & 0.108 \\ \hline  & & 10 & 0.6136 \\  & & 20 & 0.6286 \\  & & 30 & 0.6396 \\ Water & & 40 & 0.6578 \\  & & 50 & 0.6714 \\  & & 60 & 0.6825 \\  & & 70 & 0.6948 \\ \hline  & & 10 & 0.2408 \\  & & 20 & 0.2438 \\  & & 30 & 0.2459 \\ [PERSON] et al. [10] (2016) & Ethylene Glycol & 40 & 0.2479 \\  & & 50 & 0.2494 \\  & & 60 & 0.2512 \\  & & 70 & 0.2533 \\ \hline  & & 10 & 0.1389 \\  & & 20 & 0.1399 \\  & & 30 & 0.1407 \\ Engine Oil & & 40 & 0.1419 \\  & & 50 & 0.143 \\  & & 60 & 0.143 \\  & & 70 & 0.146 \\ \hline [PERSON] et al. [38] (2014) & R-134a & 27 & 0.0803 \\ \hline \end{tabular}
\end{table} 
Table 2: Thermal conductivity of base liquids. amount of surfactant on the surfaces of the nanoparticles leading to a narrower available area for heat transfer. [PERSON] et al. [18] measured the thermal conductivities of Al\({}_{2}\)O\({}_{3}\)-water and Cu-water nanofluids containing SDBS surfactant. Measurements were obtained using a hot-disk thermal constant analyzer, which worked based on the transient plane source method. It was observed that higher stability in a nanofluid leads to higher thermal conductivity.