## 3 Results and Discussion
### Effect of Different Factors on the Stability of the Hybrid Nanofluids
#### 3.1.2 Influence of Dispersant  
One of the dispersing mechanisms for nanofluid stabilization is steric stabilization, which can be attained by the addition of a dispersant [64]. In order to enhance the stability of the nanofluid, polymer chains are attached to charged particle surfaces or polyelectrolyte chains are attached onto uncharged particle surfaces to form a steric barrier between nanoparticles to screen the interparticle van der Waals attraction [67; 68]. Compared to electrostatic stabilization, steric stabilization has many advantages [64]:  
(1) It is widely used in various dispersion systems as a thermodynamic stabilization method;  
(2) It is applicable to high concentration of nanoparticles and it redisperses the temporarily aggregated particles;  
(3) The dispersion is not sensitive to categories of electrolyte;  
It shows good compatibility for simultaneously dispersing hybrid particles within the same base fluid. SDS, a hydrophilic anionic surfactant with good penetration and dispersion, which has superior compatibility with anions and no-ions, was used in this study. The hybrid nanofluid of 0.1 wt % (mass  
Figure 5: Bottom precipitation of 0.1 wt % hybrid nanofluid at different pH after 7 days.  
Figure 6: Photos of the hybrid nanofluid samples with 0.05 wt % (left side) and without (right side) SDS after: (**a**) 3 days and (**b**) 30 days.  
ratio of GO and Al\({}_{2}\)O\({}_{3}\) was 1:1), pH = 11, was ultrasonically dispersed for 90 min, and a different fraction of SDS was added. Figure 6 shows the state of the hybrid nanofluid samples with (left side) and without (right side) SDS after 3 and 30 days. It can be seen that after 30 days, the hybrid nanofluid without dispersant had more sediments and a faded color. In comparison, the hybrid nanofluid with dispersant SDS had neither obvious agglomeration nor color change. It is clearly shown that the SDS can keep the nanofluid stable for a relatively long time.  
For quantitative analysis of the stability, the absorbance of the hybrid nanofluid was measured with time using the WGZ-2000 turbidity meter. At some intervals after preparation of the nanofluid, 4 mL of the upper layer of the nanofluid was taken and diluted three times with deionized water, and the absorbance was sequentially measured. The absorbance reduction ratio (ARR) was defined as:  
\[ARR\%=\frac{A_{0}-A_{n}}{A_{0}}\times 100 \tag{1}\]  
\(A_{0}\) and \(A_{\mathrm{n}}\) are the initial absorbance and the \(n\)th hour absorbance of each nanofluid, respectively. Figure 7 indicates the influence of different concentration of SDS on ARR. It was found that the nanofluid's absorbance decreased rapidly (ARR increases) in the first four days, then decreased slowly after (slower ARR increase), as can be seen in Figure 6(a). The stability of the hybrid nanofluid was improved greatly with the addition of proper amount of SDS. Moreover, the absorbance increased (ARR decreased) at first, then decreased (ARR increased) with the addition of the SDS fraction of above 1% as shown in Figure 6(b). This is due to SDS being attached on the surface of nanoparticles to form double electric layers that increase the distance between nanoparticles, which in turn decrease the van der Waals attraction potential. The particles can be completely shielded by anti-ions with a suitable increase of SDS, and the two electric layers are in electrostatic equilibrium by repulsive force, which keep good stability of the nanofluid [68; 69]. However, if the SDS amount surpasses the critical micelles concentration (CMC), surfactant monomers can join up together to form micelles via the hydrophilic group instead of attaching on the surface of nanoparticle. These strongly charged micelles could repel the charged nanoparticles and prompt nanoparticles to agglomerate [70; 71; 72].