Figure 3: Particle Size Distribution of SiO2 Nanoparticles

2.2. The Stability of Nanofluid

In nanofluids applications, one of the crucial parameters entails stability. The parameter is worth examining and analyzing relative to different factors that could influence it; hence the nanofluids’ dispersion stability. To evaluate nanofluids stability, various techniques have gained application. Examples include Spectral Absorbency Analysis, Zeta Potential Analysis, and the sedimentation technique. In this investigation, the method that was employed to assess nanofluids dispersion stability involved the sedimentation approach [47,48]. The criterion was set in such a way that if the nanofluids had their supernatant particle sizes or concentration kept constant, they (the nanofluids) would be deemed stable. In test tubes, the nanofluids’ sedimentation photographs play a crucial role in discerning material stability [49]. In this investigation, the stabilization of SiO2/60EGW nanofluids was realized through the addition of poly vinyl pyrrolidone (PVP) at 20% [50–52]. Given the desired volume concentration for SiO2, pH values were adjusted for nearly one-and-a-half hours, coming after the ultra-sonication procedure was conducted on the respective samples.
The above procedure was achieved via the use of a sonicator. Regular intervals were set at 30 minutes. To ensure that the sonicator’s possible heating effect was countered, the respective samples were kept in water beakers. Following the ultra-sonication procedures, the samples’ pH values were assessed. To ensure that the pH values were maintained between 9 and 10, 5 mol of NaoH was added; eventually observing the behavior of the nanofluids samples for three days.
Given that the extraordinary electrical conductivity enhancement is reported, some of the major factors linked to this trend include the iso electric point (IEP), monodispersity, and surface charge (or the pH value). In some of the previous scholarly investigations, the impact of IEP and particle surface charge has also been documented (especially regarding the resultant variation in the experimental sets’ states of thermal conductivity – Lee et al. [53]. In the latter investigation, findings demonstrated that colloidal particles are more likely to enhance nanofluids thermal conductivity and also exhibit stability if the target solution’s pH exceeds the particles’ IEP.

2.3. pH Measurements

Given a homogenous mixture solution, one of the crucial factors that determine nano particle aggregation entails the pH. Furthermore, the pH shapes the suspended nano particles’ state of stability. To determine the pH of the respective nanofluids, this study relied on the Mettler Toledo pH meter. The accuracy range was set at ±0.01. For the pH meter, the initial stage involves buffer solution calibration, which is followed by the rinsing of the electrode –before placement in samples for measurement. Indeed, de-ionized water aids in the rinsing process. The read button is then placed in the meter and the measurement could only be implemented if the button is pressed. On the display appears a measurement icon and, as the decimal point starts to blink, the process suggests a measurement in progress. The resultant display reflects the target sample’s pH value. He meter’s default setting is A, an automated endpoint. Given temperatures levels between 0oC and 100oC, the pH ranges that could be determined lie between 0.00 and 14.00. In this case, the accuracy of the temperature could be set at ±0.5oC.
If nanoparticles exhibit a neutral electrically surface, the resultant pH reflects a zero point of charge. On the other hand, a solution that inclines towards the basic, the interface tends to experience a predominance of negative ions, reflecting a negatively charged surface. However, situations, where the solution’s pH tends to be more inclined towards the acidic, there is prevalence of the positive charge, implying the presence of a positively charged surface. Should the pH be far from that which is exhibited by the IEP, the eventuality is that there are stable nanofluids [27,54]. pH values are taken after the ultra-sonication, as the ultra-sonication might affect the pH of the nanofluid prepared and are formulated in Table 2 .
Table 2 pH values of SiO2 nanofluids in two different base fluids