Figure 4. pH values plotted against temperature and time

Thermal conductivity and viscosity experimental setup

As mentioned earlier, the Anton Paar MCR 302 Rheometer aided in taking the viscosity readings. These readings aided in discerning viscosity and shear stress, which were the fluid parameters at the selected shear stresses. Indeed, the tool reflects an oscillatory and rotational rheometer that gives insight into the viscoelastic or viscous characteristic of different materials; including solids, gels, and fluids.
To measure the selected samples’ thermal conductivity, the KD2 pro was used. The instrument had a lab thermal properties analyzer, as well as a fully portable field. Indeed, its functionality holds that the thermal conductivity is measured using the transient line heat source – relative to the IEEE and ASTM specifications. Also, the instrument constitutes a small single needle (6 cm) and a digital controller. Furthermore, it has special algorithms responsible for analyzing the resultant measurements gained during cooling and heating intervals. In this study, five consecutive measurements were used to obtain the calibration data. Also important to note is that a 2.0-percent deviation was observed. The times of reading the measurements ranged between one and ten minute. The type of measurement determined the interval of taking the readings.

3.1. Thermal Conductivity Measurements for Stability

Measuring the thermal conductivity of the given nanofluids is one of the known methods to keep a check on stability. To have a better understanding on knf and thermal conductivity enhancements, it is important to follow the time dependent knf,η of stable metal and metal oxide nanofluids with time [55,56]. Thermal conductivity readings were measured for the prepared nanofluids in regular intervals of time at the same temperatures every time to check the stability of the nanofluids. Here, the SiO2 nanoparticles were dispersed in the both the base fluids 60EGW and 40EGW in 0.5% volume concentration. The readings were taken in a set of three temperatures 25, 30 and 40oC, respectively.
After a gap of seven days the experiment will be repeated for the same nanofluid at the same temperatures and the readings were noted. Similarly, the experiments were carried in the same manner for the next three weeks. On a whole, for the same nanofluid, the thermal conductivity measurements were taken over period of thirty days at the same temperatures and the thermal conductivity values were analyzed with time. The values were observed have less than 1% deviation and are shown in Figure 5 . Hence, it can be concluded the nanofluid was stable.
The six samples are shown in Figure 6 are taken on the first day of preparation and the samples in Figure 7 are taken after a month gap. The samples were observed to be stabilized as seen with naked eye. As the other two samples out of eight are base fluids, they were not shown here.