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