*3.2.2 Enhancing the stability of nanofluid*

*Advances in Microfluidics and Nanofluids*

**Figure 2.**

**Figure 3.**

*3.2.1.2 Spectrophotometric analyses*

*3.2.1.3 Other monitoring methods*

tion of NP from the solvent [49].

This approach relies on the intensity of absorption when the light passes through a target sample. As shown in **Figure 3**, Ngo et al. monitored the stability of alumina-

*Monitoring alumina-based NFs stability using UV-Vis spectroscopy and 1-(-2pyridylazo)-2-naphthol, PAN.* 

Another straightforward approach for monitoring the nanofluid stability is to measure the particle size at different time intervals. This could be achieved by either using scanning/transmitting electron micro- scope (SEM/TEM) or zeta potential [48]. SEM/TEM allows to directly visualize the distribution of particle size and the evolution of particle coagulation. Easy and fast, SEM/TEM does not require separa-

As far as colloidal suspensions are concerned, Zeta potential defines the electro kinetic potential in a nanofluid. It indicates the interaction energy between

based nanofluid by combining colorimetry and spectrophotometry [47].

*Adapted with permission from [47]. Copyright (2020) American Chemical Society.*

*Sample pictures of water-based NFs prepared using alumia oxide NPs and ethylene glycol as BF.*

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The stability of NF is fully acquired when there is a minimization of the surface area of the NPs dispersed in the solution. To prevent the particle agglomeration, an energy barrier must be created to prevent them from passing the unstable to stable energy state. This could be performed by (a) by addition of acidic or alkaline materials, (b) altering the preparation step, and (c) choosing a proper BF [50–52].

**Figure 4** shows the influence of the preparation step of nanofluid stability.

The results showed that the acidity of the solution decreases regardless the preparation method. However, combing both the sonication and the magnetic stirring could prolong the stability of the nanofluid. Furthermore, Nguele et al. [43] and later Ngo et al. [47] reported that bubbling gas during the preparation could further enhance the stability regardless the type of base fluid (**Figure 5**).

The average decrease in acidity of about 20 % from the initial value (pH =5.4) was observed throughout the preparation stage when CO2 gas, which contrasts with an increase in pH twice higher when O2 was bubbled. Regardless the reason pertaining to the increase in pH (i.e., carbonation for CO2 bubbling and radical formation for O2 bubbling), the surface modification of NP and thus the stability is enhanced.

The addition of dispersants is an alternative for enhancing the stability of NFs [48, 50, 53]. These dispersants attach to the surface of the NP due to the mutual affinity. In addition, the tail of the attached dispersant works as a steric barrier, which prevents the particles from agglomerating. Such effect, known as steric hindrance, inhibits the coagulation of NPs in the suspensions (**Figure 6**).

#### **Figure 4.**

*Influence of preparation step on the stability of nanofluid; the nanofluid consists in Si-NP dispersed in an aqueous polymeric solution, Reprint with permission from [43] Copyright (2019) American Chemical Society.*

#### **Figure 5.**

*Influence of gas bubbling on the stability of nanofluid; the nanofluid consists in Si-NP dispersed in a deionized water.*

#### **Figure 6.**

*Influence of BF on the stability of nanofluid at 25<sup>o</sup> C; the nanofluid consists in Si-NP dispersed in a polymer (PVOH) solution prepared following two-step approach using CO2 bubbling.*
