*3.2.1.1 Sedimentation and centrifugation method*

Sedimentation method is the simplest and most straightforward method to investigate the stability of a nanofluid. A fixed volume of nanofluid is transferred into a graduated test tube and observed over the time. **Figure 2** shows the sedimentation of alumina-based NFs upon increasing the load in NPs.

The monitoring of the volume of deposited NPs showed that a load in alumina-NP of 0.05 wt.% was sufficient to give a stable nanofluid. The literature reports similar results, in which no or little visual sedimentation of particles can be observed from the naked eye [46].

**Figure 1.**

*Preparation of polymer-based NFs; Adapted with permission from [43]. Copyright (2019) American Chemical Society.*

**Figure 2.**

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

**Figure 3.**

*Monitoring alumina-based NFs stability using UV-Vis spectroscopy and 1-(-2pyridylazo)-2-naphthol, PAN. Adapted with permission from [47]. Copyright (2020) American Chemical Society.*

#### *3.2.1.2 Spectrophotometric analyses*

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 aluminabased nanofluid by combining colorimetry and spectrophotometry [47].

#### *3.2.1.3 Other monitoring methods*

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 separation of NP from the solvent [49].

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

**137**

**Figure 4.**

enhanced.

*Nanocomposite and Nanofluids: Towards a Sustainable Carbon Capture, Utilization, and Storage*

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

particles. High absolute value of zeta potential means stronger repulsive force

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

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**).

*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.*

between NPs, and hence indicates better stability of nanofluid.

*DOI: http://dx.doi.org/10.5772/intechopen.95838*

*3.2.2 Enhancing the stability of nanofluid*

particles. High absolute value of zeta potential means stronger repulsive force between NPs, and hence indicates better stability of nanofluid.
