# Application of Nanofluids in CO\({}_{2}\) Absorption: A Review
## 2 Nanofluid  
Nanofluids are prepared by dispersing nanosized materials (nanotubes, nanoparticles, nanorods, nanofibers, nanodroplets, and nanosheets) in a base fluid [27]. Nano-sized materials are determined according to their strict definition that specifies materials with at least one dimension (length, diameter, or thickness) in the range of 1-100 nm. This definition is widely accepted by nanotechnology researchers. The origin of this definition comes back to the fact that when the particle size is decreased, the ratio of surface area to volume increases rapidly, therefore 100 nm was set as a criterion for nanomaterials. Nanofluids are typically prepared by dispersion of ceramics (PNP, aluminum nitride, cellulose, etc. ), carbonaceous materials (fullerene, graphene, carbon nanotubes, etc. ), metals (gold, copper, aluminum, etc. ), and inorganic oxides (silicon dioxide, zinc oxide, iron oxide, etc.) in a base fluid [28]. A refrigerant, mineral oils, liquids with high viscosity like ethylene glycol, and low viscosity liquids like water, or a blend of various liquids (propylene/water, water/EG, etc.) may be used as base fluid. The researchers found that the addition of nanoparticles to the base fluid led to changes in the thermophysical properties of the nanofluid. In other words, Nanofluids caused changes in the base fluid due to their unique thermophysical properties such as density, thermal conductivity, viscosity, and other thermodynamic properties. Table 1 summarizes some of the thermophysical properties of common nanoparticles. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Nanoparticle**} & **Average Particle Size [nm]** & \multirow{2}{*}{**Morphology**} & \multirow{2}{*}{**Surface Area [m\({}^{2}\)/g]**} & \multirow{2}{*}{**Density [kg/m\({}^{3}\)]**} & \multirow{2}{*}{**Thermal Conductivity**} \\  & & & & & (W/m-K) \\ \hline Al\({}_{2}\)O\({}_{3}\) & \(<\)40 & Spherical & - & 4700 & 36–40 & [24] \\ MWCNT & 10–20 & Tubular & 200 & 2100 & - & [29] \\ ZnO & 10–30 & Nearly spherical & 20–60 & 5606 & 29 & [30] \\ TiO\({}_{2}\) & \(<\)50 & Spherical & \(50\pm 15\) & 5500–6000 & - & [29; 30] \\ SiO\({}_{2}\) & 10–15 & Spherical & 180–270 & 2200 & - & [31] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of nanoparticles. [PERSON] was the first one who proposed this definition for nanofluids in 1995 that two-phase, uniform and stable suspensions resulted from the dispersion of nanoparticles in an inorganic or organic liquid phase with a pre-specified proportion [36]. Nanofluids are theoretically classified as colloidal dispersion systems with properties that are related to the characteristics of the nanoparticles like shape, properties, scale, content, or their chemical or physical properties of nanoparticles [37]. Tests that have been carried out on nanofluids also indicated many beneficial points; when compared to particles with millimeter or micrometer sizes, nanofluids are more stable and have higher thermal conductivity, pressure drop and erosion are lower, especially in microchannels, when using nanofluids. Many industries benefit from nanofluids like the chemical industry, electronics and machinery, aerospace, biomedicine, energy, and power. Nevertheless, the application of nanofluids as mass and heat transfer media has attracted special attention due to their high mass and heat transfer coefficients [38; 39]. These advantages of using nanofluids have made researchers eager to discover other fields of application of nanofluids. Since mass transfer and heat transfer have similar basic principles, nanoparticles can also influence mass transfer by a fluid. Researchers concluded that using small quantities of soluble particles will considerably increase the gas absorption rate. The enhancement in mass transfer rate was later proved by [PERSON] et al., who assessed four probable mechanisms to improve mass transfer in gas-liquid systems [40]. [PERSON] et al., were one of the first groups that studied mass transfer improvement in nanofluids.