*3.1.9. Interfacial layering on the liquid-nanostructure interface*

**Figure 5.** Thermophoretic motion of particles. The Brownian motion brings more particles to the colder region of the

oil at different filler fractions, ranging from 0.5 to 2.0 wt.% at different temperatures. It was observed that with the increase in nanoparticle loading, the relative viscosity of ZnO-crude oil nanofluid increases. At the mass fraction lower than 1.0 wt.%, the relative viscosity of nanofluid decreases slightly with the increase in temperature and the main factor which can influence the relative viscosity is nanoparticle type. On research by Jeong et al. [181], the viscosity behavior of water-based ZnO nanofluids with two nanoparticle shapes and semirectangular and spherical at various filler fractions ranging from 0.05 to 5.0 vol.% was investigated. Their results indicated that the viscosity increased from 5.3 to ~70% with increase in the filler concentrations. Moreover, the enhancement of the viscosity of the nearly rectangular shape nanoparticles was found to be more than 7%, rather than the spherical nanoparticles.

tions (0.2–0.9 vol.%) and found a 54% increase in viscosity, when compared to pure water.

Thermophoresis or thermodiffusion is an interesting consequence of the Brownian motion of the nanostructures. High energy molecules in a warmer region of a liquid migrate in the direction opposite the temperature gradient to cooler regions; small particles tend to disperse faster in hotter regions and slower in colder regions. Thermophoresis and Brownian motion effects are the mass transfer mechanisms which also influence the convective heat transport performance of nanofluids [78–81, 183–189]. As Michaelides [189] explained, interparticle collisions in the colder regions where the nanostructure concentrations are higher, partly hinder this accumulation and a dynamic equilibrium for nanoparticle concentration is established, with lower concentrations in the hotter regions and higher concentrations in the colder regions. **Figure 5** schematically depicts the differential dispersion and the resulting thermophoresis which shows the effects of the magnitude of the molecular collisions on small particles.

Diverse theoretical and experimental investigations have been developed. For instance, Wang et al. [187] observed the effects of temperature-dependent properties on natural convection


system.

Williams et al. [182] studied ZrO2

*3.1.8. Temperature dependence*

228 Microfluidics and Nanofluidics

Interfaces are ideal templates for assembling nanoparticles into structures by the nature of the interfaces. Interfacial layering or nanolayer refers to a phenomenon at the liquid-particle interface where liquid molecules are more ordered than those in the conventional fluids; therefore, the interface effect could enhance the thermal conductivity by the layering of the liquid at the solid interface (given that crystalline solids possess much better thermal transport that liquids) [193–195], by which the atomic structure of the liquid layer is significantly more ordered than that of the conventional liquid. At the interfaces, the nanostructures are mobile, and defects could be eliminated [196]. Nanoparticles suspended in base fluids form clusters that create a low thermal resistance path which can enhance the thermal conductivity, according to Saterlie et al. [197]. The ordered structure could have higher thermal conductivity than that of the conventional, therefore an enhancement of the effective thermal conductivity. Various researchers have suggested that there is liquid layering on the nanoparticles, which helps to enhance the heat transfer properties of nanofluids [197–201]. Yu et al. [200] proved the formation of layers by the liquid molecules close to a solid surface, even though the thickness and thermal conductivity of the nanolayers are not well known yet. According to Elis Josna Mary et al. [164], a temperature-dependent, linear variation in thermal conductivity increase with filler fraction was observed, which could be attributed to liquid layering.

DiW- and EG-based nanofluids reinforced with MWCNTs, CuO and SiO<sup>2</sup>

of dispersants, temperature and the experimental errors involved as well.

systems, the addition of SiO<sup>2</sup>

maximum increase of 10 and 14% for Al<sup>2</sup>

O4

ment of 20% for TiO<sup>2</sup>

properties of Co3

Kong et al. found for the maximal enhancement for Al2

same particle size and nanoparticles filler fraction.

thermal conductivity was improved almost linearly as filler fraction increased. For DiW-based

of 3, 5 and ~12%, respectively. Also, CuO/EG nanofluid at 1.0 vol.% showed an increase of ~9%. Wen and Ding et al. [192] also investigated the effects of MWCNTs within DiW, with addition of 0.25 wt.% gum Arabic (GA) dispersant with respect to DiW. For MWCNTs at 0.50 wt.% and 1.0 wt.%, an increase in thermal conductivity enhancement was achieved up to ~30% and ~38% at 25°C, and ~35% and ~80% at 30°C, respectively. It was found that these improvements were slightly higher than the results reported by Liu et al. [14], Assael et al. [219], Wen et al. [220], and Xie et al. [221], but lower than the results showed by Choi et al. [218]. There are diverse factors that cause these discrepancies among the different groups; as mentioned by Sing [55], these discrepancies should rely on the dependency of thermal conductivity is on diverse important factors such as the structure and properties of the CNTs, their aspect ratio, clustering, addition

Nanodiamonds (NDs) dispersed in EG and mineral oil (with addition of OA) were studied by Branson et al. [222]. It was observed that addition of 0.88 vol.% of NDs enhanced the thermal conductivity by ~12%. In MO, for instance, an enhancement of ~6% and ~11% is achieved at NDs loading of 1.0 vol.% and 1.9 vol.%, respectively. According to Branson et al., the differences on enhancement efficiencies are attributable to divergence in thermal boundary resistance at nanoparticle/surfactant interfaces [222]. Research by Khairul et al. [173] on the effects of Al<sup>2</sup>

and CuO nanoparticles filler fraction and use of SDBS surfactant on viscosity and thermal conductivity of water-based nanofluids was performed. It was observed that thermal conductivity of the nanofluids increased nonlinearly with increasing nanoparticles filler fraction, with a

nanostructures, Yiamsawasd et al. [224] reported a maximum thermal conductivity enhance-

fluid and observed a temperature effect on thermal conductivity rise of 17% and ~11% at 10 and

EG/water mixtures. It was observed that thermal conductivity performance was improved by 5% at 1.5 vol.%. Mariano et al. [226] estimated thermal conductivity behavior and rheological

5.0 wt.%. Aluminum nitrides (AlN) can also find many applications in the heat exchange process. Thermal conductivity performance of AlN/ethanol nanofluid was investigated by Hu et al. [227]. Results showed a 20% increase in the thermal conductivity of ethanol with 4.0 vol.% at room temperature. Furthermore, a strong temperature dependence of the thermal conductivity was observed in this research. Yu et al. [228] thermal conductivity of AlN dispersed in two different conventional fluids, such as EG and propylene glycol (PG), was investigated. It was found a 39 and 40% thermal conductivity improvement for EG and PG, respectively, having the

A great improvement on 2D-nanostructure-based nanofluids was obtained by Taha-Tijerina et al. [3], where exfoliated h-BN and graphene were homogeneously dispersed within mineral oil with superb thermal conductivity increase up to ~80% at very low filler fractions (<0.10 wt.%)

O3

/EG nanofluids and obtained thermal conductivity enhancement of 27% at

/water nanofluid. Elis Josna Mary et al. [164] investigated CeO2

O3

30°C, respectively. Serebryakova et al. [225] investigated the effects of dispersing Al<sup>2</sup>

, CuO and MWCNTs at 1.0 vol.% filler fraction showed an increase

Thermal Transport and Challenges on Nanofluids Performance

http://dx.doi.org/10.5772/intechopen.72505

and CuO, respectively, at 0.15 wt.%, similar to what

nanofluids [223]. On the field of oxide

. It was observed that

231

O3

/EG nano-

within

O3
