TiO2 나노유체를 향하여 - 1부:준비 및 특성

Physical Properties of TiO₂ Nanofluids

The physical properties of TiO₂ nanofluids are focused on the viscosity and thermal conductivity. Also, a few papers investigated the surface tension. Using nanofluids to enhance the thermal conductivity is a typical application in heat transfer filed. Therefore, the thermal conductivity of TiO₂ nanofluids will be introduced in part 2 of the reviews. In part 1, the viscosity and surface tension are introduced as follows.

Viscosity

Viscosity is an essential parameter for nanofluids especially for flow and heat transfer applications because both the pressure drop and the resulting pumping power are depended on the viscosity. Viscosity describes the internal resistance of a fluid to flow, and it is an important property for all thermal and flow applications for nanofluids. The nanofluids with higher viscosity will result in higher flow resistance and lower flow velocity, which also induce the decrease of the heat transfer. To obtain flow velocity and heat transfer efficiency, more pumping powers are needed which induce more energy consumption. Moreover, for some mass transfer application of nanofluids, viscosity plays more important roles than thermal conductivity because the viscosity determines the mass transfer resistance of molecules entering the liquid surface and the diffusion coefficient in the liquid. Murshed and Estellé [106] provide a state-of-the-art review on the viscosity of various nanofluids. They found that the experimental data from various literatures are greatly scattered and not consistent even for the same nanofluids. This review will discuss in detail the influence factors on the viscosity of TiO₂ nanofluids to provide an exhaustive knowledge on this topic.

Particle Loading Effect

Many literatures have concerned the volume concentration effect on the viscosity of TiO₂ nanofluids. Table 3 shows the particle loading dependence of the viscosity of TiO₂ nanofluids in different research. It can be observed that the viscosity of the TiO₂ nanofluids increases with the increase of the particle loading. However, some works showed that the viscosity ratio varies linearly with variation of volume concentration, but some other results showed the viscosity ratio variation is parabolic. The viscosity enhancements of TiO₂ nanofluids were greatly distinguishing in various researches. For example, in Vakili et al. [82], Arulprakasajothi et al. [107], Duangthongsuk and Wongwises [108], Saleh et al. [109], and Mahbubul et al.’s [110] results, the increments of viscosity were below ten times of the volume percentage of the added particles. However, He et al. [111] and Turgut et al.’s [112] results showed that the viscosities were increased by more than 100 times of the volume percentage of the TiO₂ particles added. There are also many results distributed between the values in the above two extreme cases. Therefore, it can be concluded that the influence of particle loading on the viscosity of TiO₂ nanofluids is more complex than that on thermal conductivity due to the widespread data in various studies.

Temperature Effect

Besides the volume concentration effect, the temperature effect on the viscosity of TiO₂ nanofluids is also widely studied by many researchers. He et al. [111] prepared four different concentration TiO₂ –H₂ O nanofluids with 20 nm TiO₂ and measured the viscosities of TiO₂ –H₂ O nanofluids and deionized water with different temperatures. They observed that the TiO₂ –H₂ O nanofluids were Newtonian fluids, which were the same as Chang and Liu’s finding [69], and the viscosities varied inversely with the temperature of the TiO₂ –H₂ O mixture system. Ling et al. [113] also measured the viscosities of the TiO₂ –H₂ O nanofluids with different mass fractions, when temperature varied from 15 to 40 °C. They found that the viscosity of the nanofluids increased when fluids thicken and decreased with the increment of the temperature exponentially. Liu et al. [114] figured that the viscosities of TiO₂ –H₂ O nanofluids increase remarkably with the volume fraction of nanoparticles and vary oppositely to the temperature of the TiO₂ –H₂ O nanofluids greatly with similar experimental method. Based on the value of the viscosities, they also propose an amended suspension viscosity formula. Some research results showed that the viscosity of nanofluids is a function of volume loading and temperature as well as base fluid viscosity. Yiamsawas et al. [115] measured the viscosity of TiO₂ –water with a volume loading varied from 1 to 8% at a high-temperature range of 15 to 60 °C. By comparisons with previous studies, they proposed a useful correlation for practical applications which indicated that the viscosity of nanofluids is a function of volume loading and temperature as well as the base fluid’s viscosity.

Comparing with the absolute viscosity, the varieties of relative viscosity at different temperatures were more impressive for researchers. Jarahnejad et al. [93] carried out a detailed study on the effect of temperature on the viscosity and the relative viscosity of TiO₂ respectively. And the results are shown in Fig. 9. It can be found that compared to base water, the average viscosities of TiO₂ nanofluids increased by 17, 50, and 78% for 3, 6, and 9 wt.% of particles’ loading, respectively, at 20 °C. The viscosity of nanofluids with different particle loading decreased as the temperature increased, while the relative viscosity remained nearly constant with the temperature. The observation of independent of temperature can be also included in some other research. Fedele et al. [89] presented the characterization of water-based nanofluids where TiO₂ ranging between 1 and 35% in mass. They concluded that the relative viscosity was independent from temperature for all the particle loading employed. And the nanofluids at 1 wt.% exhibited a water-like behavior within the experimental error. But this observation was invalid at the higher concentrations (+243% for 35 wt.% at 343 K). Also, Silambarasan et al. [66] found that the temperature has a smaller effect on the relative viscosity since the viscosity of TiO₂ suspensions was reproducible even after repeated and alternating heating and cooling processes. And they attributed the reason to the effect of particles’ temperature-dependent intermolecular forces in the suspension. However, some different results can also be observed. Teng et al. [116] found that the relative viscosity increased from 8.2 to 16% when the temperature varied from 10 to 40 °C for the TiO₂ nanofluids with 0.5 wt.% of particle loading. Cieśliński et al. [117] found that the relative viscosity of thermal oil-based TiO₂ nanofluids remained constant when the temperature varied from 20 to 40 °C, but had a nearly linear increase with the increase of temperature when exceeding 40 °C. Yapici et al. [118] observed that the effect temperature was different for different shear rate. The relative viscosity measured was independent of the temperature at a higher shear rate region. However, for lower shear rate region, a great temperature dependency behavior of viscosity of TiO₂ nanofluids was exhibited especially at high temperatures

Dynamic viscosity (a ) and relative viscosity (b ) for TiO₂ water-based nanofluids at different temperatures [93]. Reproduced with permission from Springer

Particle Size and Shape Effect

The particle size and shape effects on the viscosity of TiO₂ nanofluids were not investigated as widely as that of particles’ loading or temperature. In particular, Chen et al. [64, 119] investigated experimentally the viscosity of spherical (25 nm) and rod-like (10 × 100) TiO₂ nanoparticle-based nanofluids with water and EG as base fluid, respectively. They found that the viscosity of TiO₂ nanofluids was more sensitive to the rod-like particles than spherical particles. It can be seen from Table 3 that the viscosity was increased by 0.5–23% when adding 0.1–1.86 vol.% of spherical TiO₂ nanoparticles, while increased by 1–82% when adding 0.1–0.6 vol.% of rod-like TiO₂ nanoparticles. The same observation can also be found for EG-based nanofluids.

Surfactant Effect

The surfactants have been observed to have great effects on the viscosity of TiO₂ nanofluids in some recent research. Jarahnejad et al. [93] investigated the effect of two kinds of surfactant trioxadecane acid and poly carboxylate on the viscosity of TiO₂ nanofluids respectively. Their results of the dynamic viscosity of 9 wt.% TiO₂ –water nanofluids with different surfactants vs. temperature are shown in Fig. 10. The results demonstrated only a very slight increase was found in the viscosity of nanofluids even with the highest particle loading viz. 9 wt.%. However, the two kinds of surfactants could greatly increase the viscosity of nanofluids in the temperature range of 20–50 °C, especially for trioxadecane acid. The similar effect of surfactant on viscosity can also be observed in Ghadimi and Metselaar’s report [90], in which they found SDS can also increase the viscosity of TiO₂ nanofluids with 0.1 wt.% particle loading. It was also observed there were important roles of SDS in the long-term dispersion stability of TiO₂ nanofluids. Therefore, they still suggested that the dispersion method of adding surfactant and ultrasonic vibration to be adopted in the preparation of nanofluids.

Dynamic viscosity of 9 wt.% TiO₂ –water nanofluids with different surfactants vs. temperature [93]. Reproduced with permission from Springer

However, the above results cannot prove that all kinds of surfactant will result in high viscosity for nanofluids. Figure 11 shows the viscosity of TiO₂ nanofluids with PEG600 as surfactant measured by Bobbo et al. [120]. It can be seen that the viscosity of base water will not increase but decrease slightly when adding PEG600 at 0.02 or 0.2% loadings. Also, the viscosity of nanofluid containing 0.01% TiO₂ nanoparticles and 0.02% PEG600 was a little lower than that of the base water. However, for higher loading of PEG, the viscosity will be greatly increased whether or not containing nanoparticles. It can be seen from Fig. 11 that the nanofluids containing 2% PEG600 and 1% TiO₂ nanoparticles showed a viscosity higher than 7% in respect to water, which was analogous at each temperature. The above observation showed the viscosity of nanofluids can be lower than the base fluid in some cases, which also occurred in SWCNT nanofluids in their experiment. The decline of viscosity of fluid when adding surfactant or nanoparticles was also been found in some other research. Yang et al. [121] found that emulsifier OP-10 can reduce the viscosity of ammonia–water in lower concentrations. Ling et al. [122] observed that adding SDBS or OP-10 in TiO₂ nanofluids with a lower loading can induce a slight drop in viscosity. Therefore, it is an important issue to choose the suitable surfactants to improve the dispersion stability without increasing the viscosity significantly.

The viscosity of TiO₂ nanofluids with PEG600 as surfactant [120]. Reproduced with permission from Elsevier

Base Fluid Effect

The information about base fluid effect on viscosity can be illuminated though Chen et al.’s study [119], in which they found the relative increments of viscosity of water-based TiO₂ nanofluids were distinctly higher than that of EG based. It seemed that the higher viscosity the base fluid could result in lower increment in viscosity. Mahbubul et al. [110] found that the viscosity of R123 was increased by only 5.2% when adding 2 vol.% TiO₂ nanoparticles. Sen et al. [78] and Yapici et al. [118] found relative increments of viscosity about 20 times of the particles’ volume percentages. It also seems that TiO₂ nanoparticles are more suitable in the organic liquid because a lower relative increment in viscosity can be obtained especially at the higher temperature. Yiamsawas et al. [123] conducted experiments on a mixture with TiO₂ nanoparticles and EG/water (20/80 wt.%) in which the volume loading ranged from 0 to 4% and temperature ranged from 15 and 60 °C. They used the experimental data to present a useful correlation to predict the viscosity.

Shear Rate Effect

Another main distinction on the viscosity of TiO₂ nanofluids in different research is that whether the fluids were Newtonian fluids in different shear rates. A typical Newtonian nanofluid can be found in foregoing Fig. 11. However, it can be observed from Table 3 that more than half of the results showed that the TiO₂ nanofluids in their work are Newtonian fluids, but some others come to the opposite conclusion. Research on rheological characteristic has demonstrated that whether or not the TiO₂ nanofluids exhibit Newtonian behavior is also affected by other factors, including the base fluid type, temperature, and particle loading. A quintessential example can be found in Chen et al.’s research [64], where they measured the viscosity of four types of nanofluids made of TiO₂ nanoparticles (25 nm) and TiO₂ nanotubes (10 nm × 100 nm) dispersed in water and EG. They found that EG–TiO₂ nanofluids exhibited Newtonian behavior, whereas water–TiO₂ , water–TNT, and EG–TNT nanofluids exhibited non-Newtonian behavior. They indicated that the rheology behavior of TiO₂ nanofluids is affected by their specific ingredient and environment, such as particles’ shape and liquid circumstance. The rheological characteristic of TiO₂ nanofluids is also related to the temperature. Yapici et al. [118] investigated the rheological characteristic of 9 wt.% TiO₂ –water nanofluids with different surfactants vs. temperature. The results are shown in Fig. 12. It can be observed that the base fluid PEG was a typical Newtonian fluid in all kinds of temperature. However, TiO₂ –PEG200 nanofluids were nearly Newtonian fluid at a lower temperature and higher shear rate, but it changed into non-Newtonian fluid at higher temperature and lower shear rates. Also, in Said et al.’s results [94], the TiO₂ nanofluid with 0.1 vol.% loading was Newtonian fluid at 55 °C, whereas it was non-Newtonian below this temperature for 0.3 vol.% particle loading.

Shear rate dependency of viscosity as a function of temperature for 5 wt.% TiO₂ –PEG200 nanofluids [118]. Reproduced with permission from Springer

Running Time Effect

When the nanofluids are actually used in a running system, the time-dependent properties of nanofluids should be a crucial issue for the sustainable application. However, this matter has not been widely studied because of the faultiness in the development of nanofluids. It is generally considered that the thermal and rheological properties of nanofluids will be deteriorated due to the aggregation of nanoparticles after running a long time in the system. However, an opposite result in the time-dependent viscosity of TiO₂ nanofluids can be observed in Said et al.’s research [94]. Their results for viscosity of TiO₂ –water nanofluid with different volume loading and temperature as well as running time are shown in Fig. 13. It can be observed that the viscosity of fresh samples and the stale samples after running in a flat plate solar collector for 1 month were distinctly different. The viscosity of TiO₂ nanofluids was decreased after undergoing the alternative variations in temperature and flow rate in the cycle. This observation was quite interesting and could not be explained anywhere else in the literature. They thought this finding could open new research scope for the applications of nanofluids for a long-term use.

Viscosity of TiO₂ –water nanofluid with different volume concentrations and different temperatures [94]. Reproduced with permission from Elsevier

An inconsistency in viscosity of TiO₂ nanofluids is quite evident. The intensities of growth in viscosity of TiO₂ nanofluids with particle loadings greatly differ in various studies. And there is not yet a universal agreement on the effect of temperature, base fluid, and surfactant on viscosity of TiO₂ nanofluids. Moreover, the biggest controversy on viscosity of nanofluid is that whether nanofluid is Newtonian fluid or not. The results in Table 3 exhibit that a substantial part of TiO₂ nanofluids in their work are Newtonian fluids, but also, some others exhibit non-Newtonian behavior. The pronounced differences in different samples are mainly due to the complex influence factors on the rheological property. The shear rate has been proved to have great effect on the rheological property, and also, it has combined effect with other factors including temperature, shearing time, particle loading, base fluid type, and particle shape [124], which make it rather difficult to predict whether a nanofluid is Newtonian fluid or not except by experimental means.

Surface Tension of Nanofluids

The research on surface tension of TiO₂ –H₂ O nanofluids is much less than that of thermal conductivity or viscosity. Some results showed that adding TiO₂ nanoparticles had little effect on the surface tension of nanofluids. Liu et al. [125] prepared TiO₂ –H₂ O nanofluids whose particle size ranged from 11 to 50 nm and the surface tensions TiO₂ –H₂ O nanofluids were investigated experimentally. They found the surface tension had no obvious change with the increase in particle loading because the surface tension of nanofluids (1% mass fraction) increased only 1.6% compared with deionized water. Hu et al. [126] found the surface tension of TiO₂ –H₂ O nanofluids increases slightly when adding nanoparticles. And the surface tension decreased as an increase in temperature. Buschmann and Franzke [127] found that no obvious variation occurs when adding a high-volume fraction (5 vol.%) of TiO₂ nanoparticles in water. Tian and Wang [128] measured the surface tension of TiO₂ –water nanofluids by Jolly balance and abruption method. They found that the surface tension behavior of TiO₂ –water nanofluid was the same as water viz. the surface tension decreased as the temperature increases. However, the variation of surface tension is related to the content of nanoparticles. When the content of nanoparticles increases rapidly, the decrease rate of surface tension of TiO₂ –water nanofluids will slow down. Yang et al. [129] observed that nanoparticles have little effect but the surfactant can greatly change the surface tension of nanofluids, when the loading of surfactant is below the critical micelle concentration (CMC). And they explained this appearance as follows:The effect of surfactant on the surface tension of liquid is much greater than that of nanoparticles. When adding nanoparticles into a fluid containing surfactant whose loading is below CMC, the “free” surfactant will be absorbed on the surface of nanoparticles and then immersed in the liquid, which can weaken the reducing effect of surfactant on the surface tension of liquids.

However, some results also revealed that the nanoparticles played an indispensable role in the surface tension of nanofluids. Chinnam et al. [130] measured the surface tensions of Al₂ O₃ , ZnO, TiO₂ , and SiO₂ nanofluids with a mixture of 60% propylene glycol and 40% water as base fluids, respectively. They only used one average particle size of 15 nm for TiO₂ nanofluid due to limiting of manufacturer. They presented a single correlation as a function of volume loading and particle size as well as temperature for all the nanofluids by statistical analysis based on the experimental results. The experimental and fitting results related to TiO₂ nanofluids are shown in Fig. 14. It was observed that the surface tension of nanofluids decreased as the temperature and particle volume loading increase and the correlation perfectly fitted the experimental data. In addition, they also observed that the surface tension decreased as the particle size decrease for a certain loading and temperature of nanofluids except the ZnO nanofluid.

Variation of measured surface tension values of the TiO₂ nanofluids with temperature [130]. For different volumetric concentrations up to 1.5% and containing 15 nm particles. Reproduced with permission from Elsevier

Although the surface tension study of nanofluid is not as prevalent as studies in thermal conductivity or viscosity, surface tension is also an important parameter which can affect the film flow especially the initial infiltration of film and the probability of forming channel flow. Due to the effect of surfactant on surface tension of nanofluids is greater than nanoparticles, some researchers thought that the reduction in surface tension by surfactant SDBS can produce a superior enhancement of pool boiling performance in R141b-based nano-refrigerant [131].