3.1 Effect of mixing temperature
The influence of temperature on the dispersion stability of nanofluids was tested and analyzed. By adjusting the temperature of the circulating water bath to keep the internal temperature of the solution at a constant temperature (30 ° C, 50 ° C, 70 ° C, 90 ° C), set the ultrasonic amplitude to 60%, that is, the output power is 93 W.
During the ultrasonic process, cavitation accumulates a lot of heat, which will lead to the increase of temperature, and the increase of temperature will also lead to the decrease of liquid cavitation strength. Too high temperature will also cause the foaming of oleic acid, which will break the bond between oleic acid and Al2O3 NPs. Therefore, in order to study the dispersion stability of Al2O3 in oil phase base solution, the most suitable temperature range should be determined in this discussion. Nanofluid samples were prepared and stability was analyzed.
Samples of Al2O3 nanofluid with 0.02wt% were prepared, Oleic acid has been used as surfactant and assumed a weight concentration of 0.5wt%.
As shown in Figure 4, nanofluids were prepared under different temperatures, and the absorbency behavior of the nanofluids was measured every day for 10 days, while pure PAO6 oil was presented as the comparison group. Besides, the samples have not shaken during all the time to really evaluate the change of natural deposition. It was found that the nanoparticles agglomerate with time and all have different degrees of settlement. Figure.4 (b) shows the settlement after ten days. The results show that the temperature plays an important role in the stability of nanofluids. The stability of nanofluids prepared at 50 ℃ is better than that of other groups. This conclusion is further confirmed by UV–Vis spectroscopy absorption coefficient analysis. As shown in Figure.5, the absorption coefficient of different nanofluids decreases with the increase of time, and it can be seen that the stability of nanofluids prepared at 50 ℃ is the best behavior. Therefore, the temperature of the preparation process is kept at 50 ℃ in the subsequent scheme experiment.
3.2 Effect of surfactant concentration
In the oil-based system, Al2O3 nanoparticles need to be coated with surface modifiers to prepare stable nanofluids. The important role of surface modifiers in the preparation of stable oil-based nanofluids has been widely confirmed and adopted by the industry. Drawing on previous research foundations and experiences, oleic acid was used as a surface modifier in this study[5, 58]. Since the alkyl chain on the other side of the oleic acid molecule is easily soluble in oil solution, the nanoparticles modified by oleic acid have lipophilicity and can be well soluble in oil. Because oleic acids contain carboxyl groups, and nano-metal oxides contain hydroxyl groups, they can undergo chemical reactions. The reaction products are linked together in the form of ionic bonds, thus forming a single molecule coating on the surface of nanoparticles, forming a microcellular shape, which can effectively avoid the contact of nanoparticles and forms a steric hindrance to prevent a large number of particles from settling. If the amount of the surfactant is too small to completely cover the surface of all particles, causing some particles to agglomerate and settle, But if the amount of the surfactant is too large, the modifier is simultaneously adsorbed on two or more adjacent Particle surfaces, bridging between particle surfaces are more likely to form larger aggregates to accelerate sedimentation and reduce the stability of the nanofluid, so matching an appropriate amount of surfactant with the nanoparticles is of great significance[41, 44, 59].
Set the mass concentration of Al2O3 nanofluid to 0.005wt%, 0.01wt% and 0.02wt%, and the corresponding oleic acid concentration gradients are (0.2wt%、0.4wt %、0.8wt%)、（0.2wt%、0.4wt %、0.8wt%）and（0.2wt%、0.4wt%、0.8wt%、1.6wt %）, respectively. The nanofluid is prepared according to the preparation process in Fig. 3, and the temperature of stirring and ultrasonic process is ensured to be 50 ℃. The ultrasonic amplitude is set to be 60% with 93 W mixing power, and the ultrasonic is uninterrupted for 2 hours.
As shown in Fig.6 and Fig.7, the nanofluids modified by different oleic acid concentrations have different degrees of sedimentation after ten days of placement, and too large or too small concentration is unfavorable to stability. It was found that the absorption coefficient of nanofluids reached the maximum value after 1 day of storage, which may be caused by the fact that the adsorption of the surface modifier on the nanoparticle is not a rapid response process, and it takes a certain time to complete the adsorption. The nanoparticle at the bottom of the tube can be suspended again by absorbing the surface modifier. The concentration of 0.2wt% oleic acid is obviously not enough to support the modification of all nanoparticles, resulting in agglomeration and serious sedimentation of particles.
Oleic acid at a concentration of 1.6wt% can adsorb on the surface of particles in a short time to avoid agglomeration of nanoparticles and thus reduce the sedimentation. However, from Figure. 7(b), it can be seen that the absorption coefficient decreased sharply on the third day, which means that the particles grew up rapidly and agglomerated, resulting in a large amount of sedimentation. This indicates that 1.6wt% oleic acid may be in excess, which makes it easier for particles to form larger aggregates. Moreover, it is difficult for particles to be broken by ultrasound, leading to rapid precipitation. It can also be seen from in Figure.7(b) that the maximum absorption coefficient of three concentrations decreased significantly in the early days after the preparation, which may be due to the insufficient ultrasonic time leading to the incomplete disaggregation of some original nanoparticles and the rapid growth of the particles. After preparation for 10 days, it can be seen from the comparison in Figure.6 (b) that the nanofluid modified by 0.8wt% oleic acid showed the least sedimentation and the best stability performance, which is also confirmed as shown in Figure.7(a) and Figure.7(b). From the above analysis, oleic acid with concentration of 0.8wt% can better modify nanoparticles with concentration of 0.02wt% NPs, played a good role of surface modifiers, and nanofluids showed good stability. According to the same test procedure and analysis method, the two Al2O3 samples with 0.005wt% and 0.01wt% were tested, and it can be concluded that the optimal oleic acid concentration for these two samples is 0.4wt%.
In order to confirm the modification effect of oleic acid on the nanoparticles, the "FT-IR" was used for analysis, as shown in Figure.8 The infrared absorption spectrum of oleic acid is presented in Fig.8(a) 2852cm-1 and 2925cm-1 represent the characteristic bands of -CH2 and -CH3 in oleic acid, respectively. The strong 1710cm-1 characteristic peak represents the C=O group of oleic acid. The broad peak bands at 1630cm-1 and 3445cm-1 in FT-IR of unmodified NPs (Fig.8b) are attributed to the stretching and bending bands of O-H groups on the surface of Al2O3NPs, respectively. In the FT-IR spectra of Al2O3 modified with oleic acid (OA) in Fig.8c. Before taking FT-IR spectra, it is necessary to remove any non-adsorbed oleic acid from the surface of alumina nanoparticles. The comparison reveals the presence of oleic acid characteristic peaks (1710cm-1，C=O bond) in the modified Al2O3 spectrum. Two absorption bands at 2852cm–1 and 2925cm–1 (Fig.8c) are due to the vibrational frequencies of CH2 and CH3 groups of oleic acid covering the surface of nanoparticles. The above observation results of FT-IR spectra confirmed that oleic acid chemically reacted with the particle surface, and the surface of Al2O3 NPs was modified by oleic acid[61, 62].
3.3 Effect of ultrasonication power
In order to reduce sedimentation and achieve long-term stability, good dispersion is an essential prerequisite. Mechanical agitation is a simple physical dispersion method to break up the agglomeration of nanoparticles, which forces the aggregated particles to disaggregate by external shear stress. In this study, all the experimental nanofluids were stirred by magnetic force for 5 hours before ultrasonication. Such operation can not only break the agglomerated nanoparticles, but also make oleic acid fully contact and collide with nanoparticles, so that it can fully wrap on the surface of nanoparticles. Currently, ultrasonication is considered an effective method for dispersing nanoparticles. Previous ultrasonication studies have shown that continuous ultrasonication is better than pulsed ultrasonication and that there are optimal ultrasonication duration. Previous studies have shown that the optimal ultrasonication period and the ultrasonication amplitude are related to the type of base fluid, the type of nanoparticles, and the particle concentration, and is also affected by other conditions[46, 64]. In order to study the effects of ultrasonication amplitude and ultrasonication time on the dispersion stability of nanofluids, the ultrasonication output power was changed by adjusting the ultrasonication amplitude. The output power is changed by adjusting 50% amplitude, 60% amplitude, and 70% amplitude, corresponding to 77.5W, 93W, and 108.5W, respectively. Continuous ultrasonication was performed for 5h under each ultrasonication amplitude, but samples were retained at time nodes of 0.5h, 1h, 2h, 3h, 4h and 5h respectively. Therefore, 18 groups of samples were generated according to the matching combination of different ultrasonication amplitudes and ultrasonication time points, which were packed in different tubes, and the dispersion stability of 18 groups of samples with time was observed and analyzed. It should be noted that it is necessary to conduct ultrasonication for 2min for 18 groups of samples again after 6 hours of preparation and dispersion, so as to suspend the precipitated but wrapped nanoparticles in the liquid again and to prolong the stabilization time.
The influence of ultrasonication amplitude and ultrasonication duration on all nanofluid samples was characterized by UV spectrum. As shown in Figure.9, after 10 days of the preparation, the absorbance coefficients of 54 samples of three concentrations (0.005wt%, 0.01wt% and 0.02 wt.% NPs) were analyzed and characterized. It can be seen that the three concentrations of nanofluids all show large absorption coefficient values at the 70% amplitude (ultrasonication output power 108.5W), and obviously the disaggregate ability of the nanofluids at the 50% amplitude (ultrasonication output power 77.5W) is weak. It shows that the larger the amplitude in the same ultrasonic time, the better the effect of de-agglomeration, and the smaller the particles.
The absorption coefficients of 0.005wt% and 0.01wt% nanofluids increased with the increase of ultrasonication time, showing good dispersion stability. The absorption coefficient of 0.02wt% nanofluid reaches the maximum after 4 hours of ultrasonication. Increasing the ultrasonication time is not conducive to reducing the cluster size. This is because for the nanofluid with high mass concentration, the ultrasonication time is too long, and the chance of particle collision increases, thus causing particle agglomeration. Therefore, according to the above analysis, there is an optimal value of ultrasonication amplitude matching with ultrasonication duration for the preparation of long-term dispersed and stable nanofluids, that is, 0.02wt% needs 70% ultrasonication amplitude for 4 hours, 0.01wt% and 0.005wt% needs 70% ultrasonication amplitude for 5 hours.
3.4 Dispersion performance
In this section, three samples were sealed in a tube to observe the dispersion stability with time, and the dispersion stability was characterized by photo capturing, UV-Vis Spectroscopy and DLS method, reflecting the longest stability time that could be achieved.
For the three samples, the digital photos on the 20 and 160 days after ultrasonication were selected to reflect the change of the deposition of nanoparticles with time, as shown in figure.10. The photo on the 20 days after sonication showed a slight sedimentation at the bottom of the test tube, but it was not obvious. It can be found that the particle deposition of samples with a concentration of 0.02wt% is obvious after 160 days of ultrasonication, but the liquid color is still darker than that of pure PAO6 samples, indicating that the liquid contains a large number of suspended nanoparticles. Compared with the photos on the 20 days, it can be found that the amount of nanoparticle deposition of 0.005wt% and 0.01wt% NPs for the two samples has increased to a certain extent, but it is not obvious and can still maintain a good stability. It should be noted that oleic acid, as a surface modifier of nanoparticles, tends to form colloids at the bottom of the tube rather than white solid precipitates.
Figure.11 shows the relationship between the maximum absorption coefficient and storage time of the three samples. It is obvious that the higher the nanoparticle concentration, the larger the absorption coefficient. The absorption coefficients of the three samples increased rapidly with the storage time, reached a peak after 2 days, and then began to decrease to a fixed value, and the values could be kept within small range of variation for a long time. In this study, ultrasonication was repeated for 2 min at the 6 hours after the preparation, so that the particles which had settled to the bottom of the test tube but had been coated with oleic acid floated up again, so the absorption coefficient of the second day test reached the maximum. It can be found that the absorption coefficient of 0.02wt% nanofluid decreases significantly after 134 days, while 0.01wt% and 0.005wt% can maintain the stability for 160 days, and the stability trend of 0.005wt is better, which is also consistent with the settlement situation in the photo in Fig.10.
In the process of preparing nanofluids, the main concern is to obtain uniform suspension of nanoparticles by reducing the particle size of agglomerated nanoparticles. Particle size test can also reflect the effect of ultrasonication cracking on agglomeration and the effect of surface modifiers, so it is necessary to characterize and analyze the particle size at different time after preparation. Based on the previous analysis, the particle size of the three samples was analyzed, and the results are shown in Figure.12.
It can be found that the effective minimum particle size of the three samples are basically the same, showing a bimodal distribution. The existence of large particles indicates that a small part of clusters had not been broken completely. It also shows that under the same ultrasonication condition, the particle size obtained has little relationship with the concentration of nanofluids. For both 0.01wt% and 0.02wt% samples, the large particle size distribution disappeared after the 10 days, because the previous large particle size nanoclusters continued to grow over time until it was difficult to overcome gravity and lead to sedimentation. Due to the steric effect of oleic acid on the surface modification of nanoparticles, the growth and agglomeration trend of clusters are weak, which can make the particle size change little in a long period of time. After the tenth day of preparation, the particle sizes of the three samples gradually increased with time, but the increase was not significant. For 134 days after preparation, the particle size of the 0.02wt% nanofluid increased greatly, and the proportion of the large particle size was large, indicating that the particles grew significantly and clusters occurred, resulting in sedimentation. For 160 days after preparation, the particle size of 0.01wt% and 0.005wt% nanofluids also increased. However, the apparent deposition phenomenon is not obvious, and it still shows good stability.
Compared with the above methods, TEM is considered as one of the important tools to determine the particle size distribution and morphology of nanoparticles. TEM micrographs were taken for the three concentrations for the samples after directly mixing and 160 days after preparation. Since the oil cannot be evaporated to a sufficient extent, it can be seen from the TEM microphotograph that all the nanoparticles are distributed within the range of the oil blot. It can be seen from the photos at the initial stage after preparation in Fig. 13 that not all the original clusters have been completely de-agglomerated, except for the large clusters, the distribution of other particle sizes has little difference, which also verifies the particle size distribution in Fig. 12. From the TEM microphotograph on the 160 days after preparation, it can be seen that the 0.02wt% nanofluid has undergone severe agglomeration, which will inevitably lead to the deposition, the decrease of absorption coefficient, and cannot maintain stability for a long time. However, although 0.005wt% nanofluid and 0.01wt% nanofluid also show a certain tendency of agglomeration, the clusters with large particle size are less and can still remain stable. The situation reflected in the TEM microphotograph is consistent with the sedimentation of the nanofluid observed in the test tube photograph, which also accords with the tendency of the absorption coefficient to change with time.