The Influence of Mixing and Nano-Solids on the Formation of

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The Influence of Mixing and Nano-Solids on the Formation of Nanobubbles Wei Xiao, Xingxing Wang, Limin Zhou, Weiguang Zhou, Jun Wang, Wenqing Qin, Guanzhou Qiu, Jun Hu, and Lijuan Zhang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b11385 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 15, 2018

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The Journal of Physical Chemistry

The Influence of Mixing and Nano-Solids on the Formation of Nanobubbles Wei Xiao, abcd Xingxing Wang, d Limin Zhou,abe Weiguang Zhou, d Jun Wang, *d Wenqing Qin, d Guanzhou Qiu, d Jun Hu, *ab Lijuan Zhang *ab

a Shanghai

Synchrotron Radiation Facility, Shanghai Advanced Research Institute,

Chinese Academy of Science, Shanghai, 201204 b Key

Laboratory of Interfacial Physics and Technology, Institute of Applied Physics,

Chinese Academy of Sciences, Shanghai 201800, China c School

of Resources Engineering, Xi’an University of Architecture and Technology,

Xi’an 710055, China d School

of Minerals Processing & Bioengineering, Central South University,

Changsha 410083, China e

University of Chinese Academy of Science, Beijing, 100049, China

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Nanobubbles (NBs) generation by replacing different dissolved gas solutions has been widely adopted. Recently, we have found that mixing solutions with different gas contents can also produce a large number of NBs. However, the mechanism of NBs formation during mixing has not been well explored. Here, we designed a series of experiments to investigate the influence of mixing of different solutions on the concentration and size contribution of formed NBs via the help of nanoparticle track analysis. The effect of nano-solids was also investigated. The pressurization and depressurization were used to produce NBs. The results indicated that NBs can be influenced by the gas contents and nano-solids. The addition of nanosolids is beneficial to the produce more NBs. Both the nano-solids and gas contents together are expected to substantially increase the concentration of NBs. Those results will be very helpful to understand the formation and stability of NBs.

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

INTRODUCTION

Nanobubbles (NBs), referred as nanoscopic gaseous domains occurred both on solid surfaces and in bulk solutions 1, have drawn increasing attention over the past decades, owing to their extraordinary properties, such as the long-lifetime 2, larger contact angles (from liquid side)

3

and high stiffness

4, 5.

Although applications of NBs in water

treatment 6, 7, mineral flotation 8, 9, surface cleaning 10, 11, biomedicine delivery 12, 13 and modern farming

14, 15

have been extensively reported, NBs themselves are still very

mysterious and controversial. NBs generally fall into two categories: surface NBs (SNBs) and bulk NBs (BNBs). SNBs were first proposed by Parker, et al. 16 in 1994 to explain the unusual long-range attractive forces while two hydrophobic surfaces were approaching. SNBs are typically characterized with a spherical cap with a height about several ten nanometers and a lateral size up to micron 17. According to the classical view of the air-water interface, the curvature radius of NBs is rather small and the Laplace pressure inside the tiny bubble would be very high and should drive gas diffusion across the interface instantaneously, thus leading to the bubble dissolution in milliseconds hypotheses were proposed

20

18, 19.

Some

to explain their stability, including the line tension

21,

impurity layer theory 22, three-phase contact line theory 23, dynamic equilibrium model theory 24, and high density inside 25. The existence of SNBs is generally accepted after more than 10-year research worldwide even though there are no theory to explain their stability perfectly until now. As for BNBs, they exist in bulk solution with a diameter less than 1 μm, also called 3



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ultrafine bubbles 26. BNBs are thought to be less stable than SNBs 27, though they can remain stable in the solution for several hours. Traditionally, BNBs can be mainly produced by hydrodynamic cavitation 26 (e.g., pressurization-decompression method 28, 29,

jet cavitation using a venturi tube

30,

and high-strength mechanical agitation

31),

ultrasonic irradiation 32, and electrolysis 33, 34. However, it is gradually realized that the formation of BNBs seems to be a spontaneous process

35

and BNBs are almost

ubiquitous in the bulk 36. Recently, we found that BNBs could also be formed by mixing ethanol and water 36. Other researchers reported that any local disturbance in the liquid can result in the formation of nanometer-sized voids, which serve as nuclei for the following generation of BNBs 37. Therefore, whether could BNBs be produced when two solutions with different dissolved gas concentration are mixed? How about some nano-solids in solution? How about the influence of homogenous and heterogeneous nucleation on the formation of BNBs? Another major factor limiting researchers’ understanding of BNBs is the lack of appropriate techniques for BNBs investigation. Unlike the detection of SNBs which can be achieved by atomic force microscopy infrared spectroscopy

38,

17,

attenuated total internal reflection

total internal reflection fluorescence microscopy

synchrotron-based scanning transmission soft X-ray microscopy microbalance

41, 42,

40

39,

and quartz crystal

the exploring of BNBs size information almost relies on dynamic

light scattering (DLS) during the first 10-year period of the new millennium

32, 43

.

Compared with the normal DLS technique, nanoparticle tracking analysis (NTA) can gather and analyze specific information from individual particle, rather than average an 4


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integral sample 44, which is very suitable for BNBs detecting. Therefore, in this paper we designed a series of experiments to investigate the influence of mixing different solutions on the production and stability of NBs as well as nano-solids. NBs were firstly produced by injecting gas at a certain pressure. The concentration and size distribution of NBs under different conditions were recorded by the NTA. The effect of degassing on the formation was also studied.



EXPERIMENTAL SECTION Materials. Rutile type titanium dioxide nanoparticles (TiO2 > 99.7%, GR) with a

size of 5-10 nm, purchased from Shanghai Maikun Chemical Co., Ltd, was prepared as the nano-solid solution (with a concentration about 2×109/mL) using sodium carbonate as dispersant. Sodium carbonate (Na2CO3 > 99.5%, GR) used was purchased from Sinopharm Chemical Reagent Co., Ltd, and its concentration was kept at 10-5 mol/L in each group of NBs solution. Ultra-pure water with a conductivity of 18.2 MΩ·cm used in the experiments was provided by an USF-ELGA Maxima water purification system. Generation of the Initial NBs. The original NBs solutions were produced by pressurizing pure nitrogen (99.937%) and then slowly depressurizing the solution, a method adopt in generating NBs 40. More details were followed. Firstly, the solution was placed in a pressurized chamber with 10 MPa for 30 min, during which the purity nitrogen was injected to form nitrogen NBs. Then, the decompression process was last for another 30 min to decrease the pressure to about 0.1 MPa. During the decompression the solution will reach to gas supersaturated state and many tiny bubbles will be formed. 5



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After 10-minute standing, the micro-sized bubbles escaped or ruptured, and almost only nanoscale bubbles stagnated in the bulk, forming the NBs solution. All the experiments were performed at room temperature. Design of Experiments. The purpose of this study is to reveal the effect of mixing and nano-solid on the formation of nanobubbles. To achieve this, the mixing processes both in the presence and absence of nano-solids acted as heterogeneous nucleation sites were designed and performed. Here, the pre-added Na2CO3 plays dual roles in our views. Firstly, it creates a weakly alkaline environment which enhances the stability of generated NBs by increasing the electrostatic repulsion force between NBs, as NBs are more negatively charged under this condition 45. Moreover, as a common dispersant, Na2CO3 helps to keep the nanoparticle solution disperse well 46. According to the flowsheet of Figure 1, nine kinds of solutions were prepared as the test samples: Na2CO3 solution (N); pressurized N (NP); (Na2CO3+titanium dioxide) solution (NT); pressurized NT (NTP); mixture of N and N (N+N); mixture of NP and NP (NP+NP); mixture of N and NP (N+NP); mixture of N and NT (N+NT); mixture of NP and NT (NT+NP). To avoid the possible interference of mechanical shaking in the experimental results, the production and mixing of the solution were well accomplished with 20s, and all the mixtures were prepared by gently mixing the component solution with a volumetric ratio of 1: 1.

6


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Figure 1. Preparation process of nanobubble solutions. Degassed Experiments. To verify whether these nanoparticles are NBs or not, degassing experiments were achieved in a vacuum desiccator. For control, the experimental solutions were put in the vacuum desiccator, and then they were decompressed at a low pressure of 0.1 atm for 10 min by a vacuum pump (2XZ-2C, Linhai Tanshi Vacuum Equipment Co., Ltd), and finally this low pressure was maintained for 2 h. Every experiment was repeated 3 times independently. NTA Measurements. The concentration and particle size distribution were acquired from the NTA (NS 300, Malvern). As an optical detection method, the measurement of NTA is based on the Brownian motion and principle of light scattering. The schematic diagram of the working principle of NTA is shown in Figure 2. During measurement, a blue laser light source (65 mW, λ = 405 nm) passes through a prism edged glass flat within the sample chamber into the nanoparticle suspension. It was equipped with a 7



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×20 magnification microscope and a high-speed camera. When the laser light struck on the particle, scattering faculae formed. The track of scattering faculae was recorded by the high-speed camera. Each result was got from the average of five measurements, and the movie was captured at 20 frame/s and last for 60 s. Camera level was usually set at 10, the threshold was set at 15 and the solution viscosity was 1CP. Optical field of view was fixed (approximately 100 μm × 80 μm) and the depth of the illuminating beam was approximately 10 μm. Here, the size of individual particle (nanobubble) could be calculated with its diffusion from Brownian motion using the Stokes−Einstein equation. The number of particles (nanobubbles) was counted by NTA, thus the particle (nanobubble) concentration could be obtained by dividing the volume of the field of view. ( x, y ) 2 

2 k BT 3rh

where kB is the Boltzmann constant, ( x, y ) 2 is the mean squared speed of a particle at a temperature T in a medium of viscosity  with a hydrodynamic radius of rh 44.

Figure 2. Schematic diagram of working principle of NTA. 8


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

RESULTS AND DISCUSSION The Formation of NBs and the Influence of Nano-Solids on Their Formation.

The production of NBs was performed by injecting pure nitrogen to Na2CO3 solution as described in the experiment part. Five independent measurements were carried out to reveal the correlations between the concentration and size of nanoparticles in different experimental groups. By comparing two solutions, Na2CO3 solution (N) and pressurized N (NP), it can be seen that the concentration of nanoparticles is tripled after the solution is pressurized. As showed in Figure 3a, the total concentration of the nanoparticles in N is about 1.8×107/mL. The measured data are close to the data of pure water 27, suggesting that N is suitable to be the background solution in our investigations. But from Figure 3b, the concentration of formed nanoparticles would increase larger and about 5.7×107/mL. It can be seen that pressurization of the solution increases the dissolved gas concentration and the following depressurization promotes the homogeneous nucleation of bubbles, which therefore leads to more BNBs production in the aqueous system 29, 47. We then compared other two solutions with titanium dioxide nano-solids before and after pressurization and depressurization. They were (Na2CO3 +titanium dioxide) solution (NT) and pressurized NT (NTP). As seen in NT curves in Figure 3c, after adding TiO2 nanoparticles into the solution, the measured nanoparticles concentration increases to 5.1×107/mL and was about three times higher than before the addition (see Figure 3a). The number was significantly lower than the concentration of titanium dioxide monomers while larger than that 9



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obtained from N. The measurable range of NTA is 10 nm to 1 μm 48, but the size of TiO2 nano-solids is about 5-10 nm, which means that if these TiO2 nano-solids are indeed dispersed completely, they should hardly be detected by NTA. Zhang et al. 1 has proved that these nanoscale solids can hardly aggregate in such high-dilute solution, due to the very high energy barrier between nano-solids. The results revealed that the added nano-solids could promote the formation NBs. It may explain that gas molecules in the solution are adsorbed on the nano-solid surface and become easier to nucleate than homogeneous nucleation.31 The NT solution were applied to a process of pressurization and depressurization. As showed in Figure 3d, the measured nanoparticles concentration rises to larger than 30×107/mL, much higher than that detected from NP and NT, suggesting that more NBs were produced. This means that the existence of nano-solids would promote largely the formation of NBs. Nano-solids are believed to play a role in stabilizing the tiny gas bubble nuclei in solution

31, 49.

We suppose that heterogeneous nucleation happened

during the formation of NBs in the case of NTP, and much easier than homogeneous nucleation in the case of NP 50. The average size of formed NBs in four solution were nearly similar and from 175 nm to 215 nm. Figure 3e provided the summary about the concentration of NBs in four solutions. When adding nano-solid in the N solution, the concentration of NBs would increase substantially.

10


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Figure 3. (a-d) The concentration of nanobubbles and images of scattering dynamic light scattering faculae (inserted) in different solutions and (e) nanoparticles’ total concentration and mean diameter. 1~5 representing 5 independent experiments, with M being the mean value; (a) N; (b) NP; (c) NT; (d) NTP. Overall, addition of gas molecules and nano-solids can increase the NBs concentration either by adding the gas bubble nucleation or by providing extra nucleation centers. The enhancement in generating NBs is promoted if both of them 11



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exist. The gas molecules are thought to be the sources for homogeneous formation of NBs, while heterogeneous nucleation (gas molecules adsorbed on nano-solid surfaces) is dominant in the presence of nano-solids. The Influence of Mixing on the Formation of NBs. It has been proposed that the gas bubble nucleation could be triggered by any local disturbance in the liquid 51. The local disturbance of the liquid in mixing is inevitable. Thus, what will happen by mixing about the formation of nanobubbles? We performed the experiments by mixing above four solutions as (N+N), (N+NP), (NT+NP) and (NP+NP), respectively. Figures 4a and 4b represented the concentration and size distribution of the “nanoparticles” after mixing solutions N and N, solutions N and NT. It was found that the finally total concentration of each mixture is close to the arithmetic mean of the concentration of their correspondingly original solutions. It suggests that there is only few NBs newly formed in the mixing process in these cases. The average size of “nanoparticles” in the mixed solutions is in the similar range. In contrast, the concentrations of “nanoparticles” in the mixture would increase if NP is involved in the mixing process, such as the cases of (N+NP) and (NT+NP) (reaching to 20.3×107/mL and 52.2×107/mL, respectively) showed in Figures 4c and 4e. So, it seems that the behavior of nanoparticles in the solution cannot be effectively explained by conventional mixing theory when the sample have been pre-pressurized before mixing. It has been reported that the gaseous content of the solution is the key to generate NBs in mixing

51,

and higher gas supersaturation of the solution generally

produce more NBs 52. This can efficiently explain why much less NBs generates in the 12


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mixture without NP. Another finding is that these nano-solids presented in the solution also play special roles in NBs generation. By comparing the two cases of (N+NP) and (NT+NP), the concentration of nanoparticles would remarkably increase in the mixture of (NT+NP) than the case of (N+NP). Heterogeneous nucleation may dominant as Figure 3d during the mixing of NT and NP. The nano-solids are only used as adsorption sites for gas molecules in the solution, and the gas molecules adsorb to form gas bubble nuclei, which in turn grow into nanobubbles. Because heterogeneous nucleation is much easier than homogeneous nucleation, the presence of nano-solids can significantly increase the number of nanobubbles in solution, and the resulting nanobubbles also be bulk nanobubbles. While for the case of (NP+NP), the results were not as expected that more gas would produce more NBs. The concentration of formed “nanoparticles” is much less than the case of (NT+NP) and also less than the case of (N+NP), which means the number of NBs newly formed is not only related with the gas content, but also related with the difference of the gas content between two original solutions.

13



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Figure 4. (a-e) The concentration of nanobubbles and images of scattering dynamic light scattering faculae (inserted) in different mixtures and (f) nanoparticles’ total concentration and mean diameter. 1~5 representing 5 independent experiments, with M being the mean value; (a) N+N; (b) N+NT; (c) N+NP; (d) NT+NP; (e) NP+NP. Effects of Degassing on the Formation of NBs in the Solutions. Many researchers have reported that degassing has great influence on the formation of NBs

53, 54.

Meanwhile, it is usually used to identify whether those measured nanoparticles are real gas NBs 55. In our study, we designed the degassing experiment to confirm the formed nanoparticles are indeed gas NBs. Also, the effect of degassing on the formation of NBs 14


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in solution before and after mixing was studied. Nine solutions of N, NP, NT, NTP, N+N, N+NT, N+NP, NT+NP, NP+NP were prepared as samples. First, we compared the concentration and size distributions of formed “nanoparticles” in those nine solutions before and after degassing. Comparing Figure 3a-3d and Figure 4a-4e with Figure 5a, it can be seen that the contribution of NBs size distributions after degassing is very similar as that before degassing, suggesting that the degassing process has little effect on the size distribution of NBs. But in Figure 5b, it is found that the concentration of NBs in these solutions decreases sharply after degassing, especially in the case of (N+NP), (NT+NP) and NTP solution. The degassing process results in a slight decrease in the concentration of NBs in the N solution, from 1.82×107/mL to 0.69×107/mL. The degassing process has a great influence on the concentration of NBs in the NP and NT solutions, which reduces their concentration from 5.70×107/mL and 5.10×107/mL to 1.08×107/mL and 1.14×107/mL, respectively. The most evident change is the (N+NP), (NT+NP) and NTP solutions, which directly reduces the concentration of NBs from 20.30, 52.20 and 31.40×107/mL to 1.21, 1.82 and 1.67×107/mL. According to the degassed experiments, we can find that the degassing process can result in the disappearance of a large number of NBs in the solution, and the concentration of NBs will be stabilized between 1.00 and 2.00×107/mL. After degassing, the significant decrease in the concentration of NBs indicates that the “nanoparticles” observed in Figures 3 and 4 are indeed NBs. The mean diameters of NBs in different solutions after degassing were shown in Figure 5c. The average sizes of NBs in nine solution are nearly similar and from 140 15



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nm to 240 nm. Comparing more details, we found that when the concentration of the NBs in the initial solution is low, the average diameter of the remaining NBs after degassing will be smaller than the average diameter of the initial NBs. When the concentration of the NBs in the initial solution is high, the average diameter of the remaining NBs after degassing will be greater than the average diameter of the initial NBs. This may cause the different change process during the degassing. Less NBs would become smaller, but more NBs will increase during the degassing.

Figure 5. The concentration and size distributions of NBs in nine different solutions. (a) The concentration vs size distributions after degassing. (b)Total concentration before and after degassing. (c) Mean diameter before and after degassing.



CONCLUSIONS

In summary, we investigated the influence of nano-solids and different mixtures of NBs 16


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solutions on the formation of NBs. The degassing process was applied to confirm the formed “nanoparticles” are gas NBs. Based on our results, we could give the following conclusions: (1)

A new method to produce NBs was presented by the mixing different solutions.

(2)

The formation of NBs is significantly affected by the gas content and nano-solids of the initial solutions in mixing. More and smaller NBs may be produced if high gas content in initial solution. Nano-solids and a larger difference between two initial solutions would strongly enhance the formation of NBs.

(3)

The addition of nano-solid can significantly increase the concentration of nanobubbles in the solution, can also increase the stability of nanobubbles.

(4)

Degassing can decrease the concentration of NBs in the solution. Moreover, it also proves that the “nanoparticles” produced are indeed gas NBs.



AUTHOR INFORMATION

Corresponding Authors *E-mail [email protected] (J.W.). *E-mail [email protected] (J.H.). *E-mail [email protected] (L.Z.). Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Nos. 11874379, 11575281, 11290165, 11305252, U1532260, 51474254), and the 17



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Knowledge Innovation Program of the Chinese Academy of Sciences (Nos. KJZD-EWM03, QYZDJ-SSW-SLH019). The authors also thank the beamline 08U1A staffs at the Shanghai Synchrotron Radiation Facilities (SSRF) for their suggestions and help.

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