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Controllable Preparation of Nanoparticles by Drops and Plugs Flow in a Microchannel Device Shaowei Li, Jianhong Xu, Yujun Wang, and Guangsheng Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing, 100084, China ReceiVed NoVember 1, 2007 Well controlled two-liquid-phase flows in a T-junction microchannel device have been realized. The system of H2SO4 and BaCl2, respectively, in two phases to form BaSO4 nanoparticles was used as a probe to characterize the microscale two-phase flow and transport conditions of a system with interphase mass transfer and chemical reaction. Nanoparticles with narrow size and good dispersibility were produced through drops or plugs flow in the microdevice. As a novel work, the influence of mass transfer and chemical reaction on interfacial tension and flow patterns was discussed based on the experiments. At the same time, the effect of the two-phase flow patterns on the nanoparticle size was also discussed. It was found that the increase of the amount of mass transfer and chemical reaction could change the flow patterns from plugs flow to drops flow. The drop diameter or plug length could be changed in a wide range. Accordingly, a new parameter of µ0uc/γ0 /Qd was defined to distinguish the flow patterns. The prepared nanoparticles ranged in size from 10 to 40 nm. Apparently, the particle size decreased with the increase of the drop diameter or plug length. Reasons were discussed based on the mass transfer direction and speed in drops and plugs flow patterns.
Introduction Recently, the rapid development of microreaction and micromixing technology has led to a considerable variety of microfluidic devices. These devices have been used in many fields for its high efficiency, safety, repeatability, and facile controllability.1,2,14-32 In particular, significant advances have been made in the use of microfluidic devices for controlling the * Corresponding author. E-mail:
[email protected]. (1) Ehrfeld, W.; Hessel, V.; Lo¨we, H. Microreactors: New Technology for Modern Chemistry; Wiley-VCH: Weinheim, 2000. (2) Pamme, N. Lab Chip 2007, 7, 1644. (3) Gu¨nther, A.; Jensen, K. F. Lab Chip 2006, 6, 1487. (4) Thorsen, T.; Roberts, R. W.; Arnold, F. H.; Quake, S. R. Phys. ReV. Lett. 2001, 86, 4163. (5) Anna, S. L.; Bontoux, N.; Stone, H. A. Appl. Phys. Lett. 2003, 82, 364. (6) Xu, J. H.; Li, S. W.; Tan, J.; Wang, Y. J.; Luo, G. S. AIChE J. 2006, 52 (9), 3005. (7) Xu, J. H.; Li, S. W.; Tan, J.; Wang, Y. J.; Luo, G. S. Langmuir 2006, 22 (19), 7943. (8) Link, D. R.; Anna, S. I.; Weitz, D. A.; Stone, H. A. Phys. ReV. Lett. 2004, 92, 054503. (9) Xu, J. H.; Luo, G. S.; Li, S. W.; Chen, G. G. Lab Chip 2006, 6, 131. (10) Xu, J. H.; Li, S. W.; Wang, Y. J.; Luo, G. S. Appl. Phys. Lett. 2006, 88 (13), 133506. (11) Guillot, P.; Colin, A. Phys. ReV. E 2005, 72. (12) Chan, E. M.; Alivisatos, A. P.; Mathies, R. A. J. Am. Chem. Soc. 2005, 127, 13854. (13) Zheng, B. L.; Roach, S.; Ismagilov, R. F. J. Am. Chem. Soc. 2003, 125, 11170. (14) Song, H.; Chen, D. L.; Ismagilov, R. F. Angew. Chem., Int. Ed. 2006, 45, 7336. (15) Burns, J. R.; Ramshaw, C. Lab Chip 2001, 1, 10. (16) Burns, J. R.; Ramshaw, C. Chem. Eng. Commun. 2002, 189, 1611. (17) Dittrich, P. S.; Tachikawa, K.; Manz, A. Anal. Chem. 2006, 78 (12), 3887. (18) Xu, J. H.; Luo, G. S.; Chen, G. G.; Tan, B. J. Membr. Sci. 2005, 249, 75. (19) Kumemura, M.; Korenaga, T. Anal. Chim. Acta 2006, 558, 75. (20) Grodrian, A.; Metze, J.; Henkel, T.; Martin, K.; Roth, M.; Kohler, J. M. Biosens. Bioelectron. 2004, 19, 1421. (21) Zheng, B.; Tice, J. D.; Ismagilov, R. F. Anal. Chem. 2004, 76, 4977. (22) Steinbacher, J. L.; Mcquade, D. T. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6505. (23) Zourob, M.; Mohr, S.; Mayes, A. G. et al. Lab Chip 2006, 6, 296. (24) Nie, Z.; Xu, S.; Seo, M. et al. J. Am. Chem. Soc. 2005, 127, 8058. (25) Quevedo, E.; Steinbacher, J.; McQuade, D. T. J. Am. Chem. Soc. 2005, 127, 10498. (26) Dendukuri, D.; Tsoi, K.; Hatton, T. A. et al. Langmuir 2005, 21, 2113. (27) Dendukuri, D.; Pregibon, D. C.; Collins, J. et al. Nat. Mater. 2006, 5, 365.
multiphase flow patterns. Crossflow and focusing flow are two main methods for controlling different multiphase flow patterns, which can be suspended drops flow, channel spanning slugs flow, or wall wetting films flow.3-8 Many researches have been carried out to investigate how the operation conditions and fluid properties as well as the channel structures affect the multiphase flow. Flow patterns can be clearly described with some dimensionless parameters such as capillary number.3,9-13 Well controlled multiphase flows in microdevices have the advantages of large interfacial area, short transfer distance, and fast mixing performance, which can reduce mass transfer limitations to achieve much better performances relative to conventional scale systems.3 Thus, well controlled multiphase flows in microdevices have been widely used in chemical reaction,14-16 liquid-liquid extraction,17-19 biological analysis,20 crystallization,21 polymer synthesis,22-25 structural material preparation,26-28 and nanoparticle synthesis.12,29-32 Especially, in nanoparticle synthesis, particle size controlling is just one of the key issues in the production of nanoparticles because many properties of the solid particles are sensitive to the particle size and particle size distribution.33 The drops or plugs flow can provide high mixing efficiency, which is very important in particle size controlling.34-39 Moreover, well controlled multiphase flows can also prevent a (28) Khan, S. A.; Gu¨nther, A.; Schmidt, M. A.; Jensen, K. F. Langmuir 2004, 20, 8604. (29) Yen, B. K. H.; Gu¨nther, A.; Schmidt, M. A.; Jensen, K. F.; Bawendi, M. G. Angew. Chem., Int. Ed. 2005, 44, 5447. (30) Hung, L. H.; Tseng, W. Y.; Choi, K.; Tan, Y. C.; Shea, K. J.; Lee, A. P. 8th International Conference on MicroTAS, Malmo, Sweden, 2004; Vol. 2, p 539. (31) Sotowa, K. I.; Irie, K.; Fukumori, T. K. Kusakabe and Sugiyama, S. Chem. Eng. Technol. 2007, 30, 383. (32) Shestopalov, I.; Tice, J. D.; Ismagilov, R. F. Lab Chip 2004, 4, 316. (33) Siegel, R. W. Sci. Amer. 1996, 275, 74-79. (34) Li, S. W.; Xu, J. H.; Luo, G. S. J. Cryst. Growth 2007, 304, 219. (35) Chen, G. G.; Luo, G. S.; Xu, J. H. et al., Powder Technol. 2004, 139, 180. (36) La Mer, V. K.; Dinegar, R. H. J. Am. Chem. Soc. 1950, 72, 4847. (37) Mantzaris, N. V. Chem. Eng. Sci. 2005, 60, 4749. (38) Gradl, J.; Schwarzer, H. C.; Schwertfirm, F. et al, Chem. Eng. Process. 2006, 45, 908. (39) Schwarzer, H. C.;Peukert, W. Chem. Eng. Commun. 2004, 191, 580.
10.1021/la800107d CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008
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Figure 1. The experimental setup for nanoparticle preparation.
Figure 2. The experimental setup for interfacial tension measurement.
liquid from direct contact with microchannel walls and thereby eliminate or reduce undesired clogging of channels due to deposition of material on wall surfaces.3,12 Most of the works on nanoparticle preparation with multiphase flows in microdevices are not closely connected with multiphase reactions. The reactants exist only in one of the phases. The other phase is inert and only provides an isolated reaction environment in the microscale. The high mixing performance for homogeneous reaction systems is mainly applied. But for heterogeneous reaction systems, there are few researches on how the mass transfer and chemical reaction affect the multiphase flows in microdevices, and the preparation of nanoparticles with heterogeneous reaction systems in microdeveices is also little mentioned in the previous work. In this work, a T-junction microchannel device was used to produce liquid-liquid drops flow and plugs flow with different sizes. The formation of BaSO4 nanoparticles was used as a probe to characterize the microscale two-phase flow and transport conditions of a system with interphase mass transfer and reaction. The influence of mass transfer and chemical reaction on interfacial tension and two-phase flow patterns in the T-junction microchannel device was investigated. At the same time, the effects of the operation conditions and the droplet size on the particle size was also investigated and discussed. Experimental Figure 1 shows the experimental setup. Nanoparticles were produced in a T-junction microchannel device fabricated on a poly(methyl methacrylate) (PMMA) sample plate (60 mm × 40 mm × 5 mm) using an end mill. The main channel dimensions were about 1000 µm × 1000 µm (width × height). The dispersed channel was about 200 µm × 200 µm (width × height) and perpendicular to the main channel. The microfluidic device was sealed using another
PMMA thin plate (1 mm in thickness) by curing at 105 °C using high-pressure thermal sealing techniques. Two microsyringe pumps and two gas-tight microsyringes were used to pump the two phases into the microfluidic device, respectively. Butyl alcohol, hexyl alcohol, or octyl alcohol solutions with sulfuric acid concentration of 0.1, 0.2, 0.4, and 0.8 mol/L were used as the continuous phase. Aqueous solutions with barium chloride concentration of 0.02, 0.05, 0.1, 0.2, 0.4, and 0.8 mol/L were used as the dispersed phase. Droplets of aqueous solution were formed by the cross-flowing rupture in the main channel. The reaction of sulfuric acid with barium chloride took place at the droplet interface and nanoparticles of barium sulfate were produced. A microscope at magnifications from 20× to 200× with a highspeed CCD video camera was used to measure the drop diameter and the plug length. The images were recorded at a frequency of 600 images/s and then analyzed using a custom-made image-analysis software. The produced nanoparticles were characterized directly by TEM pictures on JEM2010 (Japan) and the particle size was analyzed using the TEM pictures. Furthermore, the interfacial tension of the experimental systems was measured by the pendant-drop method using the DataPhysics Instruments GmbH (Filderstadt) to investigate the effect of mass transfer and interphase reaction on interfacial tension. As shown in Figure 2, a microsyringe pump was used to generate a droplet of aqueous solution in the ambience of butyl alcohol solution. Because it was a dynamic system, different growth rate of the droplets might cause different results. Therefore, we fixed the flow rate at 0.04 µL/min. Then, the change in the interfacial tension would be due to the different mass transfer and reaction conditions in different system concentrations. The data will help us to understand the flowpattern shift with changing solution concentration.
Results and Discussion Two-Phase Flow Patterns. Figure 3 shows the droplet shapes and sizes under different experimental conditions. Figure 3a,b
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Figure 3. Micrographs of droplet flow under different conditions.
shows the flow patterns of pure water and pure solvent without solutes in the both phases. All others are the results of the chemical reaction. From Figure 3a,b,d,f, we can see that the flow rates of
both phases have an obvious effect on the droplet shape and size. The increase of the continuous phase flow rate or the decrease of the dispersed phase flow rate can result in the decrease of
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Figure 4. Interfacial tension between the butyl alcohol solutions and the aqueous solutions.
Figure 5. Two-phase flow patterns.
droplet or plug size. This is consistent with the literature reports.4-9 Panels c, e, and g of Figure 3 show the effect of concentrations of the two phases on the droplets or plugs. From Figure 3c, we can see that the dispersed phase size gets smaller and shifts from plugs to drops with increase in the continuous phase concentration, while the inverse trend is observed with increase of the dispersed phase concentration. Figure 3e shows that the concentrations of the two phases have little effect on the droplet sizes in the system of octyl alcohol. Figure 3g shows a similar trend to that of Figure 3c. We can also find out that the droplet size gets more sensitive to the continuous phase concentration when the dispersed phase concentration is increased. In all experiments, we observed that the dispersed phase became turbid while the continuous phase remained clean. This indicated that the chemical reaction took place in the aqueous side of the interface and the produced particles remained in the aqueous phase. Interfacial Tension. Figure 4 shows the measured interfacial tension between the butyl alcohol solutions of sulfuric acid and the aqueous solutions of barium chloride. We can see evidence for the decrease in the interfacial tension when the concentration of the butyl alcohol solution is increased. With the increase of the aqueous solution concentration, the interfacial tension experiences a recovery after a rapid decrease. Mechanisms. The possible mechanisms of the influence of the two-phase concentration on the flow patterns could be explained based on the measured interfacial tension. The increase of the sulfuric acid concentration leads to the increase of the mass transfer from the continuous phase to the dispersed phase, which results in the decrease of the interfacial tension. Therefore, the droplet size is decreased when the sulfuric acid concentration is increased. Because the chemical reaction is in the dispersed phase, the amount of mass transfer is not influenced directly by
the concentration of the dispersed phase. But the chemical reaction should enhance the mass transfer rate, which will influence the interfacial tension too. When the sulfuric acid concentration is high, the interfacial tension decreases when the dispersed phase concentration is increased because of the mass transfer enhancement due to the chemical reaction and the droplet size decreases a little. When the sulfuric acid concentration is low, the interfacial tension first decreases with the increasing dispersed phase concentration because of the mass transfer enhancement and then increases because of the salt-out effect. When the salt-out effect dominates the process, the droplet size will increase with increasing dispersed phase concentration. Of course, it should be mentioned that the flow patterns for the system with relatively higher interfacial tension, e.g., octyl alcohol/water system, will not be influenced by the concentrations of the two phases seriously. Flow-Pattern Regions. A capillary number Ca ) µuc/γ is generally used for micro-channel flow pattern characterization.3,9-13 However, the interfacial tension measured by the pendant drop-method cannot be directly used in our experimental system because the mass transfer rates are different. In fact, the dynamic interfacial tension in the special condition of the experiment is hard to determine. For this limitation, we use the capillary number of the reaction-free system Ca ) µ0uc/γ0 to characterize the flow patterns, and the interfacial tension change is directly characterized by the two-phase concentration. We define drops flow as that in which the size of the droplet on the flow direction is smaller than the channel width, and the other situations are defined as plugs flow. Based on the above analysis and the experimental results in Figure 3, we could distinguish the two flow-pattern regions by defining a new parameter of µ0uc/γ0 /Qd, as shown in Figure 5. The abscissa µ0uc/γ0 /Qd includes the effect of the capillary number and the dispersed
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Figure 6. The factors to determine particle size.
Figure 7. Comparison of flow and resulting nanoparticles with flow conditions held constant and varying concentrations. Butyl alcohol system with Cc ) 0.1 mol/L, Qc ) 50 µL/min, and Qd ) 20 µL/min.
Figure 8. Comparison of flow and resulting nanoparticles with concentration held constant and varying flow conditions. Butyl alcohol system with Cc ) 0.4 mol/L and Cd ) 0.4 mol/L.
phase flow rate on the droplet shape and size. The solid lines are comparting borderlines of the two regions in different dispersion phase concentrations (marked at the end of the lines). We could see that the flow patterns can be distinguished as plugs flow and drops flow very well with the new defined parameter. Some examples of flow-pattern images are shown in the figure and the corresponding experimental conditions are listed below the figure. These examples support the flow-pattern region compartmentalization. Particle Size. The particle size in a precipitation process is basically decided by the competition of nucleation and growth which are controlled by the system supersaturation ratio. High supersaturation ratio generates smaller particles for a short nucleation burst, while low supersaturation ratio generates larger ones for persistent particle growth.34-39 The supersaturation ratio in our experiments is determined by the initial concentration, flow pattern, as well as the mass transfer speed. The relationships of these factors could be described by Figure 6. BaSO4 nanoparticles with size between 10 and 40 nm were successfully produced with the two-phase flow in our experiment. Some of the TEM pictures are listed in Figures 7 and 8. We could see that the particle size varies obviously when the experimental
conditions are changed. The prepared nanoparticles show good monodispersity. If the flow patterns are similar, i.e., with similar drop size or plug length, the initial concentration decides the supersaturation ratio. Therefore, high initial concentration generates smaller particles. Figure 7 shows an example for this situation. The plug lengths of these pictures are similar except the first one which is a little short. The continuous phase concentrations are all at 0.1 mol/L. We can see that the particle size decreases with the increasing dispersed phase concentration. If the two-phase flow has different flow patterns or different drop sizes, as shown in Figure 8, the nanoparticle size will vary greatly even at the same initial concentration. From Figure 8, we can see that the particle size decreases rapidly with the increase of the droplet size at same initial concentration. This phenomenon indicates that big droplet size is benefit to nucleation rather than growth. In order to know about the relationship between the droplet size and the nanoparticle size, we plot all the results of particle size and droplet size in one picture, as shown in Figure 9. We call this figure the particle size diagram.
Controllable Preparation of Nanoparticles
Figure 9. Particle size diagram.
In the particle size diagram, the abscissa is droplet size, which is the drop diameter (below 1000 µm) or plug length (above 1000 µm). The ordinate is the particle size. We can see clearly that the particle size decreases with the increase of drop diameter or plug length within a narrow belt marked in the diagram. The slope of the belts characterizes the effect of the droplet size on particle size while the width of the belts characterizes the effect of the initial concentration. Of course, as shown in Figure 5, the initial concentration has a great influence on the flow patterns and the droplet size. Although it is hard to explain the diagram clearly and there are some unclear mechanisms, we would like to explain the possible reasons for our experimental results. The particle size difference in different drop sizes and plug lengths may be explained by the forming mechanism of two-phase regular flow and the mass transfer speed. In general, from experimental conditions, we know that sulfuric acid is usually in excessive in drops flow and the supersaturation ratio is controlled by the mass transfer speed inside the drops. At the drop generating stage, there is a strong inner circulatory flow inside drops because of the action of shear force of the continuous phase, and the mass transfer speed inside the drops is very high. In some situation, 70% of the total mass transfer could be accomplished during the drop generating stage.40 Nucleation will mainly take place at this stage, and a large amount of reactants will react at this stage too. After the drops are formed and flow in the mcirochannel, the inner circulatory flow will be very weak and the mass transfer inside the drops just provides the materials for the particle growth. With this concept, we could know that a bigger drop requires a relatively longer forming time. Therefore, the nucleation process persists for a little longer time and consumes most of the reactants. The materials left for the later growth stage become less, and the finally produced particles are smaller. We could quantitatively describe the above process with some approximate assumption. The continuous flow washes the surface of the droplet, and we can suppose that the concentration of the sulfuric acid at the interface remains constant. On the other hand, the inner circulatory flow inside the drops is very strong. Therefore, the supersaturation ratio at the interface is little changed by the continuous flow-rate change. We can use (40) Xu, J. H.; Tan, J.; Li, S. W.; Luo, G. S. Chem. Eng. J. 2007, doi: 10.1016/ j.cej.2007.12.030.
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the same interface nucleation rate J (m-2s-1) for systems with same concentration despite of the continuous flow rate changing, which alters the droplet size. The generated nuclei number in an infinitesimal time dt is dn ) JAdt, where A is the interface area of a drop. Assuming the drop is spherical, we have A ) 4πr2. The volume of the drop is V ) Qdt ) 4/3 πr3, and thus dt ) 4πr2/Qd dr. Then, we have dn ) 16π2Jr4/Qd dr. Integrating this expression, we can obtain the nuclei number generated in a drop with radius R as n ) 16π2JR5/5Qd. Then the mean volume value of particles generated in the drops with radii R isVp ) VCdM/Fn ) 5QdCdM/12π FJR2, where V is the volume of drop and M and F are, respectively, the molar mass and density of barium sulfate. The mean particle size dp ∼ R-2/3, i.e., the particle size decreases with increasing drop size. Most of the plug flows are in low continuous phase concentration and high dispersed phase concentration. The phase ratio of the continuous phase to the dispersed phase is also much lower than that of the drops flow. In this case, the mass transport speed of sulfuric acid in the continuous phase may become a determining factor on the particle size. Short segments of the continuous phase will benefit from the rapid mixing and cause high supersaturation ratio. Therefore, in contrast to the effect of the segment length of the continuous phase, the longer plug length of the dispersed phase will be helpful in the preparation of smaller nanoparticles. Because the mechamisms are unclear yet, the difference between the drops flow and the plugs flow will not be explained here. We will give out our explanation after further study.
Conclusions The system of H2SO4 and BaCl2, respectively, in two phases to form BaSO4 nanoparticles was used as a probe to characterize the microscale two-phase flow and transport conditions of a system with interphase mass transfer and reaction. Nanoparticles with size between 10 nm and 40 nm were produced through the two-phase flow in a T-junction microchannel device. By adjusting the experimental conditions including the concentrations of the two phases and flow rates, droplets with different shapes and sizes were generated in the microdevice and the size of BaSO4 nanoparticles was controlled. We found that mass transfer and chemical reaction could affect the interfacial tension and flow patterns seriously. With a new parameter Ca/Qd as the horizontal axis and Cc as the vertical axis, a picture of two-phase flow patterns has been plotted. The experimental results showed that the particle size decreased with an increase in the drop diameter or plug length. The possible mechanisms have been provided to explain the effect of flow patterns. From the forming mechanisms, we might conclude that the mass transfer speed in the dispersed phase for drops flow, or the mass transfer speed in the continuous phase for plugs flow, mainly decide the particle size. Notations Ca ) capillary number Cc ) concentration of continuous phase (mol/L) Cd ) concentration of dispersed phase (mol/L) Qc ) flow rate of continuous phase (µL/min) Qd ) flow rate of dispersed phase (µL/min) uc ) average velocity of continuous phase (m/s) γ0 ) interfacial tension between the continuous phase and dispersed phase without solute (N/m) µ0 ) viscosity of pure continuous phase (Pa‚s)
Acknowledgment. We gratefully acknowledge the support of the National Natural Science Foundation of China (20490200, 20476050, and 20525622) and National Basic Research Program of China (2007CB714302) for this work. LA800107D
