- Email: [email protected]
Study on Co-flow Effect on Janus Droplet Generation Based on Step Emulsification
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 48, Issue 1, January 2020 Online English edition of the Chinese language journal
Cite this article as: Chinese J. Anal. Chem., 2020, 48(1): 57–65
RESEARCH PAPER
Study on Co-flow Effect on Janus Droplet Generation Based on Step Emulsification LIAN Jiao-Yuan1, ZHENG Su-Xia2, XU Zhong-Bin1,*, RUAN Xiao-Dong3 1
Institute of Process Equipment, College of Energy Engineering, Zhejiang University, Hangzhou 310027, China Hangzhou Vocational and Technical College, Hangzhou 310018, China 3 The State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, China 2
Abstract:
Continuous phase co-flow can increase the throughput and control the flexibility of step emulsification, and has an
important effect on droplet microfluidic. In this study, a multi-channel step emulsification device was developed to generate Janus droplets and to study the effects of both continuous phase co-flow and dispersed phase flow on the generation of Janus droplets. Janus droplets were successfully prepared by the device, and the generation stability was optimized by using glass capillaries instead of microcapillary film based on their different wall contact angles at the channel outlet. The results showed that when the continuous phase co-flow rate increased, the resulted Janus droplet diameter decreased, and the generation frequency increased. When the flow rate of dispersed phase increased, both the diameter and generation frequency of the generated Janus droplets increased. Within the scope of this study, the diameter and the generation frequency of the Janus droplets could be controlled to be 638−1640 μm and 0.02−2.2 Hz, respectively. In addition, three-component droplets were generated successfully and steadily by a chip which was prepared based on the two-component droplet generation device. By changing the flow rates of two phases, the droplet diameter and frequency could also been tuned on-line. It was found that the co-flow effect had a greater impact on the generation of three-component droplets because of the result that the three-component droplet diameter changed greater than the two-component Janus droplet when changing both the continuous and dispersed phase flows. The results of this study provided a basis and method for micro and trace research of chemical and analysis industry. Key Words:
Co-flow; Microfluidic; Janus droplet; Step emulsification; Chemical industry; Analysis
1 Introduction As an emerging technology, microfluidic has attracted wide attention in various applications[1]. Being one kind of microfluidic, droplet microfluidic technology has been successfully applied in the fields of biomedicine, medicine, chemical industry, cosmetics, food, analysis and detection due to its unique performance[2,3]. Droplet microfluidic technology includes droplet generation and droplet manipulation technologies[4,5]. The commonly used methods of droplet generation include T-type, cross-type or Y-type shear methods, flow focusing and step emulsification[6]. The T-, cross- or Y-type method is a method to disperse the dispersed phases
into micro-droplets through the strong shear action of continuous phase. This method is simple and easy to control, however, it is not suitable for materials sensitive to shear force, and the monodispersity of micro-droplets is easily affected by the flow fluctuation of two phases[7–10]. The shear action of the flow focusing method on the dispersed phase is relatively weak, and this method has strong controllability, however, the chip fabrication of this method is complex, and the monodispersity of the droplets generated is still affected by the flow of two phases[11–13]. Step emulsification method which is set up of a step at the end of capillary tube, makes the dispersed phase expand through the dimension forming micro-droplets under the effect of Laplace pressure difference.
________________________ Received 20 June 2019; accepted 15 July 2019 *Corresponding author. Email: [email protected] This work was supported by the National Natural Science Foundation of China (No. 21676244). Copyright © 2020, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(19)61210-7
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
This technique is simple and has good droplet monodispersity which is basically not affected by the flow fluctuation of continuous and dispersed phases in a certain range, but the method has the disadvantages such as poor controllability, low throughput, and dependent of droplet size on chip dimensions[14,15]. Droplet control technology includes droplet coalescence, encapsulation, splitting, positioning, capture, screening and so on[16]. Droplet coalescence technology enables two or more micro-droplets with different chemical properties, compositions, polarities, groups or other properties to fuse together to form a droplet. Janus droplet which denotes to the droplet containing two different components is widely used in biochemical reactions, cell analysis and material synthesis. In the field of analytical science, droplet coalescence technology can quantitatively fuse reagents with cells or microorganisms for biochemical analysis, and fuse different reactants for chemical quantitative analysis[17–19]; Janus droplet can act as a self-actuating micro-pump to propel drugs, cells, or microorganisms through microfluidics[20]; Janus droplets with different magnetic or optical functions can be used as magnetically or optically controlled switches for microanalysis systems[21]; and the droplets with colloidal photonic crystals (CPCs) that can be used to make display screens when solidified into Janus particles [22]. However, it is a key point for droplet coalescence technology to destabilize the interfacial tension to form Janus droplet. According to whether the external force is applied, droplet coalescence techniques can be categorized as passive and active types [23]. Active droplet coalescence technique generally employs energy generated by electric field, temperature field, magnetic field, acoustic wave or laser etc. to realize droplet coalescence process. Passive droplet coalescence technique relies on the structural design or velocity regulation of microchip to make the two droplets contact first and achieve fusion finally. In contrast, the passive coalescence technique is more applicable because it is not subject to external interference and does not require complex control system. However, its structural design requires a full understanding of the flow characteristics of two fluids and increases the complexity of the microchip[24]. Nevertheless, Janus droplet can be generated directly by the properly designing of microfluidic chip [25]. Nisisako[26] and Cheng et al.[13] prepared Janus droplets successfully by connecting a Y-shaped microchannel to a flow focusing device. Two dispersed phases fluids were injected to the two entrances of the Y-shaped microchannel and contacted each other at the junction in the channel. Nisisako et al.[27] also prepared Janus droplets by connecting the Y-shaped microchannel to a T-type shear emulsification device. In addition, Yang et al.[28] prepared Janus droplets first by using a combined device of a Y-shaped microchannel and a three-dimensional coaxial flow device, and then formed Janus particles by downstream photo curing. As studied, Y-shaped
microchannel is generally used to produce Janus droplet. However, two dispersed phases get contacted and fused at the junction of Y-shaped channel first and then flow together into the downstream channel and emulsification device. The fusion of two components in channel is not conducive to the research and application of chemical reactions. Furthermore, the monodispersity of droplets generated by T-type, flow focusing and coaxial flow emulsification devices is not high enough to meet the requirements of high accuracy chemical reaction research. Therefore, it is necessary to design a Janus droplet generation device with higher droplet produce monodispersity and applicability. Because of the high monodispersity of droplets prepared by step emulsification method, Janus droplets were directly generated by a step emulsification device in this study. Moreover, to compensate the defects of step emulsification, continuous phase co-flow was introduced into the step emulsification chip. As a method of parallel flow of dispersed and continuous phase fluids in chip [6], the mechanism of co-flow is to exert the dragging and squeezing force to dispersed phase by continuous phase co-flow to make dispersed phase unstable to pinch-off and to form a droplet[29]. Therefore, the size and generation frequency of the generated droplet can be changed on-line by adjusting the parameters of co-flow[30]. In this study, a co-flowing step emulsification chip was fabricated. The technical difficulty and the solution to the stable generation of Janus droplet by this device were explored. Moreover, the effect of co-flow on the formation of Janus droplets was investigated in detail. In addition, three-component droplet was prepared successfully, and the effect of co-flow was also studied.
2 Experimental 2.1
Chip and fabrication
Materials used for fabricating the co-flowing step emulsion chip included MCF (Microcapillary film), glass capillaries, UV adhesive, acrylic plates, UV curing lamp, fixing clip, and marker pen. The chip fabrication process is shown in Fig.1. To fabricate the chip, first of all, the MCF, glass capillaries, and acrylic sheets were washed by ultrasonic waves for 3 min in water bath at room temperature. Then the surfaces of these materials were scrubbed to remove oil blots using anhydrous ethanol. After drying, the terrace location on the acrylic sheets was marked with a water-based marker pen. Second, UV adhesive was brushed on a piece of acrylic sheet at a distance above the terrace position (to prevent capillary effect from inhaling the adhesive into the terrace and blocking the channel). Then MCF film or glass capillaries were arranged on the acrylic sheet by a tweezer, and the position was adjusted to guarantee the perpendicular of channel longitudinal axis to the marker line and coincides of the
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
channel outlet with the marker line. Third, UV adhesive was brushed on the arranged MCF film or glass capillaries above the outlet. Another acrylic sheet was placed over the MCF film or glass capillaries array, and the position was adjusted to ensure that this acrylic sheet's marker line coincided with the one below. Finally, the device was clamped and it was placed under UV curing lamp for 1−2 minutes. After removing the clip, the chip fabrication was finished. The main and top views of the chip are shown in the lower left of Fig.1, and the fabricated chips are shown in the lower right. 2.2
Materials and methods
In experiment, the dispersed phase was pure water with 1% (w/w) sodium dodecyl sulfate (SDS) (Sinopharm Chemical Reagent) and different pigments. The continuous phase was dimethyl silicon oil (PMX-200, 10 cSt, Dow Corning) with 5% (w/w) Dow corning 749 (Dow corning). A single-channel peristatic pump (Baoding Shenchen Pump, SPLab01) was used for dispersed phase injection, and a four-channel syringe pump (LongerPump, LSP04-1A) was used for continuous phase injection. The chip was connected to the syringes via Polytetrafluoroethylene (PTFE) microtubules. Micro droplets reservoir was made of bonded transparent acrylic plates. The lighting unit in experiment was a shadowless lamp (sunflower lighting company, SFL-Y-5W/7W). The droplet generation process was monitored by a digital microscope (AmScope, HVS430W) and recorded by AMCap (No Danjou) software. The setup of experimental system is shown in Fig.2 2.3
coming close to each other. At the same time, the continuous phase between the two liquid cakes is gradually discharged until two liquid cakes contact with each other and fuse together under a certain force. After fusion, the combined cake expands further in the terrace before reaching the terrace boundary. Then, the combined liquid cake flows out the terrace boundary and expands in three dimensions to form a head. Under the effect of Laplace pressure difference, necking occurs in the terrace, and the head pinches off finally to generate a Janus droplet. Because the flow in the chip is laminar, there is an obvious dividing line of two components either in the combined liquid cake or in the head out of the terrace, despite there is molecular motion. Mixing of two components starts when the neck pinch-off occurs. The pinch-off disturbance and the swirling inner the droplets cause the rapid mixing of two components in a droplet. With continuous phase co-flow, the generation process of Janus droplet is basically unchanged, whereas the volume of liquid cake generated in the terrace is significantly reduced, and the liquid cake is even elongated into a tongue shape by strong co-flow, as shown in Fig.3g. The combined liquid tongue reaches the terrace boundary more rapidly, creating smaller Janus droplets. Therefore, the size and frequency of
Characterization of Janus droplet generation
Schematic of the process to form Janus droplets in a step emulsification device is shown in Fig.3a–Fig.3f. Two dispersed phases with different components flow into the terrace from adjacent channels. When the dispersed phases expand in the terrace, two liquid cakes keep growing and
Fig.2
Fig.1
Fabrication of chip to produce Janus droplet.
Schematic of setup of experimental system
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
Fig.3
Generation procedure of a Janus droplet with two components
generated Janus droplets can be controlled by regulating the flow rate of co-flow once the chip dimension is fixed. In addition, the composition of Janus droplet can be controlled by changing the flow rates of two dispersed phases. The relation between the total dispersed phase flow, droplet size and the frequency is shown in Eq.(1)[31]. f = (Qd/6πD3) × 10‒8 (1) where, f is the droplet generation frequency (Hz); Qd is the total dispersed phase flow (mL/h); D is the droplet diameter (m).
3 Results and discussion 3.1
Janus droplet generation stability
The general generation process of Janus droplets in step emulsification is shown in Fig.3. However, in practical operation, to realize the fusion of two dispersed phase liquid cakes, the interface tensions of the two dispersed phases should be equal, and the silicon oil between them should be drained thoroughly. The experiment initially used MCF to fabricate co-flowing step emulsification chip, whereas the fusion of two liquid cakes taking place must be under certain conditions[32]. It was found in this study that this fusion condition changed with the chip dimensions and liquid physical properties, that is, each chip or fluid had its own specific fusion conditions. In addition, the range of droplet fusion was very limited, which also meant that the fusion was unstable and easy to fail under certain disturbance. The limited and unstable fusion was not suitable for large-scale application. Therefore, a method of stable and effective fusion of two dispersed phase cakes was investigated in this section. The result showed that contact angle between dispersed phases and chip surface had a significant effect on the stable fusion of two phases. As shown in Fig.4A, the surface contact angle of water on plastic (i.e. acrylic sheets) was more than 90º. Hence, when microchannels in the chip were hydrophobic MCF, the dispersed phases could not wet the outlet surface of the channel. This hydrophobic property made it difficult for the two liquid cakes to fuse by draining the silicon oil between them. As the dotted circle shown in Fig.4, there was silicon oil
between the two liquid cakes, resulting in fusion failure. In contrast, when a water droplet was in a glass surface, their contact angle was less than 90º. Therefore, the dispersed phases were easy to form wettability in glass capillary outlet. As shown in Fig.4B, the oil phase was drained completely between the two dispersed phases because that the hydrophilic glass surface made the water phase be more likely to stick than oil. Therefore, two dispersed phases could fuse rapidly at the capillary outlet. The experimental results showed that this fusion method had high stability and was neither affected by the fluctuation of two phases flows nor changed with chip dimensions. In addition, due to the wettability, the two dispersed phases could also fuse steadily under a certain interfacial tension difference. Therefore, the chip with glass capillaries was fabricated in this study to investigate the generation of Janus droplet under the action of co-flow. 4.1
Effect of continuous phase co-flow
The continuous phase co-flow can reduce the size of the fused dispersed phase cake in the terrace, making the cake expanded in tongue-shape. The tongue-shaped dispersed phase reaches the edge of terrace faster, thus smaller Janus droplets produced. In this section, the dispersed phase flow rates were
Fig.4
Effect of wall contact angle on droplet coalescence: (A) plastic surface and (B) glass surface
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
set to 0.1 mL/h, and the continuous phase flow rate varied from 0.01 mL/h to 30 mL/h. The effect of co-flow rate on the size and generation frequency of Janus droplets was investigated. The experimental results are shown in Fig.5. As the co-flow rate increased, the droplet diameter decreased. When the co-flow rate changed from 0.01 mL/h to 30 mL/h, the generated Janus droplet diameter was decreased by 61%, which indicated that co-flow had a significant influence on the generated droplet dimension. According to the fitting results shown in Fig.5A, there was a significant linear correlation between the Janus droplet size and the logarithm of co-flow rate (see the illustration in Fig.5A), with the linear correlation coefficient of R > 0.99. The fitting relation is as follows: D = 1520.26 ‒ 610.27lgQc (2) where, Qc is the continuous phase flow rate (mL/h). The relationship between the Janus droplet generation frequency and the co-flow rate is shown in Fig.5B. The results showed that the droplet generation frequency increased with the increasing co-flow rate, which was consistent with the theoretical results of Eq.(1). Figure 6A illustrates the Janus droplet generation processes under different co-flow rates. It was found that the width of the liquid tongue in the terrace decreased with the increasing co-flow rate, because the lateral expansion of liquid tongue was limited by the neighbouring co-flows. Moreover, with the increase of co-flow rate, the mixing effect of the two dispersed phases in liquid tongue gradually decreased, which was mainly because the increase
Fig.5
of generation frequency caused by co-flow reduced the contact time of the two dispersed phases in the liquid tongue, thus the effect of molecular diffusion was reduced. As shown in Fig.6B, with the increase of co-flow rate, head diameter D1 of the dispersed phase before pinching off decreased. This was because the drag force of co-flow reduced the time to reach force balance on the dispersed phase flow tip, hence, the dispersed phase flowing into the head decreased correspondingly. In addition, the mixing effect of the two dispersed phases reduced with the increase of co-flow rate. On one hand, the flow direction of the neighbouring co-flows might decrease the internal swirl flow in Janus droplets. On the other hand, the disturbance was reduced due to the decrease of the dispersed phase flowing into the "head". As to the point of pinch-off location, the height H of the pinch-off point was basically unchanged with the increasing co-flow rates. The result indicated that co-flow could not change the droplet generation mode. The conclusion was consistent with the results of the co-flow study of single dispersed phase droplets[15]. 3.3
Effect of the dispersed phase flow
Based on the characteristics of co-flowing step emulsification, the dispersed phase flow also affects the generation of Janus droplets. In this study, a same flow rate was adopted for the two dispersed phases, and as a result, the contents of the two
Effect of different co-flow flows on (A) diameter and (B) frequency of generated Janus droplet. Inset in (A) refers to the result in single logarithmic coordinate system
Fig.6
Effect of co-flow rate on (A) “tongue” in the step and (B) pinching-off during Janus droplets generation
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
components in the generated Janus droplets were equal. The effect of dispersed phase flow on Janus droplet generation is shown in Fig.7, where the continuous phase flow remains 20 mL/h. The result depicted in Fig.7A showed that, the droplet diameter increased with the increasing dispersed phase flow. When the dispersed phase flow reached 3.0 mL/h, the diameter of Janus droplet increased by 63% compared with the dispersed phase flow of 0.1 mL/h. The reason for the diameter increase lay in that, the velocity difference between the continuous phase and dispersed phase decreased when the dispersed phase flow increased, leading to the decrease of drag force. The reduced drag force made the time to get force balance be longer, resulting in the "head growth" before pinching off. The droplet diameter was logarithmically linear correlated to the dispersed phase flow with R > 0.98, as shown in Fig.7A. The relation is fitted as follows: D = 971.66 + 276.33lgQd (3) The relationship between dispersed phase flow and droplet generation frequency is shown in Fig.7B. With the increase of dispersed phase flow, the droplet generation frequency increased. Therefore, increasing the dispersed phase flow could increase the droplet diameter and generation frequency at the same time. The droplet generation processes with different dispersed
Fig.7
phase flow rates are shown in Fig.8. The morphology of "tongue" in the step basically did not change and the width of "tongue" increased slightly with the increasing dispersed phase flow, as shown in Fig.8A. Nevertheless, the time of the "tongue" spreading to the boundary of the terrace decreased significantly with the increasing dispersed phase flow rate. In Fig.8B, the dimension of head formed out of the terrace before pinch-off increased with the increasing dispersed phase flow, because the volume of dispersed phase flowing into the head increased with the increasing dispersed phase flow. In addition, with the increasing dispersed phase flow, the head pinch-off location was gradually moved to the boundary of the terrace, suggesting that the change of dispersed phase flow may changes the droplet generation mode. The experimental results showed that when the dispersed phase flow was 4 mL/h, the head pinch-off location moved out of the terrace, which mean the droplet generation mode changed from dripping to jetting. However, the diameter of droplet formed in the jetting mode was much large (2.55 mm), and the two dispersed phases were rapidly mixed in the head before pinching off. Therefore, for different chips and terrace lengths, there existed specific ranges of dispersed phase flows within which the generated droplet size was small, monodispersed, and the boundary between the two phases was clear.
Effect of different dispersed phase flows on (A) diameter and (B) frequency of generated Janus droplets. Inset in (A) refers to the result in single logarithmic coordinate system
Fig.8
Effect of dispersed phase flows on (A) tongue in the step and (B) pinching-off during Janus droplets generation
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
3.4
Three component droplets generation
By adding a capillary to the multi-channel step emulsification chip prepared in Section 2.1, a five-channel chip, which could generate three-component droplets, was fabricated. In this section, three-component droplets were successfully prepared by introducing dispersed phases with different components in adjacent three capillaries and injecting continuous phase into the two channels on both sides of the dispersed phases. As shown in Fig.9, the dispersed phases flowing out of different channels fused stably at the outlet of capillaries (a); the tip of fused tongue expanded to the step boundary under the action of co-flow (b); the tip flowed out of the terrace boundary and expands in three dimensions to be a head (c); necking occurred in the terrace (d); necking continued until the head pinching off (e); a threecomponent droplet was formed and flowed out of the chip (f). Similar to the Janus droplets generation, the co-flow and dispersed phase flow influenced the dimension and generation frequency of the three-component droplets. As depicted in Fig.10A, the mean diameter of three-component droplets decreased with the increasing co-flow rate. When the co-flow rate increased from 1 mL/h to 30 mL/h, the droplet diameter was decreased by 45%. In contrast, the droplet diameter increased with the increasing dispersed phase flow. When the dispersed phase flow increased from 0.1 mL/h to 5 mL/h, the droplet diameter was increased by 89%. Equations (4) and (5) are fitted as follows (R > 0.96): D = 4.50 × 103 ‒ 1454.56lgQc (4)
Fig.9
Fig.10
D = 2.16 × 103 + 374.53Qd (5) It is found that, compared with the results of Janus droplet generation experiments, the increase of dispersed phase capillaries made the liquid tongue in the step to become wider, prolonging the necking time before pinching off. Hence, the resulted three-component droplet diameter increased obviously. Moreover, the slope of Eq.(4) decreased significantly compared with Eq.(2). The fitted relationship of droplet diameter and dispersed phase flow changed from a single logarithmic linear relationship to a linear relation in Eq.(3) and Eq.(5). These changes indicated that co-flow had a larger effect on the generation of three-component droplets than for Janus droplets, which was because that the slower necking process led to longer interaction time between the continuous phase and dispersed phases in the terrace, resulting in the enhancement of co-flow effect. The generated three-component droplets in this study provided a strategy for the study of micro chemical reactions with catalysts or three reactants. For instance, the intermediate phase could be a catalyst. Because the flow was laminar in the chip, no significant mixing of the three dispersed phases occurred. In addition, as the intermediate phase, the catalyst could also separate the two reactants and prevent the pre-reaction. Furthermore, the content of catalyst phase could be precisely controlled by adjusting the flow rate. Therefore, with good monodispersity and small reagent consumption, the three component droplets provided an insight and new idea for chemical and analytical quantitative research.
Generation procedure of droplet with 3 kinds of components with different color
Effects of different flows on size of three-component Janus droplet with 3 structural color: (A) continuous phase flow; (B) dispersed phase flow
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
4
Conclusions
The generation and control strategy of Janus droplet was investigated in this study. At first, a co-flowing step emulsification chip was fabricated by an MCF. Whereas the results showed that the generation of Janus droplets in adjacent channels was unstable, and the range of fusion condition was limited. We found that the fusion stability of two dispersed phases could be improved by decreasing the contact angle between dispersed phases and surface. Therefore, an optimized co-flowing step emulsion chip was fabricated by replacing MCF with glass capillaries. With this chip, the dispersed phases in adjacent channels were stably fused at the outlet of capillaries, and monodisperse two-component Janus droplet was successfully prepared. Furthermore, the influences of continuous phase flow and dispersed phase flow on the generation of Janus droplet were investigated experimentally by this chip. The results showed that with the increase of continuous phase flow, Janus droplet size decreased and generation frequency increased. With the increase of dispersed phase flow, both the droplet size and generation frequency increased. In addition, three-component droplets were generated successfully and steadily by a three-component droplet generation chip which was prepared by adding a capillary to the two-component Janus droplet generation chip. By changing the two phases flow rates, the droplet diameter and frequency could also be tuned on-line. It was found that, compared with the Janus droplet, the co-flow effect had a greater impact on the generation of three-component droplet because of the result that the three-component droplet diameter changed greater than Janus droplet when changing both the continuous and dispersed phase flows. The results of this study provided a carrier and theoretical basis for quantitative analysis and catalysts or reagents involved reaction and detection in chemical and analytical industry.
[6]
Zhu P, Wang L. Lab Chip, 2017, 17(1): 34–75
[7]
Yun D, Xavier C I S, Andrew D. Analyst, 2014, 140(2): 414–421
[8]
Chen Y, Deng Z. J. Fluid Mech., 2017, 819: 401–434
[9]
Xu J H, Li S W, Tan J, Wang Y J, Luo G S. AIChE J., 2010, 52(9): 3005–3010
[10] Li Y F, Xia G D, Wang J. J. B. Univ. Technol., 2016, 42(9): 1414–1421 [11] Bai F, He X M, Yang X F, Zhou R, Wang C. Int. J. Multiphase Flow, 2017, 93: 130–141 [12] Teo A J, Li K H, Nguyen N T, Guo W, Heere N, Xi H D. Anal. Chem., 2017, 89(8): 4387-4391 [13] Cheng L, Cai B, Zuo Y, Xiao L, Rao L, He Z, Yang Y, Liu W, Guo S, Zhao X Z. Chem. Phys. Lett., 2017, 673: 93–98 [14] Maan A A, Schroën K, Boom R. J. Food Eng., 2011, 107(3): 334–346 [15] Lian J Y, Luo X Y, Huang X, Wang Y H, Xu Z B, Ruan X D. Colloids Surf. A, 2019, 568: 381–390 [16] Deng N N, Wang W, Ju X J, Xie R, Liu Z, Chu L Y. Sci. China Ser. B, 2015, 45(1): 7–15 [17] Jia P F, Jiang K M, Liu C, Zhou W P, Zhang T, Zhang Z Q, Li H W. Chem. Res. Appl., 2015, 27(8): 1097–1103 [18] Michele Z, Baroud C N, Cooper J M. Phys. Rev. E, 2009, 80(2): 046303 [19] Shestopalov I, Tice J D, Ismagilov R F. Lab Chip, 2004, 4(4): 316–321 [20] Shklyaev S. Europhys. Lett., 2015, 110(5): 54002 [21] Liu S S, Wang C F, Wang X Q, Zhang J, Tian Y, Yin S N, Chen S. J. Mater. Chem. C, 2014, 2(44): 9431–9438 [22] Kim S H, Jeon S J, Jeong W C, Park H S, Yang S M. Adv. Mater., 2008, 20: 4129–4134 [23] Shen F, Li Y, Liu Z M, Cao R T, Wang G R. Chinese J. Anal. Chem., 2015, 43(12): 1942–1954 [24] Li S J, Zeng W. Chin. Hydrau. Pneumatics, 2013, (6): 13–23 [25] Nisisako T. Curr. Opin. Colloid Interface Sci., 2016, 25: 1–12 [26] Nisisako T, Torii T, Takahashi T, Takizawa Y. Adv. Mater., 2006, 18(9): 1152–1156
References
[27] Nisisako T, Hatsuzawa T. Sens. Actuators B, 2016, 223:
[1]
Zhao L, Shen J, Zhou H W, Huang Y Y. Chinese Sci. Bull.,
[28] Yang Y T, Wei J, Li X, Wu L J, Chang Z Q, Serra C. Adv.
[2]
Liu Z M, Yang Y, Du Y, Pang Y. Chinese J. Anal. Chem., 2017,
209–216 2011, 56(23): 1855–1870 45(2): 282–296 [3]
Wang Z L, Wang Z Y, Zong S F, Cui Y P. Chinese Optics, 2018, 11(3): 513–530
[4]
Chen J S, Jiang J H. Chinese J. Anal. Chem., 2012, 40(8): 1293–1300
[5]
Song W B, Dong C Q, Ren J C. J. Anal. Sci., 2011, 27(1): 106–112
Powder Technol., 2015, 26(1): 156–162 [29] Cramer C, Fischer P, Windhab E J. Chem. Eng. Sci., 2004, 59(15): 3045–3058 [30] Utada A S, Alberto F N, Stone H A, Weitz D A. Phys. Rev. Lett., 2007, 90(9): 094502 [31] Fujiu K B, Kobayashi I, Neves M A, Uemura K, Nakajima M. Colloids Surf. A, 2012, 411: 50–59 [32] Huang X, Eggersdorfer M, Wu J, Zhao C X, Xu Z B, Chen D, Weitz D. RSC Adv., 2017, 7(24): 14932–14938
Cite this article as: Chinese J. Anal. Chem., 2020, 48(1): 57–65
RESEARCH PAPER
Study on Co-flow Effect on Janus Droplet Generation Based on Step Emulsification LIAN Jiao-Yuan1, ZHENG Su-Xia2, XU Zhong-Bin1,*, RUAN Xiao-Dong3 1
Institute of Process Equipment, College of Energy Engineering, Zhejiang University, Hangzhou 310027, China Hangzhou Vocational and Technical College, Hangzhou 310018, China 3 The State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, China 2
Abstract:
Continuous phase co-flow can increase the throughput and control the flexibility of step emulsification, and has an
important effect on droplet microfluidic. In this study, a multi-channel step emulsification device was developed to generate Janus droplets and to study the effects of both continuous phase co-flow and dispersed phase flow on the generation of Janus droplets. Janus droplets were successfully prepared by the device, and the generation stability was optimized by using glass capillaries instead of microcapillary film based on their different wall contact angles at the channel outlet. The results showed that when the continuous phase co-flow rate increased, the resulted Janus droplet diameter decreased, and the generation frequency increased. When the flow rate of dispersed phase increased, both the diameter and generation frequency of the generated Janus droplets increased. Within the scope of this study, the diameter and the generation frequency of the Janus droplets could be controlled to be 638−1640 μm and 0.02−2.2 Hz, respectively. In addition, three-component droplets were generated successfully and steadily by a chip which was prepared based on the two-component droplet generation device. By changing the flow rates of two phases, the droplet diameter and frequency could also been tuned on-line. It was found that the co-flow effect had a greater impact on the generation of three-component droplets because of the result that the three-component droplet diameter changed greater than the two-component Janus droplet when changing both the continuous and dispersed phase flows. The results of this study provided a basis and method for micro and trace research of chemical and analysis industry. Key Words:
Co-flow; Microfluidic; Janus droplet; Step emulsification; Chemical industry; Analysis
1 Introduction As an emerging technology, microfluidic has attracted wide attention in various applications[1]. Being one kind of microfluidic, droplet microfluidic technology has been successfully applied in the fields of biomedicine, medicine, chemical industry, cosmetics, food, analysis and detection due to its unique performance[2,3]. Droplet microfluidic technology includes droplet generation and droplet manipulation technologies[4,5]. The commonly used methods of droplet generation include T-type, cross-type or Y-type shear methods, flow focusing and step emulsification[6]. The T-, cross- or Y-type method is a method to disperse the dispersed phases
into micro-droplets through the strong shear action of continuous phase. This method is simple and easy to control, however, it is not suitable for materials sensitive to shear force, and the monodispersity of micro-droplets is easily affected by the flow fluctuation of two phases[7–10]. The shear action of the flow focusing method on the dispersed phase is relatively weak, and this method has strong controllability, however, the chip fabrication of this method is complex, and the monodispersity of the droplets generated is still affected by the flow of two phases[11–13]. Step emulsification method which is set up of a step at the end of capillary tube, makes the dispersed phase expand through the dimension forming micro-droplets under the effect of Laplace pressure difference.
________________________ Received 20 June 2019; accepted 15 July 2019 *Corresponding author. Email: [email protected] This work was supported by the National Natural Science Foundation of China (No. 21676244). Copyright © 2020, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(19)61210-7
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
This technique is simple and has good droplet monodispersity which is basically not affected by the flow fluctuation of continuous and dispersed phases in a certain range, but the method has the disadvantages such as poor controllability, low throughput, and dependent of droplet size on chip dimensions[14,15]. Droplet control technology includes droplet coalescence, encapsulation, splitting, positioning, capture, screening and so on[16]. Droplet coalescence technology enables two or more micro-droplets with different chemical properties, compositions, polarities, groups or other properties to fuse together to form a droplet. Janus droplet which denotes to the droplet containing two different components is widely used in biochemical reactions, cell analysis and material synthesis. In the field of analytical science, droplet coalescence technology can quantitatively fuse reagents with cells or microorganisms for biochemical analysis, and fuse different reactants for chemical quantitative analysis[17–19]; Janus droplet can act as a self-actuating micro-pump to propel drugs, cells, or microorganisms through microfluidics[20]; Janus droplets with different magnetic or optical functions can be used as magnetically or optically controlled switches for microanalysis systems[21]; and the droplets with colloidal photonic crystals (CPCs) that can be used to make display screens when solidified into Janus particles [22]. However, it is a key point for droplet coalescence technology to destabilize the interfacial tension to form Janus droplet. According to whether the external force is applied, droplet coalescence techniques can be categorized as passive and active types [23]. Active droplet coalescence technique generally employs energy generated by electric field, temperature field, magnetic field, acoustic wave or laser etc. to realize droplet coalescence process. Passive droplet coalescence technique relies on the structural design or velocity regulation of microchip to make the two droplets contact first and achieve fusion finally. In contrast, the passive coalescence technique is more applicable because it is not subject to external interference and does not require complex control system. However, its structural design requires a full understanding of the flow characteristics of two fluids and increases the complexity of the microchip[24]. Nevertheless, Janus droplet can be generated directly by the properly designing of microfluidic chip [25]. Nisisako[26] and Cheng et al.[13] prepared Janus droplets successfully by connecting a Y-shaped microchannel to a flow focusing device. Two dispersed phases fluids were injected to the two entrances of the Y-shaped microchannel and contacted each other at the junction in the channel. Nisisako et al.[27] also prepared Janus droplets by connecting the Y-shaped microchannel to a T-type shear emulsification device. In addition, Yang et al.[28] prepared Janus droplets first by using a combined device of a Y-shaped microchannel and a three-dimensional coaxial flow device, and then formed Janus particles by downstream photo curing. As studied, Y-shaped
microchannel is generally used to produce Janus droplet. However, two dispersed phases get contacted and fused at the junction of Y-shaped channel first and then flow together into the downstream channel and emulsification device. The fusion of two components in channel is not conducive to the research and application of chemical reactions. Furthermore, the monodispersity of droplets generated by T-type, flow focusing and coaxial flow emulsification devices is not high enough to meet the requirements of high accuracy chemical reaction research. Therefore, it is necessary to design a Janus droplet generation device with higher droplet produce monodispersity and applicability. Because of the high monodispersity of droplets prepared by step emulsification method, Janus droplets were directly generated by a step emulsification device in this study. Moreover, to compensate the defects of step emulsification, continuous phase co-flow was introduced into the step emulsification chip. As a method of parallel flow of dispersed and continuous phase fluids in chip [6], the mechanism of co-flow is to exert the dragging and squeezing force to dispersed phase by continuous phase co-flow to make dispersed phase unstable to pinch-off and to form a droplet[29]. Therefore, the size and generation frequency of the generated droplet can be changed on-line by adjusting the parameters of co-flow[30]. In this study, a co-flowing step emulsification chip was fabricated. The technical difficulty and the solution to the stable generation of Janus droplet by this device were explored. Moreover, the effect of co-flow on the formation of Janus droplets was investigated in detail. In addition, three-component droplet was prepared successfully, and the effect of co-flow was also studied.
2 Experimental 2.1
Chip and fabrication
Materials used for fabricating the co-flowing step emulsion chip included MCF (Microcapillary film), glass capillaries, UV adhesive, acrylic plates, UV curing lamp, fixing clip, and marker pen. The chip fabrication process is shown in Fig.1. To fabricate the chip, first of all, the MCF, glass capillaries, and acrylic sheets were washed by ultrasonic waves for 3 min in water bath at room temperature. Then the surfaces of these materials were scrubbed to remove oil blots using anhydrous ethanol. After drying, the terrace location on the acrylic sheets was marked with a water-based marker pen. Second, UV adhesive was brushed on a piece of acrylic sheet at a distance above the terrace position (to prevent capillary effect from inhaling the adhesive into the terrace and blocking the channel). Then MCF film or glass capillaries were arranged on the acrylic sheet by a tweezer, and the position was adjusted to guarantee the perpendicular of channel longitudinal axis to the marker line and coincides of the
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
channel outlet with the marker line. Third, UV adhesive was brushed on the arranged MCF film or glass capillaries above the outlet. Another acrylic sheet was placed over the MCF film or glass capillaries array, and the position was adjusted to ensure that this acrylic sheet's marker line coincided with the one below. Finally, the device was clamped and it was placed under UV curing lamp for 1−2 minutes. After removing the clip, the chip fabrication was finished. The main and top views of the chip are shown in the lower left of Fig.1, and the fabricated chips are shown in the lower right. 2.2
Materials and methods
In experiment, the dispersed phase was pure water with 1% (w/w) sodium dodecyl sulfate (SDS) (Sinopharm Chemical Reagent) and different pigments. The continuous phase was dimethyl silicon oil (PMX-200, 10 cSt, Dow Corning) with 5% (w/w) Dow corning 749 (Dow corning). A single-channel peristatic pump (Baoding Shenchen Pump, SPLab01) was used for dispersed phase injection, and a four-channel syringe pump (LongerPump, LSP04-1A) was used for continuous phase injection. The chip was connected to the syringes via Polytetrafluoroethylene (PTFE) microtubules. Micro droplets reservoir was made of bonded transparent acrylic plates. The lighting unit in experiment was a shadowless lamp (sunflower lighting company, SFL-Y-5W/7W). The droplet generation process was monitored by a digital microscope (AmScope, HVS430W) and recorded by AMCap (No Danjou) software. The setup of experimental system is shown in Fig.2 2.3
coming close to each other. At the same time, the continuous phase between the two liquid cakes is gradually discharged until two liquid cakes contact with each other and fuse together under a certain force. After fusion, the combined cake expands further in the terrace before reaching the terrace boundary. Then, the combined liquid cake flows out the terrace boundary and expands in three dimensions to form a head. Under the effect of Laplace pressure difference, necking occurs in the terrace, and the head pinches off finally to generate a Janus droplet. Because the flow in the chip is laminar, there is an obvious dividing line of two components either in the combined liquid cake or in the head out of the terrace, despite there is molecular motion. Mixing of two components starts when the neck pinch-off occurs. The pinch-off disturbance and the swirling inner the droplets cause the rapid mixing of two components in a droplet. With continuous phase co-flow, the generation process of Janus droplet is basically unchanged, whereas the volume of liquid cake generated in the terrace is significantly reduced, and the liquid cake is even elongated into a tongue shape by strong co-flow, as shown in Fig.3g. The combined liquid tongue reaches the terrace boundary more rapidly, creating smaller Janus droplets. Therefore, the size and frequency of
Characterization of Janus droplet generation
Schematic of the process to form Janus droplets in a step emulsification device is shown in Fig.3a–Fig.3f. Two dispersed phases with different components flow into the terrace from adjacent channels. When the dispersed phases expand in the terrace, two liquid cakes keep growing and
Fig.2
Fig.1
Fabrication of chip to produce Janus droplet.
Schematic of setup of experimental system
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
Fig.3
Generation procedure of a Janus droplet with two components
generated Janus droplets can be controlled by regulating the flow rate of co-flow once the chip dimension is fixed. In addition, the composition of Janus droplet can be controlled by changing the flow rates of two dispersed phases. The relation between the total dispersed phase flow, droplet size and the frequency is shown in Eq.(1)[31]. f = (Qd/6πD3) × 10‒8 (1) where, f is the droplet generation frequency (Hz); Qd is the total dispersed phase flow (mL/h); D is the droplet diameter (m).
3 Results and discussion 3.1
Janus droplet generation stability
The general generation process of Janus droplets in step emulsification is shown in Fig.3. However, in practical operation, to realize the fusion of two dispersed phase liquid cakes, the interface tensions of the two dispersed phases should be equal, and the silicon oil between them should be drained thoroughly. The experiment initially used MCF to fabricate co-flowing step emulsification chip, whereas the fusion of two liquid cakes taking place must be under certain conditions[32]. It was found in this study that this fusion condition changed with the chip dimensions and liquid physical properties, that is, each chip or fluid had its own specific fusion conditions. In addition, the range of droplet fusion was very limited, which also meant that the fusion was unstable and easy to fail under certain disturbance. The limited and unstable fusion was not suitable for large-scale application. Therefore, a method of stable and effective fusion of two dispersed phase cakes was investigated in this section. The result showed that contact angle between dispersed phases and chip surface had a significant effect on the stable fusion of two phases. As shown in Fig.4A, the surface contact angle of water on plastic (i.e. acrylic sheets) was more than 90º. Hence, when microchannels in the chip were hydrophobic MCF, the dispersed phases could not wet the outlet surface of the channel. This hydrophobic property made it difficult for the two liquid cakes to fuse by draining the silicon oil between them. As the dotted circle shown in Fig.4, there was silicon oil
between the two liquid cakes, resulting in fusion failure. In contrast, when a water droplet was in a glass surface, their contact angle was less than 90º. Therefore, the dispersed phases were easy to form wettability in glass capillary outlet. As shown in Fig.4B, the oil phase was drained completely between the two dispersed phases because that the hydrophilic glass surface made the water phase be more likely to stick than oil. Therefore, two dispersed phases could fuse rapidly at the capillary outlet. The experimental results showed that this fusion method had high stability and was neither affected by the fluctuation of two phases flows nor changed with chip dimensions. In addition, due to the wettability, the two dispersed phases could also fuse steadily under a certain interfacial tension difference. Therefore, the chip with glass capillaries was fabricated in this study to investigate the generation of Janus droplet under the action of co-flow. 4.1
Effect of continuous phase co-flow
The continuous phase co-flow can reduce the size of the fused dispersed phase cake in the terrace, making the cake expanded in tongue-shape. The tongue-shaped dispersed phase reaches the edge of terrace faster, thus smaller Janus droplets produced. In this section, the dispersed phase flow rates were
Fig.4
Effect of wall contact angle on droplet coalescence: (A) plastic surface and (B) glass surface
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
set to 0.1 mL/h, and the continuous phase flow rate varied from 0.01 mL/h to 30 mL/h. The effect of co-flow rate on the size and generation frequency of Janus droplets was investigated. The experimental results are shown in Fig.5. As the co-flow rate increased, the droplet diameter decreased. When the co-flow rate changed from 0.01 mL/h to 30 mL/h, the generated Janus droplet diameter was decreased by 61%, which indicated that co-flow had a significant influence on the generated droplet dimension. According to the fitting results shown in Fig.5A, there was a significant linear correlation between the Janus droplet size and the logarithm of co-flow rate (see the illustration in Fig.5A), with the linear correlation coefficient of R > 0.99. The fitting relation is as follows: D = 1520.26 ‒ 610.27lgQc (2) where, Qc is the continuous phase flow rate (mL/h). The relationship between the Janus droplet generation frequency and the co-flow rate is shown in Fig.5B. The results showed that the droplet generation frequency increased with the increasing co-flow rate, which was consistent with the theoretical results of Eq.(1). Figure 6A illustrates the Janus droplet generation processes under different co-flow rates. It was found that the width of the liquid tongue in the terrace decreased with the increasing co-flow rate, because the lateral expansion of liquid tongue was limited by the neighbouring co-flows. Moreover, with the increase of co-flow rate, the mixing effect of the two dispersed phases in liquid tongue gradually decreased, which was mainly because the increase
Fig.5
of generation frequency caused by co-flow reduced the contact time of the two dispersed phases in the liquid tongue, thus the effect of molecular diffusion was reduced. As shown in Fig.6B, with the increase of co-flow rate, head diameter D1 of the dispersed phase before pinching off decreased. This was because the drag force of co-flow reduced the time to reach force balance on the dispersed phase flow tip, hence, the dispersed phase flowing into the head decreased correspondingly. In addition, the mixing effect of the two dispersed phases reduced with the increase of co-flow rate. On one hand, the flow direction of the neighbouring co-flows might decrease the internal swirl flow in Janus droplets. On the other hand, the disturbance was reduced due to the decrease of the dispersed phase flowing into the "head". As to the point of pinch-off location, the height H of the pinch-off point was basically unchanged with the increasing co-flow rates. The result indicated that co-flow could not change the droplet generation mode. The conclusion was consistent with the results of the co-flow study of single dispersed phase droplets[15]. 3.3
Effect of the dispersed phase flow
Based on the characteristics of co-flowing step emulsification, the dispersed phase flow also affects the generation of Janus droplets. In this study, a same flow rate was adopted for the two dispersed phases, and as a result, the contents of the two
Effect of different co-flow flows on (A) diameter and (B) frequency of generated Janus droplet. Inset in (A) refers to the result in single logarithmic coordinate system
Fig.6
Effect of co-flow rate on (A) “tongue” in the step and (B) pinching-off during Janus droplets generation
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
components in the generated Janus droplets were equal. The effect of dispersed phase flow on Janus droplet generation is shown in Fig.7, where the continuous phase flow remains 20 mL/h. The result depicted in Fig.7A showed that, the droplet diameter increased with the increasing dispersed phase flow. When the dispersed phase flow reached 3.0 mL/h, the diameter of Janus droplet increased by 63% compared with the dispersed phase flow of 0.1 mL/h. The reason for the diameter increase lay in that, the velocity difference between the continuous phase and dispersed phase decreased when the dispersed phase flow increased, leading to the decrease of drag force. The reduced drag force made the time to get force balance be longer, resulting in the "head growth" before pinching off. The droplet diameter was logarithmically linear correlated to the dispersed phase flow with R > 0.98, as shown in Fig.7A. The relation is fitted as follows: D = 971.66 + 276.33lgQd (3) The relationship between dispersed phase flow and droplet generation frequency is shown in Fig.7B. With the increase of dispersed phase flow, the droplet generation frequency increased. Therefore, increasing the dispersed phase flow could increase the droplet diameter and generation frequency at the same time. The droplet generation processes with different dispersed
Fig.7
phase flow rates are shown in Fig.8. The morphology of "tongue" in the step basically did not change and the width of "tongue" increased slightly with the increasing dispersed phase flow, as shown in Fig.8A. Nevertheless, the time of the "tongue" spreading to the boundary of the terrace decreased significantly with the increasing dispersed phase flow rate. In Fig.8B, the dimension of head formed out of the terrace before pinch-off increased with the increasing dispersed phase flow, because the volume of dispersed phase flowing into the head increased with the increasing dispersed phase flow. In addition, with the increasing dispersed phase flow, the head pinch-off location was gradually moved to the boundary of the terrace, suggesting that the change of dispersed phase flow may changes the droplet generation mode. The experimental results showed that when the dispersed phase flow was 4 mL/h, the head pinch-off location moved out of the terrace, which mean the droplet generation mode changed from dripping to jetting. However, the diameter of droplet formed in the jetting mode was much large (2.55 mm), and the two dispersed phases were rapidly mixed in the head before pinching off. Therefore, for different chips and terrace lengths, there existed specific ranges of dispersed phase flows within which the generated droplet size was small, monodispersed, and the boundary between the two phases was clear.
Effect of different dispersed phase flows on (A) diameter and (B) frequency of generated Janus droplets. Inset in (A) refers to the result in single logarithmic coordinate system
Fig.8
Effect of dispersed phase flows on (A) tongue in the step and (B) pinching-off during Janus droplets generation
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
3.4
Three component droplets generation
By adding a capillary to the multi-channel step emulsification chip prepared in Section 2.1, a five-channel chip, which could generate three-component droplets, was fabricated. In this section, three-component droplets were successfully prepared by introducing dispersed phases with different components in adjacent three capillaries and injecting continuous phase into the two channels on both sides of the dispersed phases. As shown in Fig.9, the dispersed phases flowing out of different channels fused stably at the outlet of capillaries (a); the tip of fused tongue expanded to the step boundary under the action of co-flow (b); the tip flowed out of the terrace boundary and expands in three dimensions to be a head (c); necking occurred in the terrace (d); necking continued until the head pinching off (e); a threecomponent droplet was formed and flowed out of the chip (f). Similar to the Janus droplets generation, the co-flow and dispersed phase flow influenced the dimension and generation frequency of the three-component droplets. As depicted in Fig.10A, the mean diameter of three-component droplets decreased with the increasing co-flow rate. When the co-flow rate increased from 1 mL/h to 30 mL/h, the droplet diameter was decreased by 45%. In contrast, the droplet diameter increased with the increasing dispersed phase flow. When the dispersed phase flow increased from 0.1 mL/h to 5 mL/h, the droplet diameter was increased by 89%. Equations (4) and (5) are fitted as follows (R > 0.96): D = 4.50 × 103 ‒ 1454.56lgQc (4)
Fig.9
Fig.10
D = 2.16 × 103 + 374.53Qd (5) It is found that, compared with the results of Janus droplet generation experiments, the increase of dispersed phase capillaries made the liquid tongue in the step to become wider, prolonging the necking time before pinching off. Hence, the resulted three-component droplet diameter increased obviously. Moreover, the slope of Eq.(4) decreased significantly compared with Eq.(2). The fitted relationship of droplet diameter and dispersed phase flow changed from a single logarithmic linear relationship to a linear relation in Eq.(3) and Eq.(5). These changes indicated that co-flow had a larger effect on the generation of three-component droplets than for Janus droplets, which was because that the slower necking process led to longer interaction time between the continuous phase and dispersed phases in the terrace, resulting in the enhancement of co-flow effect. The generated three-component droplets in this study provided a strategy for the study of micro chemical reactions with catalysts or three reactants. For instance, the intermediate phase could be a catalyst. Because the flow was laminar in the chip, no significant mixing of the three dispersed phases occurred. In addition, as the intermediate phase, the catalyst could also separate the two reactants and prevent the pre-reaction. Furthermore, the content of catalyst phase could be precisely controlled by adjusting the flow rate. Therefore, with good monodispersity and small reagent consumption, the three component droplets provided an insight and new idea for chemical and analytical quantitative research.
Generation procedure of droplet with 3 kinds of components with different color
Effects of different flows on size of three-component Janus droplet with 3 structural color: (A) continuous phase flow; (B) dispersed phase flow
LIAN Jiao-Yuan et al. / Chinese Journal of Analytical Chemistry, 2020, 48(1): 57–65
4
Conclusions
The generation and control strategy of Janus droplet was investigated in this study. At first, a co-flowing step emulsification chip was fabricated by an MCF. Whereas the results showed that the generation of Janus droplets in adjacent channels was unstable, and the range of fusion condition was limited. We found that the fusion stability of two dispersed phases could be improved by decreasing the contact angle between dispersed phases and surface. Therefore, an optimized co-flowing step emulsion chip was fabricated by replacing MCF with glass capillaries. With this chip, the dispersed phases in adjacent channels were stably fused at the outlet of capillaries, and monodisperse two-component Janus droplet was successfully prepared. Furthermore, the influences of continuous phase flow and dispersed phase flow on the generation of Janus droplet were investigated experimentally by this chip. The results showed that with the increase of continuous phase flow, Janus droplet size decreased and generation frequency increased. With the increase of dispersed phase flow, both the droplet size and generation frequency increased. In addition, three-component droplets were generated successfully and steadily by a three-component droplet generation chip which was prepared by adding a capillary to the two-component Janus droplet generation chip. By changing the two phases flow rates, the droplet diameter and frequency could also be tuned on-line. It was found that, compared with the Janus droplet, the co-flow effect had a greater impact on the generation of three-component droplet because of the result that the three-component droplet diameter changed greater than Janus droplet when changing both the continuous and dispersed phase flows. The results of this study provided a carrier and theoretical basis for quantitative analysis and catalysts or reagents involved reaction and detection in chemical and analytical industry.
[6]
Zhu P, Wang L. Lab Chip, 2017, 17(1): 34–75
[7]
Yun D, Xavier C I S, Andrew D. Analyst, 2014, 140(2): 414–421
[8]
Chen Y, Deng Z. J. Fluid Mech., 2017, 819: 401–434
[9]
Xu J H, Li S W, Tan J, Wang Y J, Luo G S. AIChE J., 2010, 52(9): 3005–3010
[10] Li Y F, Xia G D, Wang J. J. B. Univ. Technol., 2016, 42(9): 1414–1421 [11] Bai F, He X M, Yang X F, Zhou R, Wang C. Int. J. Multiphase Flow, 2017, 93: 130–141 [12] Teo A J, Li K H, Nguyen N T, Guo W, Heere N, Xi H D. Anal. Chem., 2017, 89(8): 4387-4391 [13] Cheng L, Cai B, Zuo Y, Xiao L, Rao L, He Z, Yang Y, Liu W, Guo S, Zhao X Z. Chem. Phys. Lett., 2017, 673: 93–98 [14] Maan A A, Schroën K, Boom R. J. Food Eng., 2011, 107(3): 334–346 [15] Lian J Y, Luo X Y, Huang X, Wang Y H, Xu Z B, Ruan X D. Colloids Surf. A, 2019, 568: 381–390 [16] Deng N N, Wang W, Ju X J, Xie R, Liu Z, Chu L Y. Sci. China Ser. B, 2015, 45(1): 7–15 [17] Jia P F, Jiang K M, Liu C, Zhou W P, Zhang T, Zhang Z Q, Li H W. Chem. Res. Appl., 2015, 27(8): 1097–1103 [18] Michele Z, Baroud C N, Cooper J M. Phys. Rev. E, 2009, 80(2): 046303 [19] Shestopalov I, Tice J D, Ismagilov R F. Lab Chip, 2004, 4(4): 316–321 [20] Shklyaev S. Europhys. Lett., 2015, 110(5): 54002 [21] Liu S S, Wang C F, Wang X Q, Zhang J, Tian Y, Yin S N, Chen S. J. Mater. Chem. C, 2014, 2(44): 9431–9438 [22] Kim S H, Jeon S J, Jeong W C, Park H S, Yang S M. Adv. Mater., 2008, 20: 4129–4134 [23] Shen F, Li Y, Liu Z M, Cao R T, Wang G R. Chinese J. Anal. Chem., 2015, 43(12): 1942–1954 [24] Li S J, Zeng W. Chin. Hydrau. Pneumatics, 2013, (6): 13–23 [25] Nisisako T. Curr. Opin. Colloid Interface Sci., 2016, 25: 1–12 [26] Nisisako T, Torii T, Takahashi T, Takizawa Y. Adv. Mater., 2006, 18(9): 1152–1156
References
[27] Nisisako T, Hatsuzawa T. Sens. Actuators B, 2016, 223:
[1]
Zhao L, Shen J, Zhou H W, Huang Y Y. Chinese Sci. Bull.,
[28] Yang Y T, Wei J, Li X, Wu L J, Chang Z Q, Serra C. Adv.
[2]
Liu Z M, Yang Y, Du Y, Pang Y. Chinese J. Anal. Chem., 2017,
209–216 2011, 56(23): 1855–1870 45(2): 282–296 [3]
Wang Z L, Wang Z Y, Zong S F, Cui Y P. Chinese Optics, 2018, 11(3): 513–530
[4]
Chen J S, Jiang J H. Chinese J. Anal. Chem., 2012, 40(8): 1293–1300
[5]
Song W B, Dong C Q, Ren J C. J. Anal. Sci., 2011, 27(1): 106–112
Powder Technol., 2015, 26(1): 156–162 [29] Cramer C, Fischer P, Windhab E J. Chem. Eng. Sci., 2004, 59(15): 3045–3058 [30] Utada A S, Alberto F N, Stone H A, Weitz D A. Phys. Rev. Lett., 2007, 90(9): 094502 [31] Fujiu K B, Kobayashi I, Neves M A, Uemura K, Nakajima M. Colloids Surf. A, 2012, 411: 50–59 [32] Huang X, Eggersdorfer M, Wu J, Zhao C X, Xu Z B, Chen D, Weitz D. RSC Adv., 2017, 7(24): 14932–14938