Microfluidic Generation of High-Viscosity Droplets by Surface

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Microfluidic Generation of High-Viscosity Droplets by Surface-Controlled Breakup of Segment Flow Haosheng Chen, Jia Man, Zhongnan Li, and Jiang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2017 Downloaded from http://pubs.acs.org on June 11, 2017

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ACS Applied Materials & Interfaces

Microfluidic Generation of High-Viscosity Droplets by Surface-Controlled Breakup of Segment Flow Haosheng Chen,1 Jia Man,1 Zhongnan Li,1 and Jiang Li 2* 1

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State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, China.

KEYWORDS. Droplet microfluidics, high-viscosity fluids, interfaces, surface wettability, surface roughness, surface profile, electrowetting.

ABSTRACT. Fluids containing high concentration polymers, sols, nanoparticles, etc. usually have high viscosities, and high-viscosity fluids are difficult to be encapsulated into uniform droplets. Here we report a surface-controlled breakup method to generate droplets directly from various aqueous and non-aqueous fluids with viscosities of 1.0 to 11.9 Pa s and a dispersed-tocontinuous viscosity ratio up to 1,000, while the volume fraction of droplets up to 50% can be achieved. It provides a straightforward method to encapsulate high viscosity fluids, in a wellcontrolled manner in the rapid developing droplet-based applications, including materials synthesis, drug delivery, cell assay, and bio-engineering, etc.

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Monodisperse droplets can serve as ideal templates for generating well-defined particles and functional vesicles,1,2 and a variety of high-viscosity fluids, such as polymer solutions3-5 and nanoparticle suspensions,6 are involved in rapid developing droplet-based applications,7,8 such as materials synthesis,1,4,9 drug delivery,10 3D bioprinting,11 and cell assays.12 As the concentration of polymers or nanoparticles increasing, the viscosity of fluids increase significantly. However, manipulating high-viscosity fluids is always a challenge.13-15 High viscosity fluid, such as fluid with a viscosity > 1 Pa s, tends to form a long thread (Figure 1a), which may extend to several meters in length.15 Such observations16-20 also appear when it co-flows with a second immiscible fluid in microfluidics. Because the high viscosity of the fluids resists the deformations, highviscosity fluids are difficult to be encapsulated into uniform droplets.18,21 Therefore, there is a significant demand in generating monodisperse droplets from fluids with a viscosity > 1 Pa s.18,21 However, for microfluidic methods of droplet formation, the viscosity of dispersed phases is usually < 0.1 Pa s,16-18 with a more viscous continuous phase to facilitate formation of droplets,7,18 e.g. Figure 1b. When a dispersed phase with a higher viscosity is involved, current methods to form monodisperse droplets include using gas bubbles to cut the jetting thread into droplets,19 diluting the disperse phase to a lower viscosity,22-24 or synthesizing it from the reaction of two lower-viscosity components.25,26 Therefore, there is still lack of a method to form mondisperse droplets directly from fluids with inherent high viscosity, such as glycerol, polymers solutions, sols, proteins and nanoparticle suspensions. Here we report a droplet formation method based on surface-controlled breakup of a segment flow for the generation of high-viscosity droplets, > 1 Pa s, in a continuous phase with a much lower viscosity, < 0.03 Pa s. The method takes the advantage of the easy generation of a segment flow with low-viscosity segments in a high-viscosity continuous phase (low-in-high-viscosity


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segment flow), and then obtain the desired high-viscosity droplets in a low-viscosity continuous phase (high-in-low-viscosity emulsion), with the inversed encapsulation triggered by the breakup of the high-viscosity film and the consequent adhesion of the low-viscosity droplets on the modified channel surface, as shown in Figure 1c. Different surface treatments, including surface modification of wettability, roughness and profile, are demonstrated for the generation of both aqueous and non-aqueous high-viscosity droplets, while electrowetting is investigated exclusively for the generation of non-aqueous droplets. Monodisperse droplets of a variety of high-viscosity fluids with 1.0 ~ 11.9 Pa s have been obtained, such as honey, starch solution, polymer solution, nanoparticle suspension, as shown in Figures 1d - 1g, where the coefficient of variation (CV) of the droplet size, defined as the standard deviation divided by the average droplet diameter, is CV < 2 %. Details of the fluids see Supporting Information. Generation of aqueous high-viscosity droplets. The generation of monodisperse glycerol droplets, 1.4 Pa s, in paraffin oil, 0.029 Pa s, is demonstrated as a typical example. Glycerol is difficult to be directly encapsulated into monodisperse droplets by a low-viscosity continuous phase with a typical co-flow micro capillary device, as shown in Figure 2a, where a widening jet of glycerol is observed.17 However, when glycerol is used as the continuous phase, uniform paraffin oil segments, i.e. elongated oil droplets, can be generated in a well-controlled stable dripping regime with a co-flow device, where the segment size and the distance between any two adjacent oil segments keep constant with a fixed flow rate ratio. Subsequent surface-controlled breakup of the uniform segment flow will result in the desired glycerol droplets in paraffin oil. Details of the surface treatments and the structures of the devices are described in Supporting Information. It should be noted that all the devices in Figure 2 have the same co-flow structures with similar dimensions, and work under the same flow rate ratio between fluids with similar


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viscosities, consequently the resultant high-viscosity droplets from different devices, as shown in Figure S1 in the Supporting Information, have similar average diameters of 520 ± 10 µm while CV < 1% for the high-viscosity droplets from the same device. Therefore, when comparing the production rate of the droplets generated from different devices, we directly compare the flow rates of high-viscosity fluid, or the number of droplets generated per minute. We first demonstrate the surface-wettability-controlled (SWC) breakup of the oil-in-glycerol segment flow induced by changing the surface wettability of the downstream part of the channel to hydrophobic, as shown in Figure 2b. After the hydrophobic treatment, the water contact angle increase to 90° from 19° of the original untreated surface. As the oil segments flow to the hydrophobic section, the breakup of the glycerol film and the subsequent adhesion of the oil droplets to the hydrophobic surface is induced by the dewetting under the long range hydrophobic surface force, and it results in the inverse encapsulation of glycerol by oil to form glycerol-in-oil emulsions, as shown in Figure 2b, where the flow rate ratio of the oil phase QO to the water phase QW is QO : QW = 3:1 and QW = 0.7 µl/min. As long as the size of the oil segments and the distance between any two adjacent oil segments are kept unchanged, the generated high-viscosity glycerol droplets will be monodisperse. The size of the glycerol droplets is comparable to the inner size of the capillary, and the production rate, droplets per minute, will be the same as that of the oil segments before the breakup. The breakup of the oil-in-glycerol segment flow on a hydrophobic surface happens when the average velocity, i.e. the flow rate, is below a critical value,27 which limits the production rate of the glycerol droplets. However, the modification of surface topography, such as surface roughness and surface profile, in addition to the modification of surface wettability will induce the breakup at a higher velocity.


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For the surface-roughness-enhanced (SRE) breakup, the downstream surface is modified to be rough before being treated to hydrophobic, as shown in Figure 2c, where the rough hydrophobic surface has the roughness of 50.7 ± 0.5 nm and the water contact angle of 105.6 ± 0.8°. When the oil segments were flowing into the hydrophobic rough section of the channel, the breakup of the oil-in-glycerol segment flow happens under a higher flow rate comparing to that on a smooth hydrophobic surface. For example, the flow rate of the glycerol can be increased to Qw = 2 µl/min with Qo : Qw = 3:1 in Figure 2c, which means the production rate of the high-viscosity glycerol droplets can be improved by increasing the surface roughness. Detail discussions on the experimental results of the SWC method and the SRE method are shown in Figures S2 and S3 in the Supporting Information Moreover, the generation of high-viscosity glycerol droplets can be realized with the stepsurface-enhanced (SSE) breakup, where the exit of the co-flow device is treated to hydrophobic and then connected to a wider tube to form a step-down surface profile, as shown in Figure 2d. This step can be seen as a rough surface with a deep and wide groove, and the geometrical change at the step-down edge facilitates the breakup, as the oil segment will recover to its spherical shape when it is flowing into the wider tube. The curved oil-glycerol interface at the step-down edge results in the forced drainage of the surrounding glycerol film, and the subsequent adhesion of the oil segment at the thinnest part of the glycerol film, which is located at the hydrophobic step-down edge. After the adhesion, the oil segment breaks up, and encapsulates the downstream cap of the glycerol film rapidly and forms a glycerol droplet. For example, the flow rate of the glycerol can be increased to Qw = 5 µl/min with Qo : Qw = 3:1 in Figure 2d, which means the production rate of the high-viscosity droplets can be further improved by the step-down surface profile. It should be noted that the hydrophobic treatment at


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the edge of the step is necessary for inducing the adhesion of the oil droplets, or the glycerol will remain wetting the edge, so that the breakup will not happen. Generation of non-aqueous high-viscosity droplets. The generation of monodisperse mineral oil droplets in a glycerol solution is demonstrated as a typical example, where the viscosities of the non-aqueous dispersed phase, high-viscosity mineral oil, and the aqueous continuous phase, an aqueous solution of 70 w.t.% glycerol, are 1.4 Pa s and 0.023 Pa s, respectively. Figures 2e 2h show the generation of high-viscosity oil droplets with the corresponding co-flow, SWC, SRE and SSE methods, respectively. The experimental results indicate that all the three surfacecontrolled breakup methods, i.e. SWC, SRE and SSE as shown in Figures 2f – 2h, can be used to generate high-viscosity oil droplets successfully, when the upstream part of the channel is treated to be hydrophobic, while the downstream part is treated to be hydrophilic. Features of the surface-controlled methods. The size of the high-viscosity droplets can be adjusted by the flow rate ratios of the two fluids, as shown in Figure. 2i, where the droplet size decreases as the flow rate ratio of the low-viscosity phase to the high-viscosity phase increases with a fixed flow rate of the high-viscosity phase. The production rates of the aqueous (glycerol) and non-aqueous (oil) droplets, both of which have a viscosity of 1.4 Pa s, using the co-flow method and different surface-controlled breakup methods are compared in Figure 2j, where the average sizes of the droplets from all the methods are 520 ± 10 µm. The experimental results indicate that both aqueous and non-aqueous high-viscosity droplets are difficult to be obtained with the co-flow method, because either a low flow rate of the high-viscosity phase < 0.15 µl/min or a high flow rate ratio of the low-viscosity phase to the high-viscosity phase ≥ 25:1 is necessary for the droplet generation in the dripping mode, consequently, the production rate is much less than 1 drop/min and the volume fraction of the droplets is no more than 4%. By


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contrast, the surface-controlled breakup methods, i.e. the SWC, SRE and SSE methods, can realize the formation of monodisperse droplets at a volume fraction of more than 30% at a practical production rate. The adhesion of the low-viscosity segments on its wetting surface happens at higher flow rate on a rough surface than that on a smooth surface, due to the thinning of the high-viscosity film on a rough surface,28 which results in a higher production rate with the SRE method than that with the SWC method. As a single step-down profile will cause further thinning of the film thickness at the step-down edge,28 the SSE method is the most efficient one among the surface-controlled methods. Mechanism of high-viscosity droplets generation using the surface-controlled breakup methods. The high-viscosity droplets are obtained by breaking up the thin film of the highviscosity fluid surrounding the low-viscosity segments. There is a critical film thickness, below which the attractive van der Waals force can overcome the Laplace pressure to pull the lowviscosity segments onto the surface and break the surrounding thin film.27 The thickness of a lubrication film, h, for a droplet moving along a smooth cylindrical channel at an average velocity of u, as shown in Figure 3a, can be described by the Bretherton equation,29 h ~RCa2/3, where R is the radius of the channel, and Ca is the capillary number as Ca = ηu/γ, η is the viscosity of the high viscosity phase, and γ is the surface tension between the two fluids. Therefore, there is a critical capillary number, i.e. a maximum flow rate, below which the breakup of the thin film can be triggered for the generation of the high-viscosity droplets, which limited the production rate of the high-viscosity droplets. To increase the production rate, we can decrease the radius of the channel R to reduce the film thickness h, which is proportional to R. Figure 3b shows the critical Ca for the droplet


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generation, and we can find that the critical Ca is increasing as the channel radius decreases, which indicates that the production rate would be increased when smaller channels are used. Another way to trigger the breakup of the thin film at a higher flow rate to increase the production rate is to change the surface topography. According to the theoretical prediction28 of the film thinning over a grooved surface, as shown in Figure 3c, the breakup of the thin film can happen with a higher flow rate. As a step-down edge of a single step surface is a special case of a groove with a wave length of W→ ∞, the minimum film thickness can be achieved at the stepdown edge of a single step surface, as demonstrated in Figure 3d. According to the theoretical prediction28 of the film thinning shown in Figure 3e, small and dense grooves have less effect on reducing the film thickness than deep and wide groove do, which matches well with our experimental results in Figure 2j. Therefore, the single step-down profile will reduce the film thickness down to 50% at the step-down edge, so that the SSE method is the most efficient way for the generation of high-viscosity droplets from aqueous and non-aqueous fluids. The fluid viscosity did not show obvious More experimental verifications of the surface-controlled breakup methods are provided in the Supporting Information. In addition, the experimental results indicate that the production rate of the oil droplets is lower than that of the glycerol droplets of the same viscosity in the devices with same geometries, as illustrated in Figure 2j, because the critical flow rate of the breakup of oil film on a hydrophilic surface is lower than that of the breakup of aqueous film on a hydrophobic surface. This is because the attractive force between two hydrophobic surfaces is a long-range hydrophobic interaction force, of which the interaction range is larger than the van der Waals force between two hydrophilic surfaces.30 Therefore, an electrowetting technology is inverstigated to increase the interaction between the low-viscosity aqueous droplet and the hydrophilic surfaces, so that


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the oil film can be broken on a hydrophilic surface at a higher flow rate, which will subsequently facilitate the generation of the high-viscosity oil droplets, and increase the production rate of the high-viscosity oil droplets. Electrowetting-controlled generation of non-aqueous droplets. The generation of mineral oil droplets, 1.4 Pa s, in deionized water, 0.001 Pa s, is demonstrated here as a typical example. The microchannel is made from indium tin oxide (ITO) coated polyethylene terephthalate (PET), as shown in Figure 4a. A 50-µm-thick PET layer is used as an insulting layer, while the 0.3-µmthick ITO layer is divided into four electrodes, with a 5-µm-thick polydimethylsiloxane (PDMS) layer coated on top as a dielectric layer. Then, the whole surface is folded along the dashed folding lines into a square channel with an inner size of 1.0 mm. The pair of two facing electrodes in the downstream part of the channel are connected to a voltage power supply, and the inner PDMS surface will be changed from hydrophobic to hydrophilic when a voltage is applied on the electrodes. Under this condition, the aqueous droplets would adhere to the electrowetting surface, and the water-in-oil segment flow formed in the upstream would change into oil-in-water emulsions, as shown in Figure 4b. With the effect of the external electric field, the breakup of the segment flow can happen with a thicker film under a higher flow rate. This electrowetting-controlled method can also be combined with a step surface. An enameled copper wire with a diameter of 100 µm, which has an exposed tip, is inserted into the microchannel as the power supply electrode, and the ITO layer embedded on the wall was used as the ground electrode, as shown in Figures 4c and 4d. When the water-in-oil segment flow was flowing out of the step-down edge to a wider tube, the water droplet would contact to the charged copper electrode, and adhere to the channel surface rapidly. Thus, the continuous oil phase is broken and be inversely encapsulated into oil droplets. The collected oil droplets are


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shown in Figure 4e, which are monodisperse with a diameter of 519 µm and CV of 0.7 %. When the electrowetting-controlled method is combined with a step-down geometry, there is a significant increasing in the maximum production rate. As shown in Figure 4f. the maximum production rate increase as the voltage increases, and it can be 5 times higher than the stepsurface method only, where the applied voltage is 0. In addition, the high-viscosity oil droplets are generated from fluids with a dispersed-to-continuous viscosity ratio more than 1,000, which is far beyond the ability of typical droplet microfluidics, and the volume fraction of the highviscosity droplets is about 50% for all the cases shown in Figure 4f. In this work, we demonstrated a kind of surface-controlled breakup methods to fabricate monodisperse droplets from various fluids with a viscosity higher than 1 Pa s. The microfluidic device has a straightforward design of a typical co-flow channel with surface modification on the downstream part of the channel. A segment flow of low-viscosity droplets in a high-viscosity continuous phase is first generated in the co-flow structure in a well-controlled manner, and then is broken up into an emulsion of high-viscosity droplets in a low-viscosity continuous flow, induced by the adhesion of low viscosity droplets on the downstream wetting surface with an appropriate modification of surface wettability. The modification of surface roughness and surface profile in addition to the surface wettability facilitate the droplet adhesion and the subsequent breakup at a much higher flow rate, which increases the production rate of the highviscosity droplets. These surface-controlled breakup methods can be applied to both aqueous and non-aqueous high-viscosity fluids, while the electrowetting-controlled method is developed for the generation of non-aqueous droplets. We demonstrate the generation of monodisperse droplets from a variety of high-viscosity fluids up to 11.9 Pa s and the dispersed-to-continuous viscosity ratio up to 1,000, which is commonly believed impossible for microfluidic methods. As a


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volume fraction of droplets up to 50% can be achieved, the surface-controlled breakup methods can be practically applied to the applications with high-concentration polymer solutions, sol solutions, nanoparticle suspensions, which are widely used in material synthesis, drug delivery, bioengineering, etc.


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Figure 1. High-viscosity fluids and droplet formation. (a) Honey (11.9 Pa s) falling out of a 20G dispenser needle with an inner diameter of 0.6 mm at 2 ml/min. A long thread is observed. (b) Monodisperse low-viscosity oil droplets (0.029 Pa s) generated in honey in a co-flow microfluidic device. The flow rates of oil and honey are 1 µl/min and 5 µl/min, while the generation process can be well controlled in a wide range of flow rates. (c) Schematics of the surface-controlled breakup method. (d) – (g) Monodisperse droplets generated from (d) 11.9 Pa s honey, (e) 8.5 Pa s starch solution, (f) 2.5 Pa s polymer solution of polyvinyl alcohol and (g) 1.2 Pa s aqueous suspension of silica nanoparticles using the surface-controlled breakup method. The diameters and the CVs of the high-viscosity droplets are (d) 612 µm and 0.7 %, (e) 600 µm and 0.9 %, (f) 773 µm and 0.7 %, and (g) 397 µm and 1.3 %. All the scale bars are 1.0 mm.


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Figure 2. Surface-controlled breakup methods for the generation of high-viscosity droplets. (a) – (d) Glycerol droplets (1.4 Pa s, dyed blue) generated in paraffin oil (0.029 Pa s) with surfacecontrolled methods. (e) – (h) Mineral oil droplets (1.4 Pa s, dyed red) generated in an aqueous solution of 70% glycerol (0.022 Pa s) with surface-controlled methods. Scale bars are 500µm and the flow directions are from left to right. All the flow rate ratios of the low-viscosity phase to the high-viscosity phase are 3:1; and the flow rates of the high-viscosity phases are 0.7 µl/m in (a) and (b), 2 µl/min in (c), 10 µl/min in (d), 0.3 µl/min in (e) and (f), 0.6 µl/min in (g) and 5 µl/min in (h). The images and the size distribution of the generated droplets are provided in the Supporting Information. (i) droplet size from (d) could be adjusted by the flow rate ratios. (j) Comparison of the production rates of different methods for the generation of high viscosity droplets with the same average diameter of 520 µm ± 10 µm.


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Figure 3. The mechanism of the generation of high-viscosity droplet in the surface-controlled breakup devices. (a) The geometry of the thin lubrication film between the drop and the channel wall based on Bretherton equation, where h0 is the film thickness on the smooth surface and hmin is the minimum thickness over the surface topography. (b) The measured critical capillary number for the drop adhesion with the variation of the channel radius. (c) – (d), Schematics of the reduction of the film thickness on a rough surface (c) and on a step surface (d). (e), the variation of the calculated film thickness with the normalized wave length of different surfaces topographies, where H/h0=1.


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Figure 4. Electrowetting-controlled generation of high-viscosity oil droplets. (a) The fabrication process of the microchannel with electrodes. The dashed lines are folding lines. (b) The generation of high-viscosity oil droplets (mineral oil, 1.4 Pa s) in deionized water (0.001 Pa s) under the voltage of 600 V in a smooth microchannel. (c) and (d) The electrowetting-controlled step-surface method. When the flow rates of water and oil are 120 µl/min and 250 µl/min, respectively, no oil droplets were generated with a voltage supply of 0 V in (c), while monodisperse oil droplets are generated with a voltage supply of 500 V in (d). Scale bar is 1 mm. (e) monodisperse droplets generated from (d) with a diameter of 519 µm and CV of 0.7 %. (f) The variation of the maximum production rate of mineral oil droplets with the applied voltage in the device shown in (d).


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Fluids used in the droplet generation, microfluidic devices, size and size distributions of droplets generated from SWC, SRE, SSE devices, comparison of the critical capillary numbers in the SWC and SRE devices, effect of the viscosity on throughput and size of droplets in an SSE device, and throughput of SSE devices with different dimensions (PDF) Movie of droplets generated with a SWC device for Figure 2b (AVI) Movie of droplets generated with a SRE device for Figure 2c (AVI) Movie of droplets generated with a SSE device for Figure 2d (AVI) Movie of droplets generated from an electrowetting-controlled SSE device for Figure 4d (AVI)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants No. 51420105006, No. 51275266, No. 51322501). The authors thank Prof. Howard A. Stone for helpful discussions, and Bing Dong for the support on the experiments. ABBREVIATIONS ITO, indium tin oxide; OTS, octadecyltrichlorosilane; PDMS, polydimethylsiloxane; PET, polyethylene terephthalate; SRE, surface-roughness-enhanced method; SSE, step-surfaceenhanced method; SWC, surface-wettability-controlled method; TTS, triethoxysilane.

REFERENCES (1) Shah, R. K.; Shum, H. C.; Rowat, A. C.; Lee, D.; Agresti, J. J.; Utada, A. S.; Chu, L. Y.; Kim, J. W.; Fernandez-Nieves, A.; Martinez, C. J.; Weitz, D. A. Designer Emulsions Using Microfluidics. Mater. Today 2008, 11, 18-27. (2) Joensson, H. N.; Svahn, H. A. Droplet Microfluidics—A Tool for Single‐Cell Analysis. Angew. Chem. Int. Ed. 2012, 51, 12176-12192. (3) Dendukuri, D.; Doyle, P. S. The Synthesis and Assembly of Polymeric Microparticles using Microfluidics. Adv. Mater. 2009, 21, 4071-4086. (4) Park, J. I.; Saffari, A.; Kumar, S.; Günther, A.; Kumacheva, E. Microfluidic Synthesis of Polymer and Inorganic Particulate Materials. Annu. Rev. Mater. Res. 2010, 40, 415-443.


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TOC GRAPHIC 44x23mm (300 x 300 DPI)


Microfluidic Generation of High-Viscosity Droplets by Surface

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