Directional and Path-Finding Motion of Polymer Hydrogels Driven by

Letter pubs.acs.org/Langmuir

Directional and Path-Finding Motion of Polymer Hydrogels Driven by Liquid Mixing Yongxin Wang,†,‡ Xiaofang Liu,†,‡ Xiaofeng Li,† Junjie Wu,†,‡ Yuhua Long,† Ning Zhao,*,† and Jian Xu*,† †

Beijing National Laboratory for Molecular Sciences, Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: The spreading of a miscible liquid with a low surface tension on a water surface generates the directional motion of submerged polymer hydrogels, which could be attributed to convective flows resulting from the gradient of surface tension along the surface (Marangoni effect). The direction and velocity of this motion can be well controlled by altering the driving conditions. Furthermore, a spherical hydrogel can smartly find the path to walk through a microfluidic maze when liquid mixing occurs near the maze exit. This convenient chemical driving approach to transporting submerged objects in a desired way may be useful in microfluidics, micromechanics, and other applications.

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INTRODUCTION In microfluidics and micromechanics, it is of great significance to drive specified objects to move in desired way. The traditional strategies that use electric fields or mechanical devices to control movement have several inherent drawbacks. For instance, the systems are usually complicated, and the external force may do harm to the moving object, especially when it is bioactive. As alternative approaches that are relatively simple and effective, chemical driving movements have aroused increasing interest.1−3 When a drop of liquid with a lower surface tension is placed on the surface of another liquid, the former liquid will spread on the surface rapidly. The free surface of the supporting liquid undergoes a movement directed away from the liquid interface. This movement, which is the so-called Marangoni effect, results from surface tension gradients, and it is a general and important mechanism of mass transportation in nature.4−7 Osada and colleagues8−10 utilized this mechanism to guide the motion of polymer gels swollen in organic solvents. For example, controlled translational motion has been realized by pumping out the organic solvent of a THF-swollen gel through a spouting hole.9 Compared to the method utilizing temperaturegradient-induced Marangoni flow to manipulate aqueous droplets suspended in an oil film,11 the chemical approaches have advantages in driving macroscopic objects and the flexibility of choosing driving systems to meet specific requirements. Similar efforts employing the spreading of organic solvents on water surface have been made to drive the motion of floating macroscopic plastic boats,12 resin microboats,13 lithographically fabricated gels,14 and solid/liquid composites.15 This research has provided an effective way to © 2012 American Chemical Society

convert chemical free energy to mechanical work. However, previous work was mostly involved with objects floating on the surface of water, and less attention has been paid to the underwater state of the Marangoni effect, whereas it is necessary to drive submerged objects16,17 such as hydrogels and cells under many practical conditions. In this letter, we have investigated the motion of submerged hydrogels driven by the liquid-mixing-induced Marangoni effect. It was found that the direction of the motion could be well controlled without external equipment attached to the hydrogel. A plausible mechanism for the motion has been put forward on the basis of visually monitoring the Marangoni convective flows. We also studied the effect of various driving conditions, including the feeding rate and surface tension of the liquid added. Furthermore, a more sophiscated path-finding motion in a microfluidic maze could be realized using this convenient driving method.

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RESULTS AND DISCUSSION Figure 1 shows the directional motion of a spherical alginate hydrogel submerged in water. The diameter of the spherical hydrogel d is 2.74 mm, and the depth of water h is 7.00 mm. The distance between the hydrogel and the syringe needle l is 50 mm at the beginning. The motion of the hydrogel was monitored in an apparatus shown in Figure S1. When the added liquid ethanol was dropped on the water surface through Received: May 14, 2012 Revised: July 5, 2012 Published: July 24, 2012 11276

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been introduced along the trough bottom, and the liquid mixing process was observed from the side of the trough. Right after an ethanol drop entered the water surface in the center of the trough, there were near-surface currents moving away from the addition point to the end of the trough, which resulted in a hollow spot in the center. Within tens of milliseconds, we could see that convective flows in the bulk water had evoked two spirals on each side (Figure 2b). The counter flow of water layers underneath was considered to generate the motion of submerged hydrogels directly. To confirm this mechanism, we reduced the depth of water from 5 to 2 mm to prevent the formation of convective flows. When the water was too shallow, instead of convective flows, a hollow spot formed, and the hydrogel would be pushed away with the movement of the free surface (Figure 2c,d). In previous study, the solvent-driven gels had to be coupled with external equipment to control their moving direction.9 Herein, the moving direction can be steered simply by altering the feeding point of the liquid, making it much more convenient to manipulate the motion of submerged objects. As shown in Figure 2e, when ethanol is supplied at points A−C in sequence, the hydrogel can accomplish linear movement from point 1 to points 2−4 successively (image sequences in Figure S2). The displacement and velocity of the spherical hydrogel shown in Figure 1 were studied (Figure S3). There is an acceleration period in the initial stage. When the hydrogel approaches the syringe needle, the velocity declines, accompanied by a slight deviation in the original direction. It may be attributed to the interference of the upward motion of the bulk water below the syringe needle. The average velocity of the hydrogel is about 9.6 mm/s, and the energy efficiency ηeff is 8.8 × 10−9 %, which is defined as the ratio of the kinetic energy of hydrogel motion to the free-energy change due to ethanol spreading (detailed explanations in Supporting Information, SI).8 The velocity of the hydrogel can be controlled by tuning the feeding rate of ethanol as presented in Figure 3a. Because the mechanical work of motion comes substantially from the chemical free energy change during liquid mixing, it is reasonable that the hydrogel moves faster with the increase in the ethanol feeding rate. The spreading coefficient, which is the free-energy variation for the spreading of a liquid B on the surface of liquid A, can be written as

Figure 1. Image sequences of the directional motion from the video. The blue spherical hydrogel is marked with a red circle. A paper with 10 × 10 mm2 grids was placed under the glass trough as a scale.

a syringe needle at a feeding rate Q of 20 μL/s, the spherical hydrogel began to move toward the needle directionally.18 It is very interesting that the motion of the hydrogel is directed toward the needle, opposite to the moving direction of the free surface. As a completely miscible liquid with water, ethanol dissolves immediately when it is dropped on water. The liquid interface takes the shape of the contour of the meniscus below the needle, from which the free surface of water begins to move because of the Marangoni effect. Ruckenstein et al.5 reported that the drag exerted by the rapidly moving free surface would result in convective flows in the bulk water. As the arrows in Figure 2a show, the water layers beneath the

S = γA − γB − γAB where γA and γB are the surface tensions of A and B, respectively, and γAB is the interfacial tension between A and B. The spreading coefficient is a crucial factor determining the spreading behavior. For ethanol spreading on the water surface, S is about 50.4 mN/m.21,22 The ethanol spreading velocity ν is a function of the product of the surface tension variation dγ/dc and the concentration difference Δc.6

Figure 2. (a) Schematic and (b) recorded images of the convective flows. Motion of hydrogels when the convective flows (c) could and (d) could not form: the depth of water was (c) 5 and (d) 2 mm. (e) Controlled linear movement of the hydrogel.

surface sweep along with the moving free surface and acquire a downward motion with increasing radial distance from the syringe needle. Furthermore, the bulk liquid below the needle moves upward because of the suction induced by the vacancy above it. After a certain time, a steady toroidal motion pattern is developed.19,20 We have performed experiments to observe this toroidal pattern. Deionized water was contained in a narrow rectangular trough (inner thickness 2.0 mm), which would restrict the liquid mixing in 2D space and help us to visualize the convective flows. A layer of water dyed with red ink had

ν ∝ Δc

dγ dc

Because the motion of submerged hydrogels originates from the surface tension gradients in the spreading, the velocity definitely depends on the surface tension of the liquid added. Employed as reference liquids with various surface tensions, aqueous solutions of ethanol at different concentrations were added to water.23 The results indicate that the velocity shows 11277

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Figure 3. (a) Dependence of the average velocity (from l = 50 to 10 mm) on the ethanol feeding rate. (b) Dependence of the average velocity (from l = 40 to 10 mm) on the surface tension difference between the ethanol solution and water, when the feeding rate of the ethanol solution is 20 μL/s.

Figure 4. (a) Schematic of the microfluidic maze. (b) Image sequences of the hydrogel motion in the microfluidic maze. (c) Illustration of the “micropump mechanism: when liquid mixing occurs near the exit, a suction force begins to be exerted on the bulk water because of the Marangoni effect. The suction generates a pressure difference in the tube and therefore pumps the water in the tube out. As ethanol is continuously supplied, the suction acts as a micropump and allows the water to flow through the tube to the exit.

positive correlations with the surface tension difference between water and the liquids added (Figure 3b). This is consistent with the aforementioned theory. Other physiochemical properties of the spreading liquid, including the density, viscosity, and miscibility with water, will also affect the liquid spreading and mixing. However, the surface tension or “spreading coefficient” mentioned above usually has a major effect.21 For example, it has been found that dioxin and ethylene glycol, although heavier than water, spread over the water surface and behave exactly as liquids that are lighter than water.5 This is because the surface force exceeds the gravity force in the case. In our experiment, a dimethyl sulfoxide (DMSO) droplet moves downward into the water phase and the directional motion of the hydrogel does not occur. This may be attributed to the combined effect of the higher density and poorer miscibility of DMSO with water. Moreover, hydrogels with different features such as size will react nonuniformly under the same external driving condition

(Figure S4). This may be useful in the separation of hydrogels with various characteristics. Besides moving toward a specific point where ethanol was supplied, the hydrogel can even find the path to walk through a microfluidic maze. Figure 4a is the sketch of a microfluidic maze with a channel that is 800 μm in width. As shown in Figure 4b, a spherical hydrogel (d = 550 μm) is located at the entrance of the maze submerged in water. When ethanol is supplied continuously near the exit (Q = 10 μL/s), the hydrogel is capable of finding the path through the maze (video in SI). The mechanism of this path-finding motion is based on the principle mentioned above: the microfluidic maze is simplified as a submerged tube, and when liquid mixing occurs near the exit, a suction force begins to be exerted on the bulk water because of the Marangoni effect (Figure 4c). The suction generates a pressure difference in the tube and therefore pumps the water in the tube out. As ethanol is continuously supplied, the suction acts as a micropump and allows the water to flow through the tube to the exit. We performed another experiment to confirm 11278

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this mechanism. The spherical hydrogel was put in a glass tube submerged in water. When the entrance of the tube was open or partially open, the hydrogel in the tube could be driven to move by adding ethanol near the exit. In contrast, the hydrogel stood still when the entrance was closed to eliminate the pressure difference. Then we built a separation wall around the tube to avoid the effect of liquid mixing on the water area in the vicinity of the entrance, and the hydrogel still moved when ethanol was fed (Figure S5). We used the Hagen−Poiseuille law to estimate the pressure difference generated by liquid mixing in the tube. When the ethanol feeding rate is 20 μL/s, the pressure difference is about 2.5 Pa (detailed explanations in SI). Compared to former methods of solving mazelike problems including chemical waves,24 amoeboid organism,25 plasma propagation,26 and chemotactic droplets,27 our strategy is relatively simple and low-cost. Furthermore, when the maze was scaled up with a channel of 8 mm width, this approach was still effective in driving the hydrogel to find the path out (Figure S6). The study of the effect of the maze design on the hydrogel motion, such as path choice in a maze with multiple solutions, is in progress. It should be noted that besides ethanol, many other miscible liquids with low surface tension, such as methanol, isopropanol, acetone, dimethyl formamide (DMF), and tetrahydrofuran (THF), have also been proved to be effective in driving submerged hydrogels (Figure S7). Moreover, the hydrogels might be substituted with other objects, for instance, living cells. In this case, the organic solvents mentioned above are not appropriate for a cell medium, and some other liquid pairs with different surface tensions may be used. For example, we have examined whether water/NaNO3 saturated aqueous solution and even water at different temperatures could play a similar part to the ethanol/water system. These might be useful in the cell medium and other applications. Therefore, this driving mode may be employed as a general method in practical use.

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CONCLUSIONS

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ASSOCIATED CONTENT

Letter

AUTHOR INFORMATION

Corresponding Author

*(N.Z.) E-mail: [email protected]. (J.X.) E-mail: jxu@iccas. ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (grant nos. 21121001 and 50821062). We thank Professor J. T. Dickinson (Washington State University) for his kind and valuable suggestions about the observation of the convective flows.

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REFERENCES

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We have developed an effective approach to driving submerged hydrogels to move directionally by simply adding a low-surfacetension miscible liquid to water. The mechanism of the directional motion was attributed to the convective flows induced by the Marangoni effect. This driving method has several advantages over former work utilizing the chemically induced Marangoni effect: submerged objects without special modification can be driven with a well-controlled direction, and the moving velocity can be easily regulated by changing the feeding rate or the surface tension of the liquid added. On the basis of the mechanism, a path-finding motion of a hydrogel in a microfluidic maze could also be realized when a low-surfacetension liquid was continuously supplied in the vicinity of the maze exit. Becauase the driving manipulation is convenient and the liquid pair used can be diverse, this approach to transporting submerged objects in predefined paths may be useful in microfluidics, micromechanics, and other applications.

S Supporting Information *

Experimental section, additional figures, additional explanations, and a video of the path-finding motion in the microfluidic maze. This material is available free of charge via the Internet at http://pubs.acs.org. 11279

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