Marangoni effect-driven motion of miniature robots and generation of

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Marangoni effect-driven motion of miniature robots and generation of electricity on water Lidong Zhang, YIhui Yuan, Xiaxin Qiu, Ting Zhang, Qing Chen, and Xinhua Huang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03270 • Publication Date (Web): 14 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Marangoni effect-driven motion of miniature robots and generation of electricity on water †

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Lidong Zhang,*, Yihui Yuan, Xiaxin Qiu, Ting Zhang, Qing Chen, and Xinhua Huang †

Department of Chemistry and Molecular Engineering, East China Normal University, Shanghai,

200241, People’s Republic of China ‡

School of Materials Science and Engineering, Anhui University of Science and Technology, Hu

ainan, 232001, People’s Republic of China *Corresponding Author E-mail: [email protected]

ABSTRACT

The well-known Marangoni effect perfectly supports dynamic mechanism of organic solventswollen gels on water. Based on this, we report a series of energy conversion processes of concentrated droplets of polyvinylidene fluoride/dimethyl formamide (PVDF/DMF) that can transfer chemical-free energy to kinetic energy, to rapidly rotate itself on water. 22.2 mg of this droplet is capable to offer kinetic energy of 0.099 µJ to propel an artificial paper rocket of 31.8 mg to move over 560 cm on water at an initial velocity of 7.9 cm s−1. As increasing the droplet to 35.0 mg, a paper goldfish of 10.6 mg can be driven to swim longer at a higher initial velocity of

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20 cm s−1. The kinetic energy of the droplet can be further converted to electrical energy through an electromagnetic generator, in which as a 0.5-MΩ resistor is loaded, the peak output reaches 6.5 mV that corresponds to the power density of 0.293 µW kg–1. We believe this report would open up a promising avenue to exploit energies for applications in miniature robotics.

INTRODUCTION Humidity,1 light,2-4 heat,5-7 electric8-10 and magnetic11 fields have been extensively utilized as energy sources to drive mechanical work of artificial materials in various non-aqueous environments.

Alternatively energy conversion in aqueous environment has also attracted

enormous attention , and many promising designs have been demonstrated to convert energy to mechanical work for various applications on water.12-14 Typical examples include ambientpressure responsive mini-generator from biomimicking of a swim bladder that can generate electricity on water,15 and a new type of cooperating system on water that is capable of working for small robots by conversion of chemical energy to electricity.16 On the other hand, researchers have also developed a meaningful protocol for control over macroscopic supramolecular assembly on water surface based on classical Marangoni effect.17 In this work, we hope to contribute to this rapidly growing research field by reporting a new type of energy source that can be directly operated in aqueous environment, can offer powerful propulsion to drive motion of a series of artificial macro-simulators, and can power a generator for generation of electricity on water. In aqueous environment, aquatic organisms are able to harness a variety of fantastic strategies for their motion that have inspired researchers in designing artificial simulators for various applications.18-29 To mimic natural organisms with self-propulsion, 3D-printed artificial


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microfish and other simulators that move autonomously by ejecting small bubbles of gas have been reported.18,29,30 These dynamic systems above generally require chemical reactions to produce bubbles of gas for self-propulsion under the assistance of chemical catalysts. In our mechanical systems, we completely avoid usage of expensively chemical catalysts, and there are no chemical reactions for energy conversion. The driving force delivered to simulators is only from a physical “chemomechanicals” process of a polymer droplet that isothermally transformed chemical-free energy to mechanical energy through the surface spreading reaction of organic fluid that is pumped out by osmotic and hydrostatic pressures in the polymer.31-35 Such an energy-transfer process is similar to biological motors that run with the dissipation of chemical energy.36 This droplet utilized as a “propeller” on water can be conveniently equipped with other devices or actuators, and has prominent advantages, relative to existed mechanical-motion systems, including noise free, air pollution free, and without producing unnecessary exhaust products.

EXPERIMENTAL SECTION Preparation of droplets of PVDF/DMF: 1 g of PVDF powder (purchased from Solvay, America) was dissolved in 50 mL of DMF in a 100-mL beaker under vigorously mechanical stirring at 100 °C. The final concentration of the solution of PVDF/DMF was 45 mg mL−1, due to evaporation of DMF solvent by heating. According to the results we tested, the optimized concentration range was 40-60 mg mL−1. If the concentration was blow this range, the droplet would not self-rotate, but disperse quickly on water surface. When it was higher than this range, the droplet contained too less content of DMF, and thus rotated only for a short time.


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Artificial paper rockets and goldfishes: The mimetic shapes were designed with a common software, and copied to a paper with colour printing. The paper rockets and goldfishes were then cut out for various tests. Droplet-driven mechanical agitator: The agitator shown in Figure 4a was printed and cut out. The droplet (5 mg) of PVDF/DMF was coated equally at four ends of the agitator blades. Once contacted with water, the spreading reaction of the droplets provided kinetic energy to the agitator, resulting in the rapid rotation. Electric generator: The Faraday’s law of induction states “that a voltage is induced in a circuit whenever relative motion exists between a conductor and a magnetic field and the magnitude of this voltage is proportional to the rate of change of the flux”. According to this law, we designed a facile electromagnetic device as shown in Figure 4b, in which the droplet at both ends of the rotor offered rotational energy. As the rotor rotated, the magnets moved close to or away from the induction coil that continually changed magnetic flux inside the induction coil, thus resulting in the generation of electricity.

RESULTS AND DISCUSSION In our previous work, we designed the polymer films composed from polyvinylidene fluoride/dimethyl formamide (PVDF/DMF), to convert acetone vapors to mechanical energies.37 Here, we utilized PVDF/DMF composite to transfer its chemical-free energy to mechanical energy. We first prepared PVDF/DMF solution in a high concentration of 45 mg mL−1 (see experimental section). When such a droplet (22.2 mg) of the solution was placed on a plane surface of water, a rapid spreading reaction of DMF was generated due to the Marangoni effect that originated from differential surface tension of two liquid phases,38 which offers the shape-


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asymmetric droplet a force and results in its rotational motion (Figure 1a, Movie S1 in SI). The initial spreading rate of DMF is mainly dependent on the Harkins spreading coefficient,39 and it changes with release of DMF and variation of surface tension on water. As the droplet contacted with water, the surface DMF spread preferentially and quickly into water, to generate high propelling force for the rapid rotation. When the droplet-internal DMF started to spread into water, DMF/water exchange would proceed inside the droplet that to a certain degree, decreased spreading rate of DMF. In the meantime with the release of DMF into water, the surface tension of water also slightly weakened (Figure S5 in SI), which hindered DMF spreading rate as well. As the results, rotational velocity of the droplet gradually decreased with time. (Figure 1c and d). The rotational motion terminated until DMF spreading-induced propelling force was unable to overcome water resistance.

Figure 1. Time-dependence of rotational movements of the droplet on water. The droplet can generally rotate more than six minutes on water, and its velocities at fast and slow rotation


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stages are analyzed. (a) A single droplet rotates rapidly in the time course from 0 to 30.33 s with the rotational velocity-variation profile shown in (b) (n = 5). (c) The droplet rotates at the slow stage on water, and (d) its rotational velocity-variation profile from 116.66 to 146.88 s (n = 5). Scale bar: 0.5 cm. Not only single droplet of PVDF/DMF is able to rotate rapidly on water, more of them connected together are also capable of rotational movements while keeping their original rotation direction. For example, for the droplets rotating clockwise on water, when such two, three, or more connected respectively, they still rotated clockwise, but at different rotation velocities. To clarify the rotation velocities, the rotational motion on water was recorded on a video camera (JVC, GC-PX100), and analyzed by a Videomach software (v5.15.1.0) that helps to calculate the number of frames in each circle of rotational motion of the droplet. We then divided the number of frames by frame rate (50 fps) to get exact time of rotating a circle, which was then translated to revolutions per minute (rpm). To a single droplet of PVDF/DMF (weight: 22.2 mg) on water, the rotational velocity was capable to maintain at 270−355 rpm in the time course from 0 to 30.33 s, and subsequently it slowed down gradually (Figure 1b). In the period of 116.66 to 146.86 s, the velocity decreased to the range of 110−180 rpm that is comparable to the rotational motion of a polymer gel on water (Figure 1d).31 Such a droplet of 22.2 mg was capable of continuously rotating for more than 6 minutes, if there were no external influences such as air flow, vibration, and bubbles of gas on water. If more droplets of PVDF/DMF were placed closely on water, they would automatically get together and connect each other by rotational movements. When two of them connected, they showed not only rotational motion, but also translational motion on water. The rotational velocity was only 145−158 rpm that is much lower than that in a single droplet (Figure S1c). As


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the three droplets connected in a line on water, the rotational velocity further decreased to a range of 135−145 rpm (Figure S1d). As a more visualized demonstration, we placed sizedifferent eight droplets of PVDF/DMF on water. They connected and rotated at only 60−70 rpm. As more droplets connected on water, the rotational velocity decreased dramatically. The reason can be supported by more complex spreading reactions of DMF that resulted in the droplets with mainly translational motion, and thus reduced the kinetic energy for rotational motion. Any materials that are able to float on a supporting liquid surface and continuously spread another liquid along the surface, should spontaneously move driven by surface spreading reaction.38 To demonstrate the potential of the droplet of PVDF/DMF in mechanical work, we employed it as a type of “propeller” to drive motion of a series of simulators through the surface spreading reaction on water. We first designed and tailored a two-tailed paper “rocket” that is capable to rapidly move on water when the droplet was placed at its tail (Figure 2a, Movie S2). The spreading reaction of DMF from the droplet offered kinetic energy that was delivered to the rocket, resulting in the forward movement. This process resembles 3D-printed microfish that moves on water by means of the propulsion from self-produced bubbles of gas at its tail.30 Differently the motion velocity in the two-tailed rocket is adjustable. When the droplet was equipped with the rocket at position-1, the spreading reaction of DMF drove the rocket to move at 8.07 cm s−1 that is much higher than the case at position-3 (6.28 cm s−1) in the initial time course (Figure 2b and c). However, the droplet with the largest area of contacting with water at position-1 also accelerated energy consumption that made the rocket slow down quickly (Figure 2b and c). Note that the motion direction of the two-tailed rocket is random at this stage. To achieve the direction controllability in droplet-driven motion, we designed a threetailed paper rocket. As the droplet was at its right position of the tail, the spreading reaction of


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DMF propelled the rocket (weight: 31.8 mg) to move anticlockwise on water (Figure 2e). The motion velocity was up to 7.9 cm s−1 in the first two circles (≈56 cm long in one circle), and then slowed down gradually (Figure 2d, see the profile with red triangles). In this course, 22.2 mg of the droplet was able to supply kinetic energy of 0.099 µJ that could drive the rocket motion over 560 cm (The energy was calculated as 0.5mν12, where m is the rocket weight; ν1 is the average motion velocity in the first circle). When the droplet was applied to the left position at the tail, the rocket moved clockwise at an initial velocity of 6.28 cm s−1 that is a little slower than the anticlockwise motion due to operational error and external influences (Figure 2d and f). To further demonstrate capability of the droplet of PVDF/DMF on converting its chemical-free energy to mechanical work, we engineered a series of artificial goldfishes from a printing paper that initially floated or stayed randomly on water. As the droplet was applied to its surface, it was capable of swiftly swimming forwards, exhibiting vividly fishlike behaviors. The goldfish of 34.0 mg swam anticlockwise when 22.2 mg of the droplet was placed at its right fins that could offer kinetic energy of 0.043 µJ to propel it swimming over 500 cm (Figure 3a, Movie S3 in SI). The swimming direction is switchable that is subject to the position of the droplet on the paper goldfish. As the droplet at its left fins, the goldfish ran clockwise, while at the tail, it swam forwards in a straight line, but quickly became random (Figure 3b and c).


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Figure 2. Droplet working as a “propeller” to drive motion of rockets on water. (a) The twotailed rocket running anticlockwise on water driven by the droplet of PVDF/PVA. (b) Schematic diagram indicating the position of each droplet that influences motion velocity of the rockets. (c) The average velocities profiles of the two-tailed rockets running on water driven by the droplets at different positions (n = 5). The average velocity was calculated as 2πr / t, where r is the radius of each circle of the motion; t is the time of the rocket running each circle. (d) The average


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velocity profiles of the three-tailed rocket running on water, and the calculation method is the same as in (c). (e) The three-tailed rocket running anticlockwise on water driven by the droplet at its right position of the tail. (f) Clockwise motion of the three-tailed rocket with the droplet at its left position of the tail.

Figure 3. The droplet-propelled swimming of artificial paper goldfishes. The goldfish swims anticlockwise (a), clockwise (b), or in a straight line (c) on water as the spreading reaction of the droplet takes place at its right fins, left fins, or at its tailed position, respectively. (d,e,f) Sizeequal two goldfishes swimming synchronously on water demonstrate excellent capability of the droplet in control over the motion direction.


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For more visualized insights into the droplet-driven mechanical motion, two goldfishes were propelled to move synchronously on water to examine the capability of the droplet in control over the motion direction. As expected that the two goldfishes ran oppositely to each other when the droplet was placed at their symmetric positions of the fins (Figure 3d, Movie S4 in SI). The initial velocities in both were similar, ≈4.03 cm s−1, but it slowed down more quickly for the goldfish with clockwise motion due to the influence of external environment. (Figure S2a in SI). We then examined the motion behaviors of the two goldfishes driven by the droplet (22.2 mg) at their right fins, as a result the both were capable of swimming anticlockwise at a similar initial velocity of ≈4.11 cm s−1. Differently, with the droplet at their left fins, the goldfish ran clockwise. All these results above confirmed the excellent capability of the droplet in control over the motion direction (Figure 3d,e and f). The droplet of PVDF/DMF works like a fuel container and the larger the container, the longer the duration of its spreading reaction on water. Thus the larger droplet should be capable of offering more kinetic energy to simulators. To clarify this point, a paper goldfish of 10.6 mg was driven to move clockwise on water by the mass-varying droplet (Figure S2b in SI). As we increased the droplet from 10 to 35 mg, the initial velocity of the goldfish increased from 4.19 to 20 cm s−1, corresponding to a maximum kinetic energy of 0.21 µJ generated and delivered to the goldfish (This kinetic energy was calculated based on goldfish’s mass). This maximum motion velocity is four orders of magnitude faster than the swimmer driven by magnetic field-assistant reciprocal motion, and three orders of magnitude faster than the artificial microfish driven by the gas of bubbles produced from a chemical reaction.11,30 When keeping the droplet mass (22.2 mg) and other parameters constant, a small goldfish (10.6 mg) was propelled to swim at 18.8 cm s−1


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that is two-times faster than the big one (34.0 mg) swimming on water (Figure S3 and Movie S5 in SI). The droplet of PVDF/DMF as an energy source is easily available, and inexpensive, which is not only capable to power the motion of simulators above, but also can be exploited for the real-world applications. One of the examples was to utilize it in a mechanical agitator that rotated with maximum velocity of 166 rpm, and could accelerate the dispersion of a dye droplet in water (Figure 4a, Figure S4, and Movie S6 in SI). The rapidly rotational motion of the droplet of PVDF/DMF on water can also be applied for transduction of the chemical-free energy to mechanical work, and further to electrical energy. As a proof of the concept of utilizing this droplet for generation of electricity, we constructed an electromagnetic generator using the droplet that was equipped with permanent magnets, introduction coil, stator and rotor (Figure 4b). The spreading reaction of the droplet (44.4 mg) located at both ends of the rotor, induced rotor’s rotation on water that changed magnetic flux inside the induction coil to generate an alternating current to pass through a resistor (see the principles in experimental section in SI). When a 0.5-MΩ resistor was loaded onto the generator, the peak output reached ≈6.5 mV (Figure 4c). The average power output was ≈0.013 nW that corresponds to the power density of 0.293 µW kg–1 (per mass of the droplet) (Figure 4d). This device was capable of converting chemical-free energy to electromagnetic energy of 95 nJ in 33 s delivered to the resistor (Figure 4e).


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Figure 4. The droplet working for mechanical agitator and generation of electricity. (a) A droplet-powered paper agitator that is capable of accelerating the dispersion of a dye droplet in water. The dimension of each agitator blade is 1.5 cm x 3 mm. (b) A homemade generator for conversion of chemical-free energy of the droplet to electrical energy on water. The rotor is made of compacted foams, and the size is 4.5 cm x 1.5 cm x 0.8 cm. The size of magnet is 0.5 cm x 0.5 cm x 0.5 cm, and totally four were used at the both ends. An introduction coil that was connected with a 0.5-MΩ resistor, was customized with the inter radius: 16 x 30 mm, wire diameter: 0.2 mm, thickness: 2 mm, and the inductance: 345 µH. An Agilent digit multimeter (34401a) was employed to measure voltages. (c) Voltages generated as the rotor rotated. (d) Instantaneous power, and (e) electrical energy delivered to the resistor.


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CONCLUSIONS In summary, highly-concentrated droplet of PVDF/DMF is capable of converting its chemical-free energy to kinetic energy with giving rise to rapidly rotational motion on water due to Marangoni effect. Such droplets can be thus utilized as a “propeller” to offer kinetic energy for motion of some simulators on water. 22.2 mg of this droplet could generate and deliver kinetic energy of 0.099 µJ that is sufficient to propel a two-tailed paper rocket of 31.8 mg continuously running over 560 cm on water at an initial velocity of 7.9 cm s−1. If increased the droplet to 35.0 mg, it can generate kinetic energy of 0.21 µJ that is capable of propelling a smaller paper goldfish (10.6 mg) to move longer at a higher initial velocity of 20 cm s−1. On the other hand, the motion direction of the three-tailed rockets or goldfishes is controllable while driven by the droplet on water. Moreover, the droplet can also be utilized to convert its chemicalfree energy to electricity through an electromagnetic generator, in which the peak output reaches 6.5 mV and its power density is up to 0.293 µW kg–1. These results highlight the potential of the droplet of PVDF/DMF for applications in micro-robotics and energy conversion. Supporting Information. Measurements of motion velocity of artificial simulators, and movies showing rotation of the droplet and motion of simulators. “This material is available free of charge via the Internet at http://pubs.acs.org.” Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS The authors are grateful to East China Normal University, National Natural Science Foundation of China (Grant No. 51603068) and Natural Science Foundation of Shanghai (Grant No. 17ZR1440600) for financial support. REFERENCES (1) Lee, S. –W.; Prosser, J. H.; Purohit, P. K.; Lee, D. Bioinspired Hygromorphic Actuator Exhibiting Controlled Locomotion. ACS Macro Lett. 2013, 2, 960–965. (2) Lu, Y.; Xu, B.; Sun, S.; Wei, J.; Wu, L.; Yu, Y. Humidity- and Photo-Induced Mechanical Actuation of Cross-Linked Liquid Crystal Polymers. Adv. Mater. 2017, 1604792. (3) Wani, O. M.; Zeng, H.; Priimagi, A. A light-driven artificial flytrap. Nat. Commun. 2017, 8, 15546. (4) Zhou, H.; Xue, C.; Weis, P.; Suzuki, Y.; Huang, S.; Koynov, K.; Auernhammer, G. K.; Berger, R.; Butt, H.; Wu, S. Photoswitching of glass transition temperatures of azobenzenecontaining polymers induces reversible solid-to-liquid transitions. Nat. Chem. 2017, 9, 145–151. (5) Zhao, F.; Zhao, Y.; Cheng; H.; Qu, L. A Graphene Fibriform Responsor for Sensing Heat, Humidity, and Mechanical Changes. Angew. Chem. Int. Ed. 2015, 54, 14951–14955. (6) Kim, Y. S.; Liu, M.; Ishida, Y.; Ebina, Y.; Osada, M.; Sasaki, T.; Hikima, T.; Takata, M.; Aida, T. Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel. Nat. Mater. 2015, 14, 1002–1007. (7) Zhang, X.; Pint, C. L.; Lee, M. H.; Schubert, B. E.; Jamshidi, A.; Takei, K.; Ko, H.; Gillies, A.; Bardhan, R.; Urban, J. J.; Wu, M.; Fearing, R.; Javey, A. Optically- and thermally-responsive programmable materials based on carbon nanotube-hydrogel polymer composites. Nano Lett. 2011, 11, 3239–3244.


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ToC figure:


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Marangoni effect-driven motion of miniature robots and generation of

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