Water-Evaporation-Powered Fast Actuators with Multimodal Motion

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Applications of Polymer, Composite, and Coating Materials

Water Evaporation Powered Fast Actuators with Multimodal Motion Based on Robust Nacre-Mimetic Composite Film Li Zhang, Yaqian Zhang, Feibo Li, Shuang Yan, Zhaoshuo Wang, Lixia Fan, Gongzheng Zhang, and Huan-Jun Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01912 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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ACS Applied Materials & Interfaces

Water Evaporation Powered Fast Actuators with Multimodal Motion Based on Robust Nacre-Mimetic Composite Film

Li Zhang, Yaqian Zhang, Feibo Li, Shuang Yan, Zhaoshuo Wang, Lixia Fan, Gongzheng Zhang, Huanjun Li*

School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, P. R. China, 100081

KEYWORDS: actuator, multimodal motion, graphene oxide, bioinspired, humidity response

ABSTRACT: Water evaporation as a source of energy to trigger the moistureresponsive soft materials is an emerging field in a variety of energy-harvesting devices,

ACS Paragon Plus Environment

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which has attracted widespread attention. Here, we design and fabricate bio-inspired nacre-like composite film actuators consisting of graphene oxide (GO) and sodium alginate (SA), which demonstrate an obvious shrinkage in volume when their state transfers from wet to dry and the contractile stress is up to 42.3 MPa. Based on these features, the film actuators can show rapid and continuous movements under the water gradient. The flipping frequency of the actuators can reach up to 76 rounds/min, which is much faster than the previous reports. The film can flap back and forth quickly on water vapor even after loading a cargo that is 9 times its own weight. Moreover, high mobility with multimodal motion including blooming, stretching, folding and twisting can also be achieved by modulating shapes of films. Thus, the film actuators may hold a great potential in many fields, such as micro-robots, artificial muscles and sensors on grounds of its rapid response speed and adjustable motion models.

INTRODUCTION

Natural evaporation is a potential ubiquitous resource because about three-quarters of the Earth’s surface is covered with water and half of the solar energy absorbed is


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used for water evaporation.1 Moreover, the evaporation energy has the advantage of being eco-friendly and renewable compared with other energy sources. Therefore, efficient utilization of this energy has become an important subject of academic research. Currently, one promising strategy to address this problem is to develop highperformance water-responsive actuators, which can be applied to convert evaporation energy into mechanical energy through various motions like bending, walking, flipping and jumping.2-6 In nature, there exist widely water-responsive phenomena, such as the bending and twisting of wheat awns7 and the opening and closing of pine cones.8 These plants achieve complex movements, which can be attributed to the reversible absorption and desorption of water caused by changes in ambient humidity. Inspired by nature, great efforts have been made to fabricate a variety of water-responsive actuators by mimicking these organisms. Among them, the film actuators have captured special attention because of their flexibility, excellent mechanical properties and rapid humidity response performance.4-6, 9-13 For example, a composite film actuator based on polypyrrole was reported to perform water gradients-driven flipping locomotion and serve as a piezoelectric generator.5 Chen et al. demonstrated water evaporation-driven


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engines which are able to power an electricity generator to light up LEDs and move a miniature car.4 In another study, Zhang and coworkers fabricated a polymer film actuator composed of agarose and azobenzene-containing conjugate, which is capable of undergoing swift motion in response to humidity gradients and photomechanical response by light.13 Despite impressive performances of these water responsive actuators, the research of the film actuators driven by water gradients is still its infancy.917

In addition, the existing film actuators still suffer from shortcomings including slow

response rate due to the weak mechanical properties4,5 and manual humidity control rather than water evaporation as driving force.3

To this end, developing the novel water-responsive materials, exhibiting excellent mechanical properties and high absorption and desorption rate of water, is critical to overcoming the above-mentioned limitations. How one can fabricate such highperformance film actuators? To address this challenge, we focus our attention on the natural nacre, which is famous for its remarkable increase in mechanical properties from the unique layered structure.18 Based on the striking nacre model, many efforts have


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been reported on nacre-inspired artificial structural composites with the exceptional mechanical properties. Among them, the design of graphene oxide (GO) composite films with nacre-like microstructures is an appealing strategy, particularly GO/sodium alginate (SA) nacre-mimetic composite film with a perfect combination of high strength and toughness.19,20 In this case, it is believed that GO plays an important role strengthening and toughening nacre-inspired composite films due to its large surface area, superior mechanical properties, and its oxygen-containing functional groups such as hydroxyl and carboxyl groups.21-24 SA is a natural macromolecule polymer with good biocompatibility, easily accessible and environmentally friendly.25-27 And it has rapid water absorption properties, indicating potential humidity response performance. Therefore, it is expected that such nacre-like composite films base on self-assembly of GO and SA could exhibit the improved mechanical properties and rapid humidity response. To the best of our knowledge, the design of water-responsive GO/SA nacremimetic composite film actuators with multimodal motion has rarely been reported.


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Herein, we reported a GO/SA composite film actuator with a nacre-like structure prepared by evaporation-induced self-assembly process at room temperature, which has excellent mechanical properties. The prepared film actuator can achieve rapid and continuous flipping movement due to the moisture gradient on the upper and lower surfaces of the film caused by water evaporation. Moreover, we can design different shapes of films to generate a series of complex movements, including flipping, flapping, blooming, stretching, folding, twisting, and convert evaporation energy into different forms of mechanical energy. In a word, this kind of actuator material has the advantages of simple preparation method, rapid response and multimodal motion that has broad application prospects in the fields of soft robots and artificial muscles.

RESULTS AND DISCUSSION

The GO/SA nanocomposites with layered structure were formed by mixing GO aqueous solution with a certain amount of SA solution, and then pouring it into petri dishes to transform into films via an evaporation-induced self-assembly method at room temperature.22,28-29 The prepared films were submerged in calcium chloride solution for


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ionic cross-linking,20 followed by drying them for further use. The entire fabrication process is simple, convenient and time-saving. GO was prepared from natural graphite powder by a modified Hummers’ method as reported in our previous papers.30,31 In the preparation process, four kinds of weight ratios of GO to SA were used, which were 1:5, 1:10, 1:15, and 1:20, respectively. The optimized ratio of GO to SA in the film is 1:15, which has good mechanical properties and humidity response performance (Figure S1 and Figure S2). The resultant composite films are light brownish yellow and gradually become darker as the thickness increases (Figure 1a). The GO/SA films are very flexible and have good mechanical strength, that can be bent and folded into many different shapes, such as windmills and pigeons (Figure 1b-c), which is conducive to constructing different macroscopic structures in order to generate various movement behaviors under the water gradient. It is worth mentioning that in the process of making windmills, self-adhesiveness of the film is utilized. A drop of water is dropped in the middle where the four windmills coincide, and they can stick together and maintain the shape of the windmill after a few minutes of air drying.32 According to the crosssectional SEM images (Figure 1d), it can be clearly seen that the films have a thickness


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of about 10 μm and show nacre-like layered microstructure, which is in agreement with the previous report.19,20 This special structure contributes to its enhanced mechanical properties and suitable water absorption and desorption rates compared to pure SA films.

Figure 1 (a) Picture of pure SA film (Transparent) and GO/SA composite film (Brown yellow). Pictures of (b) windmill and (c) pigeon consisting of the films by bending and folding. (d) SEM images of the GO/SA composite film and the weight ratios of GO to SA is 1:15. (e) Typical stress-strain curves of pure SA film and GO/SA composite film in wet and dried state.


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FTIR and XRD tests were carried out to investigate the characteristics of GO, SA and GO/SA films. First, the FTIR spectrum of GO confirms that there are a large number of oxygen-containing functional groups on the GO surface, which can act as reactive sites with SA. In the composite film, the asymmetric and symmetrical peaks of carboxyl groups and the peaks of alkyl oxygen are shifted to 1599 cm-1, 1409 cm-1, 1027 cm-1 respectively compared to pure SA film, demonstrating that GO and SA are cross-linked by hydrogen bonds (Figure S3).33 Moreover, the XRD pattern shows the crystal structure of the films produced. As can be seen in Figure S4, the diffraction peak of GO is at 9.0°, indicating that the d-spacing obtained from Bragg equations is about 9.9 Å, which is consistent with previous report.34 SA presents two broad peaks at 15.2° and 22.2° suggesting that it has an amorphous morphology.33 It is worth noting that there is no GO peak in the composite film, which shows that adding a small amount of GO has no significant effect on the crystal structure of SA.35 The mechanical properties of resultant films were shown in Figure 1e. The tensile strength of the pure SA film is 100.7 MPa, which is consistent with the previous report.19 After being composited with GO, the ultimate strength of the resulting composite films have been significantly enhanced to


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163.1 MPa, which is attributed to hydrogen bonding between GO nanosheets and SA polymer and ionic cross-linking with Ca2+. We found that a small amount of GO can significantly improve the mechanical properties of the films in both a dry and wet state. As the amount of GO continues to increase, the mechanical strength of the composite film decreases slightly, which may be due to uneven distribution of GO (Figure S1). Moreover, the mechanical properties of the composite films in high humid atmosphere (RH≈46%) are much lower compared with films in the natural environment (RH≈26%), which may be attributed to the destruction of the original hydrogen bonds and the formation of new hydrogen bonds after water absorption of the films (Figure 2). 5


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Figure 2 Sketch map shows the cross-linking mechanism and chemical structure changes of composite film after water absorption and desorption under water gradient.

The resulting composite films have excellent hygroscopic performance and can absorb water quickly in moist environment.2,11,36 As shown in Figure S5, the film can absorb water up to saturation in 20 minutes when it is immersed into aqueous solution. The swelling ratio of the composites is about 1.38, which is slightly decreased compared with the pure SA films that may be attributed to the hydrogen bonding between GO and SA and its special layered structure. Here, it should be noted that the moderate hygroscopicity of the nacre-like composite films is crucial for achieving rapid humidity response performance, which not only requires quick water absorption in higher humidity, but also a swift dehydration in lower humidity. Humidity response requires that the two sides of the composite film have different moisture gradients, and not just absorb water. When the film is placed on the porous stainless steel net with water vapor underneath, it can continuously flip and the flipping process can be divided into four steps as shown in Figure 3a. First, the films can bend


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upwards when it is on a substrate above water vapor because the bottom surface of the films absorb more water and swell more than the top surface (I) (Figure S6). As the GO/SA film is bent upward, the center of gravity rises and the contact surface between the film and the substrate becomes smaller, leading to unstable of the center of gravity and falling on one side (II). Immediately, the bottom face of the films are far away from water vapor and lose water. Meanwhile, the upper face is close to water vapor and absorb water (III). Finally, the films become flat and the top surface down, starting a new cycle (IV).


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Figure 3 (a) Representative images of continuous flipping process of GO/SA composite film on a moisture substrate. (b) Effect of water temperature on flipping frequency when the thickness of the film is about 10 μm. (c) Relative humidity of the upper and lower surfaces of the GO/SA film at different temperatures. (d) Effect of film thickness on flipping frequency when the water temperature is constant at 40 °C.


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The water gradient can effectively drive the composite films to flip. Two influence factors were investigated on their turnover performance: water temperature and film thickness. The temperature in the laboratory was about 20 °C and the relative humidity was about 34% when the experiments were carried out. The experiments showed that the water temperature has a great influence on the flipping frequency of the films (Figure 3b). The frequency is very slow when the temperature is 20 °C, because a small amount of steam provided at a low temperature is not enough to provide the power to turn over the film. The frequency increases dramatically with increasing temperature and reaches a higher level when the temperature is 40 °C. The suitable temperature for quick flipping is between 40 °C and 45 °C, and the corresponding relative humidity of the upper and lower surfaces of the films are about 50% and 80%, respectively (Figure 3c). As the temperature continues to increase and exceeds 50 °C, the frequency does not rise but rather decreases, due to the large amount of steam that causes the film to be easily saturated with water and stick to the stainless steel mesh. In order to study the effect of temperature and humidity on the flipping frequency, we control the constant water vapor temperature and constant humidity, respectively. We found that the flipping motion of


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the film depends on the synergistic effect of temperature and humidity, and increases with increasing temperature and humidity (Figure S7). The GO/SA film with a series of thicknesses was cut into squares (1.5 cm × 1.5 cm), its turnover frequency can reach up to 76 r·min-1 when the thickness was about 10 μm (Figure 3d and Movie S1). The moisture response speed is far superior to that of the polypyrrole-based composite film (~25 r·min-1).5 The reason is that our sample show the remarkably improved contractile stress (42.3 MPa) compared to the polypyrrole-based composite films (27 MPa). When the film is thinner, the upper and lower surfaces are easily saturated with water, losing the effect of the water gradient, which is not conducive to flipping and easily adhere to the stainless steel mesh. The frequency decreases as the thickness increases, probably because the thicker the films, the stiffer the films, which is detrimental to its bending deformation. The frequency of the composite films is better than that of the pure SA films regardless of the film thickness (Figure 3d), which can be attributed to the superior mechanical properties of the composites. The composite film not only has high mechanical strength, but it also exhibits high contractile stress. The contractile stress is generated during the water absorption and dehydration of the film and is measured by


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the Shimadzu AGS-J Tester. First, the moist film was clamped on the tester and loaded with a small force of about 0.05 N to make it just tightly clamped. During the gradual loss of water of the film, the contractile stress generated gradually becomes larger and the force becomes maximum when the film is completely dried. Then the film is wetted with moist filter paper. The film swells and the force returns to zero and the next cycle is measured. A cycle takes a few minutes and the contraction stress remains basically unchanged after repeating 50 cycles. As shown in Figure 4a, b, the contractile stress of the composite film with a thickness of 10 μm is 42.3 MPa, which is almost 120 times higher than mammalian skeletal muscle (0.35 MPa)5 and 1.6 times higher than pure SA film (26.2 MPa). As the weight ratios of GO to SA varies between 1:5 and 1:20, the contractile stress is in the range of 30.5-42.3 MPa. The flipping frequency of composite film are about four times faster than the pure SA film, which may be attributed to the greater contractile stress, resulting in greater force to drive the flipping movement. The effect of different thicknesses on contractile stress was investigated and found that the stress is almost constant regardless of the thickness of the composite film (Figure 4c). However, different thicknesses have a great influence on flipping frequency, which can


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be due to the effect of thickness on its own water absorption and desorption rate. As the thicker the film, the rate of water absorption and desorption decreases, and with the thickness increasing from 10 μm to 60 μm, the time required for a complete water absorption and dehydration process increases from 2 min to 10 min (Figure 4d). One cycle time increases significantly with the thickness increasing, resulting in a decrease in frequency.

Figure 4 Mechanical properties of film actuator. (a) The contractile stress of pure SA film during water absorption and desorption. (b) The contractile stress of GO/SA composite


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film during water absorption and desorption. (c) Effect of composite film thickness on contractile stress. (d) The relationship between the different thickness of the composite film and the time required for water absorption and desorption cycles.

The composite film was cut into the shape of strip (3.5 cm × 1.0 cm × 10 μm), and the end of the strip was fixed by a clip and placed on a porous substrate above water vapor. The bottom surface of the film absorbs more water compared with the top surface and bends upwards close to 180° quickly. At this time, the original bottom surface is exposed to water vapor and drives the entire film back to the initial state (Figure 5a). The definition and measurement of the bending angle are shown in Figure S8. The whole process takes less than 1 s and can be repeated hundreds of times. When one end of the film is clamped and the other end is loaded with a weight, its movement is the same as the above process. As shown in Figure 5b, when one end of the film is fixed with a cargo that is one-time weight its own weight, the response speed will not decrease substantially compared with unload. A cycle time is very fast and within 1 s regardless of the load and unload. However, there is a certain difference between the


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two kinds of sports trends. When the film is not loaded, the process of lifting is fast and falling is slow due to its light weight. The situation is exactly the opposite when the heavy object is loaded. With the increase of the weight of the cargo, the response time is gradually increased, and the heaviest cargo lifted is 9 times as heavy as itself (Figure 5c and Movie S2). In order to further evaluate the performance of the film actuator, the bending

force

driven

by

humidity

gradient

was

also

detected,

which

remains in the variation range of 1-5 mN with the different film thickness.37


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Figure 5 The flapping of the GO/SA composite film on a moist substrate. (a) Representative images of flapping process of composite film with cargo loading on a moisture substrate. (b) The relationship between bending angle and time of GO/SA composite film with cargo loading and unloading. (c) The time required for a flapping cycle when loading different weight cargos.

To achieve the water-responsive multimodal motion of the nacre-mimetic composite actuators, designing various architectures based on the composite films is the key factor. Inspired by natural plants such as flowers and tendril, we designed and fabricated flower-like and tendril-like water-responsive actuators based on the GO/SA composite film, as shown in Figure 6a-b. The films with a thickness of 10 μm was cut into the shape of a flower with five petals. The petals can be bent upwards when placed on water vapor because the bottom surface absorbs more water than top surface, resulting in higher swelling. When the flowers are removed from the high humidity, the petals will bloom automatically due to water desorption, just like the opening and closure of flowers


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in nature. The flower actuators can close within 2 s, indicating its rapid moisture response performance (Figure 6a and Movie S3).

Figure 6 The movements of various GO/SA soft actuators in response to humidity. (a) Soft actuator that can be closed like a flower responding to humidity. (b) Schematic illustration of composite film fixed into helical structure and stretching up and down in humid conditions like tendril plants. (c) Image shows the actuator self-folds into a cube on a moisture substrate and returns to its original state when removed from the


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substrate. (d) Image of composite film twisted into helical structure on a moisture substrate.

Similarly, the composite film was made into a complex 3D spiral shape that allowed it to shrink and stretch like a spring under humidity gradient conditions. The composite film was cut into strips (80 mm × 5 mm × 10 μm) and wound on a glass tube with a diameter of 3 mm. It was then placed in an oven at 120 °C for 24 h to obtain a helical actuator. The helical actuator was placed above water vapor and can stretch up and down like a spring due to the water absorption and dehydration. A cycle took about 2 s and the elongation could reach 11%. The cycle could be repeated hundreds of times and the elongation was basically unchanged (Figure 6b and Movie S4). The film actuator can be self-folded into a cube from a development diagram under the influence of humidity gradient by a partial reduction method. As shown in Figure 6c, first, the film was cut into the shape of a cube development diagram. Then, the bottom surface of the cube and the junction between the right and top sides was reduced with


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hydroiodic acid in order to decrease its water absorption capacity (Figure S5). When the actuator is placed on water vapor, the bottom surface basically does not respond to humidity and can maintain the original state. At the same time, the part that has not been reduced can be bent upward due to excellent water absorption capacity. The actuator could quickly self-fold into the shape of a cube when placed on water vapor and can return to the original cube development when the actuator is removed from the humidity gradient environment (Movie S5). The method of reduction is to wet the filter paper with the solution of hydroiodic acid, and then covered the parts that need to be reduced. The composite film can be reduced by contacting with the wet filter paper for just a few minutes. Because chemical reduction can reduce the water absorption performance of the composite film, it can be curled into a spiral by partial reduction. The film was cut into strips (40 mm × 10 mm × 10 μm) and a small strip was reduced with hydroiodic acid at intervals of about 10 mm (Figure 6d). The reduction method was described above. The strip after reduction was placed on water vapor and the strip could be twisted into a spiral within 3 s (Movie S6). This is because the non-reduced part absorbs more water


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than the reduced part, resulting in non-uniform water absorption. The actuator can return to its original shape after being removed from the humidity environment. CONCLUSIONS

In summary, we have developed water evaporation powered fast composite film actuators by evaporation-induced self-assembly of GO and SA. The resulting composite films exhibit high mechanical strength up to 163.1 MPa due to the effect of hydrogen bonding and ionic crosslinking. Importantly, the film actuators display the excellent humidity response performance due to the ability of rapid water absorption and desorption under the influence of water gradient. Our actuators could convert common evaporation energy into mechanical energy through quick flipping and have a broad application prospect in energy conversion. Moreover, we designed composite films into different shapes and structures in order to obtain complex forms of movements, such as blooming, stretching, folding, twisting. These complex and flexible forms of motion make it possible to find applications in the fields of soft robots, artificial muscles and so on.

METHODS


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Materials. Graphite powder was purchased from Qingdao Huarun graphite Co., Ltd (Qingdao, China). Sodium alginate (SA) with a viscosity greater than 0.02 Pa·s was purchased from Sinopharm Chemical Reagent Co.,Ltd (Shanghai China). NaNO3, H2SO4 (98%), KMnO4, H2O2 (30%), CaCl2, K2S2O8, P2O5, HCl (36-38%) and HI (57%) were produced by Beijing Chem. Reagents Co., Ltd (Beijing, China). Preparation of Graphene Oxide. Graphene oxide (GO) was prepared from natural graphite powder according to a modified Hummers’ method. The detailed preparation process of GO and its characterization are consistent with our previous study.30,31

Preparation of GO/SA and r-GO/SA film cross-linked with calcium ions. The GO/SA composite films were synthesized in three steps. First, 2% solution of SA was prepared by dissolving SA in distilled water. Then, a certain amount of GO aqueous solution (5mg mL-1) was mixed with SA solution, followed by stirring for 24 h to make it evenly mixed. The obtained homogeneous mixture was poured into a Petri dish and evaporated at room temperature to form the GO/SA composite film. The thickness of the film is controlled by adjusting the volume of the solution poured into a 60 mm diameter petri dish. The film was


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directly peeled off from the substrate of the Petri dish and cross-linked with 10 wt% calcium chloride solution for 12 h, followed by washing with deionized water several times to remove the residual calcium chloride. The prepared films were cut into different shapes for further research. The reduced GO/SA (r-GO/SA) films were prepared by wetting the filter paper with the solution of hydroiodic acid, and then covering the GO/SA films that need to be reduced. The films can be reduced by contacting with wet filter paper for just a few minutes. Characterization. The mechanical properties were tested with a Shimadzu AGS-J Tester at a loading rate of 1 mm min-1 with a gauge length of 15 mm. The mechanical properties tests for each sample were repeated five times. The microtopography and thickness of the films were measured by scanning electron microscopy (SEM) with a JSM-7401. The Fourier transform infrared spectroscopy (FTIR) was characterized on a Thermo Nicolet 6700 instrument in the frequency range of 4000-400 cm-1. The X-ray diffraction (XRD) was performed on a Shimadzu X-ray diffractometer 6000 with Cu Kα radiation (λ=1.5406 Å). The photographs and videos in the experiment were recorded by a digital camera in Canon G10. Humidity-driven performance tests were conducted


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in the laboratory where the temperature and ambient humidity were approximately 16 °C and 34%, respectively. A beaker of water was placed on a microcomputer heating platform to obtain the required temperature and humidity. A porous stainless steel mesh was placed on the beaker to provide a test platform for films and generate the moisture gradient. ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge.

Typical stress-strain curves of GO/SA composite film; the flipping frequency of GO/SA composite film actuator; FTIR spectra and XRD patterns of GO, SA and GO/SA film; swelling rate of SA, GO/SA and r-GO/SA films; schematic diagram of GO/SA film actuators bending under water gradient; the effect of water vapor temperature and relative humidity on flipping frequency; schematic diagram of definition and measurement of bending angle under humidity gradient (PDF) Movie S1. The GO/SA composite film with a thickness of 10 μm is rapidly flipping on a moisture substrate, converting the evaporation energy into mechanical energy, and the water temperature is maintained at 40 °C (MPG)


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Movie S2. One end of the strip film actuator is loaded with a cargo of its own weight, rapidly flapping back and forth on a moisture substrate, and the water temperature is maintained at 40 °C (MPG)

Movie S3. The opening and closing of the composite film on a moisture substrate such as a flower (MPG)

Movie S4. The composite film is fixed into a helical structure and stretching up and down in humid conditions like tendril plants (MPG)

Movie S5. The composite film can be self-folded into a cube rapidly on a humidity substrate (MPG)

Movie S6. The composite film is twisted into a spiral shape on a humidity substrate (MPG)

AUTHOR INFORMATION

Corresponding Author


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* Phone & Fax: 86-10-68918530, E-mail: [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.

ORCID

Li Zhang: 0000-0003-2356-1620

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (21736001, 21174017), the Beijing Municipal Natural Science Foundation of China (2102040) and the Cultivation Project for Technology Innovation Program of BIT (2011CX01032).

REFERENCES


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Water-Evaporation-Powered Fast Actuators with Multimodal Motion

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