Janus Soft Actuators with On-Off Switchable Behaviors for Controllable

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Surfaces, Interfaces, and Applications

Janus Soft Actuators with On-Off Switchable Behaviors for Controllable Manipulation Driven by Oil Yunyun Song, Yan Liu, Haobo Jiang, Jingze Xue, Zhaopeng Yu, Shuyi Li, Zhiwu Han, and Luquan Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20061 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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ACS Applied Materials & Interfaces

Janus Soft Actuators with On-Off Switchable Behaviors for Controllable Manipulation Driven by Oil Yun-yun Songa, Yan Liua*, Hao-bo Jianga, Jing-ze Xuea, Zhao-peng Yub, Shu-yi Lia, Zhi-wu Hana, Lu-quan Rena

a. Key

Laboratory of Bionic Engineering (Ministry of Education), Jilin University

Changchun 130022, P.R. China *E-mail: [email protected] b.

School of Automotive Engineering, Dongnan Campus, Changshu Institute of

Technology, No. 99 Hushan Road, Changshu, Suzhou 215500, PR China

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Abstract Soft actuators have tremendous applications in diverse fields. Facile preparation, rapid actuation, and versatile actions are always pursued when developing new types of soft actuators. In this paper, we present a facile method integrating laser etching and mechanical cutting to prepare Janus actuators driven by oil. A Janus film with superhydrophobic and hydrophobic sides was fabricated successfully. By cutting the functional layer at the desired positions, a number of quintessential oil-driven soft devices were demonstrated. Furthermore, Janus actuators with surfaces of different wettability exhibited different swelling behaviors, and different media manifested different surface extensions; thus, these actuators are promising candidates for soft actuators and also realized on-off switchability between an oil/water mixture and ethanol. This study offers novel insight into the design of soft actuators, and this insight may be helpful for developing an oil-driven soft actuator that can be operated like a human finger to manipulate any object and extending stimuli-responsive applications for soft robotics. Keywords soft actuator, Janus film, oil drive, swelling behavior, recovery, surface extension

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1. Introduction Recently, considerable research efforts have been devoted to smart soft actuators due to their potential applications in various fields, such as mechanics,1,2 biomedicine,36

sensors,7,8 microfluidics,9,10 and robotics.11-13 These stimuli-responsive materials can

dramatically change their shape upon the trigger of a chemical or physical stimulus, such as temperature,9,10,14 humidity,15-17 light,18-20 electric fields,21-23 magnetic fields,2426

and pH.27 A large number of stimuli-driven platforms have been employed to

transport and manipulate cargo. For example, Wang et al. reported the facile fabrication of light-driven superhydrophobic floating devices by direct laser writing (DLW) treatment on a slice of polydimethylsiloxane (PDMS).28 By integrating the functional layer at the desired position or designing asymmetric structures, linear or rotational motions were demonstrated.28 Gao et al. fabricated a flower-like soft platform that can be deflected by magnetic force and can be used as a gripper to grasp objects of widely varying shapes in air or complex environments.29 The actions and movements of stimuli-responsive actuators are usually achieved by the asymmetrical deformation of a soft platform.30 Quintessential strategies include designing bilayers, multilayers, or other anisotropic structures.31-33 Among the construction strategies, building a Janus bilayer is particularly attractive. Oh et al. fabricated a Janus hydrogel microstrip composed of a soft, pH-responsive polymer hydrogel layer laminated with a highly cross-linked, rigid thin film, generating geometric anisotropy at the micron scale.34 The large difference in the elastic moduli between the two layers of the Janus microstrips led to a self-bending behavior in response to the pH change.34 Tong et al. previously prepared an infrared-driven bilayer 3



hydrogel actuator based on a graphene oxide-poly(N-isopropylacrylamide) (GOPNIPAm) hydrogel through a two-step synthesis, with the second layer formed on the already-formed first layer.35 Zhao et al. reported a new method for preparing Janus bilayer hydrogel actuators, which manifested rapid reversible bidirectional bending actions triggered by a change in pH or ionic strength.36 Ionov et al. demonstrated that non-cross-linked polymers, which typically demonstrate plastic deformation in a melt, possess enough elastic behavior to actuate reversibly.37 The Janus polymeric structure bent because of the contraction of the polymer and the entanglements of nanocrystallites upon cooling.37 Zhang et al. prepared a unique GO/RGO bilayer structure that showed moisture-responsive properties under humid conditions due to anisotropic water-molecule adsorption.38 Based on GO/RGO smart paper, novel graphene-based moisture-responsive actuators have been successfully developed.38 Additionally, carbon-based photothermal actuators have emerged as preferred candidates for new actuating systems.39-41 Chen et al. presented a macroscopic anisotropic composite polymeric hydrogel with synergistic shape deformation and color-changing functions, and this hydrogel can undergo complex shape deformation caused by the thermoresponsive graphene oxide-poly(N-isopropylacrylamide) (PNIPAM) outside layer; then, the inside layer of the perylene bisimide-functionalized hyperbranched polyethylenimine (PBI-HPEI) hydrogel can be unfolded to trigger the on-off switch of the pH-responsive fluorescence under green light irradiation.42 Although the abovementioned actuators demonstrate versatile actuating motions, quite a few challenges remain, including the complicated hydrogel synthesis, lagging driving 4


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response, traditional stimulant factors, and multifunctional structure design. Thus, a facile fabrication method, fast actuation, and precisely designed actuator structure are still great challenges in exploring new kinds of actuators. Herein, an effective method for fabricating Janus soft actuators driven by oil is introduced. Inspired by the photoresponsive behavior of flowers and mimosa, flowerlike and mimosa-like microrobots driven by controllable oil/water mixtures were fabricated by laser etching and mechanical cutting. First, we used a mixture of PDMS and graphene (PDMS/G precursor) to fabricate a circular soft film by a template method. After heat treatment and peeling the PDMS/G film from the template, we obtained a soft PDMS/G film with smooth opposite sides. Next, the positive side was etched by a laser to obtain a uniform superhydrophobic surface. However, the negative side was untreated and retained hydrophobicity. This laser etching technique was employed for both rough structures and surface modification; thus, the Janus film, including superhydrophobic and hydrophobic surfaces, was fabricated successfully. In fact, any desired structures could be directly cut according to predesigned patterns to realize various functions. Furthermore, Janus actuators with different wettability surfaces exhibited different swelling behaviors, and different media illustrated different surface extensions; thus, these actuators are promising candidates for soft actuators. We expect to provide a convenient way to produce an oil-driven soft actuator that not only realizes on-off reversible deformation but also controls the bending positions arbitrarily and the whole deformation structures in contrast to previous light-, moisture- or heatresponsive polymeric actuators. 5



Figure 1. (a) Optical photographs illustrate the positive superhydrophobic side (water contact angle (WCA) of approximately 157.4°) and negative hydrophobic side (WCA of approximately 90.3°); (b) the Janus soft film has stretching properties; (c) schematic illustration of the Janus soft actuator: after the PDMS/G prodrome was solidified and peeled from the culture dish template, we obtained a soft PDMS/G film with smooth opposite sides; then, the positive side was etched by a laser to obtain a uniform superhydrophobic surface, on which water drops remained as spheres; however, the negative side was untreated and retained a hydrophobic character; (d-e) SEM analysis of a fracture surface and positive side of the Janus soft film.

2. Results and Discussion Generally, the two sides of a Janus interface have distinct or even opposite properties, such as hydrophilic-hydrophobic or hydrophobic-superhydrophobic properties. In terms of wettability, the positive side was designed to be rough and highly superhydrophobic with a WCA of approximately 157.4o (the volume of the water drop 6


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was 3 μL); however, the negative side was smooth and had a hydrophobic character with a WCA of approximately 90.3° (Figure 1a, inset). The circular Janus film could be easily stretched to a certain extent (Figure 1b), and this stretching ability provided convenience for the preparation of a deformed actuator. The structural integrity of the positive side was confirmed by scanning electron microscopy (SEM) images. Figure 1d illustrates the cross-sectional morphology of the Janus film. Laser etching formed a conical structure in the vertical direction with a side chord length of approximately 67 μm. In addition, porous structures were uniformly distributed on the surface of the positive side with diameters between 228 μm and 255 μm. A rough nanostructure formed inside the micropores (Figure 1e). On the one hand, the partial thermal degradation of the PDMS by high temperatures deposited rough micro/nanostructures with hydrophobic groups; on the other hand, the multilayered graphene illustrated stronger hydrophobicity than monolayer graphene. Thus, the positive side of the Janus film illustrated excellent superhydrophobicity and superoleophilicity, where water droplets were retained as spheres while oil droplets penetrated the pores of the positive side in an overwhelmingly short time.

Figure 2. WCA on the porous positive side with different pores (a) depths and (b) distances.

In this work, the smooth PDMS/G film illustrated hydrophobicity. Thus, the rough 7



structures increased the hydrophobicity. Inspired from nature, the special nano/microstructures on the surface of lotus leaf and rose petal resulted in the superhydrophobicity. In addition, in our previous work43, we designed the porous structure on the PDMS/G surface. It was found that the rough porous structure contributed to the superhydrophobicity. Thus, we designed porous structures with different sizes on the PDMS/G surface. After a series of experiments, we found that for the soft PDMS surface, the entire structure was easily destroyed by laser especially the pores diameter smaller than 200 μm. Thus, we chose the pores with 228 μm diameter as a unite. First, we studied the effect of pores depth on surface wettability. Through control of the pores depths between 35 μm and 88 μm, we measured the WCA of samples with the diameter approximately 228 μm and the pores (center-to-center) distance approximately 255 μm (Figure 2a). The pores depth after a laser etching process was approximately 35 μm. We increased the pores depth by increasing the number of laser etching. After a series of experiments, we found that when the pores depth was 67 μm, the WCA reached the maximum. It was because with the increase of pores depth, the surface roughness increased. However, multiple laser etchings destroyed the surface structures and weakened the hydrophobicity. Thus, we fabricated the superhydrophobic pores structure by three laser etching processes with the pores depth approximately 67 μm. Second, we studied the effect of pores distance on surface wettability. We measured the WCA of samples with the diameter approximately 228 μm, pores depth approximately 67 μm, and the pores distances ranging between 90 μm and 330 μm (Figure 2b). When the pores distances ranged between 90 μm and 330 μm, 8


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the WCA increased and then decreased. When the pores distances ranged between 90 μm and 210 μm, the pores distances were smaller than the pores diameter, and the overlapping laser etching also destroyed the structure. However, when the pores distance exceeded 330 μm, the WCA dropped below 150° due to reduced rough area. As a result, for the feature sizes of nano-/microstructures to achieve the superhydrophobilicity, there existed a range.

Figure 3. (a) Oil-responsive bending of the actuator, which manifests different actuation angles in response to different oil volumes; (b) the bending angles of the actuator containing different graphene contents reacting to chloroform; (c) the bending angles of the actuator reacting to hexane, 9



petroleum ether and chloroform; (d) the bending angles of the actuator reacting to emulsions (the oil/water volume ratio decreased from 20/1 to 20/3); (e) the bending angles of the actuator with different thicknesses; (f) the surface tension of different media (air and oil) for the actuator.

In fact, the two sides of the Janus film manifested different surface wettability, including superhydrophobicity/superoleophilicity and hydrophobicity. Thus, the Janus film illustrated obvious oil-responsive bending properties. We used a syringe with scale to drop small amounts of chloroform on the positive side. The film would bend toward the negative side after absorbing oil (Figure 3a). If some of the oil was dropped on the negative side, the actuator slightly bent toward the positive side, and the bending deformation was not obvious (Figure S1) though increasing the oil volume. Because the negative side absorbed limited oil, it caused smaller surface energy difference, which was not conducive to achieve effective deformation. Figure 3a depicts the simplest oil-responsive bending of the actuator, where the actuator with 0.8 mm thickness demonstrated different actuation angles in response to different oil volumes. Obviously, with an increase in oil volume from 0.01 ml to 0.1 ml, the bending angle of the actuator increased up to 360°. The graphene played a crucial role in the oilresponsive deformation. First, the multilayered graphene illustrates stronger hydrophobicity than monolayer graphene. Second, as the matrix is transparent, multilayered graphene is crucial for constructing the superhydrophobic surface as a light absorption cutoff. Third, graphene is more sensitive to oil with strong adsorption properties than PDMS. Lastly, because of the interlayer lubrication of multilayer graphene, the flexibility and impact resistance of the PDMS/G film are better than those of pure PDMS. Accordingly, to highlight the advantages of graphene, we prepared semicircular actuators without graphene and with different graphene contents and 10


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measured the bending angles of these samples when stimulated by chloroform, as shown in Figure 3b. Driven by the fixed volume of oil, the bending angle gradually increased with the graphene content. The bending angle of the actuator gradually increased as the added oil volume increased. In addition, we measured the oilresponsive bending of the semicircular actuator without graphene, as shown in Figure S2. The bending angle was much smaller than that of the actuator with graphene (Figure 3a) when stimulated by the same oil volume. When the graphene content increased to 5%, the bending angle clearly decreased. This finding indicated that excess graphene reduced the superhydrophobic properties of the surface, thus affecting the deformation ability. The results indicated that graphene was more sensitive to oil with strong adsorption properties than PDMS. In our previous work,43 we discussed the effect of the amount of graphene on the PDMS/G surface wettability. It was found that too much or too little graphene weakened the surface hydrophobicity when the temperature changed. Thus, in this work, we chose a ratio of graphene to PDMS of 3.3%. In addition, we measured the tensile strength and modulus of the actuator containing 3.3% graphene, as shown in Table S1. Figure S3 shows the engineering tensile stress-strain curve of the Janus actuator. The results indicated that the tensile strength was ~1.05 MPa and the modulus was ~0.907 MPa. To clarify the actuation behaviors of the Janus soft film when stimulated by different oils, we compared the bending angles of the as-prepared semicircular actuator in response to hexane, petroleum ether and chloroform, as shown in Figure 3c (the densities of these three kinds of oils were arranged in order from low to high). For all oils, with the increase in 11



oil volume, the bending angle gradually increased; however, for different oils with the same volume, the bending angle was the highest for chloroform. This finding was because the different densities resulted in different swelling effects. After absorbing the heavier chloroform, the surface area of the positive side was larger. We also measured the bending angle of the actuator when driven by emulsions (Figure 3d). As the oil/water volume ratio decreased from 20/1 to 20/3, the bending angle also decreased. In addition, the thickness of the actuator greatly influenced the actuation angle. As the thickness of the actuator increased, the bending angle gradually decreased (Figure 3e) because the thicker the actuator, the greater the ability to resist the bending deformation was. Therefore, the thickness of the Janus actuator seriously affected its deformability.

Figure 4. Response rate of the semicircular actuator (a) containing different graphene contents reacting to 0.02 ml chloroform and (b) reacting to hexane, petroleum ether and chloroform with different volumes.

Additionally, the response rate was evaluated by the bending angle per unit time. Dropping a certain amount of oil droplets on the positive side of the Janus actuator, we recorded the maximum bending angle and time required to reach the bend angle. Then, we evaluated the response rate of as-prepared semicircular actuator containing different graphene contents reacting to chloroform (Figure 4a). Obviously, driven by 0.02 ml chloroform, the response rate gradually increased with the graphene content. When the 12


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graphene content increased to 5%, the response rate clearly decreased. The result was consistent with Figure 3b. The root cause was that graphene contents affected the wettability and deformability of the functional surface. Because graphene was more sensitive than pure PDMS in oil absorbing, the response rate increased with the graphene content. However, excessive graphene weakened the hydrophobicity and thus reduced the response rate. Then, we compared the response rate of the actuator in response to hexane, petroleum ether and chloroform, as shown in Figure 4b. For all oils, with the increase in oil volume, the response rate gradually increased; however, for different oils with the same volume, the response was the highest for chloroform. This result was consistent with Figure 3c. Oils with different densities resulted in different swelling effects. After absorbing the heavier chloroform, the surface area of the positive side was larger. In addition to the measurement of the bending angle and response rate, the bending forces of the semicircle actuator with 0.8 mm thickness was also quantitatively evaluated. The average bending force was measured to be ≈ 1.59 mN. In fact, the bending force was dominated by the size and the thickness of the actuator, a lager force could be obtained using thicker actuator. Two driving mechanisms exist in the actuation deformation of the Janus actuator: surface tension and absorbing swell. As shown in Figure 3f, we analyzed the surface tension of different media (air and oil) on the circular actuator. Oil droplets spread instantly on the positive side and formed a circular oil spot with a very small WCA 𝜃𝑜𝑖𝑙, which applied surface tension to the actuator as follows: 𝐹𝑜𝑖𝑙 = 𝜋𝑟2𝛾𝑜𝑖𝑙 13


(1)


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𝐹𝑜𝑖𝑙 ― 𝑥 = 𝐹𝑜𝑖𝑙cos 𝜃𝑜𝑖𝑙

(2)

𝐹𝑜𝑖𝑙 ― 𝑦 = 𝐹𝑜𝑖𝑙sin 𝜃𝑜𝑖𝑙

(3)

Where r is the radius of the circular oil spot and 𝐹𝑜𝑖𝑙 ― 𝑥 is the component of the oil surface tension in the horizontal direction, whose vector sum was zero; thus, the vertical component of oil surface tension triggered the Janus actuator to bend upwards. In addition, we analyzed the air surface tension on the negative side corresponding to the oil spot. Theoretically, air easily spread over any substrate due to larger surface tension. Similar to the force analysis of 𝐹𝑜𝑖𝑙, the vertical component of air surface tension caused the Janus actuator to bend down. However, because 𝛾𝑔𝑎𝑠 > 𝛾𝑜𝑖𝑙, 𝐹𝑔𝑎𝑠 ― 𝑦 > 𝐹𝑜𝑖𝑙 ― 𝑦, and the surface tension difference drove the Janus actuator to bend away from the positive side. After absorbing oil, different surface tension caused asymmetric surface energy on the positive side and negative side, which contributed to the bending deformation.

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Figure 5. (a) Stereoscopic microscope optical images of the actuator before and after absorbing oil; (b) optical images of the semicircular actuator in an oil-water mixture and pure ethanol; (c) reversible bending behavior of the semicircular actuator switching between an oil/water mixture and pure ethanol; (d) optical graphs of the bending motion depending on time; (e) the recovery process of the curly semicircular actuator in pure ethanol with time.

In addition, the absorption and expansion ratios of the two sides for oil were different because of the difference in surface wettability, which resulted in oilresponsive swelling-shrinking properties for the two sides. For this reason, oil molecules were adsorbed by the positive side in preference to the negative side, as shown in Figure 5a. After absorbing oil droplets, the diameter of the pores increased from 244 μm to approximately 334 μm due to volume swelling. In this regard, the swelling positive side bent toward the negative side, as shown in Figure 5b. Thus, the following theory gave an excellent explanation for Figure 3c-d: the more the oil 15



droplets were absorbed, the larger the surface area expanded, and the greater the bending angles were. The selective adsorption/desorption of oil in the Janus actuator triggered a significant expansion/contraction effect, thus leading to reversible bending and straightening performance. The reversibility of this self-organization state was achieved by removing the oil molecules on the actuator. Thus, the actuator bent when stimulated by oil and stretched in pure ethanol. The oil/water mixture could slow the deformation rate and make the deformation process clearer; thus, we chose an oil/water mixture as the stimulus. Figure 5b illustrates the optical images of the actuator driven by the oil/water mixture and pure ethanol, respectively. Here, we used low-surface-tension ethanol liquid to clean the actuator. Moreover, repeated bending results (Figure 5c) also clearly show that the actuator achieved reversible bending actuation that switched between ∼540° in the chloroform-water mixture and returned to 0° in pure ethanol. The actuator could quickly bend and recover its initial curvature as the solution-switching stimulus was turned on and off several times. All these results indicate that an actuator with good shape reversibility possesses a potential application in soft actuators. To further understand the deformation mechanism of the actuator, Figure 5d-e depicts the bending and recovery process due to the solution-switching stimulus between the oil/water mixture and pure ethanol. The reaction time of a thinner Janus semicircular actuator was so fast that it was difficult to clearly observe the whole deformation process. Thus, we chose an actuator with 0.8 mm thickness as the experimental object. The actuator bent instantly when the oil/water mixture containing 0.2 ml chloroform 16


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was dropped onto the positive side of the actuator, and the response time was approximately 23 s. Then, the deformed sample was immersed in pure ethanol, and this sample gradually expanded to a flat semicircular shape (Figure 5e) while the oil molecules were removed by ethanol. However, the recovery period was slightly longer than 100 s.

Figure 6. (a-c) The bending motion of a symmetrical serrated soft platform depending on the oil absorption; (d-f) the recovery process of the contracted serrated soft platform in pure ethanol with time; (g-i) the closure motion of a flower-like soft platform depending on the oil absorption; (j-l) the bloom of the bending bract in pure ethanol with time.

The oil-responsive property of the Janus film made it feasible to design smart soft actuators. First, the Janus film was cut into a symmetrical serrated shape and then 17



adopted as a stimulus-responsive actuator for oil and ethanol. As shown in Figure 6ac, dropping an oil/water mixture onto the serrated soft platform resulted in the immediate self-assembly formation of a centimeter microchannel in 8 s. This axial folding was orthogonal to the local elastic tensile force that drove the folding of the actuator. Video S1 in the Supporting Information shows the folding process. Then, the folded serrated soft platform spontaneously unfolded after immersion in an ethanol bath in 5 s (Figure 6d-f). Subsequent desorption of the oil on the positive side restored the symmetry of the Janus actuator surface energy and unfolded the microchannel back into its original flat shape. The unfolded Janus actuator was easily reactivated and capable of multicycle shape reversibility. This directional folding and unfolding response was reminiscent of the mimosa's tropism in nature, although the stimulus propagation mechanism of the serrated actuator was different. In fact, any desired structures could be directly fabricated by cutting according to a predesigned pattern. A flower-like soft platform was cut along the outline of a radiated pattern template (Figure 6g). On contact with the oil/water mixture, the petals of the flower-like soft platform bent up instantly (Figure 6h). The elastic force downward to the center of the flower-like soft platform caused the petals to bend up. To suggest the better stability of the closed bract, we turned the bent bract upside down (Figure 6i), and it could still stand steadily. Video S2 in the Supporting Information records the closing process. Then, dropping ethanol onto the bract surface caused the surface to recover the original shape (Figure 6j-l). In addition, the flower-like soft platform had a high capability of elastic distortion. Figure 7 shows images of the flower-like soft platform. When the pressure was removed, the 18


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petal would revert to its original shape from the bent structures. Figure 7a-b illustrates the bending state constrained by the force acting on the petals, and the corresponding shape is displayed in Figure 7b, revealing softness and good flexibility when folded. In addition, the flower-like soft platform exhibited diverse mechanical motions by control of its bending positions and response time. As a demonstration, we stimulated its “petals” by oil to create flexible bending motions. We could bend the petals in arbitrary order at any position. First, when dropping the oil/water mixture onto any petal, the curvature of the bending petal gradually increased (Figure 7c-e) until to the limit. Additionally, we could choose some of these petals as the response locations (Figure 7f). Therefore, compared with previous light, moisture or heat-responsive polymeric actuators, it was easy to control the bending positions arbitrarily and the whole deformation structures.

Figure 7. (a-b) Images of the flower-like soft platform, revealing good flexibility when folded; (cf) images of the flower-like soft platform bending in response to the location of absorbed oil spots.

Based on the bending characteristics, the flower-like soft platform could be adopted as an eight-arm gripper to mimic soft robotics. Figure 8 shows the gripping 19



process of an eight-arm gripper transforming a cotton ball. When dropping a small amount of an oil/water mixture onto the gripper, all eight arms of the gripper bent downward due to the swelling of the positive side after absorbing oil droplets. As time passed, the curvature gradually increased until the gripper was curly enough to grasp the cotton ball (namely, the bending process shown in Figure 8a-c). However, when dropping ethanol onto the back of the gripper, the gripper unfolded, changed back to its initial shape, and then released the cotton ball (namely, the unfolding process shown in Figure 8d-f). This eight-arm gripper could not only transport the cotton ball but also pick up a 3 cm cubic melamine foam (Figure S4). Video S3 in the Supporting Information shows the gripping process. Clearly, this Janus film, which combines the advantages of PDMS polymers such as flexibility, elasticity, chemical resistance, and biocompatibility with the oil-responsive properties of graphene due to the surface wettability, has broad applications in soft robotics.

Figure 8. Demonstration, including the capture process (a-c) and release process (d-f), of the flowerlike soft platform used as a gripper to transport a cotton ball with a relatively soft surface.

In addition to the “smart gripper” of an oil-controlled claw, the Janus film could 20


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also be used in other smart actuators, such as a hand-shaped soft platform. Based on a previous study, the fingers bent into a fist when driven by an oil/water mixture and stretched to the unfolded hand shape after being immersed in ethanol (Figure 9a-c). In addition, we could bend the finger in an arbitrary order at any position artificially. In this way, the hand-shaped soft platform could act as “smart fingers” for oil actuation. Thus, gestural hand signals for one-, two-, and three-finger movements were successfully achieved by selective stimulation to several of the fingers (Figure 9d-e). Video S4 in the Supporting Information shows the process of making a fist.

Figure 9. (a) Images of the fingers of a hand-shaped soft platform bending and unbending in response to an oil/water mixture (b) and ethanol (c); images of the fingers bending and unbending in response to the location of absorbed oil spots; (d) the schema with the red fingers from the oilabsorbing locations. (e) gestural hand signals for one-, two-, and three-finger movements.

Based on the above discussion, the positive side of the Janus actuator illustrated superhydrophobicity/superoleophilicity and was capable of absorbing oil droplets and dewetting water droplets. Thus, the oil-driven Janus actuator could realize oil/water 21



separation. As shown in Figure 10, we prepared Janus semicircular actuators with different thicknesses for oil/water separation. For the thinner sample, when dropping the oil/water mixture on the semicircular actuator surface, this surface can bend down instantly to the limit in 5 s after absorbing oil droplets (Figure 10a), while water droplets rolled away along the curve of the folded sample. In contrast, for the thicker sample, with the increase in oil volume, the bending angle also increased until reaching the limit after 18 s (Figure 10b). Similar to the thinner sample, oil droplets penetrated the positive side, and the water droplets moved down along the curve. Thus, compared with the thinner sample, the thicker sample was capable of absorbing more oil and realized more efficient oil/water separation.

Figure 10. The oil/water separation process for Janus semicircular actuators with different thicknesses: (a) 0.5 mm thickness and (b) 0.8 mm thickness.

3. Conclusion In conclusion, we have successfully fabricated Janus actuators driven by oil absorption by integrating laser etching and mechanical cutting. By cutting the functional layer at the desired position or designing various structures, a number of quintessential oil-driven soft devices were demonstrated. Furthermore, Janus actuators with surfaces of different wettability exhibited different swelling behaviors, and different media manifested different surface extensions; thus, these actuators are 22


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promising candidates for soft actuators and also realized on-off switchability between an oil/water mixture and ethanol. The thicker the Janus actuator was, the smaller the curvature, and the longer the response time. The serrated platform folded and unfolded when stimulated by oil and ethanol, respectively. The flower-like platform was adopted as a gripper that could grasp objects. Gestural hand signals for one-, two-, three-, and four-finger movement could be achieved on the hand-like platform. This study offers novel insight into the design of soft actuators, and this insight may be helpful for developing an oil-driven soft actuator that can be operated like a human finger to manipulate any object and extending stimuli-responsive applications for soft robotics. 4. Experimental Section Fabrication

of

Janus

soft

actuators:

First,

a

little-mixed

liquor

of

polydimethylsiloxane (PDMS) solution (10:1, PDMS: crosslinking agent, by weight) and a given mass of graphene powder (PDMS/G prodrome) were dropped in a culture dish (diameter of approximately 5 cm). The graphene powder used in this work was obtained by chemically reducing graphene oxide powder; the original graphene oxide powder was purchased from Suzhou Tanfeng Science and Technology Company. Then, the PDMS/G prodrome was solidified at 80 °C for 2 h. After peeling the PDMS/G film from the culture dish, we obtained a soft PDMS/G film with smooth opposite sides. Next, the positive side was etched with a laser to obtain a uniform superhydrophobic surface, on which water drops remained as spheres. However, the negative side was untreated and remained a smooth hydrophobic surface. Thus, we fabricated a Janus soft actuator including superhydrophobic and hydrophobic sides that was stimulated by oil 23



due to surface tension and absorbing swelling behaviors. In addition, any desired structures could be directly cut according to predesigned patterns to realize various functions, such as the serrated and flower-like soft platforms. The graphene powder used in this work increased the surface roughness and absorbance medium. A schematic illustration of the fabrication process for the Janus soft actuators is illustrated in Figure 1c. Characterization: Optical photographs were observed by a single-lens reflex camera (Canon 80D, Japan) with a timescale. The WCA was measured on an OCA20 machine (Data-Physics, Germany), and the morphology of the Janus soft film was measured by stereo microscope and SEM. The mechanical properties of the actuator were evaluated by a universal tensile tester (INSTRON 1121) with a strain rate of 1.0×10−2 s−1 at room temperature. The actuator specimens were cut into a strip shape with 30 mm length, 4 mm width, 0.65 ± 0.1 mm thickness, and a gauge length of 10 mm. It is worth to point out that the strain rate is a constant tensile rate relative to the original gauge length. The specimens were fixed in a clamp and then stretched to breakage. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. When dropping oil on the negative side, the oil-responsive bending of the semicircular actuator was stimulated by different oil volumes; oil-responsive bending of the semicircular actuator without graphene, which manifests different actuation angles in 24


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response to different oil volumes; engineering tensile stress-strain curve of the Janus actuator; demonstration of the flower-like soft platform used as a gripper to transport melamine foam, including the capture process and release process; the tensile strength, modulus and elongation-to-failure of the semicircular actuator at room temperature. (PDF) The folding process of the serrated soft platform (AVI) The closing process of the flower-like soft platform (AVI) The grabbing process of the eight-arm gripper on melamine foam (AVI) The process of making a fist for the hand-shaped soft platform (AVI) Acknowledgments The authors thank the National Key Research and Development Program of China (2016YFE0132900),

the

National

Natural

Science

Foundation

of

China

(Nos.51761135110, 51625402, 51775231), the Changjiang Scholars Program (T2017035), Natural Science Foundation of Jiangsu Province (BK20181036), Science and Technology Development Project of Jilin Province (Nos.20160204005SF, SXGJSF2017-3) and 111 project (B16020) of China. Conflict of Interest The authors declare no conflict of interest.

25



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For Table of Contents Only Janus bilayer soft actuators realized on-off switchable behaviors between oil and ethanol for controllable manipulation.

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Janus Soft Actuators with On-Off Switchable Behaviors for Controllable

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