Laser-Etched Stretchable Graphene–Polymer Composite Array for Sensitive Strain and Viscosity Sensors
A simple method to prepare flexible hydrophobic smart coatings was proposed.
The change of drop size was utilized to measure the applied strain and liquid viscosity.
The prepared composite film exhibits favorable stretchability, high flexibility, and outstanding ability of controlling the drop shape.
KeywordsHydrophobic smart coatings Flexible sensors Soft materials Controlled drops Graphene
Repellent smart coatings, which can regulate surface wettability and control drops by interacting with the external stimulus or adopting the specific design of hierarchical textures, are significant in a great variety of applications ranging from digital lab on a chip, drag reduction, separations to tactile sensing [1, 2, 3, 4, 5, 6, 7]. Various attempts have been implemented to achieve smart coatings. For example, Jiang and co-workers demonstrated a TiO2 nanorod film with switchable surface wettability, which can be transformed between superhydrophilicity and superhydrophobicity by using UV light . Wang et al.  utilized asymmetric nanostructured surfaces to realize controllable liquid spreading. Recently, the increasing complicacy of scientific research and industrial production has demanded innovative smart coatings that can combine stretchable and hydrophobic functionalities to adapt flexible elements. Such coatings should possess three significant characteristics: high extensibility, variable surface wettability, and versatility in the application.
Creating flexible smart coatings is a significant challenge, because it demands a favorable integration of flexible substrates and repellent materials. Soft materials, such as elastomers, paper, and hydrogels, offer exciting promise [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. For example, Jiang et al.  produced superamphiphobic paper by two-step method. Zhao et al.  demonstrated a smart coating with tunable wetting by elastic topologically grooved poly(dimethylsiloxane) films. Among these materials, silicone rubber (Ecoflex) films used as substrates for flexible devices have obtained extensive attention because of their high stretchability and admirable accessibility. The surface wettability of the coating is regulated by a series of characteristics including surface structures and surface chemistry [23, 24, 25, 26]. Recently, graphene and carbon nanotubes have been chiefly used because they exhibit inherent hydrophobicity, high conductivity, and surface texture, even under mechanical strain [27, 28, 29, 30]. For example, Zhao et al.  developed a smart coating with the tunable wettability by using porous graphene films. Mates et al.  presented a stretchable film with stable superhydrophobicity and the restorable electrical feature by combining Parafilm-M and carbon nanofibers. Although many researches provided advancements in flexible hydrophobic coatings, few studies presented smart coatings with flexibility for strain sensing by dynamically controlling the shape and contact angle of the droplets.
Here, we report a method to prepare flexible hydrophobic smart coatings that can control the contact angles of small water drops in a horizontal tensile range of 0–200%. The coatings can be achieved by filtering graphene/SiO2 suspension and peeling off the solidified silicone rubber (Ecoflex) films. In the composite films, we utilized laser engraving method to produce arrays of individual patterns, which were availably used to control the contact angles of the drops by pinning the contact lines. The coatings are stretchable and hydrophobic and enable a strain sensitivity by measuring the change of the drops in a horizontal tensile range. These properties make the laser-etched stretchable graphene–polymer composite films for a drop regulation in various fields.
2 Experimental Section
2.1 Materials and Methods
Graphene (~ 20 μm thick), hexadecyl trimethyl ammonium bromide (CTAB), and silica particles (diameter of ~ 5 μm) were purchased from Aladdin (purity > 99.99%). Silicone elastomer (Ecoflex 0020) was purchased from Smooth-On. Hydroxypropyl distarch phosphate, sheep blood, and phosphate-buffered saline solution (PBS) were purchased from Wuhan Chundu Biology Company.
CTAB and graphene were mixed at a ratio of 1:2.5 by weight with silica particles as follows: Graphene (0.25 g) and different amounts of silica particles were added into the CTAB (0.75 g) solution in 100 mL deionized water. Then, the obtained solution was kept sonicating for 1.5 h to allow the homogeneous dispersion of graphene and silica particle, followed by a setting time of 1 h to assure the sediment of the impurities included. Finally, the mixed solution was transferred to the container, leaving out the impurities. To make the composite film, a certain amount of the solution diluted by deionized water was filtrated in the process of vacuum filtration and washed by water and ethyl alcohol alternatively for three times, with the composite film attached on the filter paper obtained. Then, the filter paper was dried at 60 °C for 5 min. The dried paper was put into a mold, followed by the addition of Ecoflex solution which was the mixture of Part A and Part B by a ratio of 1:1 by weight. After being vacuumized, the above mold was kept at 80 °C for 1 h to let Ecoflex solution cure. Then, the filter paper was removed with the composites on the cured Ecoflex film. Finally, the composite arrays (12.8 × 13 mm2) were obtained by a laser cutting machine. The graphene/Ecoflex composite film and SiO2/Ecoflex composite film were prepared in the same way. A composite film in a larger size (12.8 × 38.8 mm2) was also made by the above procedures for the measurement of angle. To obtain sheep blood of different viscosities, the bought blood was diluted and thickened by PBS solution and hydroxypropyl distarch phosphate, respectively.
2.2 Characterization and Measurements
The structure and morphology of the composite film and the materials used were characterized by scanning electron microscopy (SEM, SU8020). And the sizes of the arrays on the film were measured by optical microscope (NiKon, DS-Ri2). The viscosities of different liquids were measured by a viscometer (NDJ-79). The droplet of about 8 μL was produced from a stainless steel needle. The strains applied on the film were created by a stretching device, which was put in front of the camera of a contact angle meter (SCI3000F). Once a strain was applied, the picture of the corresponding water contact angle was captured by the computer and analyzed later by a contact angle software program. The top views of the liquid drop at different strains were obtained by rotating the contact angle meter by 90° with the camera above the liquid drop. And then, areas of the measured liquid drop from the top view were obtained by an analysis software program (Imagine J). A fixture was also made to fix the film and create different angles on it.
3 Results and Discussion
In addition to the strain sensitivities, the area and horizontal length of the drops also serve as strain sensing performances, due to their high sensitivity. Changes in arrays of individual patterns caused the size of the drops to vary, allowing the area and horizontal length of the drops to sense tensile strain. From a top view, the drops stood on the middle of carven graphene pattern at the tensile strain of 0% (Fig. 3e). With increasing the strain, the horizontal length of the drops increased, and longitudinal length reduced because the edge of the drops pinned in the channel between two carven graphene patterns, preventing it to slide along the horizontal direction. Meanwhile, the width of the channel in the film enlarged, and larger area of drops spreads over the surface of exposed rubber film. To analyze these variations, we utilized threshold algorithm, which can convert an image to a binary image, to calculate the length and area of the drops at different tensile strains. The variable tendency of the length and area of the drops has been calculated (Fig. 3f). The strain sensitivities (S) defined as the slope of curves were 1068 μm2/%, showing an outstanding performance. The tendency was also characterized on the flat rubber film and graphene composite film without arrays of individual patterns (Fig. S7). There are negligible changes for both length and width of the drops on these films. The results further indicate that arrays of individual patterns are important to prevent drops to slide on the flexible film. To enhance the variable range of the contact angle, surface compositions are also significant. For example, the pure rubber film with the arrays can achieve the function to control the drops steadily, but the change of contact angle is small due to the lack of the graphene/SiO2 composite coatings (Fig. S8). For a larger tensile strain (for example 267%), the drops on the composite film were observed to slide on the film, where the drops shrank to the center continually, and its length and area reduced (Fig. S9). This result can be attributed to the reduction in capillary force. When the composite films were stretched over the tensile strain of about 267%, the bigger gap between graphene/SiO2 patterns could not provide adequate capillary force to impede the drop sliding.
As compared with previous rigid substrates for controlling surface wettability and drop shapes, our composite film possesses excellences in good high flexibility and dynamic control. We used graphene/SiO2 coatings to increase surface hydrophobicity and arrays of individual patterns to prevent drop sliding. The preparation process is simple and can be easily realized. For practical application, the change of drop length and contact angle can also be in turn used to estimate the applied strain and angle of rotation and to analyze liquid viscosity. Considering that traditional viscosity analysis is often complex, the composite film provides a novel simple method. Moreover, the film can be used as a strain sensor by measuring the change of the contact angle, averting abundant wires as compared with conventional electric measurements.
In summary, a flexible composite film, composed of graphene/SiO2 composite coatings with arrays of individual patterns and silicone rubber films, was developed for the effective control of the liquid drops, lateral tensile sensing, and analysis of liquid viscosity. The film possesses alterable surface wettability and favorable stability to high tensile strain. Moreover, the multifunctional functions of the composite films will expand the potential applications of other flexible hydrophobic films.
This work was supported by the National Key R&D Program of China (No. 2016YFA0202701), the National Natural Science Foundation of China (Nos. 51472055 and 61404034), External Cooperation Program of BIC, Chinese Academy of Sciences (No. 121411KYS820150028), the 2015 Annual Beijing Talents Fund (No. 2015000021223ZK32), the University of Chinese Academy of Sciences (No. Y8540XX2D2), and the “thousands talents” program for the pioneer researcher and his innovation team, China.
Supplementary material 2 (MOV 2946 kb)
Supplementary material 3 (MOV 1239 kb)
Supplementary material 4 (MOV 6739 kb)
Supplementary material 5 (MOV 7929 kb)
Supplementary material 6 (MOV 7046 kb)
- 6.L. Zheng, Y. Wu, X. Chen, A. Yu, L. Xu, Y. Liu, H. Li, Z.L. Wang, Self-powered electrostatic actuation systems for manipulating the movement of both microfluid and solid objects by using triboelectric nanogenerator. Adv. Funct. Mater. 27(16), 1606408 (2017). https://doi.org/10.1002/adfm.201606408 CrossRefGoogle Scholar
- 13.Y. Ding, J. Yang, C.R. Tolle, Z. Zhu, Flexible and compressible PEDOT: PSS@ melamine conductive sponge prepared via one-step dip coating as piezoresistive pressure sensor for human motion detection. ACS Appl. Mater. Interfaces 10(18), 16077–16086 (2018). https://doi.org/10.1021/acsami.8b00457 CrossRefGoogle Scholar
- 26.J.T. Han, B.K. Kim, J.S. Woo, J.I. Jang, J.Y. Cho et al., Bioinspired multifunctional superhydrophobic surfaces with carbon-nanotube-based conducting pastes by facile and scalable printing. ACS Appl. Mater. Interfaces 9(8), 7780–7786 (2017). https://doi.org/10.1021/acsami.6b15292 CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.