Fabrication of superhydrophobic surfaces inspired by “stomata effect” of plant leaves via swelling-vesiculating-cracking method
Graphical abstract
Introduction
The surface wetting is a vital and universal phenomenon in nature and daily life. Since the lotus effect resulting from the micro-/nano dual-scale hierarchical structures on surface was revealed by Barthlott and Neinhuis in 1997 [1], [2], the superhydrophobic surfaces with high water contact angle (>150°) and low contact angle hysteresis (<10°) have attracted tremendous attention in the past decades [3], [4]. The superhydrophobic surfaces have been widely applied in various fields including oil–water separation [5], [6], anti-icing [7], [8], self-cleaning [9], [10], and microfluidic devices [11], [12] and so on.
A large number of methods have been developed to prepare the superhydrophobic materials, such as chemical vapor deposition [13], [14], embedding nanoparticles [15], [16], dip-coating [17], [18], templating method [19], [20], laser method [21], [22], electrodeposition [23], [24], sol–gel approach [25], [26], self-assembly method [27], [28] and hydrothermal approach [29], [30]. However, most of the methods usually require complex multistep processing procedures under special conditions to construct micron-scale and nanoscale hierarchical structures. Moreover, many methods often use biological poison materials, such as fluoroalkylsilane (FAS), to achieve a low-energy surface. Therefore, it is worthy of high attention to construct microscale structure without modification of low surface energy substances to simplify the fabrication of superhydrophobic surfaces.
To our best knowledge, several micro-structured superhydrophobic surfaces have been reported [31], [32]. For example, Doo-Man Chun et al. [33] presented a combined method of UV nanosecond laser textured molding and a PDMS molding process to fabricate a superhydrophobic surface with a micro-gridded structure. Dingwei Gong et al. [34] fabricated a transparent superhydrophobic PDMS film with a micro-walls array and micro-protrusion by duplicating via a femtosecond laser-ablated template. Nevertheless, these approaches were relied heavily on the expensive equipment and the structure of template. It is still a challenge to fabricate a superhydrophobic surface with unique micron-scale structure by a simple and low-cost method.
In recent years, the special functional surfaces that mimic the unique micron-scale structure of plants have gradually appeared. For instance, a fog collection material was prepared by mimicking the microsized cones of Cactus stem [35], [36] and a unidirectional liquid transport surface without energy input was built inspired by the micrometer structure of Nepenthes Peristome [37], [38]. As well known, stomata are indispensable for plant growth, ensuring the transport of carbon dioxide and water while regulating temperature for photosynthesis, respiration and transpiration [39], [40]. Therefore, the stomata, as a typical microstructure, inevitably affect the wetting behavior of leaf surface. However, there were few researches in this field.
In the present study, it was discovered that the hydrophobicity of the leaf surface of Loropetalum chinense var. rubrum is tremendously enhanced while the stomata are opening. We call it as “stomata effect”. Then a simple and low-cost method, as called swelling-vesiculating-cracking method, was put forward to fabricate the BSSS on PDMS sheets. Tetraethyl orthosilicate (TEOS) and Ethylenediamine (EDA) were employed as key reactant and catalyst, respectively. The fabrication principle is based on the hydrolysis and condensation of TEOS and the low molecular weight non-cross-linked oligomers (LMWNO) [41] in PDMS to generated hydrophobic silica gel micro-vesicles, which were then cracked to form the stomata-like structures by heat treatment. The stomata-like structures trapped air and thus provided enough contact areas of air–liquid interface to realize the super-hydrophobic surface. The chemical composition, morphology and properties of the super-hydrophobic surface were investigated systematically. And a simplified mathematical model was further established based on the Cassie-Baxter model in combination with the experimental results. The “stomata effect” leads to the fabrication of BSSS without multistep modification of nanostructure and with free of low surface energy modification. The novel fabrication strategy is believed to have great application potential for the development of superhydrophobic materials.
Section snippets
Materials
Fresh leaves of Loropetalum chinense var. rubrum were picked from the campus of Fuzhou University, China. Chloroform was obtained from Shantou Dahao Fine Chemical Co., Ltd. PDMS, a two-component crosslinkable resin (Sylgard 184), was purchased from Dow Corning Co., Michigan, USA. TEOS was purchased from Zhiyuan Chemical Reagent Co., Ltd, Tianjin, China. EDA was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All chemicals were analytical grade reagents and were used
The hydrophobicity enhanced by stomata
The SEM images of the back of the leaves of Loropetalum chinense var. rubrum without veins are shown in Fig. 2. It can be found from Fig. 2a that the microscale stomata are distributed randomly on the leaf surface and covered with nanoscale structures. This hierarchical morphology is analogous to the surface of lotus leaf. The CA of water droplets on the fresh leaf was measured to be as high as 161.7 ± 3° (inset in Fig. 2a), indicating that the leaf surface exhibited excellent
Conclusions
It is discovered that the hydrophobicity of the leaf surface is tremendously enhanced while the stomata are opening. We call it as “stomata effect”. Inspired by the “stomata effect”, a facile, simple and low-cost method, as called swelling-vesiculating-cracking method, is developed to fabricate the BSSS with stomata-like structures and without the modification of low-surface-energy substance. Two key process parameters, swelling ratio and heat treatment temperature, are explored to control the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the Special Support Program for High-level Talents of Fujian Province, China; the National Natural Science Foundation of China, China (Grant No. 51972063), the Natural Science Foundation of Fujian Province, China (Grant No. 2019J01652), the Science and Technology Project of Fuzhou, China (Grant No. 2018-G-67) and the Fuzhou University Testing Fund of Precious Apparatus, China (Grant No. 2018 T026, 2019 T025).
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