Wetting Characterizations of Oilseed Rapes

Journal of Bionic Engineering 13 (2016) 213–219

Wetting Characterizations of Oilseed Rapes Hai Zhu1,2, Zhiguang Guo1,2 1. Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Key Laboratory for the Green Preparation and Application of Functional Materials (Ministry of Education), Hubei University, Wuhan 430062, China 2. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

Abstract Oilseed rape, widely cultivated all over the world, plays an important role for our daily life due to its high nutritional and economic values. In this paper, for the first time we discuss the surface wettability of oilseed rapes with special surface structures. It is found that the fresh rape flowers are superhydrophobic with a low Adhesion Force (AF), showing the self-cleaning properties similar to lotus leaves. In contrast, the fresh rape leaves also exhibit hydrophobicity but a high AF, which resemble rose petals. Furthermore, we study the effect of storage time on the wetting properties of rape leaves. The high hydrophobicity of rape leaves gradually switches to hydrophilicity. Meanwhile, the AF intensely increases after placement at room temperature for 10 days. This research offers a profound inspiration to artificially fabricate biomimetic materials with high hydrophobicity and different adhesion characterizations. Keywords: oilseed rape, wettability, adhesion force, lotus leaves, rose petals, biomimetic materials Copyright © 2016, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(16)60295-0

1 Introduction Oilseed rape, Latin name of Brassica campestris L., is planted all round the world. In China, it is widely cultivated in the Yangtze river basin and northwest plateau (Fig. 1a)[1,2]. With the high nutritional and economic values, oilseed rape has been widely researched[3–5]. Interestingly, the oilseed rape also shows special wettabilities, whereby water droplets on the surfaces of the rape flower and rape leaf are rendering similar spherical structures. Figs. 1b–1d show the optical images of water droplets on the surfaces of the rape flowers and leaves at room temperature. A great number of superwettable materials have been inspired to come out since the hierarchical structures and epicuticular wax of lotus leaf from nature were detailed in 1997[6]. Generally, superhydrophobicity is defined that Water Contact Angle (WCA) is larger than 150˚ and water contact angle hysteresis (the difference between the advancing and receding contact angles) is less than 10˚ on a surface[7–12]. Typically, the surface of Corresponding author: Zhiguang Guo E-mail: [email protected]

lotus leaf is superhydrophobic with the WCA exceeding 150˚. Such a leaf surface has low Adhesion Force (AF) and self-cleaning properties[13–15]. But, high AF is also found on some superhydrophobic surfaces. As an

Fig. 1 (a) The optical image of oilseed rape planting base; (b–d) the optical images of water droplets on the surfaces of flower petal (b), obverse (c) and reverse (d) leaves, respectively.

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example of rose petal surface[16–18], where water droplets are difficult to roll over and stick to the above, even, they cannot fall off the petal surface rotated by 180˚. According to our findings, oilseed rape also exhibits special wetting behaviors, however, to date, there is no any systematical research involving the wettability of oilseed rape and structure-wettability relationship. Actually, wetting characterizations have a pronounced ecological significance on the growth of oilseed rape, such as the photosynthesis, antibacterial property, foliar fertilization and so forth[19–23]. Herein, for the first time, we discuss the wettability and AF of oilseed rape by analyzing the special structures and surface chemical compositions. Similar to lotus leaf, the rape flower is featured with superhdrophobicity and low AF as well as self-cleaning ability. In contrast, the fresh rape leaf also exhibits hydrophobicity but a high AF, which resembles rose petals. However, after placement at room temperature for 10 days, the hydrophobicity of rape leaves gradually turn to hydrophilicity. More importantly, this research offers a profound inspiration to artificially fabricate biomimetic materials and exceedingly enriches the directions of improving the production of oilseed rape and generating much more values for human beings. Also, it is favorable for other fields of medicine, biological, chemical, materials, water treatment, and self-cleaning coatings.

2 Materials and method 2.1 Materials All fresh rape flowers and leaves were free from Oil Crops Research Institute of Chinese Academy of Agricultural Sciences, Wuhan, Hubei Province, China. 2.2 Dried rape leaves placed at room temperature Several full fresh rape leaves were placed at room temperature for 10 days. Then, choosing 1 × 2 cm2 of flat surface of the rape leaves, and adhering them to glass slide with double faced adhesive tape to test the WCAs every day. 2.3 Characterization The morphologies of the samples were inspected by the Field-Emission Scanning Electron Microscopes (FESEM JSM-7100F and FESEM JSM-6701F). Contact angle measurements were carried out using a POWEREACH JC2000D goniometer (China). A 5 μL

droplet (such as deionized water or glycerol) was dropped on such samples. Dropping at five different positions on the samples is to obtain the average values of contact angles. A high-sensitivity microelectro-mechanical balance system (Data-physics DCAT11, Germany) was used to acquire the AF value on different samples. 4 mg of droplet was suspended on a hydrophobic ring, then approached and retracted from the surfaces of different samples at an ambient environment with a constant speed of 0.01 mm·s−1. The AF was obtained from the peak data in the force–distance curve. To analyze the surface composition of the samples, X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250, Physical Electronics, USA) was operated through the Al Kα line as the excitation source. Fourier Transform Infrared Spectroscopy (FTIR, Nexus 870) was used to gain the FTIR signal to analyze the chemical bonds in the samples.

3 Results and discussion 3.1 Rape flower with superhydrophobicity Bright yellow rape flower has four petals, which are neatly around the blossom (Fig. 2a). The SEM images of fresh petal obverse surface are illustrated with different magnifications in Figs. 2b and 2c. It is clearly seen that the intestine-like micro-structures are interlocking with each other, where the width of “intestine” is approximately 0.3 μm and the distance of the adjacent “intestine” is about 0.5 μm. It is well known that both the hierarchical structures and the surface chemical compositions have decisive effects on the wettability and AF (a)

(b)

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Fig. 2 (a) The optical image of rape flower in nature; (b,c) SEM images of rape flower petal; (d) a water droplet on the flower petal surface with CA of 155˚.

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those of the obverse one (Fig. 3). (a)

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Fig. 3 (a–c) SEM images of the reverse flower petal at different magnifications; (d) a water droplet on the flower petal surface with CA of 154˚. (a)

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Fig. 4 SEM images of the fresh rape leaf at different magnifications. (a,b) the obverse surface and (c,d) the reverse surface. Insets show the photographs of the fresh rape leaf (a,c) and a water droplet on the surface (b,d).

of a solid surface[24–27]. The surface microstructures contribute to roughness for the superhydrophobicity of the rape flower. Moreover, carotenoids[28], as a kind of fat-soluble carotenoid pigment and insoluble in water, exist in plastid pigment of upper epidermis cells, producing not only its beautiful yellow colors but also special wetting behaviors. Water droplets can be seen to be stable as similar spheres with the WCA of 155˚ ± 1.5˚ (Fig. 2d) and easily roll over the petal surfaces with a small tilt angle. Similar to lotus leaves, the rape flower surfaces with superhydrophobicity possess self-cleaning properties (http://www.tudou.com/programs/view/55Nt POkTF_Q/). The morphology and wettability properties of the reverse flower surface are almost the same as

3.2 Rape leaf with hydrophobicity and high AF Unlike the rape flower, it can be found under the SEM observation that the obverse and reverse surfaces of the fresh rape leaves have different morphologies. The surface is covered by a host of small cylinders with the diameters ranging from 80 nm to 300 nm and the length of about 1 μm, resulting in micro/nanostructures and giving rise to a sufficient roughness (Figs. 4a and 4b). As for the reverse surface, needle-like structures fully occupy on the surface like a dense grass (Figs. 4c and 4d). The “grass” is about 5 μm long and 500 nm wide on average values, also forming a considerable surface roughness. In addition, it has been demonstrated that rape leaves contain epicuticular wax[29], which is mainly composed of long chain fatty acid, alkane, soluble alcohol, fat secondary alcohols, aldehydes, ketones, esters and so on. Consequently, the epicuticular wax combined with the micro- and nanostructures plays a key role in the wetting properties of the fresh rape leaves. Compared to the superhydrophobic rape flowers, the fresh rape leaves also have high hydrophobicity with the WCA of 135˚ ± 4.5˚ and 146˚ ± 3.4˚ for the obverse and reverse surfaces, respectively. Distinctively, water droplets on the reverse surfaces did not fall down even with a large tilt and an extra shake (http://www.tudou. com/programs/view/gavYuPDXG9Y/), showing a high AF similar to rose petals. Furthermore, we measured the AF of the fresh rape flowers and leaves as shown in Fig. 5. The fresh rape flowers have a very low AF of 4.5 μN ± 1.5 μN (Fig. 5a). In contrast, the measured AFs of the observe and reverse rape leaves are as high as 95.8 μN ± 4.2 μN (Fig. 5b) and 112.1 μN ± 3.8 μN (Fig. 5c), respectively, which are obviously higher than that of the flowers. It is suggested that the different wetting behaviors between the fresh rape flowers and leaves are attributed to their different micro-/nanostructures and hydrophobic compositions. 3.3 Other wetting properties of rape flower and rape leaf Rape flowers and leaves possess superhydrophobicity and high hydrophobicity, respectively. Besides water droplet, we studied the other wetting properties of rape flowers and leaves. A series of oils, such as dichloroethane, chloroform, hexane, n-hexadecane, pe-

F(

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Fig. 5 The adhesion force measurements on a rape flower (a), the obverse (b) and reverse (c) surfaces on a rape leaf. (a)

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Fig. 6 (a,b) A n-hexadecane droplet on flower petal and obverse leaf; (c) a glycerol droplet on reverse leaf with CA of 151˚. (d–f) AF measurements of glycerol on the surfaces of flower petal (d), obverse (e) and reverse (f) leaves, respectively.

troleum, gasoline, diesel, entirely spread over the surfaces of rape flowers and leaves, showing superoleophilicity with an oil contact angles of 0˚ (Figs. 6a and 6b), resulting from the carotenoids on the flower petals and the epicuticular wax on the rape leaves. However, both the flowers and leaves behave well for glycerol repellency (Fig. 6c). The glycerol contact angles of 129˚ ± 2.1˚, 140˚ ± 4.1˚, and 151˚ ± 3.9˚ are obtained on the surfaces of flower petals, obverse and reverse leaves, respectively. In addition, the AFs of glycerol on these surfaces are 26 μN ± 2.1 μN, 112 μN ± 3.3 μN, and 139 μN ± 4.3 μN in turn (Figs. 6d–6f). It is indicated that the water and glycerol repellency on the surfaces of the fresh rape flowers and leaves displays similar behaviors, because both water and glycerol possess high surface tensions of 63.3 mN·m−1 and 72 mN·m−1, respectively.

3.4 The wettability variation of rape leaf A piece of the fresh green rape leaf was placed for 10 days at room temperature. After 3 days, the rape leaf gradually turns yellow until only the presence of green edges (Figs. 7a – 7j). Meanwhile, the soft and flat rape leaf gradually becomes dried and slightly curly. After 10 days, the rape leaf is completely dried. Figs. 8a and 8b show the SEM images of the dried rape leaf. The microand nanostructures of the fresh rape leaf disappears after 10-day placement. The surface of the dried leaf becomes greatly smooth. Moreover, the WCA of the rape leaf changes from 135˚ ± 4.5˚ to 75.5˚ ± 3.6˚ after being dried (Fig. 8c). The AF is intensely increased to 265.6 μN ± 4.5 μN (Fig. 8d), indicating the transformation from high hydrophobicity to hydrophilicity. Besides the contribution of surface structures of the dried rape leaf to the

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hydrophilic properties, we further investigated the change of chemical compositions by the XPS measurements. In addition, XPS was also employed to analyze the chemical compositions of the fresh and dried rape leaf (Figs. 8e and 8f). It is observed that a significant increase of C=O content (binding energy of 287.8 eV) for the dried leaf[30]. The XPS results reveal the formation of hydrophilic chemical compositions[31], leading to the decreased WCA and increased AF.

4 Conclusion

Fig. 7 The photographs of rape leaf after placement for different time (1–10 day).

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70000 60000 50000 40000 30000 20000 10000 0 −10000 288 287 286 285 284 283 282 0 1 2 3 4 5 6 7 8 9 10 11 12 Binding energy (eV) Time (d) (d) (f) 300 30000 265.6 N 250 25000 200 20000

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In summary, for the first time, we investigated the wettability of rape flowers and leaves with the special structures and surface compositions. Interestingly, the fresh flower petals are superhydrophobic with a low AF of 4.5 ± 1.5 μN, showing self-cleaning properties similar to lotus leaves. In contrast, the fresh rape leaves show high hydrophobicity with a high AF, which resemble rose petals. In addition, it was demonstrated that the high hydrophobicity of the fresh rape leaves gradually switches to hydrophilicity with the increased AF when the fresh leaves are dried at room temperature for 10 days. This study could help us deeply understand the oilseed rape and heartily acquire more knowledge of improving its production and generating much more value for human beings. The cognition on the wetting properties of flower petals and leaves also offer a profound inspiration to fabricate biomimetic materials.

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Fig. 8 (a,b) SEM images of dried rape leaves (10 day) at different magnifications. The insets show the photographs of the dried rape leaf (a) and a water droplet on its surface (b). (c) The dependence of WCA on the storage time. (d) The AF of the dried leaf. (e,f) XPS spectra of fresh leaf (e) and the dried leaf (f).

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Meanwhile, it is of great significance for a multitude of modern field, such as medicine, biological, chemical, materials, water treatment and so forth.

Acknowledgment This work is supported by the National Nature Science Foundation of China (No. 51522510), the Co-joint Project of Chinese Academy of Sciences, and the “Top Hundred Talents” Program of Chinese Academy of Sciences.

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