Superhydrophobic Plant Leaves: The Variation in Surface

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The superhydrophobic plant leaves: The variation in surface morphologies and wettability during the vegetation period Xuelian Gou, and Zhiguang Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03996 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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The superhydrophobic plant leaves: The variation in surface morphologies and wettability during the vegetation period Xuelian Gou,†,‡ Zhiguang Guo,*,†,‡ †Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of Education Key Laboratory for

the Green Preparation and Application of Functional Materials, Hubei University, Wuhan, People’s Republic of China. ‡State

Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, People’s Republic of China. *Corresponding author. Tel: 0086-931-4968105; Fax: 0086-931-8277088. Email address: [email protected] (Guo)

Abstract: In this paper, for the first time the surface wettability of clover and lotus leaves with specific surface structures at different growth stages is investigated. It is found that the clover exhibit water-repellent property similar to lotus leaves. Furthermore, the alternation in wettability of clover and lotus leaves during the whole vegetation period is investigated. The water contact angles and surface morphology of the leaves are measured in means of contact angles measurements and Scanning Electron Microscopy (SEM), respectively. The chemical composition of plant leaves are analyzed utilizing Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS), respectively. It is found that the wettability heavily depends on the surface structures during their growth and ageing procedure, which enlightens us to design and fabricate biomimetic multifunctional superhydrophobic surfaces inspired by nature.

Keywords: Superhydrophobic; Plant leaves; Vegetation period; Biomimetic

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Introduction In nature, there exist numerous mysteries regarding to flora and fauna.1-5 Fascinating wetting phenomena found on various plants such as lotus leaves and rose petal inspire us to further explore the reasons giving rise to these.6,7 In fact, within millions of years of biological evolution, there are a diversity of resemblances between plant leaves and human beings for the most part. From the infancy, middle age to old age, there are great changes in physiological functions of human. During the infancy, all tissues and organs of our bodies are in the development stages. Hence, it is incapable for bodies to resist external negative attacks such as bacteria, fungi and other pathogens. After the physical development process terminated, the competence of our bodies resistant to outside interference strengthens and therefore we become more insusceptible and immune after organ ontogeny. As time goes by, however, the organ failure gradually occurs and the physiological function of human body gradually weakens, which leads to irretrievable damage and vulnerability. Similar to the life cycle of human beings, several plant leaves represent similar rules during the growth cycle from tender period, mature period to senescent period and exhibit disparate durability towards environmental variables such as UV-irradiation, climate change and atmospheric pollutants. During the growth and ageing process of lotus and cloverleaf, it involves the functional adaptation of plant cuticle sculptures to environmental change thus leading to the variation in wettability, which is similar to the life cycle of human. It has been reported that the cuticles serve as a protective layer of the plant tissues to the environment, which are mainly composed of a network of cutin and hydrophobic wax.8 The structural basis of plant surface is a combination of surface roughness in microscale and a random arrangement of three-dimensional epicuticular wax with

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different sculptures in nanoscale. Furthermore, the morphologies of plant surface epicuticular wax crystals play a key role in the wettability of plant surface during the vegetation period. Hence, it is necessary to figure out the relationship between the different staged leaves and the different wetting properties, which is beneficial to fabricate more biomimetic surfaces with different wettability such as superhydrophobicity, hydrophobicity, hydrophilicity and superhydrophilicity based on actual needs. For instance, superhydrophobicity in wetland plant surfaces can avoid pathogen attack by self-cleaning or by prevention of water film formation for the growth of pathogen microorganism, which shows huge potential in biomedical field.9 Other relevant research has been done by C. Neinhuis in 1997, who investigated the leaf surface of beech, oak and ginko concerning contamination with particles during one growing season.10 The results show that the water-repellent surfaces have more inclination to avoid lasting particulate contamination, whereas smooth surfaces short of wax crystals are less effective.

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years ago, the effects of storage time on the wetting properties of rape leaves were investigated by us.11 Several fresh green rape leaves were placed for 10 days at room temperature. During the storage process, the soft and flat rape leaves gradually become dried and curly in result of only the presence of green edges. It is observed that the highly water-repellent fresh leaves gradually switch to hydrophilicity with the reduced contact angle and increased adhesion force. The micro- and nanostructures of the fresh rape leaf disappear and get greatly smooth after storage, which verifies that the surface morphology plays an essential role in the wettability of plant leaves. Actually, not only surface morphology, the chemical composition of cuticular wax discovered on leaf surfaces is also found to have little influence on the change of wettability. It is found that the dried rape leaves contain more C=O content and possess hydrophilic chemical compositions when compared to the 3


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fresh rape leaves.12,13 Furthermore, Koch et al. have investigated the influence of air humidity during the cultivation of Brassica oleracea (Brassicaceae), Eucalyptus gunnii (Myrtaceae) and Tropaeolum majus (Tropaeolaceae) on leaf surface wettability.14 They declared that within all species the younger, not fully developed leaves had higher contact angles than the older, fully expanded leaves of the same species and air humidity resulted in increasing amounts of wax deposited to the leaf surfaces due to increasing wax crystal densities and decreasing of leaf surface wettability. The research concerning cloverleaf has been done by Pereira et al. in 2014.15 They investigated the topography and wettability of the underside of cloverleaves and of their biomimetic replicas. Difference in wetting behavior between original leaves and replicas are evident from the contact angle measurements and droplets impact experiments. The results show that the replica shows worse hydrophobicity due to inaccurate replication of the chemistry and structures of the threedimensional wax projections covering the plants surface. Different from the research implemented by Pereira et al, in this paper, the alternation in wettability of clover and lotus leaves is investigated according to different growth period. More importantly, the heterogeneity of pigments distributed on the epidermal cells of cloverleaf is also discovered having an influence on wettability. This research offers a profound inspiration to artificially fabricate biomimetic superhydrophobic surfaces in means of lithography,16-18 selfassembly,19-21 plasma treatment,22-24 and so on, which can be applied to a wide range of novel fields including heat transfer,25 sensors,26 as well as energy conversion devices.27

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Experimental Sample preparation The investigations were carried out with clover (Oxalis) and lotus leaf (Nelumbo nucifera). The plants were cultivated in adjacent lake in an area of the Shahu Park. The as-prepared samples of the plant leaves including the clover and lotus leaves at tender, mature and senescent period respectively were placed on glass slides at room temperature with double faced adhesive tape for photographing and water contact angle measurements. Surface characterization The sessile droplet method was carried out for water contact angle measurement at ambient temperature using a POWEREACH JC2000D goniometer (China). Water droplet (about 4 μL) was dropped carefully onto the surface. The average CA value was determined by measuring CAs at three different locations of the same sample, and their images were photographed using digital camera. The samples were cut into 5 mm×5 mm and sputtered a layer of gold for 90 s to enhance their conductivity for SEM characterization. The morphologies of the samples were observed using scanning electron microscopy (SEM, JSM-5600LV) and a field emission scanning electron microscopy (FESEM, JSM-6701F).

Results and discussion Lotus leaf Lotus leaf is famous for growing in mud, yet never contaminates with it, which is recognized as the symbol of nobleness and purity. The morphologies of the lotus leaf were previously reported by Barthlott and Neinhuis, who observed the surface roughness caused by microscale papillae and

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proposed that the water-repellency of lotus leaf is owing to the microscopic structures and epicuticular wax.28 Furthermore, branch-like nanostructures upon the top of the microscale papillae were observed by Jiang et al., who greatly boost the development of biomimetic superhydrophobic surface (Figure 1).29 However, investigations concerning lotus leaves at different growth stages have rarely been implemented. The optical image of the lotus leaf in nature is shown in Figure 2a. The tender leaf grows up to be mature and then senescent in accompany with the change of the leaf color from light green, bottle green and to yellow. The images of a water droplet floating on the obverse side of the lotus leaf at tender, mature and senescent period, respectively, are shown in Figure 2 (b-d). It is observed that water droplets can form nearly spherical shapes on the tender and mature lotus leaves. Whereas, the case is unusual when it comes to the senescent lotus leaf where the water droplet is in the shape of a hemisphere, which implies that there exists distinct discrepancy in wettability between the senescent lotus leaf and others at tender and mature period. The variation in water contact angles and sliding angles of lotus leaves at different period is shown in Figure 3. The average values of water contact angles in decreasing order are mature, tender and senescent lotus leaves, respectively. Except for senescent lotus leaf, the sliding angles of tender and mature lotus leaves are both below 10°. Albeit all the leaves exhibit hydrophobic property, there is still a difference in the physiological function of different staged lotus leaves such as photosynthesis. When a water film forms on the leaf surface, it will hinder the uptake of CO2 for gas exchange because that CO2 diffuses 10 000 times more slowly through water than air.30 Hence, many wetland plants have gas films on submerged leaf surfaces, which enable sustained gas exchange via stomata and thus offsetting the cuticle resistance, enhancing the exchange velocity of O2 and CO2 with the surrounding water and therefore underwater photosynthesis and respiration.31 Furthermore, 6


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superhydrophobic plant leaves prevent the formation of the water films on surfaces due to the air cushion trapped in the grooves, which contributes to the growth of the plant. Therefore, the mature lotus leaf possesses superior performance in terms of photosynthesis due to preferable hydrophobicity.

Figure 1. A simplified profile of the surface sculptures on lotus leaf. The structural basis of lotus leaf is composed of convex cell papillae and randomly oriented small hydrophobic wax tubules.

Figure 2. (a) The photograph of lotus leaf in nature; The optical images of a water droplet floating on the different staged lotus leaf including (b) tender, (c) mature and (d) senescent period. The insets of (b-d) are water droplet profiles on the surface with a value of 151.6° ± 2°, 154.0° ± 2° and 135.2° ± 2°, respectively.

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Figure 3. The variation in water contact angles and sliding angles of the different staged lotus leaf including the tender, mature and senescent period.

It has been demonstrated that the wettability of plant surface is closely related to the surface morphology and chemical composition.32 The self-cleaning property of lotus leaf is ascribed to the presence of a hydrophobic epicuticular wax crystals and the hierarchical roughness.33,34 It has been reported that the epicuticular waxes, composed of aliphatic compounds, mainly nonacosanol and nonacosanediols, are featured with a three-dimensional hydrophobic organic microstructure, which can provide a significant barrier to various environmental interferences including drought, solar radiation, pollutants, phytophagous insects and pathogens.35 When it comes to the influence surface morphology exerting on the wettability, the SEM images of the different staged leaves with low and high magnification are shown in Figure 4. It can be clearly observed that the tender lotus leaf is textured with irregular protrusions with the diameter ranging from 5 μm to 10 μm (Figure 4a). As is shown in the magnification, there are numerous wax tubules with the average diameter of approximately 100 nm distributed on the subsurface layer (Figure 4b). When the lotus leaf grows to be mature, the quantity of the protrusions augments and some protrusions accumulate into the bigger 8


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bulge with diameter of about 20-40 μm (Figure 4c). Furthermore, the arrangement of the wax tubules is much denser than that of the tender leaf, which is responsible for the superior hydrophobicity due to the sufficient hierarchical roughness (Figure 4d). When the mature leaf grows to be senescent, the big bulges almost disappear and the protrusions are seriously damaged by environmental influences (Figure 4e). From the high magnification, it can be clearly seen that the wax tubules distributed almost disappear, which is in line with the worse hydrophobicity (Figure 4f). This phenomenon can be also theoretically explained according to the classical Cassie-Baxter model.36 When the lotus leaf surface at tender and mature period contains large numbers of protrusions, air instead of liquid is trapped in grooves on subsurface, resulting in increase of liquidrepellency. While, the air cushion disappears and liquid droplet spreads out spontaneously on the surface of senescent leaf when the protrusions on the surface are largely deteriorated, which exhibits worse hydrophobicity. When it comes to the influence chemical components or groups exerting on the wettability, investigations of chemical composition have been done as shown in Figure 5 (a-b). It was found that the chemical components and groups on surface of lotus leaf at each period are almost similar, which exhibits the negligible influence on the wettability.

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Figure 4. SEM images of the different staged lotus leaves at low and high magnification show (a) and (b) tender lotus leaf with irregular protrusions and numerous wax tubules distributed on the subsurface layer, (c) and (d) mature lotus leaf with ideal hierarchical structures composed of big bulge and denser wax crystals, and (e) and (f) senescent lotus leaf with smoother surface in the result of dilapidated protrusions.

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Figure 5. (a) FT-IR spectra and (b) XPS analysis of different staged lotus leaves including tender, mature and senescent period.

Cloverleaf Clover, mainly distributed in the temperate and tropical regions, are mainly cultivated in type of T.pratense, T.repens and T.incarnatum and usually semi-spontaneously grow at wet grassland, riverbanks and roadsides, which is beneficial to soil fertility. There are generally three blades on the cloverleaf, which generally symbolize love, health and glory, respectively. In particular, there exists cloverleaf with four blades in rare cases, which represents good luck and therefore so-called “fourleaf clover”. The image of the cloverleaf in nature is shown in Figure 6a. It can be observed that water droplet is floating on the cloverleaf at tender, mature and senescent period shown in Figure 6 (b-d). In particular, there are light green stripes symmetrically located on cloverleaf at mature and senescent period due to the heterogeneous distribution of pigments. Hence, it is necessary to investigate the discrepancy in wettability of certain locations with different color on the same cloverleaf. The variation in water contact angles and sliding angles of cloverleaf at different period with different color is shown in Figure 7 (a-b). It can be seen that the tender cloverleaf with homogeneous bottle green is hydrophobic with the water contact angle of 139.6°± 2°. However, the 11


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case is different when it comes to the mature cloverleaf. When the tender cloverleaf grows to be mature, a semiellipse-like light green stripe appears on the surface resulting in heterogeneity in color, which is caused by the uneven distribution of pigments. The wettabilities of mature cloverleaf with light and bottle green are both investigated. It can be seen that the water contact angles of mature cloverleaf with light and bottle green are 140.1°± 2° and 147.3°± 2°, respectively, which implies that the bottle green location of mature cloverleaf possesses slightly superior hydrophobicity compared to the light green location. Furthermore, the bottle green cloverleaf at mature period is more hydrophobic than that at tender period with the growth of the plant leaves, which results in more vigorous photosynthesis of mature cloverleaf. When the mature cloverleaf grows to be senescent, the wettability of senescent cloverleaf is actually different. The water contact angles of senescent cloverleaf with light and bottle green are 127.2°± 2° and 144.2°± 2°, respectively, which further demonstrates the difference in wettability. Furthermore, the hydrophobicity of senescent cloverleaf at light and bottle green locations dramatically decreases when compared to that of the mature cloverleaf, which can be further substantiated from the point of the surface morphologies of cloverleaf.

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Figure 6. (a) The photograph of cloverleaf in nature. The optical images of water droplets floating on the different staged cloverleaf including (b) tender, (c) mature and (d) senescent period. The insets of (b-d) are water droplet profiles on the obverse surface of the cloverleaf with (left) light green and (right) bottle green.

Figure 7. Water contact angles and sliding angles of the different staged cloverleaf including the tender, mature and senescent period with (a) bottle green and (b) light green.

It is well-known that the surface roughness plays a key role in the wettability of plant leaves, which decreases the contact area between water and solid surface due to air trapped in cavities. As is shown in Figure 8a, there are some irregular folds with the width of approximately 2 μm to 4 μm randomly distributed on the surface of tender cloverleaf. Furthermore, there are numerous rod-like structures with the length of about 200 nm to 500 nm disorderly arranged in the magnification (Figure 8b). However, the arrangement is uneven due to somewhere ravined without rodlets oriented. 13


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As the plants draw nourishment from the soil, the plant organs are further developed and complete epidermis cell walls are obtained leading to more active cell physiology. As is shown in Figure 8 (c-d), it can be observed that there are tetragonal epidermis cells with denser rodlets irregularly distributed. When compared to the tender cloverleaf, undulations of anticlines result in compact gearing of the epidermis cells, which is beneficial to promote the mechanical stability of epidermis tissues.8 With respect to the bottle green location of cloverleaf at mature period shown in Fig. 8 (ef), it can be seen that the central field of epidermis cell shows a denser layer of cuticular wax superimposed to the cutin network than that of the anticline field. Furthermore, the distribution of rod-like epicuticular wax crystals in nanoscale on mature cloverleaf with bottle green is a little bit denser than that on light green location, accordant with the slight difference in water contact angles. However, when the mature cloverleaf grows to be senescent, there exists distinct difference with respect to the morphology of the cloverleaf. As is shown in Figure 9a, the epidermis cell walls gradually degenerate and become numerous folds on the bottle green location of cloverleaf at senescent period, which leads to the decreased roughness therefore the less water contact angle. In the magnification, it can be clearly seen that the several pits emerging on the surface exhibit more flat and smooth compared to the tender and mature cloverleaves (Figure 9b). When it comes to the light green location of cloverleaf, the wrinkles distributed on the surface completely disappear and surface becomes smoother than that of the bottle green location, which accounts for the worse hydrophobicity when compared to bottle green location (Figure 9 c-d).

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Figure 8. SEM images of the different staged cloverleaves with different color at low and high magnification including (a) and (b) tender cloverleaf with bottle green, (c) and (d) mature cloverleaf with light green, (e) and (f) mature cloverleaf with bottle green.

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Figure 9. SEM images of senescent cloverleaf with different color at low and high magnification including (a) and (b) bottle green, (c) and (d) light green.

Conclusions In this paper, the surface morphologies of the different staged leaves of two kinds of plant leaves, including the lotus and clover, are investigated. The lotus leaf at mature period is deemed to possess the better hydrophobic property than that of the tender and senescent leaf, which can be substantiated according to the densely oriented small hydrophobic wax tubules on the top of convex cell papillae. Similar to the lotus leaf, the cloverleaf at mature period possesses superior hydrophobicity when compared to the tender and senescent cloverleaf due to more sound epidermis cells and a denser layer of cuticular wax superimposed to the cutin network, which can be verified by difference in water contact angles. Except for the difference in wettability in terms of different growth stages, the influences heterogeneity in color on the same cloverleaf exerting on wettability are also investigated. 16


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It has been proved that the bottle location on the cloverleaf is more hydrophobic than light location due to more abundant pigments thus leading to more sufficient photosynthesis. More importantly, it is possible to design and mimic the hydrophobic, superhydrophobic, hydrophilic and superhydrophilic surfaces according to the surface sculptures of different staged leaves with different wetting properties.

Acknowledgements This work is supported by the National Nature Science Foundation of China (NO 51675513 and 51735013). The authors acknowledge the data support from Miss Dr Lei Shi who provided the FTIR spectra and XPS analysis.

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In this paper, for the first time the surface wettability of clover and lotus leaves with specific surface structures at different growth stages is investigated. It is found that the clover exhibited water-repellent property similar to lotus leaves. Furthermore, the alternation in wettability of clover and lotus leaves during the whole vegetation period is investigated. 75x41mm (300 x 300 DPI)


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Superhydrophobic Plant Leaves: The Variation in Surface

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