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Investigations on the fog harvesting mechanism of Bermuda grass (Cynodon dactylon)
Flora 224 (2016) 59–65
Contents lists available at ScienceDirect
Flora journal homepage: www.elsevier.com/locate/flora
Investigations on the fog harvesting mechanism of Bermuda grass (Cynodon dactylon) Vipul Sharma a , Manjul Sharma b , Suneel Kumar a , Venkata Krishnan a,∗ a b
School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi 175005, H.P., India Department of Botany, Shoolini University, Solan 173229, H.P., India
a r t i c l e
i n f o
Article history: Received 27 January 2016 Received in revised form 17 May 2016 Accepted 9 July 2016 Edited by Hermann Heilmeier Available online 13 July 2016 Keywords: Fog interception Microstructures Laplace pressure Bioinspiration
a b s t r a c t Harvesting water from the humidity in air has far reaching advantages to provide access to clean water for consumption, especially in the arid and semi-arid regions of the world. In this regard, fog harvesting properties of a handful of fauna and flora have been explored in the recent past to understand their unique characteristics. In most of the cases, the structural features at the micro- and nanoscales play a crucial role in water collection and transport. In this study, we report the fog collection mechanism in Bermuda grass, Cynodon dactylon, which is commonly found in several regions of the world. The fog collection ability of this grass can be attributed to two characteristic structural traits: well-arranged conical spines with sharp edges, wherein the deposition of fog droplets occurs, and hierarchically organized seedheads having flattened surfaces with gradient grooves that transport the coalesced water drop in a directional manner. Both the conical spines and gradient grooves have specified functions in fog interception by virtue of their structural features. The gradient of the Laplace pressure and fiber-like hanging phenomenon of the droplet provide Cynodon dactylon with an efficient fog collection system. Further research on the characteristic structural features of this and other similar plants will lead us to the fabrication of bioinspired materials and devices to harvest fog in an efficient manner. © 2016 Published by Elsevier GmbH.
1. Introduction Approximately 663 million people i.e., 1 in 10 people, live without access to safe drinking water in different parts of the world and this issue of access to clean water is one of the major global concerns in modern times (UNICEF and Organization, 2015). Interestingly, some indigenous flora and fauna found in many of the arid and semi-arid regions can readily deal with inadequate access to water for survival by collecting water through dew, fog or in general moisture present in the air (Agam and Berliner, 2006; Henschel and Seely, 2008; Schemenauer et al., 1988). Many species have adapted themselves according to the arid regions to increase their chances of survival. Inspired by survival adaptation existing in nature, widespread efforts are being made to exploit fog interception to explore the clean water supply in these water deficit regions (Lekouch et al., 2012; Olivier and De Rautenbach, 2002; Schemenauer and Cereceda, 1991). From the last few years, numerous projects have been initiated in many areas located in the arid and semi-arid climate zones to potentially use fog interception for access to clean water supply (Domen et al., 2014; Fessehaye et al.,
∗ Corresponding author. E-mail address: [email protected] (V. Krishnan). http://dx.doi.org/10.1016/j.flora.2016.07.006 0367-2530/© 2016 Published by Elsevier GmbH.
2014). The initial research and development done to collect water from atmosphere was based on different types of fog collecting devices, which are known to resist wetting by water and allow condensation to take place leading to the droplet deposition on their surface (Falconer and Falconer, 1980; Schemenauer and Cereceda, 1994). In this conventional method, the droplets increase in size and coalesce with other droplets until eventually these become large enough to get detached from the surface and fall into the collection vessel. There are many flora and fauna found in several arid and semiarid regions which harvest dew and fog to cope with their water needs and have led to bioinspired fog harvesting. For example, Stenocara gracilipes beetles have micrometer level arrangements of hydrophobic and hydrophilic areas on their backs which capture water from moisture rich air (Parker and Lawrence, 2001). Some plants have also been reported, including Stipagrostis sabulicola (Namib bushman grass) and Cottula fallax, to capture fog efficiently from the atmosphere and convert it to fresh water utilizing their three-dimensional hierarchical structures (Andrews et al., 2011; Ebner et al., 2011). A multi-structural and multi-functional integrated spine-based fog collection system in cactus has also been shown to demonstrate the natural fog harvesting system (Ju et al., 2012). Different types of cacti species have been compared in a different study and it has been proved that the efficiency of
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V. Sharma et al. / Flora 224 (2016) 59–65
Fig. 1. Morphology and surface appearance of Cynodon dactylon: (a and b) Photographs in its natural habitat, (c and d) Images of spikes arranged in the whorl, (e and f) SEM Images showing the microstructures at 1 mm and 200 m, (g and h) magnified image of spikes having conical spine clusters and gradient grooves, respectively.
the interception of fog and formation of the water droplets from fog is highly dependent on the presence of spines (Malik et al., 2015). In a very recent study, hierarchical surface architecture of Hordeum vulgare (barley awns) was found to efficiently harvest fog (Azad et al., 2015). Taking inspiration from these integrated cactus-inspired spine structures and the science of shape gradientinduced Laplace pressure difference, the fabrication of biomimetic systems has been already started to collect fog in an efficient manner (Peng et al., 2015). Spider silks have also been studied to harvest water from moist air (Zheng et al., 2010). The unique fog harvesting capability of the spider Uloborus walckenaerius is due to their characteristic fiber structure which includes the naturally occurring periodic spindle knots and joints (Zheng et al., 2010). The gradient of surface-free energy (Chaudhury and Whitesides, 1992) and gradient of Laplace pressure (Bai et al., 2010) have proved to
be the main driving forces behind the fog collection system in the spider silk. Inspired by these naturally occurring ‘Fog harvesters’, biomimetic imitation of naturally occurring fog harvesting structures has become a subject of interest in the scientific community to create new possibilities for maximizing fog-collection efficiency (Dong et al., 2012; Malik et al., 2014; White et al., 2013). Here we report the fog collection mechanism in Bermuda grass, Cynodon dactylon, which is commonly found in several regions of the world. The fog interception can be ascribed to the presence of certain characteristic surface structures possessing specific features for the deposition and growth of water droplets, directional transport of the water droplets, retention and subsequent collection of water droplets. The results have been discussed in detail along with the applicable mathematical formalisms. Research on the characteristic structural features and the underlying mecha-
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Fig. 2. (a) Schematic figure of experimental setup of the fog collection process in Cynodon dactylon. (b) Water collection dynamics of Cynodon dactylon: Amount of water collected by C. dactylon possessing a surface area of about 0.0096 m2 exposed to fog flow in intervals from 0 to 120 min with 20 min increments, shown along with the standard deviation.
nism of fog collection can lead us to the fabrication of bioinspired materials and devices to harvest fog in an efficient manner.
Barthlott (1993). The samples were then immobilized onto the carbon tape supported on an aluminum stub and coated with a 5 nm layer of gold with Quorum Q150R ES combined sputter coating unit.
2. Experimental section 2.1. Materials Cynodon dactylon (L.) Pers. is a grass that originated from the Middle East and is widely found to grow in the semi-arid and arid regions in several parts of the world (Farsani et al., 2011). Samples from C. dactylon were collected in its naturally occurring habitat locally (Mandi, Himachal Pradesh, India – 31◦ 42 25 N, 76◦ 55 54 E) in August 2015. The sample parts were carefully screened and immobilized on the sample frame for further analysis. 2.2. Preparation of samples Any moisture content on the collected Cynodon dactylon samples was dried in the open air in the laboratory prior to cutting them to equal sizes. For scanning electron microscopy (SEM) analysis, the grass samples were prepared by fixing them overnight using 2% glutaraldehyde prepared in phosphate buffer solution (Sigma Aldrich). Then the glycerol substitution process was followed to replace the water molecules using the procedure reported by Ensikat and
2.3. Fog collection experiments Water collection studies were done by exposure of dry Cynodon dactylon whorls of spikes to a fine mist of ultra-pure water (18.2 M-cm ELGA PURELAB Option-R7) generated by a cold mist humidifier (Bionaire BU1300W-I) with an air flow of 130 ± 30 mL per hour and at a distance of 15 cm from the sample. Individual specimens were fixed on the 10 mL hollow plastic vial placed inside a petri dish, and the total volume of water collected was measured at 20 min time intervals over a period of 2 h. Three sets of experiments were performed on three independent specimens and the average values along with the standard deviation were determined. The deposition and movement of the water droplets were recorded by Phoenix 300 contact angle instrument (Surface Electro Optics, South Korea). The surface area of the samples were determined using Image J software (Rasband, 1997).
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Fig. 3. In situ optical observation of the droplets on spikes of Cynodon dactylon: (a) photographs of the droplet deposition and hanging on the fiber-like spikes, (b) water droplet collected at the base of the whorl transported through the grooves on the spikes and (c) process of initial droplet deposition and growth on the spike as a function of time.
3. Results 3.1. Appearance and surface morphology The photographs of Cynodon dactylon in its native habitat and collected specimen (Fig. 1a–d) show the morphological characteristics of this plant species. The outer morphology contains the leaf blades, which are grey-green, about 4 cm long, 4 mm broad, slightly keeled structures with rough edges and a sharp tip. Their upper part contains the inflorescence-like spikes which are 4 cm in length and exist in 4–6 whorls. These spikes are actually of interest in our studies for the fog harvesting investigations. To investigate the structures of the area of interest in detail, scanning electron microscopy (SEM) was employed to observe an individual spike. Fig. 1e–h shows that the spike contains two parts with dissimilar structural features: (i) the seedheads having slightly flattened surface with gradient grooves interconnected to each other in a rope-like manner and (ii) the cluster of spines located at different positions. The individual spine in the cluster is conical in shape having an apex angle of ∼15◦ (Fig. 1g). The magnified image shows that the spines are anchored to the common base made up of epidermal cells originating from the seedhead surfaces. The subtle combination of these two morphologically different structures (gradient grooves and conical spines) provides these microstructures the
ability to harvest fog, wherein the conical spines provide a good platform for the deposition of the fog droplets, which grow larger in size by coalescence as a function of time, move in a directional way on the gradient grooves on the spikes driven by gravitational force, get detached and eventually are harvested.
3.2. Fog collection studies The fog interception and collection capability of C. dactylon was examined using a consistent fog flow with a flow rate of 130 ± 30 mL (mean ± SD) per hour. The schematic of the experimental setup is shown in Fig. 2(a). We placed a whorl of 4 spikes at an angle of 90◦ to determine the fog collection efficiency of C. dactylon. The size of the deposited droplets ranged from few m to mm. Throughout two hours in the fog flow, the C. dactylon samples collected 120 ± 6 L of water. The quantity of fog water received per sq. mm was calculated from the total spike surface area used for experiments and was found to be 1.91 L mm−2 . This value corresponds to a deposition rate of 0.2667 g m−2 s−1 . The water run-off dynamics of the samples were also studied by observing the water collected at different time intervals and the obtained results are shown in Fig. 2(b). Within 20 min in the fog flow the specimen had collected ∼28 L of water and subsequent water collections were monitored at 20 min time
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Fig. 4. The deposition, growth and movement of the water droplets at different tilt angles (a) 90◦ (b) 45◦ and (c) 0◦ .
intervals up to 2 h. The fog water deposited on the spikes run-off at a steady rate of 99 L min−1 m−2 (linear regression, r2 = 0.99). The process of the initial water droplet deposition, growth and movement were investigated under in situ conditions using contact angle instrument. Fig. 3a shows the photograph of the droplet deposition and growth on the spikes of C. dactylon. By focusing on the behavior of the water droplets on the surface of the spike containing conical spines and gradient grooves, we observed the real-time deposition, growth and transport of water droplets on the spike (Fig. 3b). As a function of time, droplet increased in size and could also be observed to move along the spike driven by gravity aided by the gradient grooves (Fig. 3b). These tiny droplets either grow in size to form a bigger drop which hangs on the spike before detachment or coalesce with other tiny droplets and move towards the base of the whorl (Fig. 3c) and roll to the ground. After the water droplets fall off from the spikes, a fresh series of droplet forms and collection begins, and the cycle continues. We also investigated the deposition and movement of the fog droplets at different tilt angles of spikes and the corresponding in situ optical observations are shown in Fig. 4. As it is evident from the images, the deposition of the droplets is not much influenced by the tilt angles and the initial deposition and growth of the droplet occurs in a similar manner in all the three different (90◦ , 45◦ and 0◦ ) investigated tilt angles. At tilt angles of 90◦ and 45◦ the droplet grows in size and swiftly moves along the spike with progression in time driven by
gravity (Fig. 4a and b). However, in case of tilt angle 0◦ , the droplet gradually increase its size with time, which makes the spike to tilt and the droplet detaches itself from the spike under the influence of gravity (Fig. 4c). It is noteworthy to mention that even in 90◦ and 45◦ oriented spikes, the tilt angle slightly changed during the growth of the droplet.
4. Discussion 4.1. Fog collection There are characteristic structural features of Cynodon dactylon including the conical spine clusters at fixed intervals complemented with the flattened structure containing gradient grooves aligned in an organized pattern in the spikes, which aid in fog harvesting. As mentioned in the Results section, the tiny droplets of fog get deposited on the conical spines, grow in size on the spikes, move along the spikes, which are comprised of gradient grooves, in a directional way driven by gravity and eventually get detached/collected. In this case, the movement/rolling-off of the droplet can be attributed to the asymmetric groove-like structure, analogous to butterfly wings and cactus spines, which enables the unidirectional rolling and collection of the water drops (Ju et al., 2012; Malvadkar et al., 2010). The amount of water collected in our fog collection experiments (1.91 L mm−2 ) is adequate to claim
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Fig. 5. Mechanism of fog collection in Cynodon dactylon: (a) outline of the whole process of fog collection (vertically oriented arrows indicate how the water droplets get dislodged from the spikes of C. dactylon), (b) sketch of investigation of the driving force originating from the Laplace pressure gradient from the individual spine in the cluster, where R1 and R2 are the local radii of the spine at the two opposite sides of the water droplet (c) water droplet travelling on micrometer-sized gradient grooves on the flattened surface and (d) water drop hanging on a fiber-like surface, where m is the contact length of fiber and water drop, is the apparent contact angle of the water droplet with fiber-like surface and ␣ is the off-axis angle.
that the grass samples have remarkable interception of the fog flow. The fog collection efficiency, expressed as deposition rate, for C. dactylon corresponds to 0.2667 g m−2 s−1 . This value compares well with the values reported for other flora and fauna with good fog harvesting properties (Azad et al., 2015; Nørgaard et al., 2012). However, the experimental conditions were different for the different the cases reported in literature (Azad et al., 2015; Nørgaard et al., 2012). Based on our results, we would like to propose a mechanism to portray the structure–function relationship in the fog collection process in C. dactylon, which is illustrated in Fig. 5. The initial capturing of fog occurs on the spine followed by the water droplets moving directionally along the spine. With time, formation of water droplets proceeds in a well-defined manner growing into a bigger drop, which detaches from the tip of the spine. Finally the bigger droplets are further transported alongside the surface of the spine and coalesce at the base of the spine as shown in Fig. 5a. Similarly other spines in the cluster also transport and deposit the drops at their base, which all merge to form a much bigger drop. There are two general forces that lead to the directional movement of the water droplets: First is the Laplace pressure gradient originating from the natural conical shape of the spines and second is the gradient of surface free energy which originates from the surface roughness (Ju et al., 2012). In the case of C. dactylon only the force corresponding to the Laplace pressure gradient will be applicable as it pertains to the conical spines. It is already described in the literature that the droplet on the surface moves from the smaller radius towards the larger radius due to the Laplace pressure gradient, which is in good agreement with our fog collection results (Lorenceau and Quéré, 2004). The illustration of the small spine having a conical shape is shown in Fig. 5b. Previous studies reported in the literature (Ju et al., 2012; Lorenceau and Quéré, 2004) show
that this type of conical shape produces a Laplace pressure difference (Pcurvature ) between the two opposite sides of the droplet as follows:
R2 P curvature = − R1
2␥ (R + R0 )2
sin ˛dz
Here R0 is the drop radius, R1 and R2 are the local radii of the spine at the two opposite sides of the droplet, ␥ is surface tension of water, dz is the incremental radius of the spine and ␣ is half-apex angle of the conical spine. The Laplace pressure acting on the area close to the spine’s tip is higher than Laplace pressure near the spine’s base. Due to this difference (Pcurvature ) within the water drop a driving force is initiated that leads to the movement of the droplet from the tip area to the base area along the small spine and gets deposited there. Cynodon dactylon exhibits two different water collection behaviors. In one case, the droplet gets directionally transported along the grooves, coalesces with other droplets at the base and gets dislodged (Fig. 3c). In the other case, the droplet grows in size in its own position, hangs from the fiber-like structure and detaches itself after reaching a critical size (Fig. 3b). Both the behaviors are gravitydriven although the surface microstructure determines the ability to capture fog and leads to the growth of water droplets, which is independent of gravity. It has been reported earlier, that when the water droplet hangs on a fiber, capillary force is counter-balanced by gravity (Padday and Pitt, 1973). As our sample can be considered as a fiber, where the drop hangs, an equation can be used to determine the maximum volume of the droplet that can hang on fiber-like structure by investigating the interaction of surface force and gravity, as illustrated in Fig. 5d (Extrand, 2002). The force
V. Sharma et al. / Flora 224 (2016) 59–65
of gravity (F) can be calculated from the volume (V) and density () of the water drop using formula, F = gV, where g is the acceleration due to gravity. Conversely, the surface force component acting in the vertical direction (f) is dependent on some important parameters which include its apparent contact angle with fiber-like surface (), the surface tension of the water (␥), the off-axis angle ␣ and the contact length between fiber and water drop (m), which is described as: f = 2␥m cos sin ␣. When we equate the component force of surface force and gravity i.e., F = f, the volume of the drop can be correlated to the off-axis angle, the actual contact angle, and the contact length between fiber and water droplet as follows (Hou et al., 2012; Lorenceau et al., 2004): V=
2cos m sin ˛ g
We have examined the fog collection abilities of C. dactylon using the mechanisms of initial water droplet formation on a spine, droplet growth, hanging and transportation. The combination of the microstructural features at different hierarchical levels and the resultant combination of the multifunctional capabilities, including the water droplet deposition, growth, transportation and collection, provide C. dactylon with efficient fog collection ability. More extensive investigation into the structure–function relationship of this and other similar plants can lead to design and fabrication of novel bioinspired materials and devices to intercept fog efficiently. Successful replication of these types of structures would be very helpful in developing new approaches for fog harvesting in arid and semi-arid regions where freshwater is not easily accessible. It can also be very helpful in the industrial applications such as in development of smart textiles and filtration units. 5. Conclusions The fog-harvesting mechanism of the common Bermuda grass, Cynodon dactylon has been investigated in detail and can be credited to the combination of the structures at multilevels (hierarchical arrangements of microstructures), which aid the deposition, growth, transport and retention of water droplets on the spike surface for prolonged periods of time. The water is deposited initially at the spine tips existing in clusters, then it moves towards the base leading to the growth of the droplet. The droplet either hangs on its position before growing bigger in size and ultimately falling off from the surface or is transported through the gradient grooves to of the base of spike before getting dislodged. This study represents a previously unexplored fog harvesting system and we hope that this understanding of the relationship between microstructure and fog collection behavior in C. dactylon would provide a promising new approach to solve the fresh water accessibility problem by efficient replication of such structures. Acknowledgements We are thankful to Advanced Materials Research Centre (AMRC), IIT Mandi for providing access to the characterization facilities. VK acknowledges the financial support from Department of Science and Technology, India under INSPIRE Faculty Award. References Agam, N., Berliner, P., 2006. Dew formation and water vapor adsorption in semi-arid environments—a review. J. Arid Environ. 65, 572–590. Andrews, H., Eccles, E., Schofield, W., Badyal, J., 2011. Three-dimensional hierarchical structures for fog harvesting. Langmuir 27, 3798–3802. Azad, M.A.K., Barthlott, W., Koch, K., 2015. Hierarchical surface architecture of plants as an inspiration for biomimetic fog collectors. Langmuir 31, 13172–13179.
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Bai, H., Tian, X., Zheng, Y., Ju, J., Zhao, Y., Jiang, L., 2010. Direction controlled driving of tiny water drops on bioinspired artificial spider silks. Adv. Mater. 22, 5521–5525. Chaudhury, M.K., Whitesides, G.M., 1992. How to make water run uphill. Science 256, 1539–1541. Domen, J.K., Stringfellow, W.T., Camarillo, M.K., Gulati, S., 2014. Fog water as an alternative and sustainable water resource. Clean Technol. Environ. Policy 16, 235–249. Dong, H., Wang, N., Wang, L., Bai, H., Wu, J., Zheng, Y., Zhao, Y., Jiang, L., 2012. Bioinspired electrospun knotted microfibers for fog harvesting. ChemPhysChem 13, 1153–1156. Ebner, M., Miranda, T., Roth-Nebelsick, A., 2011. Efficient fog harvesting by Stipagrostis sabulicola (Namib dune bushman grass). J. Arid Environ. 75, 524–531. Ensikat, H., Barthlott, W., 1993. Liquid substitution: a versatile procedure for SEM specimen preparation of biological materials without drying or coating. J. Microsc. 172, 195–203. Extrand, C., 2002. Model for contact angles and hysteresis on rough and ultraphobic surfaces. Langmuir 18, 7991–7999. Falconer, R., Falconer, P., 1980. Determination of cloud water acidity at a mountain observatory in the Adirondack Mountains of New York State. J. Geophys.Res.: Oceans (1978–2012) 85, 7465–7470. Farsani, T.M., Etemadi, N., Sayed-Tabatabaei, B.E., Talebi, M., 2011. Assessment of genetic diversity of Bermudagrass (Cynodon dactylon) using ISSR markers. Int. J. Mol. Sci. 13, 383. Fessehaye, M., Abdul-Wahab, S.A., Savage, M.J., Kohler, T., Gherezghiher, T., Hurni, H., 2014. Fog-water collection for community use. Renew. Sustain. Energy Rev. 29, 52–62. Henschel, J.R., Seely, M.K., 2008. Ecophysiology of atmospheric moisture in the Namib Desert. Atmos. Res. 87, 362–368. Hou, Y., Chen, Y., Xue, Y., Zheng, Y., Jiang, L., 2012. Water collection behavior and hanging ability of bioinspired fiber. Langmuir 28, 4737–4743. Ju, J., Bai, H., Zheng, Y., Zhao, T., Fang, R., Jiang, L., 2012. A multi-structural and multi-functional integrated fog collection system in cactus. Nat. Commun. 3, 1247. Lekouch, I., Lekouch, K., Muselli, M., Mongruel, A., Kabbachi, B., Beysens, D., 2012. Rooftop dew, fog and rain collection in southwest Morocco and predictive dew modeling using neural networks. J. Hydrol. 448, 60–72. Lorenceau, É., Quéré, D., 2004. Drops on a conical wire. J. Fluid Mech. 510, 29–45. Lorenceau, É., Clanet, C., Quéré, D., 2004. Capturing drops with a thin fiber. J. Colloid Interface Sci. 279, 192–197. Malik, F., Clement, R., Gethin, D., Krawszik, W., Parker, A., 2014. Nature’s moisture harvesters: a comparative review. Bioinspir. Biomim. 9, 031002. Malik, F., Clement, R., Gethin, D., Beysens, D., Cohen, R., Krawszik, W., Parker, A., 2015. Dew harvesting efficiency of four species of cacti. Bioinspir. Biomim. 10, 036005. Malvadkar, N.A., Hancock, M.J., Sekeroglu, K., Dressick, W.J., Demirel, M.C., 2010. An engineered anisotropic nanofilm with unidirectional wetting properties. Nat. Mater. 9, 1023–1028. Nørgaard, T., Ebner, M., Dacke, M., 2012. Animal or plant: which is the better fog water collector. PLoS One 7, e34603. Olivier, J., De Rautenbach, C., 2002. The implementation of fog water collection systems in South Africa. Atmos. Res. 64, 227–238. Padday, J., Pitt, A., 1973. The stability of axisymmetric menisci. Philos. Trans. R. Soc. London A: Math. Phys. Eng. Sci. 275, 489–528. Parker, A.R., Lawrence, C.R., 2001. Water capture by a desert beetle. Nature 414, 33–34. Peng, Y., He, Y., Yang, S., Ben, S., Cao, M., Li, K., Liu, K., Jiang, L., 2015. Magnetically induced fog harvesting via flexible conical arrays. Adv. Funct. Mater. 25, 5967–5971. Rasband, W., 1997. ImageJ, US National Institutes of Health. Bethesda, Maryland, USA. Schemenauer, R.S., Cereceda, P., 1994. A proposed standard fog collector for use in high-elevation regions. J. Appl. Meteorol. 33, 1313–1322. Schemenauer, R.S., Fuenzalida, H., Cereceda, P., 1988. A neglected water resource: the Camanchaca of South America. Bull. Am. Meteorol. Soc. 69, 138–147. White, B., Sarkar, A., Kietzig, A.-M., 2013. Fog-harvesting inspired by the Stenocara beetle—an analysis of drop collection and removal from biomimetic samples with wetting contrast. Appl. Surf. Sci. 284, 826–836. World Health Organization, UNICEF, 2015. Progress on Sanitation and Drinking Water—2015. Update and MDG Assessment. WHO, Geneva, Switzerland. Zheng, Y., Bai, H., Huang, Z., Tian, X., Nie, F.-Q., Zhao, Y., Zhai, J., Jiang, L., 2010. Directional water collection on wetted spider silk. Nature 463, 640–643.
Contents lists available at ScienceDirect
Flora journal homepage: www.elsevier.com/locate/flora
Investigations on the fog harvesting mechanism of Bermuda grass (Cynodon dactylon) Vipul Sharma a , Manjul Sharma b , Suneel Kumar a , Venkata Krishnan a,∗ a b
School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi 175005, H.P., India Department of Botany, Shoolini University, Solan 173229, H.P., India
a r t i c l e
i n f o
Article history: Received 27 January 2016 Received in revised form 17 May 2016 Accepted 9 July 2016 Edited by Hermann Heilmeier Available online 13 July 2016 Keywords: Fog interception Microstructures Laplace pressure Bioinspiration
a b s t r a c t Harvesting water from the humidity in air has far reaching advantages to provide access to clean water for consumption, especially in the arid and semi-arid regions of the world. In this regard, fog harvesting properties of a handful of fauna and flora have been explored in the recent past to understand their unique characteristics. In most of the cases, the structural features at the micro- and nanoscales play a crucial role in water collection and transport. In this study, we report the fog collection mechanism in Bermuda grass, Cynodon dactylon, which is commonly found in several regions of the world. The fog collection ability of this grass can be attributed to two characteristic structural traits: well-arranged conical spines with sharp edges, wherein the deposition of fog droplets occurs, and hierarchically organized seedheads having flattened surfaces with gradient grooves that transport the coalesced water drop in a directional manner. Both the conical spines and gradient grooves have specified functions in fog interception by virtue of their structural features. The gradient of the Laplace pressure and fiber-like hanging phenomenon of the droplet provide Cynodon dactylon with an efficient fog collection system. Further research on the characteristic structural features of this and other similar plants will lead us to the fabrication of bioinspired materials and devices to harvest fog in an efficient manner. © 2016 Published by Elsevier GmbH.
1. Introduction Approximately 663 million people i.e., 1 in 10 people, live without access to safe drinking water in different parts of the world and this issue of access to clean water is one of the major global concerns in modern times (UNICEF and Organization, 2015). Interestingly, some indigenous flora and fauna found in many of the arid and semi-arid regions can readily deal with inadequate access to water for survival by collecting water through dew, fog or in general moisture present in the air (Agam and Berliner, 2006; Henschel and Seely, 2008; Schemenauer et al., 1988). Many species have adapted themselves according to the arid regions to increase their chances of survival. Inspired by survival adaptation existing in nature, widespread efforts are being made to exploit fog interception to explore the clean water supply in these water deficit regions (Lekouch et al., 2012; Olivier and De Rautenbach, 2002; Schemenauer and Cereceda, 1991). From the last few years, numerous projects have been initiated in many areas located in the arid and semi-arid climate zones to potentially use fog interception for access to clean water supply (Domen et al., 2014; Fessehaye et al.,
∗ Corresponding author. E-mail address: [email protected] (V. Krishnan). http://dx.doi.org/10.1016/j.flora.2016.07.006 0367-2530/© 2016 Published by Elsevier GmbH.
2014). The initial research and development done to collect water from atmosphere was based on different types of fog collecting devices, which are known to resist wetting by water and allow condensation to take place leading to the droplet deposition on their surface (Falconer and Falconer, 1980; Schemenauer and Cereceda, 1994). In this conventional method, the droplets increase in size and coalesce with other droplets until eventually these become large enough to get detached from the surface and fall into the collection vessel. There are many flora and fauna found in several arid and semiarid regions which harvest dew and fog to cope with their water needs and have led to bioinspired fog harvesting. For example, Stenocara gracilipes beetles have micrometer level arrangements of hydrophobic and hydrophilic areas on their backs which capture water from moisture rich air (Parker and Lawrence, 2001). Some plants have also been reported, including Stipagrostis sabulicola (Namib bushman grass) and Cottula fallax, to capture fog efficiently from the atmosphere and convert it to fresh water utilizing their three-dimensional hierarchical structures (Andrews et al., 2011; Ebner et al., 2011). A multi-structural and multi-functional integrated spine-based fog collection system in cactus has also been shown to demonstrate the natural fog harvesting system (Ju et al., 2012). Different types of cacti species have been compared in a different study and it has been proved that the efficiency of
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Fig. 1. Morphology and surface appearance of Cynodon dactylon: (a and b) Photographs in its natural habitat, (c and d) Images of spikes arranged in the whorl, (e and f) SEM Images showing the microstructures at 1 mm and 200 m, (g and h) magnified image of spikes having conical spine clusters and gradient grooves, respectively.
the interception of fog and formation of the water droplets from fog is highly dependent on the presence of spines (Malik et al., 2015). In a very recent study, hierarchical surface architecture of Hordeum vulgare (barley awns) was found to efficiently harvest fog (Azad et al., 2015). Taking inspiration from these integrated cactus-inspired spine structures and the science of shape gradientinduced Laplace pressure difference, the fabrication of biomimetic systems has been already started to collect fog in an efficient manner (Peng et al., 2015). Spider silks have also been studied to harvest water from moist air (Zheng et al., 2010). The unique fog harvesting capability of the spider Uloborus walckenaerius is due to their characteristic fiber structure which includes the naturally occurring periodic spindle knots and joints (Zheng et al., 2010). The gradient of surface-free energy (Chaudhury and Whitesides, 1992) and gradient of Laplace pressure (Bai et al., 2010) have proved to
be the main driving forces behind the fog collection system in the spider silk. Inspired by these naturally occurring ‘Fog harvesters’, biomimetic imitation of naturally occurring fog harvesting structures has become a subject of interest in the scientific community to create new possibilities for maximizing fog-collection efficiency (Dong et al., 2012; Malik et al., 2014; White et al., 2013). Here we report the fog collection mechanism in Bermuda grass, Cynodon dactylon, which is commonly found in several regions of the world. The fog interception can be ascribed to the presence of certain characteristic surface structures possessing specific features for the deposition and growth of water droplets, directional transport of the water droplets, retention and subsequent collection of water droplets. The results have been discussed in detail along with the applicable mathematical formalisms. Research on the characteristic structural features and the underlying mecha-
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Fig. 2. (a) Schematic figure of experimental setup of the fog collection process in Cynodon dactylon. (b) Water collection dynamics of Cynodon dactylon: Amount of water collected by C. dactylon possessing a surface area of about 0.0096 m2 exposed to fog flow in intervals from 0 to 120 min with 20 min increments, shown along with the standard deviation.
nism of fog collection can lead us to the fabrication of bioinspired materials and devices to harvest fog in an efficient manner.
Barthlott (1993). The samples were then immobilized onto the carbon tape supported on an aluminum stub and coated with a 5 nm layer of gold with Quorum Q150R ES combined sputter coating unit.
2. Experimental section 2.1. Materials Cynodon dactylon (L.) Pers. is a grass that originated from the Middle East and is widely found to grow in the semi-arid and arid regions in several parts of the world (Farsani et al., 2011). Samples from C. dactylon were collected in its naturally occurring habitat locally (Mandi, Himachal Pradesh, India – 31◦ 42 25 N, 76◦ 55 54 E) in August 2015. The sample parts were carefully screened and immobilized on the sample frame for further analysis. 2.2. Preparation of samples Any moisture content on the collected Cynodon dactylon samples was dried in the open air in the laboratory prior to cutting them to equal sizes. For scanning electron microscopy (SEM) analysis, the grass samples were prepared by fixing them overnight using 2% glutaraldehyde prepared in phosphate buffer solution (Sigma Aldrich). Then the glycerol substitution process was followed to replace the water molecules using the procedure reported by Ensikat and
2.3. Fog collection experiments Water collection studies were done by exposure of dry Cynodon dactylon whorls of spikes to a fine mist of ultra-pure water (18.2 M-cm ELGA PURELAB Option-R7) generated by a cold mist humidifier (Bionaire BU1300W-I) with an air flow of 130 ± 30 mL per hour and at a distance of 15 cm from the sample. Individual specimens were fixed on the 10 mL hollow plastic vial placed inside a petri dish, and the total volume of water collected was measured at 20 min time intervals over a period of 2 h. Three sets of experiments were performed on three independent specimens and the average values along with the standard deviation were determined. The deposition and movement of the water droplets were recorded by Phoenix 300 contact angle instrument (Surface Electro Optics, South Korea). The surface area of the samples were determined using Image J software (Rasband, 1997).
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Fig. 3. In situ optical observation of the droplets on spikes of Cynodon dactylon: (a) photographs of the droplet deposition and hanging on the fiber-like spikes, (b) water droplet collected at the base of the whorl transported through the grooves on the spikes and (c) process of initial droplet deposition and growth on the spike as a function of time.
3. Results 3.1. Appearance and surface morphology The photographs of Cynodon dactylon in its native habitat and collected specimen (Fig. 1a–d) show the morphological characteristics of this plant species. The outer morphology contains the leaf blades, which are grey-green, about 4 cm long, 4 mm broad, slightly keeled structures with rough edges and a sharp tip. Their upper part contains the inflorescence-like spikes which are 4 cm in length and exist in 4–6 whorls. These spikes are actually of interest in our studies for the fog harvesting investigations. To investigate the structures of the area of interest in detail, scanning electron microscopy (SEM) was employed to observe an individual spike. Fig. 1e–h shows that the spike contains two parts with dissimilar structural features: (i) the seedheads having slightly flattened surface with gradient grooves interconnected to each other in a rope-like manner and (ii) the cluster of spines located at different positions. The individual spine in the cluster is conical in shape having an apex angle of ∼15◦ (Fig. 1g). The magnified image shows that the spines are anchored to the common base made up of epidermal cells originating from the seedhead surfaces. The subtle combination of these two morphologically different structures (gradient grooves and conical spines) provides these microstructures the
ability to harvest fog, wherein the conical spines provide a good platform for the deposition of the fog droplets, which grow larger in size by coalescence as a function of time, move in a directional way on the gradient grooves on the spikes driven by gravitational force, get detached and eventually are harvested.
3.2. Fog collection studies The fog interception and collection capability of C. dactylon was examined using a consistent fog flow with a flow rate of 130 ± 30 mL (mean ± SD) per hour. The schematic of the experimental setup is shown in Fig. 2(a). We placed a whorl of 4 spikes at an angle of 90◦ to determine the fog collection efficiency of C. dactylon. The size of the deposited droplets ranged from few m to mm. Throughout two hours in the fog flow, the C. dactylon samples collected 120 ± 6 L of water. The quantity of fog water received per sq. mm was calculated from the total spike surface area used for experiments and was found to be 1.91 L mm−2 . This value corresponds to a deposition rate of 0.2667 g m−2 s−1 . The water run-off dynamics of the samples were also studied by observing the water collected at different time intervals and the obtained results are shown in Fig. 2(b). Within 20 min in the fog flow the specimen had collected ∼28 L of water and subsequent water collections were monitored at 20 min time
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Fig. 4. The deposition, growth and movement of the water droplets at different tilt angles (a) 90◦ (b) 45◦ and (c) 0◦ .
intervals up to 2 h. The fog water deposited on the spikes run-off at a steady rate of 99 L min−1 m−2 (linear regression, r2 = 0.99). The process of the initial water droplet deposition, growth and movement were investigated under in situ conditions using contact angle instrument. Fig. 3a shows the photograph of the droplet deposition and growth on the spikes of C. dactylon. By focusing on the behavior of the water droplets on the surface of the spike containing conical spines and gradient grooves, we observed the real-time deposition, growth and transport of water droplets on the spike (Fig. 3b). As a function of time, droplet increased in size and could also be observed to move along the spike driven by gravity aided by the gradient grooves (Fig. 3b). These tiny droplets either grow in size to form a bigger drop which hangs on the spike before detachment or coalesce with other tiny droplets and move towards the base of the whorl (Fig. 3c) and roll to the ground. After the water droplets fall off from the spikes, a fresh series of droplet forms and collection begins, and the cycle continues. We also investigated the deposition and movement of the fog droplets at different tilt angles of spikes and the corresponding in situ optical observations are shown in Fig. 4. As it is evident from the images, the deposition of the droplets is not much influenced by the tilt angles and the initial deposition and growth of the droplet occurs in a similar manner in all the three different (90◦ , 45◦ and 0◦ ) investigated tilt angles. At tilt angles of 90◦ and 45◦ the droplet grows in size and swiftly moves along the spike with progression in time driven by
gravity (Fig. 4a and b). However, in case of tilt angle 0◦ , the droplet gradually increase its size with time, which makes the spike to tilt and the droplet detaches itself from the spike under the influence of gravity (Fig. 4c). It is noteworthy to mention that even in 90◦ and 45◦ oriented spikes, the tilt angle slightly changed during the growth of the droplet.
4. Discussion 4.1. Fog collection There are characteristic structural features of Cynodon dactylon including the conical spine clusters at fixed intervals complemented with the flattened structure containing gradient grooves aligned in an organized pattern in the spikes, which aid in fog harvesting. As mentioned in the Results section, the tiny droplets of fog get deposited on the conical spines, grow in size on the spikes, move along the spikes, which are comprised of gradient grooves, in a directional way driven by gravity and eventually get detached/collected. In this case, the movement/rolling-off of the droplet can be attributed to the asymmetric groove-like structure, analogous to butterfly wings and cactus spines, which enables the unidirectional rolling and collection of the water drops (Ju et al., 2012; Malvadkar et al., 2010). The amount of water collected in our fog collection experiments (1.91 L mm−2 ) is adequate to claim
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Fig. 5. Mechanism of fog collection in Cynodon dactylon: (a) outline of the whole process of fog collection (vertically oriented arrows indicate how the water droplets get dislodged from the spikes of C. dactylon), (b) sketch of investigation of the driving force originating from the Laplace pressure gradient from the individual spine in the cluster, where R1 and R2 are the local radii of the spine at the two opposite sides of the water droplet (c) water droplet travelling on micrometer-sized gradient grooves on the flattened surface and (d) water drop hanging on a fiber-like surface, where m is the contact length of fiber and water drop, is the apparent contact angle of the water droplet with fiber-like surface and ␣ is the off-axis angle.
that the grass samples have remarkable interception of the fog flow. The fog collection efficiency, expressed as deposition rate, for C. dactylon corresponds to 0.2667 g m−2 s−1 . This value compares well with the values reported for other flora and fauna with good fog harvesting properties (Azad et al., 2015; Nørgaard et al., 2012). However, the experimental conditions were different for the different the cases reported in literature (Azad et al., 2015; Nørgaard et al., 2012). Based on our results, we would like to propose a mechanism to portray the structure–function relationship in the fog collection process in C. dactylon, which is illustrated in Fig. 5. The initial capturing of fog occurs on the spine followed by the water droplets moving directionally along the spine. With time, formation of water droplets proceeds in a well-defined manner growing into a bigger drop, which detaches from the tip of the spine. Finally the bigger droplets are further transported alongside the surface of the spine and coalesce at the base of the spine as shown in Fig. 5a. Similarly other spines in the cluster also transport and deposit the drops at their base, which all merge to form a much bigger drop. There are two general forces that lead to the directional movement of the water droplets: First is the Laplace pressure gradient originating from the natural conical shape of the spines and second is the gradient of surface free energy which originates from the surface roughness (Ju et al., 2012). In the case of C. dactylon only the force corresponding to the Laplace pressure gradient will be applicable as it pertains to the conical spines. It is already described in the literature that the droplet on the surface moves from the smaller radius towards the larger radius due to the Laplace pressure gradient, which is in good agreement with our fog collection results (Lorenceau and Quéré, 2004). The illustration of the small spine having a conical shape is shown in Fig. 5b. Previous studies reported in the literature (Ju et al., 2012; Lorenceau and Quéré, 2004) show
that this type of conical shape produces a Laplace pressure difference (Pcurvature ) between the two opposite sides of the droplet as follows:
R2 P curvature = − R1
2␥ (R + R0 )2
sin ˛dz
Here R0 is the drop radius, R1 and R2 are the local radii of the spine at the two opposite sides of the droplet, ␥ is surface tension of water, dz is the incremental radius of the spine and ␣ is half-apex angle of the conical spine. The Laplace pressure acting on the area close to the spine’s tip is higher than Laplace pressure near the spine’s base. Due to this difference (Pcurvature ) within the water drop a driving force is initiated that leads to the movement of the droplet from the tip area to the base area along the small spine and gets deposited there. Cynodon dactylon exhibits two different water collection behaviors. In one case, the droplet gets directionally transported along the grooves, coalesces with other droplets at the base and gets dislodged (Fig. 3c). In the other case, the droplet grows in size in its own position, hangs from the fiber-like structure and detaches itself after reaching a critical size (Fig. 3b). Both the behaviors are gravitydriven although the surface microstructure determines the ability to capture fog and leads to the growth of water droplets, which is independent of gravity. It has been reported earlier, that when the water droplet hangs on a fiber, capillary force is counter-balanced by gravity (Padday and Pitt, 1973). As our sample can be considered as a fiber, where the drop hangs, an equation can be used to determine the maximum volume of the droplet that can hang on fiber-like structure by investigating the interaction of surface force and gravity, as illustrated in Fig. 5d (Extrand, 2002). The force
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of gravity (F) can be calculated from the volume (V) and density () of the water drop using formula, F = gV, where g is the acceleration due to gravity. Conversely, the surface force component acting in the vertical direction (f) is dependent on some important parameters which include its apparent contact angle with fiber-like surface (), the surface tension of the water (␥), the off-axis angle ␣ and the contact length between fiber and water drop (m), which is described as: f = 2␥m cos sin ␣. When we equate the component force of surface force and gravity i.e., F = f, the volume of the drop can be correlated to the off-axis angle, the actual contact angle, and the contact length between fiber and water droplet as follows (Hou et al., 2012; Lorenceau et al., 2004): V=
2cos m sin ˛ g
We have examined the fog collection abilities of C. dactylon using the mechanisms of initial water droplet formation on a spine, droplet growth, hanging and transportation. The combination of the microstructural features at different hierarchical levels and the resultant combination of the multifunctional capabilities, including the water droplet deposition, growth, transportation and collection, provide C. dactylon with efficient fog collection ability. More extensive investigation into the structure–function relationship of this and other similar plants can lead to design and fabrication of novel bioinspired materials and devices to intercept fog efficiently. Successful replication of these types of structures would be very helpful in developing new approaches for fog harvesting in arid and semi-arid regions where freshwater is not easily accessible. It can also be very helpful in the industrial applications such as in development of smart textiles and filtration units. 5. Conclusions The fog-harvesting mechanism of the common Bermuda grass, Cynodon dactylon has been investigated in detail and can be credited to the combination of the structures at multilevels (hierarchical arrangements of microstructures), which aid the deposition, growth, transport and retention of water droplets on the spike surface for prolonged periods of time. The water is deposited initially at the spine tips existing in clusters, then it moves towards the base leading to the growth of the droplet. The droplet either hangs on its position before growing bigger in size and ultimately falling off from the surface or is transported through the gradient grooves to of the base of spike before getting dislodged. This study represents a previously unexplored fog harvesting system and we hope that this understanding of the relationship between microstructure and fog collection behavior in C. dactylon would provide a promising new approach to solve the fresh water accessibility problem by efficient replication of such structures. Acknowledgements We are thankful to Advanced Materials Research Centre (AMRC), IIT Mandi for providing access to the characterization facilities. VK acknowledges the financial support from Department of Science and Technology, India under INSPIRE Faculty Award. References Agam, N., Berliner, P., 2006. Dew formation and water vapor adsorption in semi-arid environments—a review. J. Arid Environ. 65, 572–590. Andrews, H., Eccles, E., Schofield, W., Badyal, J., 2011. Three-dimensional hierarchical structures for fog harvesting. Langmuir 27, 3798–3802. Azad, M.A.K., Barthlott, W., Koch, K., 2015. Hierarchical surface architecture of plants as an inspiration for biomimetic fog collectors. Langmuir 31, 13172–13179.
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