Water collection abilities of green bristlegrass bristle

Abstract

A unique water collection ability is revealed in green bristlegrass bristle that is attributed to a barb array similar to the ratchet structure that is arranged on the cone groove surface of bristle. The multi-level geometric gradients are formed to dominate the coalescence and directionally transport the tiny droplets of condensed water.

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Water shortage has always been a substantial problem that concerns people all around the world. Every year, many cases of human suffering and misery are caused by water shortage. Recently, flora and fauna with novel and interesting properties have attracted the interest of researchers.^1–5 Among the creatures, water collection ability especially arouse people's attention. For example, cactus with conical spines and trichomes are highly efficient in water collection,^6,7 which meets its need for several days in the arid environment, spider silks with special structure which are composed of periodical spindle with rough, random nanofibrils and joint with smooth, aligned nanofibrils hang many pearl-like drops in the rainy morning.^8–10 Another interesting example are desert beetles living in the Namib, which have a pattern of hydrophilic and hydrophobic regions on their back surface that captures water from humid air^11–13 to keep them alive. All these properties are because of their fine structures, which attract people's interest to reveal the mechanism of the effect of structure for water retention in depth in order to fabricate bio-inspired smart materials.

Another plant with interesting collection property has been found. Green bristlegrass (Latin name is Setaria Grass), usually found in farms,^14,15 presents strong vitality and good mechanical property when used in reinforced adobe. Here, we describe the unique water collection ability of green bristlegrass bristle, which can be attributed to a multilevel micro-/nano-structure that effectively dominate the coalescence and transportation of tiny condensed droplets in a direction, which is considerably different with the water collection ability of spider silk and desert beetles, and has significant application in water collection devices, air filters, humidity-sensors, and other smart materials.

Green bristlegrass can be found in grassplots anywhere. Fig. 1a shows the optical image of a cluster of (nine) green bristlegrass bristles with water condensed droplets from moisture (humidity of 95–99%), where many tiny droplets surround every bristle and display the ability of water collection. To reveal the mechanism of water collection, the sample of green bristlegrass was prepared (see ESI†), and its microstructure was investigated in detail by scanning electronic microscopy (SEM). Fig. 1b shows the overall morphology of a single green bristlegrass bristle, from top to the bottom. It can be a conical rough thin spine with a length of 3–6 mm, where the top is ∼28–40 μm and the bottom is ∼60–70 μm in diameter. There are some barbs uniformly distributed along the green bristlegrass bristle, as shown in Fig. 1c–e and S1.† The barbs growth is oriented towards the top in a certain degree to the base, similar to ratchet with α = ∼42–44°. The barbs also have a conical structure (apex angle 23–26°), and their length is ∼23–31 μm, while the bottom length connected with green bristlegrass bristle is ∼20–50 μm (Fig. 1c, see yellow line). Fig. 1e reveals that at the bottom of the barbs, there are folding sculptures of the epidermal cells, which are the nano-structures. Fig. 1c and d show the micro-grooves, that are distributed along green bristlegrass bristle. The fine structure is slightly similar to cactus,⁶ although not exactly the same. It is this uniqueness in the structures that endow green bristlegrass its interesting water collection ability.

Fig. 1 Optical images and SEM images of the green bristlegrass. (a) Water collection on green bristlegrass (b) a single green bristlegrass bristle (c–e) the micro-structure of green bristlegrass bristle. The bristle has aligned micro-grooves and oriented barbs (c), the magnified image of barb (d), and nano-fold structure at the bottom of the barb (e).

Fig. 2 shows the dynamic processes of water collection on green bristlegrass bristle in details at a humidity of 95–99%, which is recorded by an optical microscope and CCD components of the OCA 40 contact angle meter. To investigate how its structure affects the droplets' behaviour, the bristle was differently placed in three orientations. Fig. 2a shows the optical images when the tiny condensed droplets are coalesced and directionally transported on a single green bristlegrass bristle at 0° (defining the bristle top upward at 0°). At the saturated fog atmosphere, water droplets condense constantly, and at ∼1.6 s, there are two tiny droplets (numbered 1, 2) on the green bristlegrass bristle. Droplet 1 directionally coalesces toward droplet 2, forming a new droplet (1 + 2). The coalesced 1 + 2 droplet reaches a stable status at the original position of the droplet 2 (at ∼5.0 s). Then, it is quickly transported toward the bottom of the green bristlegrass bristle, with a transport length of ∼1000 μm during the whole process. In Fig. 2b, the water collection process with the single green bristlegrass bristle placed at 180° is shown. Three droplets (numbered 1, 2, 3) appear on a green bristlegrass bristle (at ∼2.5 s), and droplet 1 coalesces with droplet 2, leaving its original position. The new formed droplet further coalesces toward droplet 3, and droplet 1 + 2 + 3 is temporarily in a stable state. Subsequently, it moves upward (at ∼7.2 s). Droplet 1 is transported ∼800 μm. In this case, even a droplet is transported vertically upward; the droplet's gravity plays little role in its transport process. When the green bristlegrass bristle is placed at 270° (Fig. 2c), we can see droplets 1, 2, 3, 4 and 5 directionally move from left (i.e., top of bristle) to right (i.e., bottom of bristle). As mentioned above, droplets condense, coalesce and transport for ∼900 μm in ∼40.3 s. In all the three cases, no matter how the green bristlegrass bristles are placed, water condensed droplets are always transported from the top to the bottom of the bristles, even when the green bristlegrass bristle is vertically upward.

Fig. 2 Optical images of droplets transport on green bristlegrass bristles placed with different orientations. (a) At 0°, the bristle is top upward; the droplet is transported along the bristle a distance of ∼1000 μm in ∼5.8 s. (b) At 180°, the bristle is bottom upward, the droplet is transported along the bristle a distance of ∼800 μm in ∼7.2 s. (c) At 270°, the bristle is horizontal, and the droplet is transported along the bristle a distance of ∼900 μm in ∼40.3 s. In all the cases, droplets are always transported directionally from the top to the bottom of the green bristlegrass bristles.

To reveal the droplet forming dynamic process, the condensation, coalescence and transport of droplets on the top and middle of the green bristlegrass bristles are observed in detail. Fig. 3a shows that the top of green bristlegrass bristle distributes several conical barbs. Because of the Laplace pressure,^16–18 water first condenses at the tip of barb (at ∼0.42 s), then it is rapidly transported to the base (at ∼2.36 s). At the top of green bristlegrass bristle, a tiny droplet forms and grows bigger (at ∼4.72 s), which will transport to the bristle bottom. Fig. 3b shows the observation on the middle part of a green bristlegrass bristle with two barbs at the side fog flows. Collecting water fills the micro-groove aligned on the green bristlegrass bristle, and at the two barbs and micro-groove form a water film (at ∼1.86 s), which grows to a big droplet where water constantly condenses (at ∼5.64 s). Detailed observation showed that water condenses at the tips of barbs and then is transported to the base with the nano-folding epidermal cell, which boost the droplet fast transport. As shown in Fig. 3c, observed on the barbs, the droplet 1, 2 form on barb, and then they coalesce and move to base of the barb (see arrow) at ∼0.1 s, and droplet 3 appears after droplet 1 move away at ∼0.23 s. Form the above observation, apparently, the aligned micro-grooves not only form a water film which reduces the water transport resistance, but also guide the droplets transport in a definite direction.

Fig. 3 Optical images of how a droplet forms on the top and middle of green bristlegrass bristle and on barb in details. (a) A droplet formed on the top of bristle has the tendency to move in one direction. (b) A coalesced droplet covers two barbs at the middle part of a bristle. (c) Observation of droplets formation on barb. The droplet 1, 2 forms on barb, and then they coalesce and move to the base of barb (see arrow) at ∼0.1 s, droplet 3 appears after droplet 1 moves away at ∼0.23 s.

Fig. 4 illustrates the mechanism of water collection on the green bristlegrass bristle with multi-gradients, where Laplace pressure¹⁷ would dominate the behaviors of droplets in a directional transport. The whole water collection process can be regulated by different stages: condensation → coalescence → transport → collection. Moisture first condenses at the tip of the barb, and then micro-droplets move directionally from the tip to the base of the barb to coalesce into a droplet. Next, the big droplet is transported with coalescing small droplets or asymmetric growing between the barbs along the green bristlegrass bristle, and finally water is collected at the bottom of the bristle. The big droplet realizes the coalescence and transport process by “step over” the barbs with asymmetric growth and TCL (three phase contact line) spreading (Fig. S2†). Fig. 4a illustrates the directional coalescence and transport of droplets at multi-level structures. The special structure of green bristlegrass with oriented barbs, aligned micro-grooves and whole conical structure assist droplets to be directionally transported from the top to the bottom. Initially, at the condensation stage, (Fig. 4b), the tiny droplet is condensed on the barbs with a curvature difference resulted from radii of r^′₁ and r^′₂ (where r^′₁ and r^′₂ are the radius of the barb at the two sides of a tiny droplet). The Laplace pressure of barb ΔP_barb can be expressed as follows:

where γ is the surface tension of water; r′ is the local radius of barb; R′ is the droplet radius on barb; β′ is the half apex-angles of the conical barb structure and dz′ is the integration variable along the axis of conical barbs. Moreover, the nano-folding structure at the barb base is similar to a pump that draws the water from the barb tip to the base and assists the droplet to “step over” the barb due to the capillarity caused by nano-folding.

Fig. 4 Mechanism of water collection on the bristle of green bristlegrass. (a) Directional coalescence and transport of droplets at the multi-level structures of bristle. The special structure of the bristles of green bristlegrass with oriented barbs, aligned micro-grooves and its whole conical structure assist droplets to be directionally transported from the top to the bottom. The water collection process can be regulated in four stages: condensation → coalescence → transport → collection. (b) Difference of Laplace pressure resulted from curvature radii. Water firstly condenses at the tip of the barb, then it is transported to the base of the barb due to Laplace pressure. (c) Asymmetric growth and directional coalescence of droplets. Droplets directionally coalesce into big droplets or asymmetrically grow between the barbs, being transported with coalescing small droplets along the green bristlegrass bristle. (d) Anisotropic groove for directional transport of droplets. The surface on the bristle is smooth parallel to the grooves, while it is rough perpendicular to the grooves; moreover, the multi-level Laplace pressure results from the grooves, which make the droplets transport directional from the top the bottom.

For the droplet growth, the condensed droplets covering a barb directionally coalescence with other droplets, asymmetrically growing while droplets are transported between barbs (Fig. 4c). When the droplets are continuously coalescing into bigger ones on the bristle in the transport stage, the Laplace pressure ΔP_bristle resulted from curvature radii of the bristle would drive the droplet to move directionally, which is expressed as:

where r₁ and r₂ are the radii of the bristle at the two sides of a droplet; β is half the apex-angles of the conical bristle and dz is the integration variable along the axis of conical bristles.

Apart from the role of the barbs and bristles, cone groove surface base on the bristles plays an important role (Fig. 4d). The aligned micro-grooves where a water film firstly forms can reduce the water transport resistance, such as the droplet “step over” barbs, which is in contrast with butterflies wing rolling and pinning states.^19,20 Moreover, the grooves on the bristle surface create a multi-Laplace pressure that enhance the droplets directional transport behavior, as the Laplace pressure difference of this small area on a gradient grooved surface is larger than that of a smooth surface. The aligned micro-grooves can also generate an anisotropic contact angle hysteresis in a parallel or perpendicular direction to the grooves, enhancing the directional movement of the droplets along the grooves while weakening other directions due to the smooth structure parallel to the grooves direction and the rough structure perpendicular to the grooves.

Conclusion

In conclusion, we find that droplets directionally transport water from the top to the bottom of green bristlegrass bristles due to the micro-nano multilevel structure combined whole conical shape, aligned micro-grooves, oriented barbs and nano-folding. Regulated through a process of condensation → coalescence → transport → collection, the green bristlegrass efficiently collects water. This interesting water collection ability will be a guideline to design significant structured materials for applications such as water collection devices,²¹ air filters,²² humidity-sensors,²³ and others.

Acknowledgements

This work is supported by National Research Fund for Fundamental Key Project (2013CB933001), National Natural Science Foundation of China (21234001), and Doctoral Fund of Ministry of Education of China (20121102110035).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06661h

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