Mangifera indica Leaf (MIL) as a Novel Material in Atmospheric Water Collection.

Here, Mangifera indica leaves (MILs) have been used to collect atmospheric water for the first time. This novel material has been viewed by mankind as environmental waste and is mostly discarded or incinerated, causing environmental pollution. By turning waste into wealth, MILs have proven resourceful and can help ameliorate the water crisis, especially in tropical countries. The unprecedented water collection result is enough to describe MILs as an ideal material for atmospheric water collection when compared to other natural plants. Both the physical and chemical surface morphologies were extensively characterized. This comparative study shows that MIL surface droplet termination and hydrophilic nature differ from those of other materials, with the apex playing a key role in the roll-off of the droplet. The surface wettability and its interaction with the droplet are of keen interest in this study.

Introduction

The Mangifera Indica leaf (MIL) is one of the most common tropical wastes in the world, which is normally discarded or burnt. Several attributes such as the fruit and trunk of the MI have been studied, with little research on the leaves because they are considered as an environmental nuisance or waste, especially in tropical regions. Basically, “waste is what is left behind when imagination fails”,[1,2] and this expression demonstrates the need why waste should be given a desirable consideration to make it resourceful. The generation of waste has always been a burden to the environment, and the MIL is not an exception. These MILs are primarily disposed in the landfill or incinerated, which normally results in air pollution, landfill leachate, the killing of microorganisms in the soils, and other environmental problems. The MIL is described as a lanceolate with a sharp angle at both the base and apex and with protruding major veins, especially at the abaxial epidermal surface.[3,4]

Recently, atmospheric water collection has received considerable attention worldwide with the potential to transform the hidden treasure from the air into a resourceful and sustainable adventure at a low cost when compared to other water harvesting mechanisms. The MIL exhibits an intriguing surface structure that can facilitate water collection from the atmosphere. Although water sources are ubiquitous and remain untapped, the atmosphere contains a large volume of moisture that can be harnessed to ameliorate the excruciating water demand, especially when freshwater shortage is a global concern. Water shortage has greatly threatened the survival of organisms and even human beings in the world, especially in developing countries.[5] Over 2 billion people of the 7.7 billion people living in the world experienced high water stress, while 4.5 billion people have inadequate sanitation and clean water sources.[6] This perennial problem has been exacerbated by river siltation, climate change, population growth, and pollution, thus leading to water scarcity and prolonged pressure on the ecosystem.[7,8] Collecting water from the atmosphere can greatly alleviate the water shortage problem, especially for those living in arid and other crisis areas. Due to the evaporation of oceans, rivers, and lakes, the air is rich in water vapor, which flows to arid areas and encounters cold air that condenses it into tiny droplets.[9] Water collection studies have always focused on fog and dew harvesting, which emanate from water vapor in the air and condense to form water droplets. Dew droplet size varies with rain droplets, ranging from 0.5 to 5 mm, while fog droplets range between 1 and 40 μm, with a visibility of less than 1 km being the standard for the presence of fog.[9,10] Fog harvesting involves the collection of microscale water droplets suspended in the air without any heat transfer,[11,12] and dew harvesting involves obtaining the liquid state of water by condensing water vapors on a cold surface with heat transfer.[13,14] Because of the tiny line differentiating dew from fog, in this script, we decided to describe the two as atmospheric water or atmospheric liquid droplets. According to Volmer’s nucleation theory, droplets form many orders of magnitude faster on a hydrophilic surface than on a hydrophobic surface.[15,16] Atmospheric water droplets captured on a biological surface can be transported from one point to another, and this process is known as directional movement of droplets.[17] Many natural materials have intriguing surface structures that are capable of capturing and transporting liquid with special wettability features.[18,19]

By transforming “waste into gem”, we used a fresh MIL as a novel material to harvest atmospheric water. In this paper, we seek to give meaning to this MIL by using it to collect atmospheric water from the air. In achieving this dream with this novel material, we classified the harvesting process into three key components, namely, capture, coalescence, and transportation. These three components play a key role in the atmospheric water collection process, and investigating them are imperative to this study. The capture of droplets is achieved when the droplets interact with the leaf surface through the aerodynamics process. Atmospheric droplets need to be large enough to be harvested as evaporation and atmospheric droplet capturing are mutually competitive. In order to reduce re-evaporation time, it is important to achieve fast coalescence of water droplets. For fast coalescence to be enhanced, the distance between tiny droplets should be close to each other. If these tiny water droplets are not quickly collected, they will be immediately lost to heat, winds, and other environmental parameters.[20] These droplets must reach a critical size to begin movement on the surface, thereby potentially triggering the faster rate of water collection.[21,22]

The literature suggests that natural plants and biomimetic surfaces such as the Dryopteris marginata surface,[23] natural hierarchical surface,[24] biomimetic coating surface,[25] microstructured surface and mesh,[26] micropatterned bioinspired surface,[27] and Nepenthesalata surface[28] have all been studied as water collection materials. Inspired by these, fresh MILs have been selected as a novel material to harvest atmospheric water. Currently, engineers all over the world are attempting to mimic the water collection surface of various creatures in nature by employing biomimetic principles.[29,30] The main goal of using MILs is to help ameliorate the growing global water shortage. Important among the goals of this research is to address the efficient collection of water from the atmosphere using MILs. In this paper, we seek to compare the wettability and water collection efficiency of MILs with those of other reported materials. Similarly, we investigate the physical and chemical morphologies of the surface of MILs. Finally, in this paper, we will also study the atmospheric water behavior on the surface of the MIL.

Experimental Section

Materials

Commercial humidifier D20 was purchased from ANPEL laboratory Technologies, Shenzhen-China. An anemometer (Testo 416 Lenzkirch, Germany) and a hydrometer (TH603A) were purchased from South Huadi Avenue, Guangzhou, China, while fresh MILs were obtained from Lijiang, southwest of China. The equipment used for the surface morphological study and characterization of the sample included a scanning electron microscope (S-4800 HITACHI), a Fourier transmission infrared (FTIR) spectrometer (PerkinElmer Spectrum Two), an optical contact angle (OCA) goniometer (JC 2000D-1), and an optical microscope (VHX-900F). Ionized water was used in the water collection process, while a measuring tape and an electronic scale (JY501) were used to measure the distance and amount of harvested water, respectively. A transparent acrylic chamber was used to serve as a micro-weather station in the water collection process, while Fiji (ImageJ) and origin Pro were used for detailed analysis.

Characterization

A scanning electron microscope (S-4800 HITACHI) was used to investigate the surface morphological features of the samples. The water contact angle (WCA) was determined using an OCA goniometer (JC 2000D-1), using the sessile drop of a 5 μL droplet. The image of the droplet silhouette was captured using an inbuilt device camera. The surface chemical composition and morphology of the sample were ascertained using an FTIR spectrometer (PerkinElmer Spectrum Two). An optical microscope (VHX-900F) was used to study the process of the atmospheric water spreading and coalescence behavior on the MIL, while commercial humidifier D20 was used to spray the artificial atmospheric water.

Water Collection Setup

The atmospheric water collection experiment was performed in the lab, and the designed setup was in line with a previous research work.[26] The setup was housed in a transparent acrylic chamber to prevent external environmental influences. Commercial humidifier D20, a thermometer, a hygrometer, and an anemometer were all part of the microclimate setup in the transparent acrylic chamber. The temperature and humidity inside the chamber were maintained at 24.0 ± 1.0 °C and 90–95%, respectively, as measured using a TH603A hygrometer. The atmospheric water velocity was measured using an anemometer and was approximated to be 2.6 ms–1 with a distance of 5 cm between the leaves and the humidifier. The MILs were tilted at an angle of 25–30°. All used leaves had a uniform size of about 5 cm × 2.5 cm and were horizontally positioned with respect to the commercial humidifier, as shown in Figure . The distance between the leaves and the humidifier was maintained to be 5 cm. The anemometer, hygrometer, and thermometer were placed in the chamber to monitor the water velocity, temperature, and humidity, respectively, during the experiment. A 100 mL container was placed directly under the leaves to help collect the water droplets dripping from the leaves’ surface. The leaves were hierarchically placed with a 1 cm interval between the leaves. The distance between the MILs and the 100 mL container was 10 cm. The atmospheric water droplets dripped into the container with the help of the gravitational force, and the quantity of dripped water droplets was measured over a 20 min cycle. An electronic scale was used to weigh the harvested water droplets. A digital camera (D90 Nikon) and an optical microscope were used to video record and study the capturing, coalescence, and transportation behavior of the water droplets, as shown in the Supporting Information (Movie S1). The sizes of the droplets were measured using ImageJ, while the surface area of the leaves used to harvest atmospheric droplets was determined to be about 39.3 cm using the formula πab (where a = radius and b = length).

Figure 1

Schematic illustration of the water collection experimental setup.

Results and Discussion

To understand the surface morphology, scanning electron microscopy (SEM) was conducted to investigate the surface structure of the fresh MIL before (Figure a,b) and after (Figure c,d) the collection of atmospheric water. To study the surface, we divided the leaf into two parts, referred to as the base (Figure a,c) and the apex (Figure b,d). In Figure a,c, the base of the MIL surface before and after water collection was studied, respectively. As seen in Figure a, the surface possesses protruded microgrooves when compared to Figure c. These images justify that the roughness and wettability of MILs decrease after water collection. Similarly, according to Ting Wang and team,[31] the apex structure enhances water shedding with high dripping frequencies and low retention volumes, and as such, the apex role is imperative to this study. As can be seen from the apex surface (Figure b,d), the surface possessed a ridge-valley-like surface, which facilitates the accumulation and pinning of droplets, as shown in Figure S1. It is also evidence that the surface morphology of the apex accelerates the roll-off of droplets in harnessing efficient water collection. The width of the ridge-valley-like surface was 10 ± 2 μm, the length was 13 ± 2 μm, and the depth was approximated to be 2 μm. Before water collection, as seen in Figure b, the ridge-valley-like structures on the surface were sharper when compared to the surface after water collection in Figure d. The ridge-valley-like surface of the apex was further zoomed-in, as seen in Figure e,f, and they were found to have microscale structures that also play a key role in droplet absorption and surface hydrophilicity. Furthermore, to determine the wettability mechanism of MILs, the WCA before and after the collection was determined. According to Lee et al.,[32] liquid droplets show more affinity with a hydrophilic surface with easy water droplet penetration into its microgrooves, causing a low contact angle (CA) value. The CAs before and after water collection were 66.3 and 68.9, respectively, which resonates with the assertion of Lee et al. about the hydrophilic surface.

Figure 2

(a–f) SEM surface morphologies of MILs before and after water collection, (a,c) base—before and after, (b,d) apex—before and after, and (e,f) zoomed-in image of the microscale on the apex before and after, respectively. (g) FTIR spectra before and after water collection.

The surface chemistry of the fresh MIL before and after atmospheric water collection was also examined using FTIR spectroscopy, as shown in Figure g. The result showed that before water collection, the peak at 3416 cm–1 suggests strong hydroxyl (O–H), which is attributed to the (O–H) functional group. The peak region between 2933 and 3050 cm–1 agreed with the C–H functional group (alkane and alkene groups, respectively), while the peaks between 1629 and 1739 cm–1 agreed with the (C=O) carboxyl group, thereby indicating stretching in the unconjugated ketone. Peak 1465 cm–1 agreed with the (C–H) group with the alkane compound class, while peak 1049 cm–1 is attributed to the (CO–O–CO) group. Similarly, after atmospheric water collection, there were slight changes in the peaks compared to “before water collection”. The intense peaks after water collection included peak 3383 cm–1, which belongs to the (O–H) group, while peaks between 2857 and 3043 cm–1 are also attributed to the (C–H) functional group (alkane and alkene groups, respectively). Peaks in between 1443 and 1619 cm–1 are attributed to the (C=O) carboxyl group, thereby indicating stretching in the unconjugated ketone.

Efficient Atmospheric Droplet Collection

Commercial humidifier D20 was used to generate atmospheric droplets at a room temperature of 24.0 ± 1.0 °C with a relative humidity of 90–95%. Fresh MILs were titled at 25–30° to collect atmospheric water, as shown in Figure and the Supporting Information (Movie S1). This sensational innovation was experimentally observed using a digital camera (D90 Nikon). For this research, we narrow our observation to the capture, coalescence, and transportation during the collection process, while a transparent acrylic chamber was used as a mini weather station. Figure a–c shows snapshot images and graphical representations of the droplet collection process on the functional surface. Figure a depicts a detailed explanation of the three stages (capture, coalescence, and transportation), and at the capture stage, tiny water droplets are deposited on the functional surface, which move toward each other to form mass droplets. The droplet movement occurs as a result of the symmetric nature and curvature of the leaf causing coalescence of droplets, while a tiny amount of droplets is lost to evaporation and other environmental influences. Figure a(i) depicts the initial spray of droplets (assigned 1, 2, 3, 4, and 5) on the functional surface, and in Figure a(ii), the liquid droplets assigned 1 and 2 start to merge with the neighboring droplet assigned 3. The droplets assigned 3, 4, and 5 merged and coalesced in situ, forming a larger mass of droplets on the surface, as seen in Figure a(iii). As the harvesting process continues, the large mass of droplets shown in Figure a(iv) triggers droplet transportation along the drainage path. Under these combined actions of titled angle and force of gravity, the water droplets slide along the drainage path into the collection container [Figure a(v)]. During this process, the fresh MIL surface is flooded with many droplets that favor an increase in the droplet collection efficiency. The quantity of water droplets in the container was measured per gram centimeter (gcm) every 20 min for 1:40 h. The water collection efficiency (e) was determined using the equation belowwhere w, s, and t represent the weight of the water collected (per gram centimeter), sprayed atmospheric droplets, and the collection time respectively. The water collection efficiency (e) using MILs with respect to time (t) was recorded. Similarly, the weight (w) of the collected droplets was recorded with respect to the various heights (h) (15, 20, 25, 30, and 35 cm) between the leaves and the water collection container at a constant time (t) 30 min (Figure S2).

Figure 3

(a) Droplet collection process of the MIL. (b) Amount of water harvested from a functional surface with the collection time. (c) Total amount of collected water at different heights for 30 min each.

As shown in Figure b, it can be seen that the more the increase in time (t) of sprayed atmospheric droplets, the more atmospheric water was collected in the container. For example, the water collected in the container was at its best at 100 min with a weight of 5.22 g cm–2 h–1. Figure c shows that the closer the distance (h) between the MIL and the container, the more the atmospheric water was collected at a constant time (t). For example, at the height (h) of 15 and 35 cm, the water collection efficiency was 3.73 and 2.28 g cm–2 h, respectively, at a constant time of 30 min. This was because the droplet has little or no time to interface with the ambient air, and as such, environmental factors could not have much influence on the droplet before reaching the container. At a tilted angle of 25°–30° and with the help of gravitational force, the water film or droplets on the surface rolled off to be collected. More intriguing phenomena emerge as the collection process continues; when there is little space between neighboring droplets, the captured droplets were quick to coalescence to form large droplets compared to those far apart. During this process, the coalesced droplet moves in the direction of a more wettable gradient surface, which favors an increase in coalescence driving force (FD).

Atmospheric Water Behavior on Fresh MILs

The atmospheric water behavior on a fresh MIL surface was visualized using an optical microscope VHX-900F, taking note of the migration path lines, coalescence time (tc), and droplet dynamic behavior on a microscale surface. The surface interaction with the droplet exhibits a hydrophilic feature because of its WCA. Based on the droplet dynamics on the microscale surface, it was realized that the MIL surface greatly appreciates and loves liquid droplets (Figure a,b). An example of this assertion is visible in Figure a, where compacted tiny droplets occur on the MIL surface with a 200% more frequency than on the hydrophilic surface, as studied by Dai.[33] This study is in agreement with the recent theory put forward by Liu and Cheng,[34] which says that a hydrophilic surface can lead to a higher magnitude of tiny droplet density than the hydrophobic surface. The microscopic images (VHX-900F) and schematic illustration of the droplet behavior on the MIL surface are shown in Figure a,b.

Figure 4

Atmospheric water behavior on fresh MILs, ((a) i-iii) optical images depicting the three collection stages (capture, coalescence, and transportation), and (b) schematic diagram describing droplet behavior on the functional surface.

We started with the dynamic droplet description on the functional surface, and we used a commercial humidifier to spray water vapor at an adjusted temperature of 10° ± 2 °C. The microgroove surface enables water droplets to be absorbed into the surface faster as it is no hidden secret that water molecules coalesce faster on a hydrophilic substrate than on a hydrophobic substrate due to its lower energy barrier. Therefore, less hydrophilic regions on the leaf are less attracted to successive coalescence formation, and after directional movement along the drainage path, a new collection circulation starts all over again as long as the atmospheric water is being generated. The coalescence rate was calculated by measuring the onset or initial time of the droplet merging to the time the largest droplet was observed, known as critical size. At the onset or initial stage, as shown in Figure a(i), we spray the water vapor on the surface, and the liquid droplet disappears to either evaporation or absorption, which reflects Beysens’ view,[35] which says that individual or coalesced droplets can be greatly influenced by aerodynamic parameters such as ambient air velocity.

In the second spray, the surface was flooded with compacted droplets, which resulted in the coalescence of droplets, as shown in Figure a(ii). We also notice that droplet coalescence was fast on the hydrophilic surface over time. Most of the coalesced droplets were deformed, having a dumbbell or spherical shape, as shown in Figure a(iii), and this was as a result of the pinning behavior of droplets, as shown in the Supporting Information (Figure S1). This finding has been understood in three ways; first, the MIL surface has a large contact area, which defines its hydrophilic nature, thus reducing the distance between neighboring droplets. Second, the shorter distance between droplets (closer to each other) easily promotes fast droplet coalescence on the surface. Finally, the microscale, curvature, and gravity aid the transformation of droplets to a larger size before they drip off the surface.

To further understand the droplet dynamics on the MIL, the schematic diagram in Figure b describes the droplet behavior on the surface. During the coalescence of droplets, the first phase captures water droplets and at the same time penetrates the microgrooves of the leaf. As seen in the second phase, droplets start to grow and coalesce before finally forming a large water film. These growing and coalescing neighbor droplets gradually move over the already filled microgroove toward a more wettable region before reaching the drainage path or track with a time duration of 3 s. At this stage, there is some loss of liquid droplets to air and other environmental influences. Droplets merge into a large water film in the third phase along the drainage path. It remains static for 2 s before the final roll-off for droplet collection. The static nature has been described here as the “take-off stage for collection”. It is at this stage that other neighboring droplets collide with the static large water film before gravity overcomes the retention force. The increase in the volume of water droplets from phases 1, 2, and 3 reach a critical size for departure, and with the help of the titled angle (25–30°) and gravity, the accumulated droplet rolls off to be collected, as seen in phase four. The largest coalesced droplet size observed during the process was about 4150 μm in diameter, and it takes 1.43 s to collect the droplet from the take-off stage. Details are given in the Supporting Information (Figure S3). Similarly, the tangential sweeping behavior of the coalesced droplet was another reason for efficient and effective droplet removal. The downward movement of a droplet on a surface is mathematically defined aswhere mg, α, and γwater define the gravitational force from water mass, the tilting angle of the substrate, and the water surface tension, respectively. θR and θA are water receding and advancing CAs on the surface, respectively. To apprehend this exceptional phenomenon, we try to analyze the force interaction between the liquid droplets in motion with the leaf surface. After the capture of atmospheric water droplets on the surface, the coalescence process starts with the wettability gradient force (FW), hysteresis force (FH), and coalescence driving force (FD), which are the three main forces that are dominant during the coalescence of liquid droplets on a solid surface.[36,37] The difference of wettability between the inside and outside of the circle shape is termed as the “wettable different force (FWD)”, and because of the insignificant effects it has on the self-driven motion of droplets, the force is little considered. The essence of FWD as a functional role is to improve the capturing ability of atmospheric droplets. During this process, the unstable coalescence droplet’s interface free energy (IFE) is more than its corresponding equilibrium value. By decreasing its “IFE” through radius base reduction, the droplet will gradually alter its equilibrium state. During the coalescence process on a wettable surface, the merged droplet tends to form an equal pair of CAs on both sides due to Laplace pressure. This means equal wettability on the left and right sides of the MIL, and the coalescence driving force (FD) can be defined as.[38,39]where L, R, and k represent the length of O1O2 (O1O2 means the centerline of the circles), the radius (R) of the circle pattern, and the average wettable gradient (k), respectively, while γ and θ define the surface tension of the liquid droplet and the center position-responsive sessile CA of the coalesced droplet, respectively. Liquid droplets will drive toward a highly wettable region due to the existence of a wettable gradient force (FW) than the low region, and it can be defined as

The resistance on the natural MIL surface is known as Hysteresis force (FH), and this is a result of the exerted force applied by the coalesced droplet and can be written aswhere FD, FH, and FW denote the coalescence driving force (FD), the hysteresis force (FH), and wettability gradient force (FW), respectively. The total force exerted on the coalesced droplet is represented as FH.[40,41] The surface gradient energy (Figure a) and Laplace pressure (Figure b) are the key factors to overcome gravity for directional fluid transport,[17,42] while the MIL surface exhibits both attributes. The surface was also known to be in the Wenzel state, as shown in Figure c. We implicitly related this phenomenon in situ during this water collection process.

Figure 5

Basic surface wettability theories: (a) surface energy gradient, (b) Laplace pressure, and (c) Wenzel model.

Surface Wettability

The surface wettability of the MIL was measured using a 5 μL sessile droplet. The CAs before and after water collection are 66.3 and 68.9°, respectively, as shown in Figure , which exhibit hydrophilic features.

Figure 6

Droplet (5 μm) of distilled water on the MI functional leaf surface: (a) before water collection and (b) after water collection.

Before water collection, as shown in Figure a, it exhibits more surface wettability than “after water collection”, as can be seen in Figure b. It can be understood that the surface wettability decreased with an increase in water collection by washing off the rough surface. The change in the CA value can possibly be attributed to the washing away of the topmost layer (wax) of the fresh MIL. The cleaning and smoothing of the topmost surface of the MIL after 1:40 hrs of exposure to atmospheric water under a constant temperature and humidity greatly influence the significant changes in the CA. Furthermore, the dynamic feature of the unstable droplet depicting the advancing and receding CAs was experimentally determined. The contact angle hysteresis (CAH) was determined from the difference between the advancing and receding CAs. All results are shown in the Supporting Information (Table S1). However, we examined the surface roughness of the leaf before and after water collection using optical microscopy. The visualized images (Figure ) show a microgroove surface with a depth of approximately 25–45 μm. This is one of the main factors for the fast disappearance of the droplet as this resonates with Comanns and team.[43] For this research, we define surface roughness (Sa) of a leaf surface as the mean altitude of the surface asperities relative to the reference plane. The surface roughness can be mathematically expressed as

Figure 7

Surface roughness of the MIL: (a) 3D surface image before water collection and (b) 3D surface image after water collection.

As shown in Figure a, before water collection, the sample possessed sharp microgroove peaks when compared to the sample after water collection in Figure b. The microgroove peak heights before water collection range between 35 and 45 μm, while the microgroove peak heights after water collection range between 25 and 35 μm. It can be concluded that MILs have high sharp peaks before water collection and blurred peaks after water collection. Furthermore, the surface was graphically profiled to ascertain its skewness, as shown in Figure S4a. The functional surface shows an anisotropy wetting behavior similar to the one described by Chen et al.[44] During the microscopic visualization, it was realized that protruded veins (secondary and tertiary) at the abaxial epidermal surface aided droplet coalescence. The angular bearing of secondary and tertiary veins are approximated to be around 30–40° and marked with a red line labeled as (ϕ) in Figure . The coalescence driving force (FD) generated during the coalescence process decreases the movement from both sides (radius) of the leaf to the drainage path or track indicated by yellow arrows. In Figure S4b, the yellow lines indicate the path line of the droplet migration process, which has been called surface tributaries and water channels. The surface tributaries enhance the larger spread of droplets, and this is a result of the pinning behavior of the droplet at the contact line at the edge of the channel while spreading on the surface.

Figure 8

Snapshot image of an MIL showing directional droplet channels.

The process leads to the pressing and narrowing of the droplet, thus forcing the droplet to form a water film on the surface. It can be opined that the MIL surface has proper surface chemistry, capable of exhibiting unique wettability features, and with the help of the microgroove surface and curvature and the force of gravity, atmospheric water collection was harnessed. The durability of a droplet on an MIL was investigated similar to the studies of the air layer formed around the lotus leaf that was postulated by Sheng and Zhang,[45] Cheng,[46] and Jiang.[47] A single drop of a 5 μL sessle droplet was deposited on an MIL under a constant temperature and humidity, and within 12 s, the droplet dissipated to either air or absorption. This paradigm justifies the need for fast water collection from both natural and biomimetic materials as aerodynamic parameters such as air and temperature are quick to influence droplets. Another intriguing phenomenon that was observed shows that the quantity of collected water increases with time as the surface of MIL gets smoother, possibly leading to the change in CA.

Comparison of Water Collection Efficiency of Different Natural and Biomimetic Surfaces with the MIL

Comparing the MIL to previously reported water harvesting materials, as stated in Table , shows a very preferable water collection potential compared to most biomimetic and natural surfaces.

Table 1

Comparison of Water Collection Efficiency

surfaces	vol of harvested water (g cm–2 h–1)	materials	refs
D. marginata surfaces	0.72	natural leaf	(23)
natural hierarchical surfaces	0.96	natural wax	(24)
biomimetic surface coatings	3.40	polystyrene	(25)
microstructured surfaces and mesh	0.18	epoxy and polyolefin	(26)
micropatterned bioinspired photolithography surfaces	1.69	polyurethane	(27)
N. alata surface	2.58	natural leaf	(28)
M. indica surface	5.22	natural leaf	this work

The better water collection efficiency of the MIL surface over other reported materials can be possibly attributed to the orientation of the size of the surface and the microgroove channel, as stated in the literature. MILs can be considered a good candidate for water collection over other materials based on the results and the consistent rate of harvested water. In agreement with the hydrophilic theory, water is adsorbed enough onto the surface of the MIL, and due to the mentioned forces in the above literature, water is properly channeled toward the apex before finally dropping off for collection. In summary, the MIL has shown an excellent water collection ability when compared to the other reported biomimetic and natural surfaces listed in Table .

Conclusions

By turning waste into useful material, we exposed the surface of MILs to atmospheric vapor sprayed using commercial humidifier D20. The surface of the MIL has an amazing ability to capture and channel water efficiently for collection, which can be attributed to the surface microstructures, vein, apex, and curvature. The constant washing of the topmost layer (wax) of the MIL surface resulted in a change of the CA, thus exhibiting hydrophilic features both before and after water collection. In addition, water collection using MILs shows higher water collection efficiency than that using other reported surfaces. Overall, our results show that MILs can be a good candidate for water collection over other reported materials. In conclusion, when compared to other reported biomimetic and natural surfaces, the MIL provides an ideal water collection ability. The current results obtained from this novel material may be useful in harvesting atmospheric water in xeric regions, particularly in tropical regions where the MIL is regarded as an environmental waste or nuisance.