Gladiolus dalenii based Bioinspired Structured Surface via Soft

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Gladiolus dalenii based Bioinspired Structured Surface via Soft Lithography and its Application in Water Vapor Condensation and Fog Harvesting Vipul Sharma, Daniel Orejon, Yasuyuki Takata, Venkata Krishnan, and Sivasankaran Harish ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00815 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Sustainable Chemistry & Engineering

Gladiolus dalenii based Bioinspired Structured Surface via Soft Lithography and its Application in Water Vapor Condensation and Fog Harvesting Vipul Sharma,1# Daniel Orejon,2,3# Yasuyuki Takata,2,3 Venkata Krishnan1† and Sivasankaran Harish2* 1

School of Basic Sciences and Advanced Materials Research Center, Indian Institute of

Technology Mandi, Kamand campus, Mandi 175005, Himachal Pradesh, India. 2

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. 3

Department of Mechanical Engineering, Thermofluid Physics Laboratory, Kyushu

University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan # Both authors contributed equally to the work. Corresponding authors: † [email protected]; *[email protected]

Abstract Water collection via heterogeneous condensation and fog harvesting has important implications in everyday life and in several industrial applications. Recently, the unique combination of surface morphology and wettability exhibited by natural and biological species is receiving increasing attention from the scientific community. Surface morphology of such species exhibit unique micro- and nano-structure arrangements, which play a paramount role in water vapor condensation and fog harvesting. In this work, we focus on the design and replication of the bioinspired surface Gladiolus dalenii (G. dalenii) using inexpensive, facile and scalable soft lithography fabrication technique. The extent of microand nano-structure surface replication is evaluated using scanning electron microscopy and 3D laser optical microscopy. In addition, we compare the performance of G. dalenii leaf and Page 1 of 38 ACS Paragon Plus Environment

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its bioinspired replica during droplet condensation at the microscale using environmental scanning electron microscopy and optical microscopy and also its fog harvesting behavior. Droplet nucleation and growth is investigated in detail and correlated with the unique surface micro- and nano-structures arranged in a hierarchical manner on such surfaces when compared to smooth control sample. In addition, the different water collection performance on fixated and on replicated G. dalenii, as well as on the smooth control sample is compared and demonstrated by the surface energy analysis proposed. To conclude, by taking advantage of the unique G. dalenii surface morphology, this work successfully demonstrates the excellent condensation heat transfer and fog harvesting behavior of bioinspired functional surfaces fabricated using soft lithography when compared to the flat configuration. In addition, we also demonstrate the near-accurate replication of the micro-surface structures and of the governing mechanisms behind condensation and fog harvesting. Keywords: Bioinspired surfaces; Gladiolus dalenii; soft lithography; vapor condensation; heat transfer, fog harvesting

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Introduction In the past decade, the scientific community has devoted great interest to the study of natural and biologically active surfaces for several applications ranging from microfluidics to water-oil separation, anti-biofouling, cell culture, fog harvesting, and heat transfer, amongst others.1,

2, 3, 4, 5, 6, 7

Ever since their creation, biological surfaces have been evolving and

adapting as consequence of their interactions with the environment and with the biological matter present in the surroundings; achieving a higher state of “intelligent functionality”.1 Inspired by these surfaces, researchers have discovered unique topographies, functionalities and surface chemistries that can be applied to a wide range of applications.4,

7, 8, 9

For

example, bionic superhydrophobic surfaces with low adhesion enabling the self-propelling of droplets upon coalescence are included in the work of Gong et al., while one dimensional (1D) directional transport on natural and on artificial bioinspired surfaces has been reviewed in the works of Ju et al.10, 11 Recently, bioinspired surfaces have received increasing attention for their enhanced capabilities for water collection from fog as an alternative to conventional water harvesting technologies.3, 12, 13 A comprehensive review on the water collection on biomimetic surfaces inspired by nature can be found in the work of Zhu et al..14 In ambient environments, fog is a potential source of water that could be utilized by looking into the fog harvesting mechanisms of biological entities.5,

7

For example, there are many flora and fauna that can efficiently

harvest fog for their survival in the arid regions.15,

16

More specifically, looking into the

interactions between the Namib desert beetle and the surrounding environment, bioinspired surfaces with enhanced water harvesting capabilities have been developed.12 The shell of the Namib desert beetle possess a unique hierarchical arrangement of hydrophobic and hydrophilic areas enhancing the water harvesting from the environment.12,

17

Furthermore,

due to their multi-structural and multi-functional integrated spine collection system at the



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microscale plants such as grasses18, different varieties of cacti19 and other fog collecting plants15 have also been reported to possess excellent capabilities for fog and moisture water harvesting. In addition to these latter organisms, certain varieties of spider also produce spider silk that can efficiently harvest fog due to their characteristic fiber structure, which consists of naturally arranged periodic spindle-knots and joints.20 Typically, the driving force mechanisms governing the fog harvesting, on the natural systems described above, is the gradient of surface-free energy (difference in wettability) and the gradient of Laplace pressure induced by both surface structure and different wettability.10, 21, 22 By studying their performance during fog harvesting, researchers have developed many bioinspired surfaces and materials with improved water collection performance.23,

24, 25, 26, 27, 28

For instance,

through the balanced design and fabrication of hierarchical micro-/nano-structures and lowsurface energy, good progress has been made in the fabrication of robust surfaces with excellent fog and moisture harvesting capabilities.14, 29 Besides water harvesting, condensation heat transfer is another important area that has benefitted from studying the interactions between biological systems and the environment.30, 31, 32, 33

Water vapor heterogeneous condensation is a ubiquitous phenomenon observed in

nature and is present in many industrial applications such as: power generation34, thermal management and heat transfer35, water desalination36, organic solvent evaporation37, as well as in building environmental control38. . Depending on the wettability and structure of the surface, the two widely accepted condensation mechanisms are filmwise condensation (FWC) and dropwise condensation (DWC).39,

40

More recently, a simultaneous DWC and

FWC was achieved on a completely hydrophilic micropillared structure by carefully selecting the aspect ratio and the spacing between pillars.41 The use of patterned wettability has also been reported to prompt the co-existence of both DWC and FWC behaviors.42,

43

On a

wettability patterned micropillared surface, Orejon et al. achieved the migration of the


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condensate from the sides of a hydrophobic micropillar to their hydrophilic tops, which could be effectively utilized to shift the condensate form between pillars to the pillars’ tops.42 Current condensation surfaces used in industry relay in FWC as the condensation mechanism.34 However, the thickness of the liquid film increases the thermal resistance prompting the degradation of the heat transfer performance with the consequent increase in operational costs .34, 41 Hence, in the past decade, numerous studies have been dedicated to the design of surfaces that can efficiently attain the continuous nucleation, growth and departure of small droplets in a DWC fashion.41,

44

Amongst these surfaces,

superhydrophobic surfaces (SHSs), which are created by a combination of surface structure and hydrophobic wettability, have been found to display very low adhesion, i.e., very low contact angle hysteresis, and hence excellent droplet self-detachment properties.

44, 45

The

role of surface structure (shape and size of micro- and/or nanostructures), patterned wettability, presence or absence of non-condensable gases, have received increasing attention aiming for the enhancement of the condensation heat transfer performance.44, 45, 46, 47, 48, 49 The main focus of this work is the effective replication of both the surface structure and the mechanisms of water harvesting and water condensation of the biological specimen Gladiolus dalenii (G. dalenii). The G. dalenii surface was chosen on the basis of its unique wettability and droplet shedding behavior as a model surface after screening the different hydrophobic natural surfaces. Moreover, G. dalenii ornamental plant is found in many regions throughout all the seasons. Natural G. dalenii surface is hydrophobic and consists of a hierarchical array of micro- and nano-structures arranged in a well-defined manner. Researchers have developed various approaches for the fabrication of bioinspired artificial surfaces with structural and functionalized properties similar to those displayed by the natural surfaces.4, 8



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To meet the fast growing demand for efficient, inexpensive, scalable, durable and accurate replication of natural surfaces, novel approaches and techniques are required.27, 50 Templating is a well-established and efficient method for the replication of both micro- and

nano-structures present on natural surfaces.51 Amongst templating methods, soft-lithography based approach appears to be promising for the mass production of polymer surfaces with complex micro- and nano-scale structures displaying high aspect ratio.16, 20, 52 In addition, soft lithography provides with inexpensive and high precision replication procedure when compared to sophisticated photolithography or electron beam lithography techniques in which photoreactive surfaces are actually required. This technique is very well suited for different applications such as plastic electronics, biotechnology and applications involving the design of non-planar surfaces.53, 54 Soft lithography also offers easier pattern transferring approaches and choice of various materials when compared to other patterning techniques.1, 52 In this work, we replicate with reasonable accuracy the unique micro-and nanostructures of the G. dalenii using a facile soft lithography technique.52 We have used commercial epoxy resin polymer because of its anti-corrosion properties and its stability at moderate temperatures ca. 100°C, which could be applied for condensation and for fog collection in harsh environments.55 We then studied the vapor condensation on to the natural G. dalenii, on its replica, and on a flat smooth control sample, using environmental scanning electron microscopy (ESEM) and optical microscopy. ESEM technique was applied in light of its excellent spatial resolution, whereas optical microscopy was adopted as commonly well-stablished condensation technique for experimental observations. ESEM and optical experimental observations were used to establish the droplet growth before coalescence in the presence and absence of non-condensable gases, respectively. For the sake of rigorousness, the droplet nucleation and the droplet growth dynamics reported by both experimental techniques were satisfactorily replicated when comparing fixated G. dalenii and its replica. In Page 6 of 38 ACS Paragon Plus Environment

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addition, the excellent agreement between our experimental results on droplet growth with those of literature in the presence and in the absence of non-condensable gases is reported. On other hand, the fog collection efficiency of the G. dalenii surface, on the G. dalenii replica, and on the smooth control sample, were studied in detail and correlated with the surface micro- and nano-structures arranged in a well-defined manner. Fog harvesting experimental observations were also carried out to observe the droplet shape of millimeter droplets as they grow on the proposed surfaces. The shape of the droplets was found to orient/stretch differently depending on the surface studied. A surface energy analysis is proposed to demonstrate the different fog collection performance. The utility of natural G. dalenii surface and its replica for condensation heat transfer and fog harvesting applications when compared to the smooth control sample is then demonstrated for the first time. Knowledge envisage can be utilized for the design of efficient vapor condensation systems and for a variety of applications including fog harvesting.

Experimental Section Materials Gladiolus dalenii (G. dalenii) is an ornamental plant originated in the Middle East, which is widely found to grow in semi-arid and arid regions in several parts of the world. Samples from G. dalenii were collected in Mandi, Himachal Pradesh, India – (31° 42′ 25″ N, 76° 55′ 54″ E). Leaves samples were carefully screened and only the best and least damaged specimens were chosen. Then, to preserve the original properties of the natural G. dalenii, fresh samples were immobilized and fixated using glutaraldehyde phosphate buffer solution and glycerol from Sigma Aldrich, India. For the surface replication, polyvinylsiloxane (PVS), Coltene® light body (ISO 4823, Type 3, low consistency) and epoxy resin were purchased from Araldite, India.



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Fixated Gladiolus dalenii sample preparation Natural G. dalenii samples were freshly collected in the morning, washed with deionized water in the laboratory and then cut in equal sizes of 3 x 2 cm2. Due to the loss of water molecules, the original wetting properties and the morphology of the micro- and nanofeatures of the fresh G. dalenii sample will degrade shortly in time.56, 57 Then, in order to preserve the original properties of the G. dalenii from ambient exposure and time, a fixation procedure where leaf samples were immersed overnight in a 2% glutaraldehyde phosphate buffer solution was followed.58 Then, glycerol substitution process was applied to replace the water molecules using the procedure reported in the literature.58 Due to the limited electrical conductivity of the samples, to be able to perform Scanning Electron Microscopy (SEM) imaging, a few nanometers gold layer was deposited on the samples after the fixation procedure. Nonetheless, to preserve the intrinsic wettability of the fixated G. dalenii sample during condensation experiments, no coating of the leaves was carried out for experiments under Environmental Scanning Electron Microscopy (ESEM).

Replicated G. dalenii surface by soft lithography For the replication of G. dalenii surfaces, small pieces of 3 x 2 cm2 of previously fixated G. dalenii were attached to a glass slide using double sided tape. Then PVS was dispensed onto the sample followed by careful application of acute pressure over the PVS using another glass slide. PVS was allowed to cure for 15 minutes before peeling away the PVS molds with the help of tweezers. Special care was taken to make sure that any plant matter was also removed from the mold or negative replica. Molds were then washed with DI water and allowed to cure for further 3h. For the fabrication of the positive replica, epoxy resin and hardener were mixed using a mechanical mixer IKA RW-20 (Germany), and then the mix was evenly applied over the negative replica. Thereafter, a clean glass slide was


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carefully placed over the spread epoxy resin and allowed the resin to cure for 25 h. The positive replica was then removed from the negative replica using tweezers and washed with deionized water to remove any dust particles. The positive replica was then transferred to a desiccator before any further use. Control samples consisting of a planar (smooth) surface were fabricated by following the very same soft lithography methodology described above using a smooth silicon wafer instead.

Surface characterization Scanning electron microscopy (SEM) on the fixated G. dalenii and on the replicated G. dalenii samples coated with a nanometer layer of gold was carried out in a scanning electron microscope FEI Nova Nano SEM-450 from FEI Company (Hillsboro, Oregon, U.S.A.). In addition, in order to quantify the extent of surface replication by our soft lithography procedure of our proposed biological sample G. dalenii, 3D laser scanning microscopy was carried out in a LEXT OLS4000 Olympus 3D laser scanning microscopy (Japan). Topography data was then exported to Origin.

Condensation phase-change experiments Condensation experiments under both environmental scanning electron microscopy (ESEM) and optical microscopy were carried out. In the case of ESEM, experiments were carried out in a FIB-ESEM Versa 3D from FEI Company (Hillsboro, Oregon, U.S.A.). ESEM is equipped with a gaseous secondary electron detector (GSED) to rule out the interactions between the electrons and the water vapor molecules, and a Peltier stage to accurately control the temperature of the sample. The procedure is as follows. Firstly, the sample was fixed to a sample holder using double side copper tape. Thereafter, sample holder with the plant sample were placed on the Peltier stage. Then, ESEM chamber was closed and pump to high vacuum to remove any presence of non-condensable gases. At the same time and prior to



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experimental observations, the temperature of the Peltier stage was set at 1 °C for 10 minutes to ensure homogeneous temperature between the sample and the stage. Then, ESEM mode is initiated and water vapor pressure is increased at a rate of 50 Pa/min until first droplets nucleate on the surface, typically at 700 Pa. xT Microscope Control software was used to record the dynamics of condensation at a frame rate of ca. 0.3 fps during 30 minutes. On the other hand, experimental observations of condensation by optical microscopy were carried out in an environmentally controlled chamber PK-3KT (ESPERC Corp.). Environmental chamber has a high precision operation range of temperature between 10 °C and 70 °C, and of relative humidity from 30% to 100%.37 A custom-built Peltier stage connected to a cooling bath and to a PID controller was used to accurately control the temperature of the sample. Additional temperature measurements on the surface of the sample using an external thermocouple yielded differences less than ±1 °C. A high resolution zoom lens Keyence VH-Z500R coupled with a CCD camera were used for the optical microscopy observations. In the case of optical microscopy experimental observations, the procedure was as follows: G. dalenii sample of 1 x 1 cm2 was fixed on a copper block of 1 x 1 cm2 embedded on a PTFE block to ensure one dimensional (1D) heat transfer conduction. The ambient temperature and the relative humidity were set at 30 °C and 90%, respectively, for 30 minutes. Then, environmental chamber was turned off to avoid any vibrations, Peltier stage temperature was set at 10 °C and recording at 4.8 fps was initiated. Differences in ambient temperature and relative humidity observed along the duration of the experiments were within ±2 °C and ±5%, respectively. Schematic of the optical microscopy setup including environmental chamber, Peltier stage, PTFE-copper block, optical microscope, and CCD camera can be found in Figure 1a.

Fog collection experiments


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Fog collection experiments were done by exposure of the fixated and replicated G. dalenii samples and of the smooth control sample to the fine mist of ultra-pure water (18.2 MΩ-cm ELGA PURELAB Option-R7) generated by a cool mist ultrasonic humidifier (Bionaire BU1300W-I). Samples were suspended vertically and facing the humidifier at a distance of 15 cm. Air flow was estimated as ca. 130 ± 30 mL/h. The size of the droplets ranged from 1 μm to 5 μm,59 which are within the same range to those reported by Gürsoy et al.60 and comparable to natural fog.61 Then, the volume of water was collected and measured with a weight balance at intervals of 20 minutes over a period of 2 h. Three sets of experiments were performed on the three independent specimens and the average values along with the standard deviation were determined. Furthermore, to ensure the repeatability and the stability of the fabricated samples, all samples were subjected to 5 consecutive fog collection experiments. Schematic of the fog harvesting setup can be found in Figure 1b.

Figure 1 – Schematic of (a) environmental chamber, Peltier stage and optical microscopy setup for condensation experiments and (b) fog harvesting setup for water collection.



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Results and discussion Surface appearance and morphology G. dalenii is an ornamental plant of the iridaceae family, which is native to Africa and the sub-Saharan region. It typically presents sword-like leaves and it can reach heights ranging from 1.0 m to 1.2 m.62 Generally, the leaves of G. dalenii show unique hydrophobicity, which is due to the interplay between the intrinsic wettability of the cuticle film covering the epidermis leaf and the presence of surface micro-/nano-structures.58, 63 To characterize the surface morphology of fixated and replicated G. dalenii leaves, Figure 2 shows SEM images at different magnifications. Moreover, sessile contact angle snapshots and contact angle values on both fixated and replicated G. dalenii are also included as insights within Figure 2a&b, respectively. On both fixated G. dalenii and replicated G. dalenii, an intriguing array of micro- and nano-structures can be clearly observed (Figure 2a&b). The epidermis cells present at the top of the leaf form papillae of similar height and regular shape. These epidermal cells form random folds at some fixed intervals where the papillae apex becomes spherical giving rise to ogive-like morphologies (Figure 2c&d). On both fixated and replicated G. dalenii, ogive-like morphologies are regularly arranged in a unique hierarchy forming rows and columns. In addition to the micro-scale papillae structures, secondary nano-structures are also exhibited throughout the surface of both fixated and replicated G. dalenii leaves, as shown in Figure 2c,d,e,f. Magnified SEM images (Figure 2e&f) further confirms that the nano-structure consists of interlaced nano-plates arranged in a vertical position which range from several tens of nanometers in thickness to few hundreds of nanometers in breadth. The two-tier hierarchical roughness is composed by the micro-scale papillae and the vertical nano-plates, which provide the hydrophobicity to the surface.64, 65


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Figure 2 - SEM images of (a, c & e) fixated G. dalenii leaf and (b, d & f) replicated G. dalenii leaf. Insets in (a) and (b) show sessile droplet contact angle snapshots and measurements on fixated and on replicated G. dalenii, respectively. (c & e) Highresolution SEM images of fixated and (d & f) high-resolution SEM images of replicated G. dalenii.

Since micro- and nano-structures play a pertinent role in the functionality, robustness, and on the wettability of the surfaces, next we qualitatively compare the surface structure of the fixated to that of the replicated G. dalenii leaves. When comparing the surface microstructures, the excellent performance of the soft lithography replication procedure adopted is emphasized. Ogive-like structures and randomly arranged trenches with sizes in the order of few to tens of micrometers were satisfactorily reproduced on the surfaces in a well-defined



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array (Figure 2a&c versus Figure 2b&d). Furthermore, the curvature of these ogive-like structures was maintained, which is proven to be very important in inducing droplet nucleation.42,

66

Apart from the microstructures, the qualitative moderate replication of the

nano-plates arranged in a vertical position ranging from several tens of nanometers in thickness to few hundreds of nanometers was also observed. Nonetheless, the density of the nano-plates on the replicated G. dalenii was found to be considerably reduced when compared to the fixated one (Figure 2e-f). Although surface replication via soft lithography has been reported to reproduce features with vertical dimensions as small as few nanometers of a poly(methyl methacrylate) (PMMA) surface,52 the soft nature of the fixated G. dalenii makes infeasible such an accurate replication of the secondary nano-structures. A moderate replication of the number and the size of the nanostructures is reported. We note here that SEM snapshot included in Figure 2f was carried out without an additional gold coating layer. Despite the qualitative good accuracy on the replication of the surface microstructures, sessile droplet contact angle measurements differed from each other. Nonetheless, the hydrophobic nature of both fixated G. dalenii and replicated G. dalenii is emphasized. Sessile droplet contact angle measurements were carried out on a Phoenix 300 goniometer (Surface Electro Optics Co., Ltd, Korea). Three independent sessile droplet measurements were carried out and analyzed using Surfaceware 7 software. The equilibrium contact angles reported on fixated G. dalenii was 135° ± 4°, whereas on replicated G. dalenii was 109° ± 5° and on the smooth control sample was 91 ± 4°. Differences observed between fixated and replicated G. dalenii samples shall be attributed to the different intrinsic wettability of the cuticle film covering the epidermis of the fixated G. dalenii leaf when compared to the wettability of the epoxy resin and hardener used for the replication procedure. On other hand, since the surface chemistry of replicated G. dalenii and smooth control sample is the same,


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differences reported on the equilibrium contact angles shall be only attributed to the inclusion of micro- and nano-structures when compared to the smooth sample. The size, the aspect ratio and the arrangement of the micro- and nano-structures play an important role on surface wettability, and hence on the dynamics of condensation and fog harvesting.40,

41

To be able to quantitatively characterize the extent of microstructure

replication between fixated and replicated G. dalenii leaves, 3D laser scanning microscopy was carried out. Data extracted from 3D optical laser microscopy was imported into Origin. 3D surface color map and 2D surface profiles are shown in Figure 3:

Figure 3 – 3D optical laser microscopy topography on (a) fixated G. dalenii and (b) replicated G. dalenii. And 2D topography profiles on (c) fixated G. dalenii and (d) replicated G. dalenii. From topography data extracted by 3D optical laser microscopy, the size of the ogive-like micro-structures (diameter, d, and height, h), the depth of the trenches, tt, and the spacing in both the longitudinal, sl, and the transversal direction, st, were extracted for both fixated and replicated G. dalenii leaves, which are included in Table 1.



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Table 1 – Includes height, h (µm), and the diameter d (µm), of the ogive-like microstructures, the depth of cavities, tt (µm), and the spacing between ogives in the longitudinal direction, sv (µm), and in the transversal direction, sh (µm), for both fixated and replicated G. dalenii.

Height, h (µm) Diameter, d (µm) Cavity depth, tt (µm) Spacing longitudinal, sl (µm) Spacing transversal, st (µm)

Fixated G. dalenii

Replicated G. dalenii

10.2 ± 1.5 5.5 ± 0.4 6.2 ± 0.5 18.3 ± 2.5 36.6 ± 2.9

8.5 ± 0.5 4.8 ± 0.4 6.5 ± 0.5 18.6 ± 3.4 38.6 ± 4.7

The height and the diameter of the replicated G. dalenii were found to be 16% and 13% smaller when compared to the fixated one. Nevertheless, such differences are well within the calculated standard deviation. On the other hand, the depth of the cavities was replicated within a deviation of less than 5%. When comparing the arrangement of the microstructures in both the longitudinal and in the transversal directions, the agreement between fixated and replicated G. dalenii is remarkable. The calculated deviations were less than 2% and 6%, respectively. From Figure 2, Figure 3 and Table 1, the excellent microstructure surface replication and the moderate replication of the nanostructures is demonstrated. Besides the excellent micro- and moderate nano-structure surface replication performance of our soft lithography procedure, it is equally important to validate to which extent the mechanisms of condensation and those of fog harvesting are reproduced when comparing the fixated and replicated G. dalenii leaves.

Water condensation dynamics We now evaluate the condensation performance on both fixated and replicated G. dalenii by experimental observations of droplet nucleation and growth under environmental scanning electron microscopy (ESEM) and optical microscopy. On both fixated and Page 16 of 38 ACS Paragon Plus Environment

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replicated G. dalenii, nucleation takes place mainly at the base of the ogives-like structures where the energy for nucleation is minimized due to the characteristic concave curvature (Figure 4a&b).42,

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In addition, since nanoroughness also decreases the activation energy

required for nucleation, droplets also nucleate at the bottom of the surface.42 The initial droplet nucleation density calculated from 10 different independent experimental observations and regions was found to be (6.5 ± 2.1) x 109 and (5.2 ± 0.8) x 109, on fixated G. dalenii and on replicated G. dalenii, respectively. The initial droplet nucleation density on fixated and replicated G. dalenii surfaces agrees fairly well with each other and is found within the standard deviation. The slightly higher droplet nucleation density reported in the case of fixated G. dalenii is attributed to the greater density of nanostructured features at the base of the leaf and at the sides and tops of the ogive-like structures, which act as nucleation sites.42, 66 We note here that ESEM experimental observations were undertaken based on the high spatial resolution of this technique, whereas optical microscopy was adopted since it is the most widely accepted technique for imaging condensation. In addition, optical microscopy and ESEM were used to compare droplet growth in the presence and in the absence of non-condensable gases, respectively. Next, the droplet growth performance on fixated and on replicated G. dalenii leaves under ESEM in the absence of non-condensable gases is evaluated. Experimental observations of droplet growth as droplet diameter, D (µm), versus time, t (s), for several droplets on fixated G. dalenii, on replicated, G. dalenii and on the smooth control sample are included in Figure 4.



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Figure 4 – Characteristic ESEM snapshots of droplet growth during condensation on (a) fixated G. dalenii, (b) replicated G. dalenii and (c) smooth control sample. The droplet shape has been highlighted for more clarity. Log-log representation of droplet growth as droplet diameter, D (µm), versus time, t (s), on (solid symbols and solid lines) fixated G. dalenii, on (open symbols and dashed lines) replicated G. dalenii and on (crosses and dotted lines) smooth control sample, for different independent experimental observations at (d) bottom of the surface and (e) at the sides of the ogivelike structures. Insets in d & e show schematics of the droplet morphology. Solid yellow represents the region at which the growth rate D ≈ tµ was calculated, with µ as the power law exponent of droplet growth. Droplet growth rate was only compared within the time intervals at which experimental observations were clear. Measurements in the droplet diameter are considered within 20% error. Characteristic snapshots of individual droplets growing on fixated and replicated G. dalenii, and on control sample are included in Figure 4a,b,c, respectively. From Figure 4, it can be seen that after nucleation, droplets grow bigger displaying a similar spherical shape on Page 18 of 38 ACS Paragon Plus Environment

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all samples studied. We note here that despites the presence of a wettability gradient between the sides and the tops of the ogive-like structures,67 no major differences were observed when comparing droplets growing in different regions of the structured surface, i.e., side or bottom of the surface. As droplet grows with diameters above tens of micrometer they coalesce with each other and the droplet growth can be anticipated by water condensation breath figures.68 When comparing the time lapse evolution, growth rates are comparable on both surfaces. Figure 4d includes droplet diameter versus time for droplets growing at the bottom of the leaves, whereas Figure 4e accounts for droplets growing at the sides of the microstructures. On both cases (Figure 4d&e), the droplet growth follows the expected power model D ∼ Atµ 68, 69, 70

, where A is a constant and µ is the power law exponent. Typically µ ranges from 0 to 1

depending on wettability, surface structure, thermal conductivity and thickness of the surface, presence or absence of non-condensable gases, as well as on condensation conditions, i.e., temperature difference between ambient and the surface.49 When comparing the growth exponents of droplets at the bottom of the leaves, the average power law exponents were found as µfixated = 0.63 ± 0.18 and µreplicated = 0.87 ± 0.08. The greater droplet growth rates reported in the case of replicated G. dalenii are attributed to the more wetting behavior and hence the lower heat transfer resistance through the condensing droplets, which in turn enhances the condensation rate and the heat transfer.40,

49, 71

In the absence of non-

condensable gases, the good agreement between µreplicated = 0.87 ± 0.08 and µ = 1 suggests that droplet growth is limited by the heat transfer resistance at the liquid-vapor interface.40, 49 On the other hand, the droplet growth rate on the smooth control sample was µcontrol = 0.70 ± 0.04. In this case, when comparing replicated G. dalenii with control sample, greater droplet growth rates on the replicated G. dalenii can be attributed to the greater surface area for heat transfer due to the presence of micro- and nano-structures. Droplet growths reported in both Figure 4 and Figure 5 include different condensation/evaporation cycles, which demonstrates



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the reproducibility of the results under the condensation conditions studied. In addition, some of the data included in Figure 4e for replicated G. dalenii was successfully reproduced after 8 months from the initial experimental observations. We must note here that droplet growth rates reported along the manuscript could be influenced by the temperature difference between the ambient and the substrate.40 Nonetheless, no major differences were observed on experimental growth rates measured on two different replicated G. dalenii with different sample thicknesses. The different sample thickness should in turn induce a different temperature at the surface top and hence different evaporation rates. In addition, our condensation setups were designed in order to ensure homogeneous conditions during our experimental observations. We now compare the growth of droplets at the sides of the ogive-like microstructures included in Figure 4e. At the microstructures sides, the droplet growth rate is considerably decreased. The power law growth rate exponent reported for fixated G. dalenii and for replicated G. dalenii are almost half of those previously reported and equal to µfixated = 0.42 ± 0.14 and µreplicated = 0.41 ± 0.07. The deviation between fixated and replicated samples is within less than 5%. The low growth rates in the case of droplets growing at the sides of the microstructures are attributed to the greater heat transfer resistance across the ogive-like structures. In addition, we also compare experimental observations of water vapor condensation by optical microscopy carried out in an environmental controlled chamber in the presence of non-condensable gases (schematics of the setup can be found in Figure 1a). Figure 5 includes experimental observations of droplet growth as droplet diameter, D (µm), versus time, t (s), and the droplet growth analysis for several condensing droplets. We must note here that observations by optical microscopy did not allow for the differentiation between droplets growing at the bottom of the substrate and those growing at the sides of the microstructures.


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Figure 5 – Characteristic optical microscopy snapshots of droplet growth during condensation on (a) fixated G. dalenii and (b) replicated G. dalenii. The droplet edges have been highlighted for more clarity. (c) Log-log representation of droplet growth as droplet diameter, D (µm), versus time, t (s), on (solid symbols and solid lines) fixated G. dalenii and (open symbols and dashed lines) replicated G. dalenii. Solid yellow represents the region at which the growth rate D ≈ tµ was calculated, with µ as the power law exponent of droplet growth for early growth rate regime and developed growth rate regime. Diameter measurements deviation is considered within 10% error.

Figure 5a&b shows characteristic time lapse optical microscopy snapshots of droplets growing on fixated and on replicated G. dalenii, respectively. The qualitative behavior during droplet growth under optical microscopy is similar to that reported under ESEM conditions where droplets keep a spherical shape throughout condensation for the time intervals analyzed. Figure 5c represents the evolution of the droplet diameter in time for both fixated and replicated G. dalenii. In this case, we report two different regimes of droplet growth. At the early regime of condensation, the power law growth exponents reported are quite close to each other as: µfixated = 0.33 ± 0.06 and µreplicated = 0.31 ± 0.16. Results remarkably agree with Page 21 of 38 ACS Paragon Plus Environment


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droplet growth scaling limited by diffusion in the presence of non-condensable gases for small droplets: D ∼ t1/3.49 For longer condensation times, the droplet growth exponent increases to µfixated = 0.80 ± 0.15 and µreplicated = 0.74 ± 0.14, which are very close to each other and are well within the standard deviation. From the above results and analysis under both ESEM and optical microscopy, we report the excellent replication of the mechanisms of droplet condensation and of the dynamics of droplet growth when comparing fixated and replicated G. dalenii surfaces. Since the heat transfer performance of a condenser surface is typically proportional to both the nucleation density and the heat transfer through the droplet40, 45, 49, we can safely assume that the heat transfer performance on both surfaces can be considered analogous. Even though the surface wettability of fixated and replicated G. dalenii are different, we can conclude that the soft lithography replication method used in this study can physically replicate the surface structure and the condensation mechanisms with quite excellent accuracy. We also must note the faster droplet growth rates upon the inclusion of microstructures on the replicated G. dalenii sample when compared to the smooth control sample under ESEM conditions. Since the surface chemistry of the replicated G. dalenii is the same as that of the smooth control sample, the greater droplet growth rates reported shall be attributed solely to the presence of micro- and nano-structures. To compare the efficacy on water droplet removal we also compare the fog harvesting performance on the fixated and on the replicated G. dalenii leaves, as well as on the smooth control sample. Next, fog harvesting experimental results and surface energy analysis demonstrating the better performance upon the inclusion of micro- and nano-structures when compared to the smooth control sample are presented.


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Fog harvesting performance To evaluate the water droplet removal efficiency, fixated G. dalenii, replicated G. dalenii and smooth control epoxy resin samples were subjected to fog harvesting experiments. Details and schematic representation of the experimental setup used for fog harvesting can be found in Figure 1b. Characteristic snapshots of fog harvesting experiments on fixated and on replicated G. dalenii leaves and on the smooth control sample at different time intervals are represented in Figure 6a,b,c respectively.

Figure 6 – Snapshots of fog harvesting experiments on (a) fixated G. dalenii, (b) replicated G. dalenii, and (c) smooth control sample at 0, 1, 3 and 5 minutes. Scale bars are 1 cm.



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Upon impact of the water droplets suspended in the fog with the surface, first droplets are collected on the surface by heterogeneous coalescence. Then, as fog harvesting experiment develops, suspended droplets reaching the surface now coalesce with already collected droplets. After 1 minute, droplets with sizes ranging from hundreds of micrometers to few millimeters can be observed. Typically, the shape of a droplet on an inclined surface is dictated by the contact angle hysteresis, i.e., by the advancing and the receding contact angles, which in turn depends on the surface wettability and on the surface structure.72 As reported during droplet condensation experiments (Figure 4 and Figure 5), small droplets can be considered almost spherical in shape. However, as droplets grow with sizes similar and above the capillary length, lc (lc = (γlg/ρg)0.5 where γlg is the surface tension liquid-gas, ρ is the density of water and g is the gravity), the effect of gravity comes into play and the droplet shape deforms.73 From figure 6b&c, it can be appreciated that droplets with sizes above few millimeters deviate considerably from the ideal spherical cap shape. It is worth mentioning, that these droplets orientate differently on fixated G. dalenii when compared to replicated G. dalenii and to control sample. In the case of fixated G. dalenii droplets orient/stretch in the vertical direction under the effect of gravity, whereas on replicated G. dalenii droplets orient/stretch in the horizontal direction. The different shape adopted by the droplets when comparing fixated and replicated G. dalenii is attributed, presumably, to the different hydrophobicity induced by the cuticle layer of the fixated G. dalenii leave when compared to the polymeric nature of the replicated one. Furthermore, droplets bigger in volume can be appreciated on fixated G. dalenii, which will have direct implications on the forces acting on the droplet, as it will be further discussed. As fog harvesting develops once droplets become large enough, they eventually detach with the aid of gravity.39, 40The different droplet shapes reported will require further exploration to be able to assess the impact of vertical and horizontal forces action on the drops.


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The water removal performance on the different surfaces as amount of water collected, is directly related to the different forces acting on the droplet and to the droplet adhesion to the surface. In order to quantify the fog harvesting performance on fixated and on replicated G. dalenii leaves, the amount of water detaching from the surface is collected in a flask and weighted every 20 minutes as included in figure 7.

Figure 7 - Water harvesting dynamics as volume of water collected per square meter, V (mL m-2), in time, t (min), over a surface area of 6 cm2 for (squares) fixated G. dalenii, (circles) replicated G. dalenii and (c) smooth epoxy resin control sample. Standard deviation of three independent measurements is also included along. Throughout two hours of fog harvesting experiments, fixated G. dalenii sample collected 3.3 ± 0.1 mL of water, whereas the water collected by replicated G. dalenii was 2.2 ± 0.2 mL. For the sample size of 6 cm2 studied in our experiments, the amount of water collected per square meter was found to be equivalent to 5.5 L/m2 and to 3.7 L/m2 for fixated G. dalenii and for replicated G. dalenii, respectively. On the other hand, the smooth epoxy resin control sample collected ca. 1.4 L/m2. The better performance of the fixated G. dalenii surface is attributed to the greater hydrophobicity inducing lower adhesion force and hence Page 25 of 38 ACS Paragon Plus Environment


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lower contact angle hysteresis when compared to the replicated one. The more hydrophobic nature of the cuticle covering the epidermis of the leaf when compared to the polymer used for the replication is considered the main reason for the different water shedding and collection behavior. The different material and wettability alter considerably the shape of the droplets and hence the detachment behavior. Nonetheless, when comparing replicated G. dalenii sample to smooth control sample made of the same epoxy resin, we emphasize that the 230% greater fog harvesting performance (g m-2 h-1) on replicated G. dalenii is solely due to the inclusion of the micro- and nanostructures. The greater fog collection performance on replicated G. dalenii is experimentally demonstrated, however we must note here that the collection performance of replicated G. dalenii is decreased by 33% when compared to fixated one. In order to improve the fog collection performance of the replicated G. dalenii, different approaches such as surface functionalization by initiated chemical vapor deposition (iCVD) or functional nanoimpriting could be applied.74, 75 In water collection and condensation heat transfer applications, the surface should perform over a prolonged period of time and over a number of cycles. Therefore, we tested our substrates for repeatability studies. Same substrates were utilized for water collection over a number of cycles, which are shown in Figure 8. After each cycle, all substrates were dried naturally at room temperature in a desiccator. We note here that we did not considered the natural G. dalenii sample, since drying procedure would lead to the shrinkage of the micro- and nano-features and to the degradation of the original wetting properties.56, 57 The data presented in Figure 8 shows that the overall quantity of water collected over 2 h remains quite uniform even after 5 cycles


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Figure 8 – Reusability of the surfaces for water harvesting studies over a surface area of 6 cm2 as water collected (mL m-2) during 2 h over cycle number for fixated G. dalenii, replicated G. dalenii and smooth control sample.

Energy Analysis Next, we evaluate the mechanisms of droplet shedding during fog harvesting experiments. Droplet growth takes place by impingement of the droplets suspended in the fog with the surface and with already nucleated droplets. As droplets grow bigger, coalesce with neighboring ones takes place. We report that the shape of the new coalesced droplet is highly dependent on the wetting behavior of the surface. The water removal performance, i.e., amount of water collected from the fixated and from the replicated G. dalenii surfaces, is then directly related to the wetting properties, to the surface structure and to the different forces acting on the droplet. For a droplet to detach, the gravitational force acting on the droplet, Fg can be calculated as72, 76:



Fg = ρV g s i n α

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(1)

must overcome that of adhesion, Fa calculated as72, 76: Fa = γ l g D ( c o s θ r e c − c o s θ a d v )

(2)

which is proportional to the contact angle hysteresis,16, 20, 72 where γlg is the surface tension liquid-gas, ρ is the density, and V and D are the droplet volume and the droplet diameter, respectively. θadv and θrec are the advancing and receding contact angles, and α is the inclination angle, which in our case is 90°. Due to the different surface chemistry, which cannot be replicated, the contact angle hysteresis on fixated G. dalenii (θadv,fixated = 146° ± 4°, θrec,fixated = 119° ± 4°) is considerably lower than on replicated G. dalenii (θadv,fixated = 122° ± 4°, θrec,fixated = 91° ± 4°). This suggests that for the same droplet size, the greater contact angle hysteresis exerts a greater force of adhesion on the replicated G. dalenii leaf when compared to fixated one, hindering droplet self-removal.

Besides the rudimentary force analysis based on the contact angle hysteresis at the droplet triple contact line presented above, we propose a comparison of the surface energy of adhesion, Eadh, between the different surfaces studied and for a given droplet volume. The energy of adhesion based on surface energy principles for a partially wetting droplet is calculated as 45, 77: Ea d h = π γ s l rb 2

(3)

where γsl is the surface tension solid-liquid and rb is the droplet base radius in contact with the surface. Based on the advancing contact angle (rb = sinθadvR), Equation 3 can be rewritten as: Ea d h = π γ s l s i n 2 θ a d v R 2

(4)

where R is the radius of the droplet and θadv is the advancing contact angle. Then the ratio energy of adhesion on the fixated G.dalenii to that of the replicated, Eadh,fix / Eadh,rep can be expressed as follows: Page 28 of 38 ACS Paragon Plus Environment

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Ea d h, f i x Ea d h ,r e p

π γ s l , f i x s i n 2 θ a d v, f i x R 2f i x φ = π γ s l ,r e p s i n 2 θ a d v ,r e p Rr2e p

(5)

where Ø is the ratio of effective surface area on fixated G. dalenii to replicated G. dalenii. Ø was calculated from 2D topographic profiles included in Figure 3 and from Table 1 and equals 1.13. Ø is greater than one due to the slightly bigger size of the microstructures on fixated G. dalenii inducing a greater effective surface area. Then, assuming that droplets display spherical cap shape and for a same given droplet volume, i.e., Vfix = Vrep, and hence Fg,fix = Fg,rep, the radius ratio Rfix to Rrep can be obtained as: Rfix / Rrep = 0.959. We note here that we approximated the droplet volume to a spherical cap using the advancing contact angles reported. Then, Ea d h , f i x Ea d h , r e p

=

γ s l, f i x 0.452 γ s l ,r e p

(6)

Typically, γsl,fix and γsl,rep depends on the surface energy balance of the binary interactions solid-liquid, liquid-gas and solid-gas at the triple contact line73. Since the surface tension solid-gas is unknown, we cannot, at the current stage, estimate γsl,fix and γsl,rep. We note here that if assuming a similar value for γsl,fix and for γsl,rep, for the same droplet volume, droplets collected on replicated G. dalenii display up to 120% greater adhesion than on fixated one. The above analysis supports the reported 50% decrease on the fog collection performance on replicated G. dalenii sample when compared to fixated one.

Next, following the same energy of adhesion approach reported from Equation 3 to Equation 5, we estimate the ratio energy of adhesion on replicated G.dalenii to that of the epoxy resin control sample, Eadh,rep / Eadh,cont. In this case, Ø equals 1.214 due to the presence of microstructures when compared to absence of structures on the control sample. And Rrep / Rcont = 0.88 for an advancing contact angle of the control sample of 96º. Then



Ea d h ,r e p Ea d h , c o n t

=

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γ s l ,r e p 0.684 γ s l ,c o n t

(7)

When comparing replicated G. dalenii and control sample, since the surface chemistry is the same, we can then assume the same solid-liquid surface tension, i.e., γsl,rep = γsl,cont. Then, the adhesion on the control sample is up to 46% greater when compared to replicated G. dalenii. The greater adhesion of droplets on the control sample supports the lower fog collection values reported on the control sample when compared to the replicated surface. The 230% enhancement in the fog collection performance reported upon the inclusion of micro- and nanostructures inspired on the replicated G. dalenii leaf when compared to the smooth control sample is highlighted and demonstrated by our surface energy analysis. Next, in Table 2 we compare recent reported bioinspired surfaces including material and fabrication procedure, type of experiment and experimental conditions, and fog collection or condensation efficiency.

Table 2 - Comparison of fog collection and/or condensation performance of different bioinspired surfaces reported in the literature No.

Bioinspired Surface

1

Microstructured surfaces and mesh

Polyolefin mesh and resin epoxy plus hydrophobizing agent

Fog harvesting with fog flux of 400 ml/h at 8 cm distance

2

Bioinspired surfaces with star-shaped wettability patterns

Fog harvesting 1 - 40 μm dropletss at 0.75 m/s

3

Inkjet-printed micropatterned superhydrophobic surface Biomimetic coatings on surfaces

TiO2 slurry and Heptadecafluorodecyltrimethoxysilane, then photomask and UV light for hydrophilic spots Spin coating Polystyrene plus hydrophobic coating, then polydopamine by inkjet printer Spin coating Polystyrene followed by a layer of poly(4-vinylpyridine) then annealing

4

5

Bioinspired micropatterned surfaces using roll-type photolithography

Material & Fabrication

UV curable-polyurethane functionalized by vapor phase hydrophobic coating and DUV roll type

Type of Experiment

Fog harvesting & condensation

Fog harvesting with fog flow from 84 L/h to 600 L/h & condensation at 20 K subcooling Condensation at 22 K subcooling at ambient temperature and 70% RH and fog harvesting

Fog collection (g cm -2 h-1)

Ref.

~ 0.107 for hydrophobic ~ 0.199 for superhydrophobic ~ 2.78 Superhydrophobic star pattern treated by UV light

78

~ 0.062 for patterned superhydrophobic 500 μm - 1000μm ~ 0.1 for P4VP dewetted surface at 84 L/h ~ 0.34 at 600 L/h

26

~ 0.04 for condensation ~ 0.171 fog harvesting

80

25

79


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6

7

8 9

10

11

Bioinspired micropatterned surfaces using photolithography Heterogeneous rough conical wires Bioinspired hybrid micro/nanopatterned surface Biological hierarchical surface

Biological and bioinspired asymmetric-anisotropic hierarchical surface Biological and bioinspired micro/ nanopatterned surface

Si etched by CF4 plasma then Hexamethyldisiloxane coating. Superhydrophilic plus air plasma 3 min Cu wire 800 μm polished, then electrochemical corrosion by periodic current Polytetrafluoroethylene by femtosecond laser of PTFE nano particles on Cu mesh Cynodon dactylon dried and glycerol substitution

Erempophyrum Orientale plant, replica by epoxy resin and replica and iCVD Soft lithography epoxy replication of G. dalenii natural surface

Condensation with ca. 12 K subcooling at 90% relative humidty Fog harvesting velocities from 0.8 m/s to 2.4 m/s Fog harvesting velocities from 0.1 m/s at 8 cm Fog harvesting with same experimental conditions as in the present work Fog harvesting at 600 L/h of mist droplets at 11 cm distance at 23 ºC Fog harvesting

~ 0.016 superhydrophobic ~ 0.024 smooth hydrophilic ~ 0.27 at 0.8 m/s ~ 0.6 at 2.4 m/s ~ 0.08 Cu mesh ~ 0.2 Cu mesh PTFE ~ 0.096 on natural biological sample

81

~ 0.006 – natural sample ~ 0.011replica treated by iCVD ~ 0.183 for replica G. dalenii ~ 0.275 for fixated G. dalenii

60

29

13

16

This work

From Table 2, the better performance of our replicated G. dalenii when compared to other approaches such as patterned hydrophilic-superhydrophobic surface fabricated by inkjet printing26, to functionalized UV curable-polyurethane80, to Cynodon dactylon biological sample16 or to Erempophyrum Orientale biological and replicated samples treated by iCVD60, is reported. In other cases, the fog collection efficiency is comparable to our replicated sample. However, such fabrication approaches require the functionalization of the surface using a hydrophobic coating, which stability and durability in time are an issue.13, 41, 79, 80, 81 The highest of the values reported in Table 2, ca. one order of magnitude to that reported in our study, requires the functionalization of the surface by a hydrophobic coating and then the induction of the superhydrophilic spots by UV light.25 However, if not continuously exposed to UV light superhydrophilicity is lost over time, hence such surfaces cannot maintain its fog collection efficiency in time. In view of the above studies, we report that the surfaces based on G. dalenii without the assistance of a hydrophobic coating are highly efficient surfaces for water collection in comparison to the other reported bioinspired surfaces. Furthermore, the improved fog collection efficiency upon the inclusion of micro-/nano-structures and for the same surface Page 31 of 38 ACS Paragon Plus Environment


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chemistry, when comparing replicated G. dalenii and smooth control sample is also demonstrated. In summary, surfaces inspired on G. dalenii offers very good fog collection efficiency per unit area at a steady rate, which makes them a very promising candidate to be utilized for fog harvesting devices.15, 16, 26, 78

Conclusions Bioinspired functional surfaces based on the Gladiolus dalenii plant and its replica were studied in detail for the fog harvesting and condensation heat transfer applications. The soft lithography replication procedure adopted was found to successfully reproduce the unique surface topology of the micro- and nano-structures. In addition to the effective surface topology replication, the condensation mechanisms, i.e., nucleation and droplet growth, on fixated and replicated leaves were found mostly identical. The similar droplet nucleation density and droplet growth rate on both fixated and replicated G. dalenii suggests the same heat transfer performance during condensation phase-change. Both experimental deviations of droplet nucleation density and droplet growth were found within the standard deviation of the observations. Furthermore, a direct correlation was established between the unique surface micro- and nano-structures of these surfaces and the fog collecting efficiency. A 230% enhancement on the fog harvesting performance is reported upon the inclusion of the unique arrangement of micro- and nano-structures inspired on the G. dalenii leaf when compared to the smooth control sample. In addition, our surface energy analysis supports the greater fog collection performance of G dalenii replica when compared to smooth control sample solely due to the inclusion of the unique micro- and nano-structures. Up to 46% greater droplet adhesion is reported in the case of the smooth control sample when compared to replicated G. dalenii. We propose the use of soft lithography procedure and the replication


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of G. dalenii bioinspired leaves to create functional surfaces for enhanced condensation and fog harvesting performance without the assistance of a hydrophobic coating.

Acknowledgements We are thankful to Advanced Materials Research Center (AMRC), IIT Mandi and International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Japan for characterization facilities. V.K. acknowledges financial support from Department of Science and Technology, India under Young Scientist Scheme (YSS/2014/000456). V.S. acknowledges doctoral scholarship from Ministry of Human Resource Development (MHRD), India. S.H. acknowledges the support of JSPS KAKENHI JP16H07043 and D.O. acknowledges the support of JSPS KAKENHI JP16K18029.

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TOC

Bioinspired surfaces based on Gladiolus dalenii were studied for their utilization in water vapor condensation and fog harvesting applications


Gladiolus dalenii based Bioinspired Structured Surface via Soft

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