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Fabrication of Biomimetic Fog-collecting SuperhydrophilicSuperhydrophobic Surface Micropatterns Using Femtosecond Lasers Elisabeth Kostal, Sandra Stroj, Stephan Kasemann, Victor Matylitsky, and Matthias Domke Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03699 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018
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Fabrication of Biomimetic Fog-collecting SuperhydrophilicSuperhydrophobic Surface Micropatterns Using Femtosecond Lasers Elisabeth Kostal*,a, Sandra Strojb, Stephan Kasemanna, Victor Matylitskyc, Matthias Domkeb a Research Center for Microtechnology, Vorarlberg University of Applied Sciences, Hochschulstr. 1, Dornbirn, 6850, Austria b Josef Ressel Center for material processing with ultrashort pulsed lasers, Research Center for Microtechnology, Vorarlberg University of Applied Sciences, Hochschulstr. 1, Dornbirn, 6850, Austria c Spectra-Physics, Feldgut 9, Rankweil, 6830, Austria The exciting functionalities of natural superhydrophilic and superhydrophobic surfaces served as inspiration for a variety of biomimetic designs. In particular, the combination of both extreme wetting states to micropatterns opens up interesting applications, as the example of the fog-collecting Namib Desert beetle shows. In this paper, the beetle’s elytra was mimicked by a novel three-step fabrication method to increase the fog-collection efficiency of glasses. In the first step, a double hierarchical surface structure was generated on Pyrex wafers using femtosecond laser structuring, which amplified the intrinsic wetting property of the surface and made it superhydrophilic (water contact angle < 10°). In the second step, a Teflon-like polymer (CF2)n was deposited by a plasma process that turned the laser-structured surface superhydrophobic (water contact angle > 150°). In the last step, the Teflon-like coating was selectively removed by fs-laser ablation to uncover superhydrophilic spots below the superhydrophobic surface following the example of the Namib Desert beetle’s fogcollecting elytra. In order to investigate the influence on the fog-collection behavior, (super)hydrophilic, (super)hydrophobic, low and high contrast wetting patterns were fabricated on glass wafers using all reasonable combinations of these three processing steps and exposed to fog in an artificial nebulizer setup. This experiment revealed that high contrast wetting patterns collected the highest amount of fog and enhanced the fog-collection efficiency by nearly 60% compared to pristine Pyrex glass. The comparison of the fog-collection behavior on the six samples revealed that the superior fog-collection efficiency of surface patterns with extreme wetting contrast is due to the combination of water attraction and water repellency: the superhydrophilic spots act as drop accumulation areas, whereas the surrounding superhydrophobic areas allow a fast water transportation caused by gravity. The presented method enables fast and flexible surface functionalization of a broad range of materials including transparent substrates, which offers exciting possibilities for the design of biomedical and microfluidic devices. Introduction Water scarcity is one of the major global problems, affecting every continent and more than 700 million people today.1 Interestingly, precipitation occurs in many arid regions mainly in the form of dew and fog;2 a circumstance to which the local flora and fauna has adapted by developing functional surfaces.3 Besides several plants4,5 and reptiles6, some species of Darkling beetles (Tenebrionidae) are known for their fog-harvesting ability.7 The beetles are native to the Namib Desert, where the cold Benguela current runs across the coast line and regularly causes fog banks rolling over the dunes.8 In order to harvest water from fog, the beetle tilts its back towards the fog-laden wind7, droplets attach to the hydrophilic bumps on its elytra, coalesce to large water drops until they are detached by gravity, and roll to the beetle’s mouth directed by the hydrophobic valleys (see Figure 1).9
Figure 1. Fog-collecting principle of the Namib Desert beetle: droplets from the fog-laden wind accumulate on the hydrophilic peaks until they reach a critical size and roll down the hydrophobic valleys towards the beetle’s mouth.
Most literature dealing with the topic of mimicking the beetle’s functional surface, refer to the work of Parker and Lawrence10, who reported on the active fog-harvesting behavior of Stenocara gracilipes. For the sake of completeness it should be mentioned that they referred to the wrong beetle, the only species known to assume a fogbasking stance is Onymacris7,11, which has a slightly different surface pattern.7
Nevertheless, many research groups picked up the general idea of using wettability patterns to improve fogcollection efficiency and investigated the influence of different surface structures and wettability properties.9,12– 20 However, the results of these studies are contradictory. On the one hand, Garrod et al. found that patterned surfaces with high wettability contrast outperform surfaces with uniform wetting properties by several orders of 1
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magnitude12, which was also confirmed by Bai et al.18 On the other hand, Azad et al. measured only an increase by a factor of 219. Besides, White et al. could not measure any benefit at all for their hydrophilic-hydrophobic patch, dot and channel patterns.14
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pulse durations and high intensities, thermal side effects are reduced to a minimum and non-linear absorption can be initiated in nearly any material, respectively. Consequently, transparent and brittle materials such as glasses can be processed with minimal thermal side effects, e.g. cracks and melt burrs. Moreover, LIPSS tend to melt together to larger structures when using longer pulse durations.59 Summarized, ultrafast lasers seem to be an ideal choice for the generation of double hierarchical surface structures on any substrate.
Complementary to the exploitation of fog as a source for drinking water, several authors concentrated on the collection of dew.21–25 In contrast to fog, where droplets simply collide with the surface, dew involves the condensation of water vapor.21,23 Since the drop removal mechanism for fog and dew collection is very similar, some interesting findings can be deducted from these studies. Choo et al. for example found that the collection efficiency of high contrast wetting patterns shows little sensitivity to the angle of inclination and that dot patterns are most effective.26 A common concept that is agreed upon is that the combination of efficient drop accumulation and fast drop removal is crucial to achieve high fog- or dew-collection rates,12,23,27 which can be achieved by combining superhydrophilic and superhydrophobic surfaces to patterns of extreme wetting contrast.12
Contrasting (super)hydrophilic-(super)hydrophobic wetting properties can be assigned to the structured surface by applying a coating that switches its wetting state locally. This can be done either in a single step by selective deposition9,12,13,15,27 or in two steps by coating the complete surface followed by selective etching.60–62 The latter has the advantage that it can be performed using maskless methods such as laser ablation14,60, which provide also more flexibility in the design of micropatterns. Because most substrate materials are intrinsically hydrophilic, the coatings should typically be hydrophobic. Besides other low surface energy layers63,64 it has been demonstrated that fluorinated carbon layers are well suited for the fabrication of superhydrophobic surfaces.65–67
In order to generate micropatterns with contrasting wetting properties on the same surface, a local change of morphology, chemistry, or both is necessary according to the Wenzel28 and the Cassie-Baxter-Model.29 While surface chemistry determines whether a surface tends to be hydrophobic or hydrophilic, its morphology is usually responsible for the degree of hydrophobicity or hydrophilicity.30 This concept is underpinned by natural examples like the superhydrophobic Lotus leaf (Nelumbo nucifera), which combines double hierarchical surface morphologies with low surface energy coatings.31 In contrast, superhydrophilic leafs are covered with high surface energy layers.32 Interestingly, both kinds of plant surfaces have similar surface morphologies, the only decisive difference is the coating.32,33 This indicates that superhydrophilicsuperhydrophobic micropatterns can be fabricated on the same (hierarchical) surface morphology only by changing surface chemistry.34–36
The ability to fabricate surface micropatterns with extreme wetting contrast e.g. on polymers or glasses offers exciting possibilities68 in particular for biomedical and microfluidic applications.69 Besides polymers, glass is the material of choice70 due to its chemical inertness, low permeability and optical transparency.71 However, a laserbased method for the generation of extreme wetting contrasts on glass substrates has not yet been achieved to the knowledge of the authors. The goal of this study was to mimic the Namib Desert beetle’s elytra by fabricating high contrast wetting patterns on glasses and to evaluate their fog-collection efficiency. For this purpose, the patent pending ClearSurface™ process, which combines fs-laser micromachining and surface coating was applied. Fs-laser microstructuring was used to generate double hierarchical surface structures, then a Teflon-like coating was applied to switch the wetting state from superhydrophilic (CA < 10°) to superhydrophobic (CA > 150°), and finally selective ablation was used to recover the superhydrophilic state locally. Further samples were fabricated by leaving out one, two or all of these process steps. To evaluate the relation between surface wettability and fog-collection efficiency, a total set of six samples, was compared using an artificial nebulizer setup.
Various methods to mimic hierarchical surface morphologies can be found in literature, e.g. chemical etching techniques37–39, deposition of nano-spheres onto microstructures40,41, and ultrashort pulse laser ablation.42–49 Among all these methods, laser processing has several advantages. First of all, it is a maskless process which enables direct writing of arbitrary geometries. Moreover, double hierarchical surface structures are very easy to fabricate. The focus diameter, which is in the order of a few to tens of µm, defines the track width, and selforganizing laser-induced periodic surface structures (LIPSS)50–52 with a typical size and period in the sub-µm range39 form inside the laser tracks. Therefore double hierarchical structures can be fabricated just by translating the beam in parallel or crossed lines across the sample. This feature has already been applied to manufacture superhydrophobic steel53, glass42,54, silicon55–57 and superhydrophilic surfaces.58 Especially, the use of ultrafast lasers shows several advantages. Due to their ultrashort
Experimental Section Fabrication of superhydrophilic-superhydrophobic micropatterns. The substrates used were four-inch borosilicate glass wafers with a thickness of 500 µm (BOROFLOAT®33). Because the wafers were directly taken out of the cleanroom packaging, they did not undergo any cleaning treatment prior to machining. 2
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Micropatterned surfaces were fabricated in three process steps, as illustrated in Figure 2. In the first step (see Figure 2 a), the surface was structured by a fs-laser (Spirit®, Spectra-Physics) with a pulse duration of 380 fs and a repetition rate of 200 kHz. The linearly polarized laser was operated at a wavelength of 520 nm. A 100 mm telecentric lens was used to focus the beam onto the sample surface, the resulting focus radius was determined to be approximately 6 µm using the method of Liu.72
ure 2 b). The passivation step of a standard BOSCH® DRIE plasma etching process was applied using an ICP silicon dry etching system (adixen AMS 100 iSpeeder). The C4F8 plasma was ignited at a pressure of 2.2·10-2 mbar. RF (13.56 MHz) discharge power was 1800 W, substrate power was 100 W (RF). Deposition time was set to 10 s. Because of the clamping ring of the dry etching system an uncoated edge of 4 mm remained around the wafer.
a. Fabrication of double hierarchical surface structures
b. Surface coating of laser structured surfaces
c. Surface patterning by selective laser ablation
Figure 3. Optimum peak fluence level F0 and hatch distance Δx for the fabrication of double hierarchical surface structures.
Figure 2. Process steps to generate a. superhydrophilic surfaces by fs-laser structuring, b. superhydrophobic surfaces by deposition of a Teflon-like polymer layer and c. high contrast micropatterns by selective laser ablation.
For the generation of high contrast wetting patterns, the Teflon-like polymer layer was selectively ablated using the above mentioned fs-laser (see Figure 2 c). Superhydrophilic spots with a diameter of 500 µm and 1000 µm center-to-center distance were generated on a superhydrophobic background. Garrod et al. found that these values are optimal12 and interestingly they come quite close to the dimensions of the Namib Desert beetle’s bumps.10 In order to ensure the complete removal of the layer, the laser beam was translated in parallel lines at a speed of 500 mm/s across the sample. The line distance was set equal to the pulse distance of 2.5 µm, and the peak fluence level was set to 2.1 J/cm². This fluence is well above the single pulse ablation threshold of the Teflon-like polymer coating (0.56 J/cm2) but below the threshold of Pyrex (3.3 J/cm2). Preliminary experiments showed that the initial contact angle of the uncoated blank and laserstructured samples could be recovered by fs laser ablation. Finally, the samples were thoroughly rinsed with deionized clean water and dried by N2 gas. Between surface machining and the experiments the samples were stored in a wafer box exposed to open air atmosphere.
An IntelliScan Galvano-scanner system (Scanlab AG, Puchheim) was used to translate the Gaussian laser beam in horizontal and vertical parallel lines with a speed of 1000 mm/s across the sample within a scan field of 60x60 mm. The hatch distance Δx and the peak fluence F0 were adjusted such that the fluence was close to the ablation threshold Fth between two adjacent scan lines (see upper image in Figure 3). Therefore, the peak fluence level and hatch distance were set to F0= 6.2 J/cm² and Δx = 16 µm, respectively. The hatch pattern was scanned 20 times. The laser processing was performed in air at atmospheric pressure. In order to remove the debris, the wafers were cleaned in a 10 min ultrasonic acetone bath (Sonorex Digital 10P, Bandelin Electronic GmbH Co. KG), followed by a 5 min isopropanol bath directly after laser processing. The SEM image in the lower part of Figure 3 shows the generated microstructure. The period of the bumps was equal to the hatch distance and their height was measured to be 10 µm. The bumps and valleys in between were covered with LIPSS, which had a period of about 700 nm. In this way, the combination of bumps and LIPSS formed a double hierarchical surface structure.73
Fabrication of samples with different wetting properties. In this study, six samples with different wetting properties were fabricated by applying all possible and reasonable combinations of the processing steps described in Figure 2.
To render the fs-laser structured surface superhydrophobic, a 50 nm thick layer of Teflon-like polymer ((CF2)n) was deposited onto the laser-machined samples (see Fig3
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I. Blank
II. Laser structured
III. Blank + Coated
IV. Laser structured + Coated
V. Blank + Coated + Patterned
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mm vertical distance from the outlet of a commercial humidifier (Honeywell, BH-860E, fog output max. 0.4 l h-1, size of the nebulizer outlet: 51 x 8.5 mm). As preliminary experiments have shown, the distance of the sample to the nebulizer nozzle is crucial. If the sample was placed in a distance enabling a homogeneous fog distribution across the sample, the fog stream was too weak to clearly observe differences in the fog-collection behavior. Therefore, the sample had to be positioned close to the nozzle. The drawback of this configuration was that the aerodynamic conditions varied across the surface, making a comparison of the fog-collection rates normalized by effective area difficult. However, a comparative study is possible if the sample size and position are the same. Therefore, special care was taken to ensure the same experimental conditions for every measurement cycle.
Figure 4. Overview of the processing steps used to fabricate samples with uniform (I-IV) and contrasting (V, VI) wetting properties for the fog-collection experiments. As shown in Figure 4, hydrophilic (I), superhydrophilic (II), hydrophobic (III), superhydrophobic samples (IV), low contrast wetting patterns (V) and high contrast wetting patterns (VI) could be generated in this way. This sample selection enabled to investigate the influence of different wetting properties and to evaluate the benefit of using micropatterns on fog-collection efficiency.
In addition, it was observed that the water drops accumulate on the lowest point of the wafer edge before they drip into the collection vessel. Thus the above mentioned uncoated edge around the wafer might influence the size of the falling drops. However, the rate of drop accumulation at the edge is determined by the wettability of the whole wafer surface, thus it may be reasonably assumed that edge effects are negligible.
Contact angle measurements. All contact angle measurements were performed using a drop shape analyzer system (Krüss DSA25S, Krüss GmbH) in a cleanroom environment. De-ionized water was used as test liquid. The sessile drop measurements of Sample I-III were performed by placing drops with a volume of 3 µl on the surface. The contact angles were determined using the circle fitting method. Due to the extreme superhydrophobicity of Sample IV the only possibility to transfer a drop from the syringe to the surface was by using gravity. For this reason drop volume was increased here to about 10 µl, and the shape of the drop was fitted using the YoungLaplace method to take gravity into account. Because the drop took a few seconds to stabilize, the contact angle was averaged over five measurements that were taken at a rate of 2 s-1 three seconds after drop deposition. To measure the dynamic contact angle, the advancing and receding contact angle was evaluated. For this purpose the volume of a sessile drop was enlarged to 6 µl at a dosing rate of 0.02 ml/min, then kept fixed for 30 s to stabilize the drop shape and finally decreased again until the drop detached from the surface or the drop volume fell below 2 µl.
The wind speed of the fog stream was measured using an anemometer and determined to be approximately 5 m/s, which is comparable to the fog wind in the Namib Desert.10 In the experiments, the samples were exposed to the fog stream for 30 minutes, drops detaching from the sample were caught in the collection vessel, and the mass of the collected water was measured using a precision scale. Five measurements were performed for each sample. For the first of the five measurement cycles the mass of the collected amount of fog in g was logged every minute. In addition, the number of collected drops per minute was counted. In this way, the average drop mass and frequency could be determined. For the remaining four cycles only the total quantity was evaluated. The difference between the weight of the vessel before and after exposure to the fog stream, yielded the amount of collected fog for every sample. The fog-collection rate was then calculated by dividing the average total quantity of collected fog by the exposure time. Before each measurement, the sample clamping was thoroughly dried to ensure the same conditions for every measurement cycle. To avoid surface contaminations, the samples were carefully rinsed with de-ionized water and dried by N2 gas in a cleanroom environment after each measurement. All images related to this experiment were captured using an Olympus E-P1 camera.
Determination of the fog-collection rate. The experimental setup for the determination of the fog-collection rate is shown in Figure 5. Throughout the rest of this paper the term “droplet” refers to liquid in the fog stream of the nebulizer, whereas “drops” denote the fog droplets after they had collided with the sample surface. Similar investigations placed the setup either inside a closed box14,19 or used it unconfined27 in a laboratory. In this study, the nebulizer setup was operated in a closed room at ambient conditions (T=26.8 ± 1.6°C, RH=55.4 ± 3.3 %). This approach offers the advantage that only water dripping directly from the sample is collected in the vessel and the influence of condensate originating from a supersaturated environment is minimized. As shown in Figure 5, the samples were mounted vertically (flat of the wafer pointing upwards) at 10 mm horizontal distance and 100 4
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Fog-collection efficiency. The fog-collection efficiency of the six samples (see overview in Figure 4) was determined in five measurement cycles. Throughout the rest of this paper, the samples are named according to their wettability properties, to make clear how surface wettability is related to fog-collection efficiency. For one cycle, the amount of collected fog was measured every minute, yielding the fog-collection curve. As can be seen in Figure 6, all curves increase exponentially during the first eight minutes, which is related to the initial wetting phase. Then, the collected amount of fog per time was constant, which is indicated by the good agreement of the data points with the fitted linear function. This behavior was also observed by Davis et al.17 The average fog-collection rates of all tested samples are listed in Table 2. These results clearly show that both, surface wettability and patterning strongly influence fogcollection efficiency. In general, (super)hydrophobic surfaces performed better than (super)hydrophilic surfaces, which is in well agreement with similar investigations.12,18,23 Moreover, the fog-collection efficiency increased as the wetting state was driven to the extremes by fs-laser structuring. What concerns the patterned samples, an interesting behavior was observed. In case of the low contrast wetting pattern (sample V), the collection rate was comparable to the one obtained by the blank surface, which could be an explanation why White et al. could not measure any benefit for their patterned samples.14 In contrast, the high contrast wetting pattern (sample VI) achieved the highest fog-collection efficiency of all samples. An enhancement of about 60% compared to the blank Pyrex surface was measured.
ultrasonic nebulizer
Figure 5. Setup to determine the fog-collection efficiency, drawing not to scale. Results and Discussion Surface wettability. The static and dynamic contact angles of blank and laser-structured samples before and after the deposition of the Teflon-like polymer layer are shown in Table 1. The surface of blank Pyrex was found to be intrinsically hydrophilic with a contact angle of approximately 28°. By introducing a double hierarchical surface structure using fs-laser machining the contact angle was reduced to a value below 10°, classifying the surface as superhydrophilic. The deposition of a Teflon-like polymer layer onto the laser structured sample lowered the surface free energy and the roughened surface turned superhydrophobic: a static contact angle of 167° and a hysteresis value of about 5° was measured. These results are comparable to the water-repellent properties of the Lotus flower, displaying high static contact angles and high drop mobility.31 In comparison, coating a blank Pyrex wafer only yielded a static contact angle of 111° and a hysteresis value of approximately 36°, which underlines the significant contribution of surface roughness to superhydrophobicity.57 Table 1. Static (Θs), advancing (Θadv) and receding (Θrec) contact angles for the samples tested in the nebulizer setup.
Sample after different process steps
Θs(°)
Blank (Hydrophilic)
27.9 0.2
±
Laser structured
7.3 0.6
±
111.3 0.5
±
I II III
IV
(Superhydrophilic) Blank + Coated (Hydrophobic) Laser structured + Coated (Superhydrophobic)
167.0 ± 0.3
Θadv(°)
Θrec(°)
n/a
n/a
n/a
n/a
118.3 ± 1
82.6 ± 1
162.3 ± 0.7
157.3 ± 0.7
Figure 6. Collected mass of fog as function of time. The straight lines represent linear fits to the data points after the initial wetting phase (thus fitted between minute 8 and 30). Table 2. Average fog-collection rate, drop weight (at the time of collection in the vessel) and dripping frequency of all samples.
Sample number
5
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Collection
Average
Frequency
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(surface wettability)
rate (g/min)
drop weight (g)
(drops/min)
I (Hydrophilic)
0.44 ± 0.04
0.1
4.0
II (Superhydrophilic)
0.49 ± 0.04
0.1
4.7
III (Hydrophobic)
0.55 ± 0.04
0.06
9.4
IV (Superhydrophobic)
0.58 ± 0.02
0.04
15.4
V (Low contrast pattern)
0.45 ± 0.02
0.07
6.7
VI (High contrast pattern)
0.70 ± 0.02
0.1
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I. Hydrophilic
II. Superhydrophilic
III. Hydrophobic
IV. Superhydrophobic
V. Low contrast pattern
VI. High contrast pattern
7.2
Fog-collection mechanism. Having a closer look at the fog-collection mechanisms of the different surfaces makes the results described above more comprehensible. Therefore, the photographs of the wetted surfaces in Figure 7 and drop weight and frequency from Table 2 should be considered.
Figure 7. Photographs of the wetted surfaces, after 30 min exposure to an artificial fog stream: I. Hydrophilic, blank Pyrex surface covered by a liquid film and some large drops, II. Superhydrophilic surface displaying film-wise spreading of the fog droplets, III. Hydrophobic surface covered by large drops, IV. Superhydrophobic surface covered by homogeneous tiny drops, V. Liquid concentration on the hydrophilic areas of the patterned surface, VI. Large drops grow on the superhydrophilic areas until they are detached by gravity, I.-VI. Scale bars denote 3 mm.
The blank hydrophilic Pyrex surface (Figure 7, I) was covered with large and small drops; larger drops were observed in the center of the fog stream. Drops detached from the surface at a mass of about 0.1 g with a frequency of about 4 min-1. The strong drop adhesion of the (super)hydrophilic surface, which hinders liquid removal could be an explanation for the comparably low fogcollection efficiency.18 In contrast, the fs-laser structured superhydrophilic sample (Figure 7, II) was completely covered by a homogeneous water film. Here, the drops detached from the surface at about the same average mass of 0.1 g, but at a slightly higher rate of about 5 drops min-1. The larger surface area combined with faster drop spreading and transport could explain the higher fog-collection efficiency of the superhydrophilic surface.
Surprisingly, the low contrast pattern (Figure 7, V) achieved the lowest fog-collection rate besides the blank glass surface. On this pattern, large drops spread across several hydrophilic spots. The drops detached at comparably high masses of about 0.07 g at a low frequency of about 7 min-1. Obviously, the water adhesion of the hydrophilic spots dominated the hydrophobic properties of the surrounding areas, resulting in a poor drop mobility and thus a relatively low fog-collection rate.
Fog droplets colliding with the coated hydrophobic blank surface (Figure 7, III) coalesced to drops of about 0.06 g before they were detached by gravity at rate of approximately 9 drops min-1. The higher mobility of drops on hydrophobic surfaces could explain the higher fogcollection efficiency compared to hydrophilic surfaces. In contrast, the superhydrophobic fs-laser-structured surface (Figure 7, IV) was covered by much smaller drops that were sliding down on innumerable tiny paths. The drops detached at comparably lower masses of about 0.04 g but higher frequencies of about 15 min-1. This result indicates that higher mobility of smaller but more numerous drops leads to higher fog-collection rates. This finding is in well agreement with the suggestions of similar studies.12,23,27
The highest fog-collection efficiency was achieved as the wetting properties of the micropatterns were driven to the extremes by using fs-laser structured substrates (Figure 7, VI). In contrast to the low contrast pattern, fog droplets accumulated here only on superhydrophilic spots. After reaching a critical size, the drops did not take vertical paths down the sample surface, instead they were literally ripped off the superhydrophilic circular spots. This different drop transport behavior must be related to the extreme water-repellency of the superhydrophobic areas surrounding the drop-shaping superhydrophilic spots. Moreover, the drops detached from this sample at a mass of about 0.1 g at a frequency of about 7 drops min-1. Thus, the superior fog-collection efficiency of high contrast wetting patterns is related to the ability to generate drops of comparably high masses in combination with a high drop removal rate. 6
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Discussion of results. The contact angle measurements of the laser-structured glass surface demonstrated that both superhydrophobic and superhydrophilic properties can be achieved using the same surface morphology, while the wetting state is determined by the presence or absence of a (CF2)n coating, respectively. In order to understand how the laser-generated geometry affects wettability, the roughness ratio of the generated surface r must be calculated. The SEM image in Figure 3 indicates that the generated surface morphology can be approximated by a double-hierarchical sawtooth-like profile. Using the Pythagorean Theorem, the roughness ratio of our fs-laser fabricated surfaces can be estimated by = = 1 +
1 +
provides the overall collection rate of the different surfaces. Transient mechanisms of fog-collection are shown by data on drop weight and frequency. In addition, photographs of the wetted surfaces show the local distribution of fog droplets. The superior fog-collection efficiency of the high contrast wettability patterns fabricated in this study confirmed the findings of Azad et al.19 and Garrod et al.12 However, the gain measured in the study of Garrod et al. was significantly higher. This difference could be explained as follows. Garrod et al. reported that fog droplets being collected on the hydrophobic areas were blown across the surface towards the hydrophilic spots.12 Oyola-Reynoso et al. found that wind influences the fog-collection efficiency.15 In contrast, in the present study drop movement was only determined by surface wettability and gravity. Since the drops remained on the (super)hydrophobic areas, the lack of wind stream might explain the comparably lower difference between uniform and patterned samples. Another possibility is that the confinement of the setup in a box, which generates a super-saturated environment, is responsible for the strong discrepancy. Azad et al., who used a more similar unconfined nebulizer setup, measured that microstructured surfaces increase the fogcollection efficiency by about a factor of 2,19 which is in well agreement with the gain of 60% achieved in this study. The large variation between the fog-collection efficiencies measured in different studies suggests that future studies should address also the influence of different test and environmental conditions and define standardized measurement methods.
(1)
where rµ and rn are the roughness ratios, aµ and an are the periods, and hµ and hn are the depths of the generated micro- and nano-structure, respectively. The period of the microstructure aµ is given by our chosen hatch distance (hd=aµ=16 µm) and the height hµ was determined by confocal microscopy (hµ=10 µm). The geometrical values of the nanostructure (an=hn=0.5 µm) were estimated by SEM images. Thus the roughness ratio of the doublehierarchical surface morphology calculates to r=3.58. The Wenzel model28 implies that the contact angle of the roughened surface (θW) is given by the ratio between the roughened and projected surface (r) and the contact angle of the flat surface (θ0): =
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For our superhydrophilic surfaces (blank Pyrex, θ0=27.9°), the Wenzel model predicts that laser structuring leads to full spreading (θW=0°) which corresponds well with the measured value (θ=7.3°). For the hydrophobic sample (blank+coated Pyrex, θ0=111.3°), a maximum contact angle of 180° is obtained by the Wenzel model. Thus, the extreme wetting contrast achieved in this study can already be explained sufficiently just by using the Wenzel model. The reason is the extreme roughness ratio generated by laser structuring combined with the high and low surface energy of the Pyrex wafer and the (CF2)n coating, respectively. Thus, we can neither distinguish between Wenzel and Cassie-Baxter state from this calculation. However, the low contact angle hysteresis of our superhydrophobic samples suggests that the droplet is in the Cassie-Baxter state.75
The fabrication process presented in this study, which is based on fs-laser scribing, coating, and selective coating ablation, is the first laser-based process to the knowledge of the authors to generate high contrast wetting patterns on glass surfaces. Moreover, due to the use of ultrafast lasers, it was possible to fabricate such high contrast wetting patterns on nearly any substrate. The specific processing rate to fabricate the structure used in this study was 35 mm²/min/W, which means that a state of the art 100 W laser system would still take about 5 h to apply the pattern to a glass with an area of 1 m². In the present form, this processing technique is rather interesting for application on small devices. Nevertheless, there are several ways to improve the processing speed by using larger focus radii, less overscans and larger line distances. Moreover, further investigations should address long-term stability of the wetting properties, mechanical stability and wear resistance.
The results of our fog-collection experiments are in good agreement with other studies on biomimetic19,27 and natural fog-collecting surfaces3,10, and support the common observations that the combination of efficient drop collection on superhydrophilic spot and the high water repellency of superhydrophilic areas leading to fast and frequent drop removal is crucial to improve fog-collection efficiency.18,23,27 In contrast to others, this study provides detailed information about the fog-collection mechanism of surfaces with (super)hydrophilic, (super)hydrophobic, low and high contrast patterned surfaces. The fogcollection rate averaged over five measurement cycles
Applications such as condenser surfaces for vapor chambers76, cell cultivation77 and offset printing plates78 have shown the enormous potential of surface wettability patterns. The ability to fabricate high contrast wetting patterns on transparent substrates could be of particular interest for lab-on-a-chip devices, sensors, and microfluidics, and thus opens up a broad field for further research opportunities. 7
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Summary and Conclusions In this study a process combination of fs-laser machining and surface coating was used to fabricate high contrast wetting patterns on glass surfaces. As experiments in an artificial nebulizer setup have shown, such micropatterns enhance the fog-collection efficiency by nearly 60% compared to blank glass. The superior fog-collection rate of patterned surfaces is attributed to the combination of water-attracting and water-repelling areas: on the superhydrophilic spots the fog droplets accumulate until they grow to large drops and rip off the surface due to the strong water-repellency of the surrounding areas. The presented method enables the surface functionalization of a broad range of materials, including brittle substrates, like glass. This opens up exciting possibilities for fogcollection surface micropatterns, in particular in the field of biomedical and microfluidic devices.
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AUTHOR INFORMATION (8)
Corresponding Author *E-mail:
[email protected] (9)
Author Contributions M.D., S.S. and S.K. developed the idea of the project. E.K. did the experiments and the first draft of the paper. M.D. supervised the work. S.S. designed the graphics of the paper. S.K. did the surface passivation of all samples. V.M. directed the content of the work from the industrial perspective. All authors have given approval to the final version of the manuscript.
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Funding Sources The financial support by the Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development is gratefully acknowledged. Spectra-Physics is acknowledged for the close collaboration and the financial support of the Josef Ressel Center for material processing with ultrashort pulsed lasers.
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ACKNOWLEDGMENT (15)
Thanks to all colleagues at the Research Center for Microtechnology at the Vorarlberg University of Applied Sciences for the technical support and the interesting discussions.
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ABBREVIATIONS fs, femtosecond; CA, contact angle; LIPSS, laser-induced periodic surface structures; SEM, scanning electron microscopy.
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