Cassie-State Stability of Metallic Superhydrophobic Surfaces with

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Cassie-State Stability of Metallic Superhydrophobic Surfaces with Various Micro/Nanostructures Produced by a Femtosecond Laser Jiangyou Long,† Lin Pan,† Peixun Fan,† Dingwei Gong,† Dafa Jiang,† Hongjun Zhang,† Lin Li,‡ and Minlin Zhong*,† †

Laser Materials Processing Research Centre, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China ‡ Laser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester M13 9PL, England S Supporting Information *

ABSTRACT: The Cassie-state stability plays a vital role in the applications of metallic superhydrophobic surfaces. Although a large number of papers have reported the superhydrophobic performance of various surface micro/nanostructures, the knowledge of which kind of micro/nanostructure contributes significantly to the Cassie-state stability especially under low temperature and pressure is still very limited. In this article, we fabricated six kinds of typical micro/nanostructures with different topography features on metal surfaces by a femtosecond laser, and these surfaces were modified by fluoroalkylsilane to generate superhydrophobicity. We then systematically studied the Cassie-state stability of these surfaces by means of condensation and evaporation experiments. The results show that some superhydrophobic surfaces, even with high contact angles and low sliding angles under normal conditions, are unstable under low temperature or external pressure. The Cassie state readily transits to a metastable state or even a Wenzel state under these conditions, which deteriorates their superhydrophobicity. Among the six micro/nanostructures, the densely distributed nanoscale structure is important for a stable Cassie state, and the closely packed micrometer-scale structure can further improve the stability. The dependence of the Cassie-state stability on the fabricated micro/nanostructures and the laser-processing parameters is also discussed. This article clarifies optimized micro/nanostructures for stable and thus more practical metallic superhydrophobic surfaces.

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INTRODUCTION Metallic superhydrophobic surfaces have been widely studied for their potential applications in self-cleaning, anticorrosion, oil−water separation, friction reduction, and so on.1,2 For most applications, a stable Cassie state is essential because the Wenzel state causes high adhesion to water droplets, which significantly deteriorates the self-cleaning property or other performance.3,4 Although a large number of papers have been published regarding various methods used for fabricating metallic superhydrophobic surfaces, systematic research on the influence of surface micro/nanostructures on the Cassie-state stability is relatively limited. Most of the published works concerning the stability issue normally discussed few micro/nanostructures.5−7 Liu et al. studied the Cassie-state stability of nanotube arrays under dynamic impact and static pressure.8 The influences © XXXX American Chemical Society

of the nanotube diameter and the average groove width on the Cassie-state stability have not yet been covered. Tuvshindorj et al. examined the Cassie-state stability of superhydrophobic coatings against the external pressure by applying compression/ relaxation cycles to water droplets sitting on the surfaces with three limited micro/nanostructures.9 Because of the fact that numerous micro/nanostructures have been fabricated for metallic superhydrophobic surfaces in recent years, it is rather meaningful to clarify the influence of various micro/ nanostructures on the Cassie-state stability. In nature, many environmental conditions can result in the transition from the Cassie state to the Wenzel state. The most Received: November 25, 2015

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DOI: 10.1021/acs.langmuir.5b04329 Langmuir XXXX, XXX, XXX−XXX

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different surface micro/nanostructures. The repetition rate of the laser pulse was 200 kHz. The average laser fluence ( f), laser scanning speed (v), interval of adjacent laser scanning lines (d), and number of laser scans (n) were different for different samples. A detailed schematic of the laser irradiation setup is shown in Figure S1 (Supporting Information). The morphologies of the surfaces were analyzed using a scanning electron microscope (SEM) (Tescan Mira 3 LMH). Topography measurements were performed using a three-dimensional (3D) laser microscope (Keyence VK-X200 series). Wettability measurements of samples were performed by evaluating apparent CAs using a video-based optical contact angle measuring device (OCA 15 Plus from Data Physics Instruments), and the sessile drop technique. The sliding behavior of water droplets on the sample was evaluated by measuring the SAs using the tilting plate method. The selected water droplets for CA and SA measurements were 4.5 μL. An average of five readings on different areas was used for the CAs and SAs. A computer-controlled cooled platform was mounted on an inclinable plate for the condensation experiment. Samples were put on the platform, and this device was placed in a closed room. The humility and temperature of the room were 50% RH and 25 °C, respectively. When the temperature of sample surfaces were lowered to 10 °C for 5 min, CAs and SAs were recorded. An optical microscope (Olympus BX51M) was used to observe the surface-condensed water droplets. These obtained micrographs were then processed by an imageprocessing program to calculate the size of condensed water droplets. Average diameters were used to represent the size of condensed water droplets. An example of this image processing can be found in Supporting Information (Figure S2). For each sample, the average size of all condensed water droplets was obtained by analyzing five micrographs obtained on different areas. The evaporation experiments were also conducted by the contact angle measuring device. A 10 μL purified deionized (DI) water droplet was first put on the sample surface, and then the water droplet was allowed to evaporate in still air in the closed room mentioned above. Images of water droplets during evaporation were recorded every 30 s by the contact angle measuring device. Then water volumes, CAs, contactline diameters (CDs), and droplet radii (R) were calculated by software. A schematic of the calculation process can be found in the Supporting Information (Figure S3).

common condition is low temperature or heavy rain. Low temperature causes water condensation under high humidity. However, most superhydrophobic surfaces cannot prevent water from condensing on their surfaces.10 Even the surface of a lotus leaf become wet after water condensation.11 Some papers have confirmed that severe water condensation changes a Cassie state to a Wenzel state for some superhydrophobic surfaces.12 However, a stable Cassie state under condensation plays a key role in the applications of superhydrophobic surfaces in sustained dropwise condensation or anti-icing areas.13−15 During heavy rain, water droplets impact a surface with high velocity, which leads to high pressure on the surface. If the pressure caused by the droplet impact is larger than the maximally allowable pressure for a given surface structure, then the liquid will intrude the surface structure and the surface will lose its superhydrophobicity.8 Many methods have been used to characterize the Cassie-state stability under external pressure, such as droplet impact experiments,16 compression/relaxation cycles,6 and evaporation experiments.17−19 Among these proposed methods, the evaporation experiment is a simple and useful one. When the water droplet becomes smaller during evaporation, the Laplace pressure increases dramatically, which can induce the transition of the surface wettability state. In this article, the influence of surface micro/nanostructures on the Cassie-state stability was systematically studied by condensation and evaporation experiments. Six kinds of typical micro/nanostructures with different topography features were fabricated on copper surfaces by a femtosecond (fs) laser, and then these surfaces were modified by fluoroalkylsilane to induce superhydrophobicity. The fs laser is a novel tool for producing metallic superhydrophobic surfaces. This technique, being maskless, controllable, and flexible, can fabricate various kinds of micro/nanostructures on almost any kind of metal, including iron,20 copper,21 aluminum,22 titanium,23 and their alloys. Quite a lot of papers concerning fs-laser-fabricated metallic superhydrophobic surfaces have been published in recent years.24 Most of these studies normally focus on the fabrication of different kinds of surface micro/nanostructures for superhydrophobicity. Because the height and interval of surface micrometer-scale structures and the density of nanoscale structures can be regulated over wide ranges, this technique provides a perfect tool for studying the influence of various surface micro/nanostructures on the Cassie-state stability. Our results indicate that the stability of various superhydrophobic surfaces differs greatly even with good contact angles (CAs) and sliding angles (SAs). Surfaces with only micrometer-scale structures easily lose their superhydrophobicity. By increasing the height of surface micrometer-scale structures, the Cassie-state stability can be improved but still not satisfied. Surfaces with abundant nanoscale structures demonstrate stable Cassie states. Moreover, by increasing the density of micrometerscale structures on these surfaces, their stability can be further improved. These results may help to further understand the Cassie-state stability of different micro/nanostructures under condensation and evaporation conditions.

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RESULTS AND DISCUSSION Laser-Structured Surfaces. To understand the influence of micrometer-scale structures and nanoscale structures on the Cassie-state stability, we fabricated three different types of micro/nanostructures with the fs laser, and each type contains two different structures. They are named RM (regular micropillars), RMN (regular micropillars with nanoscale structures on it), and MN (random hierarchical micro/nanostructures). Laser-processing parameters for these structures are listed in Table 1. The first type is a periodic micropillar structure as shown in the first and second lines in Figure 1. These samples are named RM1 and RM2. As shown in Figure 1a4,b4, the periods of these micropillars are 30 μm, and the average heights of the micropillars in

Table 1. Laser-Processing Parameters

EXPERIMENTAL SECTION

Methods used for sample preparation, including the material used, laser irradiation process, and surface chemical modification method, are the same as in our previous works.25 The cleaned copper samples are irradiated with an fs laser at first and then modified with a layer of fluoroalkylsilane to reduce the surface free energy. In this experiment, some of the laser processing parameters were changed to fabricate B

sample

laser fluence ( f) (J/cm2)

scanning speed (v) (mm/s)

scanning interval (d) (μm)

scanning number (n)

RM1 RM2 RMN1 RMN2 MN1 MN2

1.27 1.27 2.69 2.69 7.08 2.69

200 200 20 5 30 20

30 30 30 30 20 10

16 32 1 1 1 1

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Figure 1. Surface topographies and morphologies. Each line shows one type of micro/nanostructure, and the type codes are shown in the top right corner. (a1−f1) SEM images of the surface. The scale bars are 20 μm. (a2−f2) Corresponding high-magnification SEM images. The scale bars are 5 μm. The red circle in c2 represents an area without nanoscale structures. (d3−f3) Surface 3D height map obtained by the 3D laser microscope. The black lines show the corresponding areas for the topography measurement as shown in a4−f4. (a4−f4) Corresponding surface profiles.

samples RM1 and RM2 are 22.1 and 51.2 μm, respectively. Only a few nanoripples exist on these micrometer-scale structures as shown in Figure 1a2,b2.

The second type is a regular micropillar structure with nanoscale structures on pillar surfaces as shown in the third and fourth lines in Figure 1. These samples are named RMN1 and RMN2. C

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the laser spot and the interval of adjacent laser scanning lines. When the interval of adjacent laser scanning lines is near the diameter of the laser spot (30 μm in our experiment), then the surface tends to form periodic micrometer-scale structures with a period similar to the interval of adjacent laser scanning lines. For samples RM1, RM2, RMN1, and RMN2, the used scanning intervals are all 30 μm, so the period of micrometer-scale structures on these samples is about 30 μm. However, decreasing the interval of adjacent laser scanning lines leads to random and denser micrometer-scale structures as shown in samples MN1 and MN2. The scanning intervals used for samples MN1 and MN2 are 20 and 10 μm, respectively, so the micrometer-scale structures become denser for these samples. Condensation Experiments. Figure 2 shows the changes in CAs and SAs under cooled conditions. At room temperature

The micro/nanostructures on these samples are very similar to the structures in our previously published works; they are composed of periodic micropillars with abundant nanoscale structures.26 The period of these micropillars is also 30 μm. As shown in Figure 1c4,d4, the heights of the micropillars in RMN1 and RMN2 are 33.1 and 54.5 μm, respectively. For RMN1, some areas (red circle in Figure 1c2) are not covered by nanoscale structures. But for RMN2, the surfaces are fully covered by dense nanoscale structures (Figure 1d2). The third type is a random hierarchical micro/nanostructure as shown in the fifth and sixth lines in Figure 1. These samples are named MN1 and MN2. The micrometer-scale structures on these samples show no obvious periodicity but are distributed more closely compared to sample RMN2. For sample MN1, some micropillars can also be found on the surface (Figure 1e3), and the distance between two neighboring micrometerscale structures is about 20 μm as shown in Figure 1e4. But for sample MN2, the micrometer-scale structures are relatively flat (Figure 1f4) and are distributed more closely. However, the nanoscale structures on samples MN1 and MN2 (Figure 1e2,f2) have features similar to those of sample RMN2 (Figure 1d2). These structures represent basic forms of the fs-laser-fabricated micro/nanostructures on metal surfaces. The structural patterns and parameters are related to the laser-processing parameters and can be modulated precisely over a wide range. Because this article mainly focuses on their superhydrophobicities, the formation mechanism of these structures will be discussed only briefly. The obvious advantage of an fs laser compared to other lasers is its rapid and precise energy deposition into materials, which results in high precision and a limited heat-affected zone in materials processing. The unique threshold effect of the fs laser provides some special possibilities for micro/nanofabrication.27 When the laser fluence is below the material’s ablation threshold, no damage will be done to the material surface. However, when the laser fluence is near the threshold, quasi-periodic surface structures, such as nanoripples, will be induced on the surface.21 The nanoripples existing on the top of micropillars, as shown in Figure 1a3,b3, are related to this phenomenon. Further increasing the laser fluence leads to precise material removal.28 In our experiments, the material removal amount is related to the laser fluence (f), laser scanning speed (v), and number of laser scans (n). With increasing laser fluence, the number of laser scans or decreasing laser scanning speed can increase the amount of material removal. The fs laser pulses used in the experiments have a Gaussian energy distribution, and the fluence is higher in the center of the circular spot. Hence, more material will be ablated in the center of the laser-beam-scanned paths, which results in the pillared micrometer-scale structure. Another important phenomenon is that when the input laser energy density is high, which means high laser fluence or low laser scanning speed, abundant nanoscale structures can be induced. For samples RM1 and RM2, multiple fast scanning was used to increase the height of the micrometer-scale structure but to avoid the formation of nanoscale structures (Table 1). On the contrary, for samples RMN1 and RMN2, the laser scanning speed was decreased to induce more nanoscale structures. The scanning speed for sample RMN1 was 2 times that for sample RMN2, so the input laser energy density for sample RMN1 was much lower, which leads to insufficient nanoscale structures on sample RMN1 compared to the number on sample RMN2. Samples MN1 and MN2 were both fabricated at low scanning speed or high laser fluence to induce enough nanoscale structures. Finally, the period of micrometer-scale structures is related to the diameter of

Figure 2. (a) Contact angles and (b) sliding angles on different samples at 25 °C (blue) and 10 °C (red).

(25 °C), all samples show superhydrophobicity with CAs above 150° and SAs lower than 10°, indicating that water droplets on these surfaces are typically in the Cassie state.29,30 However, the wettability of these surfaces differs greatly at low temperature. The CAs of samples RM1 and RM2 decrease significantly, and these surfaces also show a strong adhesion to water. A tiny water droplet cannot roll down the inclined surfaces (shown in Supporting Information video S1). In particular, water droplets on the cooled sample RM1 surface do not exhibit shape sphericity because of the dramatic decrease in CA. The water droplets cannot roll down the surface even with an inclination angle of 45°. These phenomena indicate that the superhydrophobicity is unstable under water condensation for surfaces mainly composed of micrometer-scale structures. Under such conditions, water droplets contact the surface in the Wenzel state or the metastable state, which results in high water adhesion. When the height of the micrometer-scale pillar increases, as shown in the case of sample RM2, the stability of its superhydrophobicity improves somewhat under condensation, but it cannot prevent the transition from a Cassie state to a metastable state or a Wenzel state. Samples RMN1, RMN2, MN1, and MN2, all with hierarchical micro/nanostructures, maintain high CAs and low SAs even under cooled conditions. As shown in video S2 (Supporting Information), after being cooled to 10 °C for 5 min, these D

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Langmuir surfaces still show low adhesion to water droplets and water droplets can roll down quickly and remove the condensed water film from the surfaces. More specifically, the CAs of these samples follow the order MN2 > MN1 > RMN2 > RMN1, and the SAs of the samples follow the order MN2 < MN1 < RMN2 < RMN1. These results indicate that increasing the density of nanoscale structures (RMN1 and RMN2) and microscale structures (MN1 and MN2) can increase the Cassie stability under condensation. During the condensation of water droplets on the surface, a thin layer of tiny water droplets appears on the surface, which leads to the change in surface color from black to gray. The surface become rough as shown in Figure S4 (Supporting Information), and then the measurement of the contact angle is not as precise as before. Therefore, on such cooled surfaces, SAs can represent the superhydrophobicity better. So even the CAs of samples RMN2, MN1, and MN2 are less than 150°, and the low SAs ( MN1 > RMN2 > RMN1 during evaporation, and sample MN2 shows the best stability. The CAs of samples MN1 and MN2 are

Figure 5. Schematic illustration of a water droplet on the cooled inclined surfaces. (a) Surface mainly with micrometer-scale structures. (b) Surface with hierarchical micro/nanostructures.

surfaces mainly composed of micrometer-scale structures. When external water droplets contact the surface, they will partially contact the surface-condensed water film, which leads to the adhesion of external droplets. However, for surfaces with densely distributed nanoscale structures, only small water droplets exist on the cooled surfaces. These condensed water droplets show ultralow adhesion to the substrate. When external water droplets contact the surface, they will maintain the Cassie state and roll down the surface, taking away these condensed tiny droplets at the same time (Figure 5b). More specifically, the stability of the Cassie state under cooled conditions is strongly related to the fs-laser-fabricated surface micro/nanostructures. Dense nanoscale structures and closely packed micrometer-scale structures help to restrain the growing of condensed droplets, which helps the surface remain in the Cassie state even under cooled conditions. Evaporation Experiments. Figure 6 shows the image evolution of water droplets during evaporation on different sample F

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Figure 7. (a) Contact angles and (b) contact-line diameters as a function of the volume of droplets during droplet evaporation. (c) Contact angles as a function of the Laplace pressure of droplets during droplet evaporation.

higher than 150° even when the droplet volume decreases to 0.5 μL. This demonstrates that the densely distributed nanoscale structure is important for a stable Cassie state during evaporation and that the closely distributed micrometer-scale structure can further improve the Cassie-state stability. The results in the above evaporation experiments correspond well to the results in condensation experiments. The samples, showing better superhydrophobicity during evaporation, also maintain their Cassie state better under cooled conditions. In fact, the limited decrease in CAs and the dramatic decrease in CDs during evaporation illustrate that the surfaces have low water adhesion even for small droplets. The low water adhesion for small droplets is crucial to the “coalescence-induced droplet jumping” during condensation. Therefore, on the sample surfaces having better superhydrophobicity during evaporation, the condensed small droplets leave the surfaces easily, which helps to maintain the Cassie state even under cooled conditions.

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected]. Tel: +86-10-62772993. Fax: +86-10-6277-3862.

CONCLUSIONS Several superhydrophobic metallic surfaces with typical micro/ nanostructures have been fabricated with an fs laser. The stability of their Cassie states was systematically studied by condensation and evaporation experiments. The surfaces with only micrometer-scale structures exhibit superhydrophobicity with even high CAs and low SAs under normal conditions, and they show poor stability under low temperatures or external pressures. The transition from the Cassie state to the metastable state or the Wenzel state can happen, which deteriorates their superhydrophobicity. The densely distributed nanoscale structure is important for a stable Cassie state. Moreover, the density of micrometer-scale structure also influences the stability. Closely distributed micrometer-scale structure is helpful to further improving the Cassie-state stability. The relationship between the final stability and the laser processing parameters was clarified. The formation of nanoscale structures is mainly related to the laser fluence and laser scanning speed. Relative high laser fluence or low laser scanning speed is necessary to ensure enough nanoscale structures. Moreover, the density of micrometer structures is mainly related to the interval of adjacent laser scanning lines, with a relatively small interval increasing the density of micrometer-scale structures, which is helpful in further improving the stability of the Cassie state.

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Movies S2 showing droplet movement on cooled inclined sample surfaces (RMN1, RMN2, MN1, and MN2). The inclination angle of all samples is 10°. (AVI) Schematic of the laser irradiation setup, example of image processing in the condensation experiment, schematic for the calculation of the contact-line diameter and droplet radius in the evaporation experiment, surface changes after condensation, diameter distribution of the condensed water droplets on the sample surfaces, contact angles and contactline diameters as a function of the volume of droplets during droplet evaporation, and detailed statistical data of the surface-condensed water droplets in the condensation experiment (PDF)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the funding support of the National Natural Science Foundation of China (51210009), the National Key Basic Research and Development Program of China (2011CB013000), and Tsinghua Fudaoyuan Research Fund.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04329. Movies S1 showing droplet movement on cooled inclined sample surfaces (RM1 and RM2). The inclination angle of all samples is 10°. (AVI) G

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