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A new method for producing “Lotus Effect” on a biomimetic shark skin
Journal of Colloid and Interface Science 388 (2012) 235–242
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
A new method for producing ‘‘Lotus Effect’’ on a biomimetic shark skin Yunhong Liu, Guangji Li ⇑ School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 14 April 2012 Accepted 14 August 2012 Available online 27 August 2012 Keywords: Biomimetic surface Shark skin Microreplication Lotus effect Superhydrophobicity
a b s t r a c t Nature has long been an important source of inspiration for mankind to develop artificial ways to mimic the remarkable properties of biological systems. In this work, a new method was explored to fabricate a superhydrophobic dual-biomimetic surface comprising both the shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures. The biomimetic surface possessing shark-skin pattern microstructure was first fabricated by microreplication of shark-skin surface based on PDMS; and then it was treated by flame to form hierarchical micro/nano-structures that can produce lotus effect. The fabricated biomimetic surfaces were characterized with scanning electron microscopy (SEM), water contact angle measurements and liquid drop impact experiments. The results show that the fabricated dual-biomimetic surface possesses both the vivid shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures. It can exhibit excellent superhydrophobicity that the contact angle is as high as 160° and maintain its robustness of the superhydrophobicity during the droplet impact process at a relatively high Weber number. The mechanism of the micromorphology evolution and microstructural changes on the biomimetic shark-skin surface was also discussed here in the process of flame treatment. This method is expected to be developed into a novel and feasible biomimetic surface manufacturing technique. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Nature provides abundant examples of structures, materials and surfaces which can be investigated to understand the basic principle and subsequently developed into fascinating technical applications [1]. The term biomimetic, which means learning from nature as an impulse for an independent technical design [2] is already popular in the field of materials science and engineering. There are many examples of biomimetic design originated from the investigation and copy of the special properties and mechanisms of natural plants and animals [3,4]. Such an example is the ‘‘shark skin effect’’, which is defined as a mechanism of wall friction reduction of a fluid resulted from a riblet structured surface similar to that of shark skin [5]. Shark skin has been widely studied for decades due to its drag reduction and antifouling properties [6–8]. Micron-sized grooved scales growing on shark skin, which are called dermal denticles, are interlocked to form a natural non-smooth surface; and the grooves between adjacent riblets on the scales are directed almost parallel to the longitudinal body axis of the shark. It has been reported that the grooved scales can reduce vortice formation or lift the vortice off the surface, so resulting in water moving easily over the skin ⇑ Corresponding author. Fax: +86 20 87113949. E-mail addresses: [email protected] (Y. Liu), [email protected] (G. Li). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.08.033
surface [9–11]. Besides, the rough texture formed by dermal denticles can reduce the adhesion area available to aquatic organisms and keep the surface clean. It is exciting that the principle has been adapted to aeroplane surfaces and achieved fuel-saving by about 1.5% [12]. Speedo invented the full-body swimsuit called ‘‘Fastskin’’ for elite swimming, which mimicked the shark-skin V-shape ridges [13]. ‘‘Sharklet’’ is another commercial product inspired by the overlapping, ridged platelet structures of shark scales. It can display excellent microbe resistant properties, which is very encouraging results to date [14,15]. Another well-known example is to design and fabricate biomimetic surface possessing ‘‘Lotus Effect’’, which is defined as the self-cleaning properties (phenomenon) and highly superhydrophobic surface like a lotus leaf [16]. It has been reported that the surface of a lotus leaf is covered with wax and has an intrinsic microscale and nanoscale hierarchical structures, providing superhydrophobicity, self cleaning, low adhesion and drag reduction [17–19]. Model proposed to interpret superhydrophobic phenomena was published by Cassie and Baxter [20], as well as Wenzel [21,22]. In the past decades, designing artificial superhydrophobic surfaces has become one of the top issues due to their potential applications in different realms, and numerous techniques have been developed to mimic lotus effect, including electrospinning [23], plasma treatment [24,25], chemical vapor deposition [26], molding [27] and phase separation [28,29]. However, there is still
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a long way to go for meeting the requirements for practical applications. Based on the understanding of the multi-level structures of multifunctional biological surfaces, the future research into bioinspired multi-functional surfaces can focus on the combination of various biomimetic structures by incorporating multiple technologies of forming surface topology so as to make the prepared surface exhibit excellent comprehensive properties close to the real biological surfaces as far as possible. The artificial surfaces inspired by the shark skin or the lotus leaf have showed unique properties and broad application prospects. Although the relationship between the nano- and microscale topographies and the surface properties of real shark skin and lotus leaf has not been fully understood, it can be predicted that a biomimetic multi-functional surface bearing the characteristics of both drag reduction and anti-bioadhesion may be produced by combining the directional microscale pattern of shark-skin surface with the nanoscale structure observed on the lotus leaf. And such a biomimetic surface can be used to optimize the surface design of underwater vehicles and fluid transportation pipelines, thus enhancing efficiency or reducing energy consumption; and to inhibit bioadhesion to the surface of underwater facilities in harsh water environments. Therefore, it is necessary and significant to develop a new method for achieving the particular combination of shark skin effect and lotus effect. In this work, we developed an original and efficient method to fabricate superhydrophobic dual-biomimetic surface comprising both the vivid shark-skin surface morphology and the lotus leaf-like hierarchical structures. In view of the fact that polydimethylsiloxane (PDMS) is a malleable material for developing topographies and its low surface energy is a key property for achieving a superhydrophobic surface state, PDMS containing nano-silica was chosen as a substrate material, the biomimetic shark-skin surface having micron-sized pattern structure was first fabricated by microreplication of shark-skin surface; and then, it was treated by flame to form hierarchical micro/nano-structures that can produce lotus effect, thereby constructing a dual-biomimetic surface. Scanning electron microscopy (SEM) was used to observe the surface morphology of the samples in the form of a PDMS sheet prepared via different routes, including microreplication, flame-treatment and microreplication followed by flame-treatment, respectively. Furthermore, their surface properties were characterized with water contact angle measurements and liquid drop impact experiments. The results were compared and analyzed using a flat PDMS sheet with a smooth surface as a control sample. The mechanism of the micromorphology evolution and microstructural changes on the dual-biomimetic surfaces in the process of flame treatment was also discussed within the paper.
2.2. Fabrication of the surfaces with special micro/nano-structures 2.2.1. Microreplication of shark-skin surface The microstructure of the shark-skin surface can be replicated to the surface of PDMS sheet by PDMS replica molding process. Due to its excellent processability, ease molding, wide spectrum of physical and mechanical properties, as well as excellent dimensional stability when curing, PDMS elastomer that can be crosslinked via the addition mechanism is chosen as the mold material to create topographical patterning of micron-scale features. The specific process involves the following steps. First of all, the treated shark skin was taken out from a refrigerator and left at room temperature for several hours, and then the shark skin was carefully fitted on a plate as a microreplication template. The plate with shark skin pattern was made into a mold for casting as shown in Fig. 1, which was designated as Mold-A. Secondly, a mixture of precursor and curing agent (10:1 by weight) was poured into Mold-A and cured for 20 min at 90 °C so as to transfer the surface microstructures of the shark skin to the PDMS surface contacting with the shark skin; and then the cured PDMS sheet was separated from Mold-A, thus obtaining the PDMS template for microreplication. The profile of the PDMS template was a counter-shape of the shark-skin pattern. Thirdly, the as-prepared PDMS template was used to manufacture a cavity block or negative mold in the same way as described in the first step, which was designated as Mold-B. Through the same microreplication process as the preceding step, the desired shark-skin replica (SSR), PDMS sheet with the microstructure of shark-skin surface, was fabricated. 2.2.2. Preparation of hierarchical structured surfaces The prepared shark-skin replica and the flat PDMS sheet without microstructure (F-PDMS) were used as substrates. The hierarchical morphological structures on the surfaces of both samples were prepared via flame treatment. Alcohol lamp was used in the flame treatment due to its consistent flame suitable for laboratory preparation. The surface of a substrate was close to the outer edge of the flame from an alcohol lamp and moved back and forth at a predetermined speed. After being treated for prescribed time, the samples were properly preserved for surface characterization and measurements. 2.3. Surface characterization 2.3.1. Observation of surface morphology The surface morphology of the prepared PDMS sheets was observed by a scanning electron microscope (S-3700N, Hitachi, Japan) and a field emission scanning electron microscope (LEO 1530 VP, Oberkochem, Germany). The sample surfaces to be observed were gold-coated using a sputter coater beforehand.
2. Experimental 2.1. Materials Fresh shark skin from a great white shark (Carcharodon carcharias), which is one of the fastest swimming sharks, was purchased from a fisherman. The subcutaneous fat was removed from fresh shark skin first. The shark skin was then washed several times with deionized water, carefully flattened, cut into the required shape and dried. The treated shark skin was stored in a refrigerator before use. A two-component room temperature vulcanizable liquid silicone rubber including precursor of PDMS and curing agent, which were purchased from Zhejiang Runhe Silicone New Material Co., Ltd., was used as received. The low viscosity precursor is composed of vinyl-polydimethylsiloxane (PDMS), which contains 25 wt.% of fumed silica (300 m2/g) filler treated with –Si(Me)2-O-oligomers.
Fig. 1. Schematic diagram of the microreplication process of shark-skin surface.
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Fig. 2. The SEM images of the shark-skin surface and the surfaces of PDMS sheets prepared via microreplication.
2.3.2. Measurements of contact angle and sliding angle Contact angle (CA) and sliding angle (SA) were measured with a Contact Angle Goniometer (DSA 100, Krüss GmbH, Germany) at ambient temperature. Each sample was measured for three times at three random locations and the average values of contact angle and sliding angle were calculated.
2.3.3. Liquid drop impact experiment In order to evaluate the robustness of various superhydrophobic solid surfaces, a very helpful method is to investigate the liquid drop impact dynamics onto the surfaces via the water drop impact experiment. The experimental procedure generally consists in releasing a water droplet with a millimeter-level diameter from a microsyringe at different heights to vary its impact velocity onto the solid surface. The falling liquid drops can be accelerated by gravity, thus hitting the solid surface at a certain impact velocity. The vertical impact velocity of the droplet released at the height h above the surface, V, can be calculated according to the following equation:
V¼
pffiffiffiffiffiffiffiffi 2gh
ð1Þ
where g is the acceleration of gravity (g 9.8 m/s2). The physical parameters of the water droplet are as follows: droplet radius r is approximately equal to 1.1 mm with 6% deviation, surface tension r is 0.0728 N/m and density q 1000 kg/m3 at 20 °C. The process of drop impact was recorded by a high-speed camera (pco.dimax HD, CooKe) with a recording rate of 1500 fps (frames per second) to obtain the information on the drop impact dynamics.
3. Results and discussion 3.1. Analyses of surface morphology Fig. 2 illustrates the SEM images of the surfaces of the real shark skin (Mold-A) and the PDMS sheet samples prepared by the microreplication technique. Compared with the real shark skin in Fig. 2a, the SSR sample in Fig. 2c possesses the almost same surface microstructure as that of the dermal denticles on shark skin. The crosslinking mechanism of additive cure type PDMS is illustrated in Fig. 3 [30]. It involves the addition of a silicon hydride (Si–H) to an unsaturated double bond in the presence of a noble metal catalyst such as platinum. Since the Mold-B and SSR are both made of PDMS, a technical key for preparing the Mold-B is to adjust the ratio of the PDMS precursor to its curing agent and to control reaction conditions, so as to make the reactive groups (vinyl groups or Si–H bonds) on the surface of the Mold-B exhausted as far as possible. When the Mold-B with no reactive groups on its surface is used to prepare SSR via the microreplication process described above, the addition curing reaction only occurs within the PDMS material that makes up SSR and makes SSR exhibit good elasticity, thus preventing the interface reaction between the Mold-B and the SSR. For this reason, the SSR can be easily separated from the MoldB with keeping the high-precision surface patterns. It indicates that the surface morphology of the dermal denticles on shark skin can be replicated with silicone rubber by the way of microreplication. In this process the properties of PDMS as a molding material plays an important role. The precursor of PDMS has excellent flowability due to its low viscosity, which makes the precursor fill into the interstice of the mold; and the subsequent crosslinking process can transform the flowable precursor into a solid
Fig. 3. The crosslinking mechanism of additive cure type silicone rubber.
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Fig. 4. (a) The SEM images (shown at four magnifications (I)–(IV)) of the flame-treated SSR sample and (b) image of a water droplet sitting on the flame-treated SSR surface.
PDMS sheet so as to maintain a conformal contact with the mold cavity and faithfully replicate its fine structure. The relatively low surface free energy of PDMS itself (csv = 21.6 dynes/cm2) [31] and the elasticity, and extremely chemical inertness of the cured PDMS will enable the prepared PDMS sheet to demold easily. Therefore, the silicone rubber replica molding process is a low-cost and reliable way for microstructure replication. The other polymers can also be used in such a microreplication with appropriate modifications. The further research into the surface morphology of the SSR samples treated by flame was conducted by SEM. The results are shown in Fig. 4. It can be seen from the SEM images (I) and (II) in Fig. 4a that the fine surface profiles of the SSR sample can be retained upon treated by flame. Moreover, it is very interesting that in Fig. 4a, the high-magnification SEM images (III) and (IV) of the flame-treated SSR surface display a unique hierarchical rough structure comprising a sub-microstructure and a nano-structure, in which the uniformly distributive micron-level aggregates (300–500 nm) constitute the sub-microstructure, and this is composed of nano-silica particles (20–50 nm). Accordingly, it can be said that the unique hierarchical micro/nano-structures observed on the flame-treated SSR surface bear much resemblance to those of natural lotus leaves. What is consistent with the above observation and analysis is that as shown in Fig. 4b, the flame-treated SSR surface can exhibit considerable superhydrophobicity, or water on this surface forms a spherical droplet. These results implies that the dual-biomimetic surface possessing the surface profile of shark skin and the hierarchical micro/nano-structures similar to those of a lotus leaf can be successfully fabricated through the microreplication of shark skin followed by flame treatment.
It should be noted that the duration of flame treatment is an important factor affecting and controlling the hierarchical micro/ nano-structures of SSR surface. To clarify the effect of flame treatment on the surface morphology of SSR, a few of SSR samples were treated by flame for different preset time, respectively; and then the flame-treated surfaces were observed with SEM. The SEM images are presented in Fig. 5. It can be clearly observed from Fig. 5 that the surface morphology and roughness of the used SSR samples change dramatically as the flame-treatment time increases. On the untreated surface there is no discernable feature. Once the surface is treated by flame for 5–10 s, some relatively regular and discrete embossment-like wrinkled surface structures along different directions begin to appear as shown in Fig. 5b and c; and the number of the regular wrinkled surface structures on the treated surface increases with an extension of the flame-treatment time. However, on the surface treated by flame for 30 s the ridges and troughs of the wrinkled surface structures disappear instead of a uniformly rough micro/ nano-structures as shown in Fig. 5d. The mechanism of the above-described micromorphology evolution and microstructural changes can be explained according to the principle of flame treatment and the analysis of the change in the physical properties of surface material during this process. Due to the action of the heat from flame, the flame treatment of surface conducted in air will lead to the surface oxidation of treated object in essence, especially for the surface of organic material. On the other hand, there may exist the thermal stress of surface generated by the non-uniform heating in the initial stage of flame treatment. Based on this principle, it can be speculated that while the surface of PDMS sheet is subjected to flame treatment, the ef-
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Fig. 5. The comparison of the SEM images of the SSR surfaces treated by flame for [(a) and (b)] 0 s; [(c) and (d)] 10 s; [(e) and (f)] 20 s; and [(g) and (h)] 30 s, respectively.
fect of the surface thermal stress and the oxidative activation of flame will induce the micro-deformation of the surface layer; and simultaneously, the non-uniform heating in the initial stage of flame treatment makes the polysiloxane located in the hightemperature region of the surface layer break down and partially degrade into inorganic SiO2 networks. Consequently, the relatively regular embossment-like wrinkled structures form on the surface of PDMS sheet. With the continuance of the flame treatment, the hydrophobic nano-silica particles previously embedded in the PDMS matrix are gradually exposed to generate flower-like nanoscale aggregates made from nano-silica. Finally, a uniformly rough surface possessing the unique hierarchical micro/nano-structures forms with increasing nano-silica aggregates on the sample surface. 3.2. Wettability of various surfaces It has been found that the surface wettability or hydrophobicity strongly depends on its topology besides its chemical nature. Thus, the water CA values of the different microstructured surfaces of PDMS sheets were measured to evaluate their hydrophobicity. The tested PDMS sheet samples included (a) F-PDMS sample with smooth surface, (b) SSR sample with vivid shark-skin morphology,
(c) the flame-treated F-PDMS sample with nano-structured surface, and (d) the flame-treated SSR sample with a hierarchical micro/nano-structured surface. The results are illustrated in Fig. 6 and the data on the water contact angle and sliding angle on the various surfaces are summarized in Table 1. The CA on the smooth surface of F-PDMS as a control sample is 103° (Fig. 6a), showing the hydrophobicity of PDMS as a low-surface-energy material. Compared to it, various microstructures fabricated on the sample surface can make the CA increase significantly, as shown in Fig. 6b–d. On the microstructured surface of SSR, the CA is 120°, which indicates that the SSR surface possesses stronger hydrophobicity than F-PDMS surface, but it is still not superhydrophobic. However, on the nano-structured surface of the flame-treated F-PDMS sample and the hierarchical micro/nano-structured surface of the flame-treated SSR sample, the measured CA values reach 153° and 160°, respectively. In other words, the CA of both the surfaces increases by 50° and 40°, respectively, as compared with that on the untreated surface of the corresponding sample. This implies that the surfaces of both samples (c) and (d) are superhydrophobic and the reason for it is that on the surfaces there form the hierarchical micro/nano-structures produced by flame treatment. And according to the experimental observation, the water droplet on the superhydrophobic surfaces is unstable and
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Fig. 6. The CA of water droplet on the surfaces of PDMS sheets fabricated via different routes and the corresponding schematic diagrams.
Table 1 Water contact angle and sliding angle measured on the various surfaces. Sample
CA (°)
SA
F-PDMS SSR The flame-treated F-PDMS The flame-treated SSR
103 120 153 160
90° Stick to the surface <1° <1°
can roll back and forth with no visible distortion, exhibiting ultralow adhesion to the superhydrophobic surfaces. In addition, as shown in Table 1, the water droplet on the FPDMS surface does not move when the slide angle is smaller than 90°; and the water droplet on the SSR surface can stick to it even though the surface is turned upside down. However, the SA for water droplet on the superhydrophobic surfaces having nanostructures or hierarchical micro/nano-structures merely reaches a limiting value of 1°. This result suggests that the lotus leaf-like hierarchical structured surfaces can be created through a simple flame treatment on the biomimetic shark-skin surface made of polymeric materials. And what is more important, just 20 s of flame treatment on the biomimetic shark-skin surface can make it exhibit superhydrophobicity. The high contact angles on the hierarchical structured surface can be easily interpreted by the Cassie’s theory [20] which has been widely used to study liquid drops in contact with microstructured surfaces. Two different wetting states can be observed on microstructured surfaces: (i) Wenzel state: the liquid droplet retains contact at all points with the solid surface below it and (ii) Cassie state: the liquid drop sits on the highest parts of the rough solid with trapped air underneath. The prepared dual-biomimetic surface possessing both the vivid shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures in this work can be considered as a typical air–solid composite surface, and the wetting behavior can be determined using the following Eq. (2) derived by Cassie and Baxter [20]:
cos h ¼ fS cos hS fG
contact the very top of the surface and does not penetrate the interspaces among the microstructures. These results indicate that the surface morphology plays an important role in the formation of a superhydrophobic surface. To investigate the durability of the biomimetic superhydrophobic surface, the flame-treated SSR sample with superhydrophobicity was either immersed in water for 2 month or continuously exposed to the water flow from the laboratory faucet at 1.0 m/s for 2 h and then the contact angle was measured. The test results show that the sample surface still maintains almost the same hydrophobicity. That is to say, there is little damage of the hierarchical structures on the surface and the dual-biomimetic surface can exhibit excellent long-term stability. The reason for it is partly due to the strong bonding of nano-silica particles with the PDMS substrate. 3.3. Liquid drop impact on various surfaces From the energy considerations [33–35] for both the wetting states, Wenzel state and Cassie state, the former is stable and the latter is metastable. It means that the water droplet will remain in Cassie state only if it is not subjected to external disturbances. There exists a free-energy barrier DG for the transition from the metastable Cassie state to the stable Wenzel state, as shown in Fig. 7 [36,37]. So it is quite necessary to consider this transition when the superhydrophobic surface possessing special surface
ð2Þ
where h is the apparent contact angle on the rough surface (160°), hS is the intrinsic contact angle on the smooth surface of an original FPDMS as a control sample (103°), fS is the total liquid/solid contact area divided by the projected area, and fG is the total liquid/vapor contact area divided by the projected area (i.e., fS + fG P 1) [32]. It is easy to deduce from Eq. (2) that increasing the value of fG, that is, the larger fraction of air on the surface, will lead to the increase of h. The rough structure on the surface can give the comparatively strong ability to trap large fraction of air within the interstices of the microstructures so that the water droplets are inclined to
Fig. 7. Schematic view of the free-energy barrier DG separating the Wenzel and Cassie state.
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topology is applied to practical dynamic water environment. The transition making a non-stick droplet sticky contradicts what we expect from a superhydrophobic surface. As the Wenzel state may not be suitable for many industrial applications involving drag reduction or self-cleaning, it is of great importance to predict the transition between Cassie and Wenzel state. The transition between different wetting states on the solid surface can be evaluated by analyzing the dynamic responses during the liquid drop impact, which is influenced by several parameters such as droplet size and impact velocity. These effects can be described in terms of dimensionless number, the Weber number We [38,39], which is defined as the ratio of kinetic energy to surface energy, characterizing the deformability of the droplet. The Weber number We can be calculated using the following equation:
We ¼
qV 2 r r
ð3Þ
here r is the radius of the liquid drop, V is the impact velocity, q is the liquid density, and r is the surface tension. Obviously, the
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Weber number, We, can be altered or adjusted by simply changing the impact velocity of the liquid drop, V, for the given liquid. In this work, water was used in liquid drop impact experiment. To figure out how the impact velocity influences the transition from the composite solid–air–liquid interface to the homogeneous solid–liquid interface during the water droplet impact, we performed bouncing droplet experiments on various surfaces by releasing droplets at different heights. Fig. 8 shows the snapshots of water droplet with 1.1 mm radius on various surfaces at different time intervals, including the smooth surface of F-PDMS, the microstructured surface of a SSR sample and the hierarchical micro/nano-structured surface of the flame-treated SSR sample. As observed from snapshots in Fig. 8, the water droplet impacting upon the F-PDMS surface and the SSR surface do not bounce off even though the impact velocity applied is up to 0.9 m/s, which means that the wetting states on the smooth PDMS surface and the microstructured surface similar to shark skin are in the Wenzel state. It is different in the case of the hierarchical micro/nanostructured surface exhibiting superhydrophobicity. Upon falling
Fig. 8. The snapshots of a droplet with 1.1 mm radius hitting various surfaces.
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on the hierarchical micro/nano-structured flame-treated SSR surface at the impact velocity of 0.5 m/s and 0.9 m/s, respectively, whose corresponding Weber numbers are 3.8 and 12.2, respectively, the water droplet first deforms followed by spreading and retracting, and finally rebounds off the surface within 10.98 ms and 10.54 ms. The Cassie model is usually used to explain this phenomenon. In the Cassie model, air can be trapped underneath the droplet and in the interstices of the microstructures, so that the droplet bounces off and cannot wet the surface. However, when the impact velocity reaches 1.5 m/s or the corresponding Weber number is 34.0, the droplet first deforms, then develops outward to form water ‘‘ring’’ with wavy bumps, and eventually merges together at the center to form an elongating water fountain. The bottom of the elongated droplet irreversibly adheres to the solid surface, which cause the droplet not to bounce off completely and just to be pinned on the partly wetted surface with maintaining small contact area after 3000 ms. The abovementioned results indicate that a transition from the composite interface to the homogenous interface can occur as the kinetic energy overcomes the surface energy and the liquid surface tension. For the flame-treated SSR sample, if the Weber number that depends on the features, size and impact velocity of liquid droplet reaches 34 or high, the Cassie state is broken and the droplet turns to the wetting Wenzel state, in which it is pinned on the surface. 4. Conclusion A brand new method was successfully developed using PDMS containing nano-silica as a substrate for producing a dual-biomimetic surface structure comprising both the shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures. It involves the PDMS microreplication processes using shark skin as a template and the subsequent flame treatment. The SEM observations show that the biomimetic shark-skin surface fabricated by the way of microreplication possesses vivid shark-skin surface morphology, or micron-sized shark-skin pattern structure; and the subsequent flame treatment makes it possess hierarchical micro/nano-structures with no damaging its shark-skin pattern structure, thereby constructing the dual-biomimetic surface as expected. The duration of flame treatment is an important factor affecting and controlling the hierarchical micro/nano-structures of the treated surface. The dual-biomimetic surface or the flametreated biomimetic shark-skin surface exhibits excellent superhydrophobicity with a low SA. Its CA reaches 160°, which increases by 40° as compared with that of the untreated biomimetic sharkskin surface. This implies that the flame treatment producing hierarchical micro/nano-structures on PDMS surfaces is a key process to fabricate superhydrophobic surface. According to the results of liquid drop experiment and their analyses, the robustness of the dual-biomimetic superhydrophobic surface is confirmed and the transition from the Cassie state to the Wenzel state arises and the phenomenon of pining a droplet on the surface occurs when the impact velocity exceeds a threshold velocity (V 1.5 m/s), at which the corresponding Weber number We reaches 34.0. Summarily, the novel method developed in this study can not only fabricate a superhydrophobic dual-biomimetic surface as expected, but also is characterized by simplicity, high efficiency and low cost. What is more significant, combined with the other techniques for fabricating biomimetic surfaces, it is expected to be
developed into a novel and feasible biomimetic surface manufacturing technique, that can create multifunctional biomimetic structured surfaces providing a better performance on self-cleaning, antifouling, drag reduction, antireflection, and so forth, thereby satisfying the requirements for practical applications in different fields. The further study on the drag reduction and anti-bioadhesion of the fabricated dual-biomimetic surface is in progress and will be reported in our future papers. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC, Grant No. 50873039) and the Foundation of Key Laboratory of Surface Functional Structure Manufacturing of Guangdong Higher Education Institutes, South China University of Technology (SFS-KF201011). The authors would like to thank Peng Xinyan, Su Dong, and Li Shuai of South China University of Technology. The authors also grateful acknowledge Mr. Wu Chaomao and his fellow workers from Yuan Ao International Trade Co., Ltd. (Hongkong) for the friendly supply of the high-speed camera. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
A new method for producing ‘‘Lotus Effect’’ on a biomimetic shark skin Yunhong Liu, Guangji Li ⇑ School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 14 April 2012 Accepted 14 August 2012 Available online 27 August 2012 Keywords: Biomimetic surface Shark skin Microreplication Lotus effect Superhydrophobicity
a b s t r a c t Nature has long been an important source of inspiration for mankind to develop artificial ways to mimic the remarkable properties of biological systems. In this work, a new method was explored to fabricate a superhydrophobic dual-biomimetic surface comprising both the shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures. The biomimetic surface possessing shark-skin pattern microstructure was first fabricated by microreplication of shark-skin surface based on PDMS; and then it was treated by flame to form hierarchical micro/nano-structures that can produce lotus effect. The fabricated biomimetic surfaces were characterized with scanning electron microscopy (SEM), water contact angle measurements and liquid drop impact experiments. The results show that the fabricated dual-biomimetic surface possesses both the vivid shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures. It can exhibit excellent superhydrophobicity that the contact angle is as high as 160° and maintain its robustness of the superhydrophobicity during the droplet impact process at a relatively high Weber number. The mechanism of the micromorphology evolution and microstructural changes on the biomimetic shark-skin surface was also discussed here in the process of flame treatment. This method is expected to be developed into a novel and feasible biomimetic surface manufacturing technique. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Nature provides abundant examples of structures, materials and surfaces which can be investigated to understand the basic principle and subsequently developed into fascinating technical applications [1]. The term biomimetic, which means learning from nature as an impulse for an independent technical design [2] is already popular in the field of materials science and engineering. There are many examples of biomimetic design originated from the investigation and copy of the special properties and mechanisms of natural plants and animals [3,4]. Such an example is the ‘‘shark skin effect’’, which is defined as a mechanism of wall friction reduction of a fluid resulted from a riblet structured surface similar to that of shark skin [5]. Shark skin has been widely studied for decades due to its drag reduction and antifouling properties [6–8]. Micron-sized grooved scales growing on shark skin, which are called dermal denticles, are interlocked to form a natural non-smooth surface; and the grooves between adjacent riblets on the scales are directed almost parallel to the longitudinal body axis of the shark. It has been reported that the grooved scales can reduce vortice formation or lift the vortice off the surface, so resulting in water moving easily over the skin ⇑ Corresponding author. Fax: +86 20 87113949. E-mail addresses: [email protected] (Y. Liu), [email protected] (G. Li). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.08.033
surface [9–11]. Besides, the rough texture formed by dermal denticles can reduce the adhesion area available to aquatic organisms and keep the surface clean. It is exciting that the principle has been adapted to aeroplane surfaces and achieved fuel-saving by about 1.5% [12]. Speedo invented the full-body swimsuit called ‘‘Fastskin’’ for elite swimming, which mimicked the shark-skin V-shape ridges [13]. ‘‘Sharklet’’ is another commercial product inspired by the overlapping, ridged platelet structures of shark scales. It can display excellent microbe resistant properties, which is very encouraging results to date [14,15]. Another well-known example is to design and fabricate biomimetic surface possessing ‘‘Lotus Effect’’, which is defined as the self-cleaning properties (phenomenon) and highly superhydrophobic surface like a lotus leaf [16]. It has been reported that the surface of a lotus leaf is covered with wax and has an intrinsic microscale and nanoscale hierarchical structures, providing superhydrophobicity, self cleaning, low adhesion and drag reduction [17–19]. Model proposed to interpret superhydrophobic phenomena was published by Cassie and Baxter [20], as well as Wenzel [21,22]. In the past decades, designing artificial superhydrophobic surfaces has become one of the top issues due to their potential applications in different realms, and numerous techniques have been developed to mimic lotus effect, including electrospinning [23], plasma treatment [24,25], chemical vapor deposition [26], molding [27] and phase separation [28,29]. However, there is still
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a long way to go for meeting the requirements for practical applications. Based on the understanding of the multi-level structures of multifunctional biological surfaces, the future research into bioinspired multi-functional surfaces can focus on the combination of various biomimetic structures by incorporating multiple technologies of forming surface topology so as to make the prepared surface exhibit excellent comprehensive properties close to the real biological surfaces as far as possible. The artificial surfaces inspired by the shark skin or the lotus leaf have showed unique properties and broad application prospects. Although the relationship between the nano- and microscale topographies and the surface properties of real shark skin and lotus leaf has not been fully understood, it can be predicted that a biomimetic multi-functional surface bearing the characteristics of both drag reduction and anti-bioadhesion may be produced by combining the directional microscale pattern of shark-skin surface with the nanoscale structure observed on the lotus leaf. And such a biomimetic surface can be used to optimize the surface design of underwater vehicles and fluid transportation pipelines, thus enhancing efficiency or reducing energy consumption; and to inhibit bioadhesion to the surface of underwater facilities in harsh water environments. Therefore, it is necessary and significant to develop a new method for achieving the particular combination of shark skin effect and lotus effect. In this work, we developed an original and efficient method to fabricate superhydrophobic dual-biomimetic surface comprising both the vivid shark-skin surface morphology and the lotus leaf-like hierarchical structures. In view of the fact that polydimethylsiloxane (PDMS) is a malleable material for developing topographies and its low surface energy is a key property for achieving a superhydrophobic surface state, PDMS containing nano-silica was chosen as a substrate material, the biomimetic shark-skin surface having micron-sized pattern structure was first fabricated by microreplication of shark-skin surface; and then, it was treated by flame to form hierarchical micro/nano-structures that can produce lotus effect, thereby constructing a dual-biomimetic surface. Scanning electron microscopy (SEM) was used to observe the surface morphology of the samples in the form of a PDMS sheet prepared via different routes, including microreplication, flame-treatment and microreplication followed by flame-treatment, respectively. Furthermore, their surface properties were characterized with water contact angle measurements and liquid drop impact experiments. The results were compared and analyzed using a flat PDMS sheet with a smooth surface as a control sample. The mechanism of the micromorphology evolution and microstructural changes on the dual-biomimetic surfaces in the process of flame treatment was also discussed within the paper.
2.2. Fabrication of the surfaces with special micro/nano-structures 2.2.1. Microreplication of shark-skin surface The microstructure of the shark-skin surface can be replicated to the surface of PDMS sheet by PDMS replica molding process. Due to its excellent processability, ease molding, wide spectrum of physical and mechanical properties, as well as excellent dimensional stability when curing, PDMS elastomer that can be crosslinked via the addition mechanism is chosen as the mold material to create topographical patterning of micron-scale features. The specific process involves the following steps. First of all, the treated shark skin was taken out from a refrigerator and left at room temperature for several hours, and then the shark skin was carefully fitted on a plate as a microreplication template. The plate with shark skin pattern was made into a mold for casting as shown in Fig. 1, which was designated as Mold-A. Secondly, a mixture of precursor and curing agent (10:1 by weight) was poured into Mold-A and cured for 20 min at 90 °C so as to transfer the surface microstructures of the shark skin to the PDMS surface contacting with the shark skin; and then the cured PDMS sheet was separated from Mold-A, thus obtaining the PDMS template for microreplication. The profile of the PDMS template was a counter-shape of the shark-skin pattern. Thirdly, the as-prepared PDMS template was used to manufacture a cavity block or negative mold in the same way as described in the first step, which was designated as Mold-B. Through the same microreplication process as the preceding step, the desired shark-skin replica (SSR), PDMS sheet with the microstructure of shark-skin surface, was fabricated. 2.2.2. Preparation of hierarchical structured surfaces The prepared shark-skin replica and the flat PDMS sheet without microstructure (F-PDMS) were used as substrates. The hierarchical morphological structures on the surfaces of both samples were prepared via flame treatment. Alcohol lamp was used in the flame treatment due to its consistent flame suitable for laboratory preparation. The surface of a substrate was close to the outer edge of the flame from an alcohol lamp and moved back and forth at a predetermined speed. After being treated for prescribed time, the samples were properly preserved for surface characterization and measurements. 2.3. Surface characterization 2.3.1. Observation of surface morphology The surface morphology of the prepared PDMS sheets was observed by a scanning electron microscope (S-3700N, Hitachi, Japan) and a field emission scanning electron microscope (LEO 1530 VP, Oberkochem, Germany). The sample surfaces to be observed were gold-coated using a sputter coater beforehand.
2. Experimental 2.1. Materials Fresh shark skin from a great white shark (Carcharodon carcharias), which is one of the fastest swimming sharks, was purchased from a fisherman. The subcutaneous fat was removed from fresh shark skin first. The shark skin was then washed several times with deionized water, carefully flattened, cut into the required shape and dried. The treated shark skin was stored in a refrigerator before use. A two-component room temperature vulcanizable liquid silicone rubber including precursor of PDMS and curing agent, which were purchased from Zhejiang Runhe Silicone New Material Co., Ltd., was used as received. The low viscosity precursor is composed of vinyl-polydimethylsiloxane (PDMS), which contains 25 wt.% of fumed silica (300 m2/g) filler treated with –Si(Me)2-O-oligomers.
Fig. 1. Schematic diagram of the microreplication process of shark-skin surface.
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Fig. 2. The SEM images of the shark-skin surface and the surfaces of PDMS sheets prepared via microreplication.
2.3.2. Measurements of contact angle and sliding angle Contact angle (CA) and sliding angle (SA) were measured with a Contact Angle Goniometer (DSA 100, Krüss GmbH, Germany) at ambient temperature. Each sample was measured for three times at three random locations and the average values of contact angle and sliding angle were calculated.
2.3.3. Liquid drop impact experiment In order to evaluate the robustness of various superhydrophobic solid surfaces, a very helpful method is to investigate the liquid drop impact dynamics onto the surfaces via the water drop impact experiment. The experimental procedure generally consists in releasing a water droplet with a millimeter-level diameter from a microsyringe at different heights to vary its impact velocity onto the solid surface. The falling liquid drops can be accelerated by gravity, thus hitting the solid surface at a certain impact velocity. The vertical impact velocity of the droplet released at the height h above the surface, V, can be calculated according to the following equation:
V¼
pffiffiffiffiffiffiffiffi 2gh
ð1Þ
where g is the acceleration of gravity (g 9.8 m/s2). The physical parameters of the water droplet are as follows: droplet radius r is approximately equal to 1.1 mm with 6% deviation, surface tension r is 0.0728 N/m and density q 1000 kg/m3 at 20 °C. The process of drop impact was recorded by a high-speed camera (pco.dimax HD, CooKe) with a recording rate of 1500 fps (frames per second) to obtain the information on the drop impact dynamics.
3. Results and discussion 3.1. Analyses of surface morphology Fig. 2 illustrates the SEM images of the surfaces of the real shark skin (Mold-A) and the PDMS sheet samples prepared by the microreplication technique. Compared with the real shark skin in Fig. 2a, the SSR sample in Fig. 2c possesses the almost same surface microstructure as that of the dermal denticles on shark skin. The crosslinking mechanism of additive cure type PDMS is illustrated in Fig. 3 [30]. It involves the addition of a silicon hydride (Si–H) to an unsaturated double bond in the presence of a noble metal catalyst such as platinum. Since the Mold-B and SSR are both made of PDMS, a technical key for preparing the Mold-B is to adjust the ratio of the PDMS precursor to its curing agent and to control reaction conditions, so as to make the reactive groups (vinyl groups or Si–H bonds) on the surface of the Mold-B exhausted as far as possible. When the Mold-B with no reactive groups on its surface is used to prepare SSR via the microreplication process described above, the addition curing reaction only occurs within the PDMS material that makes up SSR and makes SSR exhibit good elasticity, thus preventing the interface reaction between the Mold-B and the SSR. For this reason, the SSR can be easily separated from the MoldB with keeping the high-precision surface patterns. It indicates that the surface morphology of the dermal denticles on shark skin can be replicated with silicone rubber by the way of microreplication. In this process the properties of PDMS as a molding material plays an important role. The precursor of PDMS has excellent flowability due to its low viscosity, which makes the precursor fill into the interstice of the mold; and the subsequent crosslinking process can transform the flowable precursor into a solid
Fig. 3. The crosslinking mechanism of additive cure type silicone rubber.
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Fig. 4. (a) The SEM images (shown at four magnifications (I)–(IV)) of the flame-treated SSR sample and (b) image of a water droplet sitting on the flame-treated SSR surface.
PDMS sheet so as to maintain a conformal contact with the mold cavity and faithfully replicate its fine structure. The relatively low surface free energy of PDMS itself (csv = 21.6 dynes/cm2) [31] and the elasticity, and extremely chemical inertness of the cured PDMS will enable the prepared PDMS sheet to demold easily. Therefore, the silicone rubber replica molding process is a low-cost and reliable way for microstructure replication. The other polymers can also be used in such a microreplication with appropriate modifications. The further research into the surface morphology of the SSR samples treated by flame was conducted by SEM. The results are shown in Fig. 4. It can be seen from the SEM images (I) and (II) in Fig. 4a that the fine surface profiles of the SSR sample can be retained upon treated by flame. Moreover, it is very interesting that in Fig. 4a, the high-magnification SEM images (III) and (IV) of the flame-treated SSR surface display a unique hierarchical rough structure comprising a sub-microstructure and a nano-structure, in which the uniformly distributive micron-level aggregates (300–500 nm) constitute the sub-microstructure, and this is composed of nano-silica particles (20–50 nm). Accordingly, it can be said that the unique hierarchical micro/nano-structures observed on the flame-treated SSR surface bear much resemblance to those of natural lotus leaves. What is consistent with the above observation and analysis is that as shown in Fig. 4b, the flame-treated SSR surface can exhibit considerable superhydrophobicity, or water on this surface forms a spherical droplet. These results implies that the dual-biomimetic surface possessing the surface profile of shark skin and the hierarchical micro/nano-structures similar to those of a lotus leaf can be successfully fabricated through the microreplication of shark skin followed by flame treatment.
It should be noted that the duration of flame treatment is an important factor affecting and controlling the hierarchical micro/ nano-structures of SSR surface. To clarify the effect of flame treatment on the surface morphology of SSR, a few of SSR samples were treated by flame for different preset time, respectively; and then the flame-treated surfaces were observed with SEM. The SEM images are presented in Fig. 5. It can be clearly observed from Fig. 5 that the surface morphology and roughness of the used SSR samples change dramatically as the flame-treatment time increases. On the untreated surface there is no discernable feature. Once the surface is treated by flame for 5–10 s, some relatively regular and discrete embossment-like wrinkled surface structures along different directions begin to appear as shown in Fig. 5b and c; and the number of the regular wrinkled surface structures on the treated surface increases with an extension of the flame-treatment time. However, on the surface treated by flame for 30 s the ridges and troughs of the wrinkled surface structures disappear instead of a uniformly rough micro/ nano-structures as shown in Fig. 5d. The mechanism of the above-described micromorphology evolution and microstructural changes can be explained according to the principle of flame treatment and the analysis of the change in the physical properties of surface material during this process. Due to the action of the heat from flame, the flame treatment of surface conducted in air will lead to the surface oxidation of treated object in essence, especially for the surface of organic material. On the other hand, there may exist the thermal stress of surface generated by the non-uniform heating in the initial stage of flame treatment. Based on this principle, it can be speculated that while the surface of PDMS sheet is subjected to flame treatment, the ef-
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Fig. 5. The comparison of the SEM images of the SSR surfaces treated by flame for [(a) and (b)] 0 s; [(c) and (d)] 10 s; [(e) and (f)] 20 s; and [(g) and (h)] 30 s, respectively.
fect of the surface thermal stress and the oxidative activation of flame will induce the micro-deformation of the surface layer; and simultaneously, the non-uniform heating in the initial stage of flame treatment makes the polysiloxane located in the hightemperature region of the surface layer break down and partially degrade into inorganic SiO2 networks. Consequently, the relatively regular embossment-like wrinkled structures form on the surface of PDMS sheet. With the continuance of the flame treatment, the hydrophobic nano-silica particles previously embedded in the PDMS matrix are gradually exposed to generate flower-like nanoscale aggregates made from nano-silica. Finally, a uniformly rough surface possessing the unique hierarchical micro/nano-structures forms with increasing nano-silica aggregates on the sample surface. 3.2. Wettability of various surfaces It has been found that the surface wettability or hydrophobicity strongly depends on its topology besides its chemical nature. Thus, the water CA values of the different microstructured surfaces of PDMS sheets were measured to evaluate their hydrophobicity. The tested PDMS sheet samples included (a) F-PDMS sample with smooth surface, (b) SSR sample with vivid shark-skin morphology,
(c) the flame-treated F-PDMS sample with nano-structured surface, and (d) the flame-treated SSR sample with a hierarchical micro/nano-structured surface. The results are illustrated in Fig. 6 and the data on the water contact angle and sliding angle on the various surfaces are summarized in Table 1. The CA on the smooth surface of F-PDMS as a control sample is 103° (Fig. 6a), showing the hydrophobicity of PDMS as a low-surface-energy material. Compared to it, various microstructures fabricated on the sample surface can make the CA increase significantly, as shown in Fig. 6b–d. On the microstructured surface of SSR, the CA is 120°, which indicates that the SSR surface possesses stronger hydrophobicity than F-PDMS surface, but it is still not superhydrophobic. However, on the nano-structured surface of the flame-treated F-PDMS sample and the hierarchical micro/nano-structured surface of the flame-treated SSR sample, the measured CA values reach 153° and 160°, respectively. In other words, the CA of both the surfaces increases by 50° and 40°, respectively, as compared with that on the untreated surface of the corresponding sample. This implies that the surfaces of both samples (c) and (d) are superhydrophobic and the reason for it is that on the surfaces there form the hierarchical micro/nano-structures produced by flame treatment. And according to the experimental observation, the water droplet on the superhydrophobic surfaces is unstable and
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Fig. 6. The CA of water droplet on the surfaces of PDMS sheets fabricated via different routes and the corresponding schematic diagrams.
Table 1 Water contact angle and sliding angle measured on the various surfaces. Sample
CA (°)
SA
F-PDMS SSR The flame-treated F-PDMS The flame-treated SSR
103 120 153 160
90° Stick to the surface <1° <1°
can roll back and forth with no visible distortion, exhibiting ultralow adhesion to the superhydrophobic surfaces. In addition, as shown in Table 1, the water droplet on the FPDMS surface does not move when the slide angle is smaller than 90°; and the water droplet on the SSR surface can stick to it even though the surface is turned upside down. However, the SA for water droplet on the superhydrophobic surfaces having nanostructures or hierarchical micro/nano-structures merely reaches a limiting value of 1°. This result suggests that the lotus leaf-like hierarchical structured surfaces can be created through a simple flame treatment on the biomimetic shark-skin surface made of polymeric materials. And what is more important, just 20 s of flame treatment on the biomimetic shark-skin surface can make it exhibit superhydrophobicity. The high contact angles on the hierarchical structured surface can be easily interpreted by the Cassie’s theory [20] which has been widely used to study liquid drops in contact with microstructured surfaces. Two different wetting states can be observed on microstructured surfaces: (i) Wenzel state: the liquid droplet retains contact at all points with the solid surface below it and (ii) Cassie state: the liquid drop sits on the highest parts of the rough solid with trapped air underneath. The prepared dual-biomimetic surface possessing both the vivid shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures in this work can be considered as a typical air–solid composite surface, and the wetting behavior can be determined using the following Eq. (2) derived by Cassie and Baxter [20]:
cos h ¼ fS cos hS fG
contact the very top of the surface and does not penetrate the interspaces among the microstructures. These results indicate that the surface morphology plays an important role in the formation of a superhydrophobic surface. To investigate the durability of the biomimetic superhydrophobic surface, the flame-treated SSR sample with superhydrophobicity was either immersed in water for 2 month or continuously exposed to the water flow from the laboratory faucet at 1.0 m/s for 2 h and then the contact angle was measured. The test results show that the sample surface still maintains almost the same hydrophobicity. That is to say, there is little damage of the hierarchical structures on the surface and the dual-biomimetic surface can exhibit excellent long-term stability. The reason for it is partly due to the strong bonding of nano-silica particles with the PDMS substrate. 3.3. Liquid drop impact on various surfaces From the energy considerations [33–35] for both the wetting states, Wenzel state and Cassie state, the former is stable and the latter is metastable. It means that the water droplet will remain in Cassie state only if it is not subjected to external disturbances. There exists a free-energy barrier DG for the transition from the metastable Cassie state to the stable Wenzel state, as shown in Fig. 7 [36,37]. So it is quite necessary to consider this transition when the superhydrophobic surface possessing special surface
ð2Þ
where h is the apparent contact angle on the rough surface (160°), hS is the intrinsic contact angle on the smooth surface of an original FPDMS as a control sample (103°), fS is the total liquid/solid contact area divided by the projected area, and fG is the total liquid/vapor contact area divided by the projected area (i.e., fS + fG P 1) [32]. It is easy to deduce from Eq. (2) that increasing the value of fG, that is, the larger fraction of air on the surface, will lead to the increase of h. The rough structure on the surface can give the comparatively strong ability to trap large fraction of air within the interstices of the microstructures so that the water droplets are inclined to
Fig. 7. Schematic view of the free-energy barrier DG separating the Wenzel and Cassie state.
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topology is applied to practical dynamic water environment. The transition making a non-stick droplet sticky contradicts what we expect from a superhydrophobic surface. As the Wenzel state may not be suitable for many industrial applications involving drag reduction or self-cleaning, it is of great importance to predict the transition between Cassie and Wenzel state. The transition between different wetting states on the solid surface can be evaluated by analyzing the dynamic responses during the liquid drop impact, which is influenced by several parameters such as droplet size and impact velocity. These effects can be described in terms of dimensionless number, the Weber number We [38,39], which is defined as the ratio of kinetic energy to surface energy, characterizing the deformability of the droplet. The Weber number We can be calculated using the following equation:
We ¼
qV 2 r r
ð3Þ
here r is the radius of the liquid drop, V is the impact velocity, q is the liquid density, and r is the surface tension. Obviously, the
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Weber number, We, can be altered or adjusted by simply changing the impact velocity of the liquid drop, V, for the given liquid. In this work, water was used in liquid drop impact experiment. To figure out how the impact velocity influences the transition from the composite solid–air–liquid interface to the homogeneous solid–liquid interface during the water droplet impact, we performed bouncing droplet experiments on various surfaces by releasing droplets at different heights. Fig. 8 shows the snapshots of water droplet with 1.1 mm radius on various surfaces at different time intervals, including the smooth surface of F-PDMS, the microstructured surface of a SSR sample and the hierarchical micro/nano-structured surface of the flame-treated SSR sample. As observed from snapshots in Fig. 8, the water droplet impacting upon the F-PDMS surface and the SSR surface do not bounce off even though the impact velocity applied is up to 0.9 m/s, which means that the wetting states on the smooth PDMS surface and the microstructured surface similar to shark skin are in the Wenzel state. It is different in the case of the hierarchical micro/nanostructured surface exhibiting superhydrophobicity. Upon falling
Fig. 8. The snapshots of a droplet with 1.1 mm radius hitting various surfaces.
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on the hierarchical micro/nano-structured flame-treated SSR surface at the impact velocity of 0.5 m/s and 0.9 m/s, respectively, whose corresponding Weber numbers are 3.8 and 12.2, respectively, the water droplet first deforms followed by spreading and retracting, and finally rebounds off the surface within 10.98 ms and 10.54 ms. The Cassie model is usually used to explain this phenomenon. In the Cassie model, air can be trapped underneath the droplet and in the interstices of the microstructures, so that the droplet bounces off and cannot wet the surface. However, when the impact velocity reaches 1.5 m/s or the corresponding Weber number is 34.0, the droplet first deforms, then develops outward to form water ‘‘ring’’ with wavy bumps, and eventually merges together at the center to form an elongating water fountain. The bottom of the elongated droplet irreversibly adheres to the solid surface, which cause the droplet not to bounce off completely and just to be pinned on the partly wetted surface with maintaining small contact area after 3000 ms. The abovementioned results indicate that a transition from the composite interface to the homogenous interface can occur as the kinetic energy overcomes the surface energy and the liquid surface tension. For the flame-treated SSR sample, if the Weber number that depends on the features, size and impact velocity of liquid droplet reaches 34 or high, the Cassie state is broken and the droplet turns to the wetting Wenzel state, in which it is pinned on the surface. 4. Conclusion A brand new method was successfully developed using PDMS containing nano-silica as a substrate for producing a dual-biomimetic surface structure comprising both the shark-skin surface morphology and the lotus leaf-like hierarchical micro/nano-structures. It involves the PDMS microreplication processes using shark skin as a template and the subsequent flame treatment. The SEM observations show that the biomimetic shark-skin surface fabricated by the way of microreplication possesses vivid shark-skin surface morphology, or micron-sized shark-skin pattern structure; and the subsequent flame treatment makes it possess hierarchical micro/nano-structures with no damaging its shark-skin pattern structure, thereby constructing the dual-biomimetic surface as expected. The duration of flame treatment is an important factor affecting and controlling the hierarchical micro/nano-structures of the treated surface. The dual-biomimetic surface or the flametreated biomimetic shark-skin surface exhibits excellent superhydrophobicity with a low SA. Its CA reaches 160°, which increases by 40° as compared with that of the untreated biomimetic sharkskin surface. This implies that the flame treatment producing hierarchical micro/nano-structures on PDMS surfaces is a key process to fabricate superhydrophobic surface. According to the results of liquid drop experiment and their analyses, the robustness of the dual-biomimetic superhydrophobic surface is confirmed and the transition from the Cassie state to the Wenzel state arises and the phenomenon of pining a droplet on the surface occurs when the impact velocity exceeds a threshold velocity (V 1.5 m/s), at which the corresponding Weber number We reaches 34.0. Summarily, the novel method developed in this study can not only fabricate a superhydrophobic dual-biomimetic surface as expected, but also is characterized by simplicity, high efficiency and low cost. What is more significant, combined with the other techniques for fabricating biomimetic surfaces, it is expected to be
developed into a novel and feasible biomimetic surface manufacturing technique, that can create multifunctional biomimetic structured surfaces providing a better performance on self-cleaning, antifouling, drag reduction, antireflection, and so forth, thereby satisfying the requirements for practical applications in different fields. The further study on the drag reduction and anti-bioadhesion of the fabricated dual-biomimetic surface is in progress and will be reported in our future papers. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC, Grant No. 50873039) and the Foundation of Key Laboratory of Surface Functional Structure Manufacturing of Guangdong Higher Education Institutes, South China University of Technology (SFS-KF201011). The authors would like to thank Peng Xinyan, Su Dong, and Li Shuai of South China University of Technology. The authors also grateful acknowledge Mr. Wu Chaomao and his fellow workers from Yuan Ao International Trade Co., Ltd. (Hongkong) for the friendly supply of the high-speed camera. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
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