A new method for producing “Lotus Effect” on a biomimetic shark skin

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Abstract

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.

Highlights

► The dual-biomimetic surface comprises both shark-skin and lotus leaf-like hierarchical structures. ► Flame treatment is used for the first time to fabricate superhydrophobic surfaces on PDMS. ► The flame treatment time greatly influences the structure on the PDMS surface. ► The biomimetic surface exhibits excellent superhydrophobicity with a low sliding angle.

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], [7], [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 surface [9], [10], [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], [18], [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 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.

Section snippets

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

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

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

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

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