Short communication
A novel flexible micro-ratchet/ZnO nano-rods surface with rapid recovery icephobic performance

https://doi.org/10.1016/j.jiec.2018.01.024Get rights and content

Highlights

  • ZnO nano-rods are uniformly planted on the flexible PDMS surface.

  • Fabrication of the surface integrates both crystal growth and physical vapor deposition methods.

  • The superhydrophobic surface successfully realizes fast-recovery from freezing state.

  • In freezing process, the nanorods can hardly cracked due to the flexible basement.

Abstract

In this communication, we present a significant method to fabricate a robust icephobic surface on a flexible polymer substrate. The flexible functional surface is obtained by integrating both soft-lithography and crystal growth methods. Modified by the materials with low surface free energy, the composite surface exhibits a robust superhydrophobicity at not only room temperature but also low temperature. After freezing test, the surface can recover to its original function quickly, which has achieved the level for industrial application, and also performed significant role for enhancing the icephobic theories.

Graphical abstract

The liquid droplet can recover to sphere state (Cassie's state) after melting from low temperature (for example, 20 °C).

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Introduction

Superhydrophobic substrates, which are characteristics of a high water contact angle (more than 150°) and a low sliding angle (less than 5°), have been studied in recent years for the water-repellence, anti-icing, anti-fogging, and self-cleaning applications [1], [2], [3], [4], [5]. Some plants, such as lotus leaves and rose petal, exhibit excellent superhydrophobic properties owing to their micro/nanostructure and the plant wax coating on the surface [6], [7], [8], [9]. Attracted by the natural plants, scientists have paid much attention to mimic the micro-nanostructure for getting the hydrophobic functions like the natural surfaces. However, limited by preparation technologies [10], [11], [12], [13], the bio-fabricated substrate materials are mainly rigid materials, which cannot remain the superhydrophobic property well in low temperature [14], [15], [16]. The condensed droplets easily penetrate into the valley of the topography and extend their volume during freezing process, because the rigid micro-nanostructure cannot deform and match the increasing volume of droplet, resulting in failure of the functions due to the continuous creak of topographies. Furtherly, the pocketed air at the solid/liquid interface is removed and replaced by the freezing droplet. Unsurprisingly, the creaked topography cannot recover to its initial state, even when the temperature raises to room temperature. To deal with this problem, researchers have pay much attention on the robust flexible materials such as liquid metal, polyvinylidene fluoride (PVDF), polydimethylsiloxane (PDMS) and polyethylene glycol, because of their easy-acquisition and excellent shape memory functions [17], [18], [19].

PDMS, as one of the most important and common materials for mimicking the structure of nature plants, has been widely used in bio-fabrication field [20], [21]. However, due to its distinctive chemical stability, PDMS is hard to be used as substrate for fabricating nanostructures on it. Although some researchers have prepared nanostructure on PDMS surface, for example, ZnO nanoparticles, the nanostructure could not be distributed uniformly on PDMS surface, which has made adverse impact for the function and applications [22], [23]. Meanwhile, the developments of new growth methods and new applications of ZnO nanorods are becoming more and more popular [24], [25], [26].

In our experiment, we fabricated ZnO nano-rods-coated PDMS surface by an enhanced crystal growth method. After modified by fluoride (FAS-17), the PDMS surface presents excellent superhydrophobic and icephobic properties. The surface recovers its initial superhydrophobic performance in a short time from the freezing state (in low temperature). This finding reveals an important issue for realizing robust recovery function from low temperature.

Section snippets

Materials

Zinc acetate dehydrate (A.R.), heptadeca fluorodecyltri-propoxysilane (FAS-17), hexamethylene tetramine (A.R.), monoethanolamine (A.R.), polydimethylsiloxane (PDMS, Sylgard 184, product of USA) and zinc nitrate hexahydrate (A.R.) were purchased from Sigma–Aldrich. And ethylene glycol monomethyl ether (A.R.) was received from Beijing Chemical Plant.

Preparation of the basement

The growth crystal seed was prepared by mixing and stirring 5.0 g zinc acetate dehydrate, 1.0 g monoethanolamine and 25 mL ethylene glycol monomethyl

Results and discussion

According to thermodynamic Yong's equation (Eq. (1)), roughness is the key factor for getting superhydrophobic performance on the hydrophobic material surface. In order to minimize the surface free energy, and obtain larger contact angle and small sliding contact angle, the roughness factor (r) becomes the most significant issue.cosθ=rcosθ0where, θ0 represents the contact angle of liquid droplet on the flat smooth surface, and θ is the contact angle on the roughness surface. We define that

Conclusions

In conclusion, the robust icephobic surface with fast recovery is successfully achieved by integrating the methods of soft-lithography and crystal growth. The surface performs excellent icephobicity at higher than −30 °C, and fast recovery of the robust performance from freezing state due to the elasticity of the basement material. This finding is significant to design novel surfaces for anti-icing, superhydrophobic, and deicing applications.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (Grant No. 21376030), the research grants of NRF funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (NRF-2015H1D3A1036078, and NRF-2017R1A2B2002607), the International Collaborative Energy Technology R&D Program (20178530000140) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), the KIST institutional program (2E21841), and Korea Institutional Program

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    These authors contributed equally to this work and consider to be first co-authors.

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