Regular Article
Clay-based superamphiphobic coatings with low sliding angles for viscous liquids

https://doi.org/10.1016/j.jcis.2019.01.024Get rights and content

Abstract

Although significant attention has been paid, most of superamphiphobic surfaces suffer from high sliding angles (SA) for liquids with low surface tension and complicated preparation methods. Also, superamphiphobic coatings with high repellency to the liquids with very high viscosity and low surface tension are rare. Here, we report preparation of clay-based superamphiphobic coatings with low SAs for viscous liquids. A homogeneous suspension was prepared by hydrolytic condensation of 1H,1H,2H,2H-perfluorodecyltriethoxysilane and tetraethoxysilane in the existence of attapulgite, a kind of natural clay mineral with nanorods-like microstructure. The superamphiphobic coatings were readily prepared by spray-coating the suspension onto substrates. The effects of attapulgite on microstructure and superamphiphobicity of the coatings were studied. Also, the static and dynamic superamphiphobicity were investigated. The attapulgite concentration has great influences on superamphiphobicity and solid-liquid adhesion force of the coatings, as it determines microstructure of the coatings. The superamphiphobic surfaces feature high contact angles and low SAs for various liquids including those with extremely high viscosity and low surface tension, e.g., hydroxyl-terminated polybutadiene (HTPB) and the HTPB/Al mixture (1:1, w/w). The coating also shows low solid-liquid adhesion force, high impact resistance and fast rolling of various liquids.

Introduction

Superhydrophobic surfaces have high water contact angle (CA > 150°) and low sliding angle (SA), but can be easily wetted by liquids with low surface tension [1], [2]. Differently, superamphiphobic surfaces feature high CAs and low SAs for both water and organic liquids with low surface tension [3], [4], [5], [6], [7]. Thus, superamphiphobic surfaces have very wide potential applications in many fields, such as self-cleaning surfaces, anti-oil climbing, anti-oil contamination, chemical shielding and anti-icing, etc. [8], [9], [10], [11], [12]. It has been well demonstrated that the combination of hierarchical microstructures and materials with low surface energy is a successful way to prepare superhydrophobic surfaces [13], [14], [15]. However, it is not that easy to fabricate superamphiphobic surfaces, because the surface tension of organic liquids is much lower than that of water [7]. In spite of high CAs, most of the reported superamphiphobic surfaces do not have low SAs for liquids with low surface tension, i.e., the liquids adhere stably on the surfaces even when the surfaces were turned upside down and thus the surfaces do not have the unique self-cleaning properties [4], [16], [17], [18], [19]. Although some of the superamphiphobic surfaces achieved low SAs for the liquids with low surface tension [4], the methods are complicated and expensive in order to design sophisticated re-entrant or double re-entrant microstructures. These bottle necks seriously hindered superamphiphobic surfaces from practical applications. Up to now, preparation of superamphiphobic surfaces with low SAs for diverse liquids with low surface tension via simple methods remains challenging [14], [20].

On the other hand, the wettability of a solid surface is closely related to properties of the liquids, mainly the surface tension. The solid-liquid dynamic interaction is also highly dependent on the viscosity and density of liquids [21], [22], [23]. In the field of bioinspired superhydrophobic and superamphiphobic surfaces, we simply used pure liquids like water and n-hexadecane as the probes to study their wettability [24], [25], [26]. This is important but insufficient to demonstrate the wettability of super anti-wetting surfaces, as liquids of diverse compositions, e.g., solutions, suspensions and emulsions, are used in academic research, industrial production and our daily life. For example, viscous liquids like polymer solutions and suspensions with high solid contents are frequently encountered, whereas their interaction with super anti-wetting surfaces is yet to be studied. A drop’s impact, motion, and rebound on a superamphiphobic surface rely on wettability of the superamphiphobic surface, impact velocity and inherent properties of the liquid drop. Compared to the liquids with high surface tension and low viscosity, it is challenging to prepare superamphiphobic surfaces with excellent static and dynamic repellency to the liquids with low surface tension and high viscosity. Until now, we can only found limited examples about the interaction between super anti-wetting surfaces and liquids with relatively high viscosity including super-cooled water (2.66 mPa·s at −10 °C) [22], [27], water/glycerol mixture (0.89–150 mPa·s) [23] and shampoo [28]. For example, Chen and Bonaccurso studied the effects of surface wettability and liquid viscosity on the dynamic wetting of individual drops by using water/glycerol mixtures that have comparable surface tensions (62.3–72.8 mN m−1) but different viscosities (1.0–60.1 mPa·s) [29]. Bhushan and his colleague fabricated superamphiphobic surfaces with repellency to shampoos [28].

Here, we report fabrication of superamphiphobic coatings with low SAs for the liquids with extremely high viscosity and low surface tension by the combination of organosilanes and attapulgite (APT). APT is a kind of phyllosilicate clay with a unique nanofibrous or nanorod-like microstructure [30].

Section snippets

Materials

Tetraethoxysilane (TEOS, 99.9%) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES, 97.0%) were purchased from Sigma-Aldrich. APT was supplied by Jiuchuan Technology Co. Ltd., Jiangsu, China. Ammonia (25.0–28.0 wt%), anhydrous ethanol (>99.7%), glycerol (98.0%), and n-hexadecane (>99.0%) were purchased from China National Medicines Co. Ltd. Hydroxyl-terminated polybutadiene (HTPB, >99.8%) was purchased from China Haohua Chemical Group Co., Ltd, with an average molecular weight of 3000. Al

Preparation of APT@F-POS superamphiphobic coatings

The preparation of the APT@F-POS superamphiphobic coatings is schematically shown in Fig. 1a. First, the APT@F-POS suspensions were prepared by ammonia-catalyzed hydrolytic condensation of TEOS and PFDTES in the presence of APT in ethanol. The APT nanorods are ∼1 μm in length and ∼50 nm in diameter (Fig. 1b) [31]. After hydrolytic condensation of the silanes, film-like F-POS was generated on the surface of the APT nanorods and among the nanorods (Fig. 1c). Subsequently, the superamphiphobic

Conclusion

In summary, superamphiphobic coatings with high repellency to various liquids including water, n-hexadecane, viscous HTPB and HTPB/Al are prepared by the combination of natural nanoclay and organosilanes. The introduction of the nanoclay generated a two-tier hierarchical micro-/nanostructure of the coatings. The superamphiphobicity and solid-liquid adhesion forces rely on microstructure of the coatings, which can be easily regulated by the nanoclay content. The superamphiphobicity reached the

Acknowledgements

This work was supported by the Presidential Foundation of China Academy of Engineering Physics (YZ2015010), National Natural Science Foundation of China (51873220 and 51503212), Funds for Creative Research Groups of Gansu, China (17JR5RA306), Major Projects of the Natural Science Foundation of Gansu, China (18JR4RA001) and the Talents of Innovation and Entrepreneurship Project of Lanzhou, China (2016-RC-77).

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    These authors contributed equally to this work.

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