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Particuology 11 (2013) 361– 370

Contents lists available at SciVerse ScienceDirect

Particuology

jou rn al h om ep age: www.elsev ier .com/ locate /par t ic

eview

anoparticles assembly-induced special wettability for bio-inspired materials

huai Yanga, Xu Jina,c, Kesong Liua,∗, Lei Jianga,b

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of the Ministry of Education, School of Chemistry and Environment, Beihang University,eijing 100191, ChinaBeijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, ChinaResearch Institute of Petroleum Exploration and Development, Beijing 100191, China

a r t i c l e i n f o

rticle history:eceived 12 December 2012eceived in revised form 7 January 2013ccepted 4 February 2013

a b s t r a c t

Through billions of years of evolution, nature has optimized the programmed assembly of the nano- andmicro-scale structures of biological materials. Nanoparticle assembly provides an avenue for mimick-ing these multiscale functional structures. Bio-inspired surfaces with special wettability have attracted

eywords:anoparticle assemblyio-inspired materialsettability

urface

much attention for both fundamental research and practical applications. In this review, we focuson recent progress in nanoparticle assembly-induced special wettability, including superhydrophilicsurfaces, superhydrophobic surfaces, superamphiphobic surfaces, stimuli-responsive surfaces, and self-healing surfaces. A brief summary and an outlook of the future of this research field are also provided.

© 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of

Sciences. Published by Elsevier B.V. All rights reserved.

ontents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3612. Superhydrophilic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3623. Superhydrophobic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3624. Superamphiphobic surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3635. Stimuli-responsive surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3656. Self-healing surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3677. Conclusions and outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

. Introduction

Nanoparticle assembly plays a vital and functional role in almostvery related field (Bonderer, Studart, & Gauckler, 2008; Li & He,012; Luo, Du, Wang, Lu, & Xu, 2011; Omenetto & Kaplan, 2010;heng et al., 2010). To construct ordered structures with specialunctions, it is crucial to control and manipulate the assembly ofanoparticles, where the building blocks spontaneously organize

nto ordered structures through thermodynamic processes and

improve nanoparticle performance (Andrews, Eccles, Schofield, &Badyal, 2011; Chen, Gao, Song, Smet, & Zhang, 2010; Gao & Yan,2012; Li, Wang, & Song, 2011). Although the field of nanoparticleassembly is in its infancy, it is rapidly growing and offers enormouspromise (Grzelczak et al., 2010).

The wetting behavior of solid surfaces is a very important aspectof surface chemistry and has a wide variety of practical applica-tions in industry, agriculture, and daily life (Liu, Yao, & Jiang, 2010).Usually, surface wettability is governed by surface geometric struc-

ther constraints (Grzelczak, Vermant, Furst, & Liz-Marzan, 2010).anoparticle assembly is an effective approach for preserving thehemical properties of nanoparticles that can also be applied to

∗ Corresponding author. Tel.: +86 1082316066.E-mail address: liuks@buaa.edu.cn (K. Liu).

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674-2001/$ – see front matter © 2013 Chinese Society of Particuology and Institute of Process Ettp://dx.doi.org/10.1016/j.partic.2013.02.001

ures and surface chemistry. Through billions of years of evolution,ome biological surfaces, including lotus leaves, rice leaves, butter-y wings, mosquito compound eyes, cicada wings, red rose petals,

ecko feet, desert beetles, and spider silk, have been imparted withxcellent surface wettability (Askarieh et al., 2010; Feng et al., 2010;ao & Jiang, 2004; Jiang, Zhao, & Zhai, 2004; Liu, Du, Wu, & Jiang,012; Parker & Lawrence, 2001; Prakash, Quéré, & Bush, 2008;

ngineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

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62 S. Yang et al. / Particu

u et al., 2011; Zhang, Zhang, Xie, Liu, & Shao, 2006; Zhang, Guo,hang, Wu, & Zhang, 2012; Zheng, Gao, & Jiang, 2007). In theseaterials, hierarchical nano- and micro-structures play important

oles in determining wettability. Nature has long been a sourcef bio-inspiration for scientists and engineers (Liu & Jiang, 2011;ang et al., 2012). Biomaterials with desirable surface wettability

roperties have inspired scientists to discover special wettability.n the past decade, nanoparticle assembly proved to be an effec-ive method for constructing functional surfaces with multiscaletructures (Bhushan & Jung, 2011; Crick & Parkin, 2010; Du et al.,010a; Liu & Jiang, 2011; Liu & Jiang, 2012; Liu et al., 2010, 2011b;apadopoulou, Zorba, Stratakis, & Fotakis, 2012; Quéré, 2008; Sun,eng, Gao, & Jiang, 2005; Xin & Hao, 2010; Yang, Zhang, Men, Xu,

Zhu, 2011; Zhang, Xia, Kim, & Sun, 2012). In recent years, a greateal of research has focused on constructing functional surfacesith special wettability through nanoparticle assembly, as well as

n combination with surface functionalization.This review will focus on recent developments in nanoparti-

le assembly-induced, bio-inspired special wettability, particularlyuperhydrophilic surfaces, superhydrophobic surfaces, superam-hiphobic surfaces, stimuli-responsive surfaces, and self-healingurfaces. A brief summary and outlook of recent research progressn bio-inspired special wetting via nanoparticle assembly will beresented in the final section.

. Superhydrophilic surfaces

Superhydrophilic biomaterial surfaces are usually found inquatic environments, such as shorebird beaks (Prakash et al.,008). These superhydrophilic surfaces help the organisms survivend adapt to their peripheral environment. The most intriguingunction of superhydrophilic surfaces is their ability to anti-fogven in moist environments (Gan et al., 2007). This phenomenons dictated by water contact angles of less than 5◦ that enable

ater droplets to spread flat within 0.5 s or less (Dong et al.,010). These special nano-scale particle surfaces that cooperateith micro-scale tissues have inspired scientists to fabricate hier-

rchical surface structures via nanoparticle assembly (Kou & Gao,011; Zhong, Liu, Chen, Wang, & Yang, 2006).

TiO2 is one of the most important oxides (Fujishima, Zhang, &ryk, 2008). Since the discovery of light-induced amphiphilicityboth hydrophilicity and oleophilicity) of TiO2 by Fujishima et al. in997 (Wang et al., 1997), titania has been widely used as a super-ydrophilic surface due to its photocatalytic and photo-induceduperhydrophilic properties. The cooperation of photocatalysisnd photo-induced superhydrophilicity provides a self-cleaningunction that can be used for a wide variety of practical applica-ions. In the past decade, a wide variety of TiO2-based hydrophilicurfaces have been fabricated using nanoparticle assembly onifferent substrates (Blossey, 2003; Fujishima et al., 2008; Liut al., 2005b, 2006a; Liu, Zhang, Shi, & Fu, 2005; Liu et al., 2005a,006b; Parkin & Palgrave, 2005; Xu et al., 2010; Zhou et al.,008).

Using layer-by-layer assembly, Liu and He (2009) coated a trans-arent substrate with silica nanoparticles and polyelectrolytesontaining poly(diallyldimethylammonium) chloride (PDDA) andodium poly(4-styrenesulfonate) (PSS) (Fig. 1(A)). It took 0.28 s forhe droplets to spread flat and coat the glass, with a maximumransmittance approaching 98.5%. In contrast to layer-by-layerssembly, one-step synthesis can fabricate hierarchical mesostruc-

ured silica nanoparticles. Du and He (2011) dip-coated theseierarchical mesostuctures on glass substrates (Fig. 1(B)). Theoatings increased the transmittance of glass from 90% to 96%, andecrease the spreading time to approximately 0.3 s.

schs

11 (2013) 361– 370

Taking these various structured morphologies into consider-tion, Du, Liu, Chen, and He (2009) generated a raspberry-likeopology via a single-step sol–gel method (Fig. 1(C)). Raspberry-likearticles with hierarchical surface morphologies have high surfaceoughness that can provide desirable wettability. These surfacesllow droplets to spread flat in 0.5 s. Although their maximumransmittance is not as good as other surfaces, these transparentubstrates with superhydrophilicity have an important applicationn the field of anti-fogging (Fig. 1(E) and (F)).

Nanoparticle assembly can also be used to modify the surfaceettability of natural clothing materials. For example, environ-entally stable superhydrophilic wool fabrics were fabricated by

oating an ultrathin silica layer onto natural wool fabrics (Chent al., 2010a) (Fig. 1(D)). The ultrathin silica layer increased both theurface roughness and the surface energy of the fibers and endowedhe wool fabrics with excellent water absorption. Due to the opti-al transparency, chemical stability, and nontoxicity of silica, theesulting fabric maintained its color and morphology.

. Superhydrophobic surfaces

Superhydrophobic surfaces are often found on plant leaves,nsect wings, and insect eyes (Liu et al., 2010). These specialiomaterial surfaces have low surface energies and possessierarchical structures ranging in scale from nano to micro toacro (Steele, Bayer, & Loth, 2009). Inspired by biomaterials with

uperhydrophobicity, a wide variety of superhydrophobic surfacesave been fabricated (Bhushan & Jung, 2011; Callies & Quéré,005; Crick & Parkin, 2010; Feng & Jiang, 2006; Feng et al., 2002;enzer & Marmur, 2008; Liu, Zhang, Zhai, Wang, & Jiang, 2008;iu et al., 2010; Liu, Li, Wang, & Jiang, 2011; Liu et al., 2012a; Ma

Hill, 2006; Quéré, 2008; Roach, Shirtcliffe, & Newton, 2008; Sunt al., 2005; Voronov, Papavassiliou, & Lee, 2008; Xia & Jiang, 2008;in & Hao, 2010; Yao, Song, & Jiang, 2011; Zhang, Shi, Niu, Jiang, &ang, 2008; Zhang, Chen, Shi, Li, & Guo, 2012; Zhao, Liu, Li, Wang,

Jiang, 2009). In this section, we mainly focus on nanoparticlessembly-induced special wettability.

Usually, silica coatings with hierarchical structures exhibitxcellent superhydrophilicity. Using mesoporous silica nanopar-icles as building blocks, Du, Li, & He (2010). fabricateduperhydrophilic coatings (Fig. 2(A)). After modification, theoatings exhibited hydrophobic properties. Furthermore, the sil-ca coatings enhanced long wavelength transmittance and reducedhort wavelength transmittance as compared with uncoated glass.rzh, Genish, Klein, Solovyov, & Gedanken (2010) used microwavelasma to fabricate ZnO and Zn nanoparticles coatings on glassFig. 2(B)). The hydrophobic ZnO increased the water contactngle.

Superhydrophobic surfaces with very high static water contactngles and very low sliding angles can be used for self-cleaning.u, Karunakaran, Guo, & Yang (2012) fabricated transparent,uperhydrophobic surfaces by one-step spin-coating assembly ofuorosilane modified silica nanoparticles. These surfaces, pos-essing high static water contact angles and low sliding angles,ould be fabricated on various substrates (Fig. 2(C)). The superhy-rophobic films were highly transparent in the visible spectrum,ith greater than 95% transmittance. Inspired by the special wet-

ing of biomaterials, Liu, Zhai, & Jiang (2008) used nanoparticlessembly to fabricate superhydrophobic Sb2O3 films with nano-nd micro-scale hierarchical structures. These films exhibited high

tatic water contact angles and low sliding angles (Fig. 2(D)) thatould be useful for self-cleaning. The low adhesivity and high super-ydrophobicity can be attributed to the cooperation of multiscaleurface structures and residual surface alkyl chains.

S. Yang et al. / Particuology 11 (2013) 361– 370 363

Fig. 1. SEM images of (A) layer-by-layer (LBL)-deposited (PDDA/S-150)2/(PDDA/S-30) coatings (Liu & He, 2009), (B) mesostructured silica nanoparticles with polycations(Du & He, 2011), (C) raspberry-like silica shells (Du et al., 2009), and (D) wool fibers coated with silica nanoparticles (Chen et al. 2010a). Insets represent magnified views.( a parta glass d

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E) Comparison of the fogging behavior of a bare glass slide (right-hand slide) with

superhydrophilic film showing a comparison of normal glass wetting and coated

Nanoparticle assembly-induced superhydrophobic surfacesave important applications in other fields. For example, Lee, Kwak,

Yong (2011) fabricated superhydrophobic nanocoatings by spin-oating ZnO nanoparticles. This coating can be applied to metals,emiconductors, paper, cotton fabric, and even flexible substrateso impart a high static water contact angle with a small contactngle hysteresis (Fig. 2(E) and (F)). Moreover, this superhydropho-ic coating is highly stable in thermal and dynamic conditions,uggesting that it may be potentially useful for industrial appli-ations.

Controlling the wettability of solid surfaces is a key issuen surface engineering. To achieve stable bio-inspired super-ydrophobicity, it is essential to consider the cooperation of

urface chemistry and surface roughness on multiple scales. Bio-nspired superhydrophobic materials can be applied for corrosionesistance, microfluidic systems, oil–water separation, liquid trans-ortation, liquid painting and reprography, and antireflection

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ially coated glass slide (left-hand side). (F) A slide glass half-coated (left side) withewetting (Cebeci, Wu, Zhai, Cohen, & Rubner, 2006).

Hancock, Sekeroglu, & Demirel, 2012; Liu & Jiang, 2011; Yao et al.,011; Zhang, Liu, Li, & Jiang, 2013).

. Superamphiphobic surfaces

Superhydrophobicity can be found in many biological mate-ials; however, superoleophobic properties are extremely rare.his rarity can be attributed to the fact that oils and other organiciquids possess lower surface tensions than water. Therefore, it is

uch more difficult to fabricate superamphiphobic (both super-ydrophobic and superoleophobic) surfaces than it is to fabricateuperhydrophobic surfaces. Through rational design of the surface

hemical composition and surface geometry, a wide variety ofuperamphiphobic surfaces have been recently constructed (Liu,ian, & Jiang, 2013; Tuteja et al., 2007; Tuteja, Choi, McKinley,ohen, & Rubner, 2008; Zhao et al., 2009). This section will focus on

364 S. Yang et al. / Particuology 11 (2013) 361– 370

Fig. 2. SEM images of (A) hierarchically mesoporous silica nanoparticles (Du et al., 2010b), (B) zinc nitrate ethanol solution coatings (insets are magnified views) (Irzh et al.,2010), and (C) spin-coated fluorosilane modified silica nanoparticles (inset is a digital photo of coatings with water (blue)) (Xu et al., 2012). (D) Optical image of waterd y (Liu

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roplets of different sizes (17–22 �L) on Sb2O3 film exhibiting superhydrophobicitroplets showing extreme superhydrophobicity (Lee et al., 2011). (For interpretationf this article.)

ecent progress in the development of superamphiphobic surfaceshrough nanoparticle assembly.

Fluorinated silica nanoparticles can be used not only for theabrication of superhydrophobic coatings but also for the prepa-ation of superamphiphobic surfaces. Sheen, Huang, Liao, Chou,

Chang (2008) fabricated an extremely superamphiphobic sur-ace using fluorinated silica nanoparticles (Fig. 3(A)). The contactngles of water and diiodomethane reached 167.5◦ and 158.6◦,espectively. These superamphiphobic organic–inorganic hybridlms can be applied as functional coatings to various substrateshrough a simple coating process. He et al. (2011) fabricated

transparent superamphiphobic coating with improved stabil-ty by coating perfluorooctyl-trichlorosilane (PFTS) on a sinteredase composed of polydimethylsiloxane (PDMS) and hydrophobicilicon dioxide nanoparticles (Fig. 3(B)). Because of their trans-arency, superamphiphobic properties, and improved stability,hese superanphiphobic coatings have many outdoor applications.

uperamphiphobic diblock copolymer coatings on silica particlesere fabricated by Xiong, Liu, Hong, & Duncan (2011) using a

ol–gel approach (Fig. 3(C)). These films can be coated on var-ous substrates, including paper, to obtain superamphiphobicity

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et al., 2008a). (E)–(F) Digital images of coatings on different substrates with watere references to colour in this figure legend, the reader is referred to the web version

Fig. 3(E)). These films are also resistant to aqueous NaOHtching. Using a simple one-step dip coating method, silica-ased coatings were fabricated on glass substrates to provideater- and oil-repellent properties with stable weathering abil-

ty (Nimittrakoolchai & Supothina, 2012). One-pot co-condensationethodology was proposed to fabricate superamphiphobic cot-

on textiles using hydrophobic mesoporous silica nanoparticlesPereira et al., 2011). These organic–inorganic hybrid nanopar-icles can be used to design high-performance multifunctional

aterials.Using one-step vapor-phase polymerization of polypyrrole

PPy) in the presence of a fluorinated alkyl silane (FAS), Wang, Xue, Lin (2011) fabricated patterned, electrically conductive, super-mphiphobic coatings on fibrous materials. The contact anglesf water and hexadecane reached 165◦ and 154◦, respectively.atterned PPy – FAS coatings were generated by screen-printingo quench the polymerization reaction in fabric areas that did

ot require functionalization (Fig. 3(D)). These patterns can besed to form circuits that drive electronic devices with applica-ion including multi-functional protective clothing and electronicsextiles.

S. Yang et al. / Particuology 11 (2013) 361– 370 365

Fig. 3. (A) SEM images of fluorinated silica nanoparticle films (inset is the magnified view) (Sheen et al., 2008). (B) SEM images perfluorooctyl-trichlorosilane coatings (insetis the AFM image) (He et al., 2011). (C) Silica particle coatings with diblock copolymers (inset is the AFM image) (Xiong et al., 2011). (D) Procedure for making PPy – FASpatterns on fabrics (a) with examples of PPy – FAS patterns (b), a PPy – FAS letter pattern (c), and a simple PPy – FAS circuit for lighting a LED device (d). Colored water(green) and hexadecane (red) droplets are shown on the working PPy – FAS surface (Wang et al., 2011b). (E) Photograph of cooking oil and water droplets on the surfaceso etatiov

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Recently it was demonstrated that the rational design of over-ang and re-entrant structures is essential for the reproducibleonstruction of superamphiphobic surfaces (Ahuja et al., 2008;ao, Price, Weiss, & Gao, 2008; Nosonovsky, 2007). Nanoparticlessembly represents an effective approach for the constructionf functional surfaces with overhang and re-entrant structures inombination with low surface energy coatings for applications inhe field of superamphiphobicity.

. Stimuli-responsive surfaces

Wetting structured surfaces respond to stimuli instantaneouslyy reversing their wetting ability to adjust to particular changes,imilar to gecko feet (Koch & Barthlott, 2009; Liu et al., 2012a).n recent years, functional surfaces with reversibly tunable wett-bility have received much attention due to their important appli-ations in fundamental research and industry. Usually, surfaces

rafted with functional polymers are responsive to one or morenvironment conditions (Zhang & Han, 2010). Stimuli-responsiveurfaces can be divided into different categories based on theiresponse to various types of stimuli (e.g., pH, light, temperature,

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n of the references to colour in this figure legend, the reader is referred to the web

nthalpy, magnetic field, selected solvent, and electrical poten-ial) (Liu et al., 2013; Stuart et al., 2010). The functional surfaceslter their structures and properties in response to changes in theirnvironment, resulting in reversible switching of wettability.

Polymer brush–nanoparticle assemblies have been used forensors, medical diagnostics, and information storage (Chen,erris, Zhang, Ducker, & Zauscher, 2010; Orski, Fries, Sontag, &ocklin, 2011). Lim et al. (2008) prepared smart surfaces withuperhydrophobic to superhydrophilic wetting transition throughrogrammable ion-pairing interaction of gold micro/nanotexturedubstrates (Fig. 4(A)). The counteranion plays a key role in deter-ining the hydrophobic/hydrophilic character of the switchable

urface. Such materials have a wide range of practical applicationsn the fields of biosensors, microfluidics, and smart coatings.an, Kim, & Karim (2007) reported UV-ozone (UVO)-tunable

uperhydrophobic to superhydrophilic wetting transition on aontinuous biomimetic nanostructured hybrid film. The film

as fabricated using LBL assembly of negatively charged silicaanoparticles and positively charged poly(allylamine hydrochlo-ide) (Fig. 4(B)). The roughness and nanoporosity of these surfacesan be controlled. Furthermore, it was possible to modify the

366 S. Yang et al. / Particuology 11 (2013) 361– 370

Fig. 4. (A) SEM images of the gold micro-/nanotextured substrates produced by the galvanic cell displacement reaction and photographs of water droplets on a smoothsubstrate and a rough substrate, showing reversible changes in wettability (Lim et al., 2008). (B) An illustration of roughness enhancement and nanopore generation duringl top an( lity ofe

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ayer-by-layer assembly of poly(allylamine hydrochloride) and silica nanoparticles (C) A diagram showing the molecular mechanism of thermally-responsive wettabilectrospun composite films (Gu et al., 2010).

urface sensitivity using different monochlorosilane monolayerolecules applied by vapor deposition. The wetting transition was

ependent on the surface roughness and nanoporosity generatedfter LBL assembly of the silica nanoparticles and the polyca-ions. Poly(N-isopropylacrylamide) (PNIPAAm) is one of the mostopular temperature-responsive polymers owing to its sharphase transition in water at approximately 32 ◦C. This respon-ive polymer undergoes thermo-responsive switching betweenydrophilicity and hydrophobicity, arising from competitionetween intermolecular and intramolecular hydrogen bondingelow and above the lower critical solution temperature (LCST).hen PNIPAAm was grafted on a rough substrate, reversible

witching between superhydrophilicity and superhydrophobicityas achieved due to amplification by the surface roughness (Sun

t al., 2004). Electrospinning is a promising approach for fabricat-ng functional nanomaterials. Gu, Wang, Li, & Ren (2010) preparedhermo-responsive biocompatible nanofibrous films with switch-ble wettability made from PNIPAAm/poly(l-lactide) (PLLA). Theseanofibrous films had tunable surface morphologies and superioriocompatibility. The wettability of electrospun PNIPAAm/PLLAomposite films could switch from superhydrophilicity to super-ydrophobicity when the temperature increased from 20 to 50 ◦CFig. 4(C)). Recently, Zhang et al. (2012a) fabricated superhy-rophobic and superoleophilic nanoparticle films using a one-steppray deposition process. These films exhibited responsiveettability that transitioned between superhydrophobicity andydrophilicity according to the temperature.

Among the wide variety of external stimuli that can be usedor switching, light is one of most promising. A large numberf photo-responsive inorganic oxides and organic polymers haveeen developed for constructing functional surfaces with reversibleuperhydrophobicity to superhydrophilicity transitioning (Sun &ing, 2011; Sun, Qing, Su, & Jiang, 2011; Xin & Hao, 2010). Usu-

lly, these surfaces exhibit superhydrophilicity after ultravioletllumination and revert to their original superhydrophobic statefter dark storage. Lim, Kwak, Lee, Lee, & Cho (2007) prepared

rose-like porous nanostructured V2O5 film by drop-casting an

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d left) with photographs of the water contact angles on the films (Han et al., 2007). PNIPAAm/PLLA composite films (top), and photographs of water-droplets on the

thanolic suspension of V2O5 particles synthesized using a sol−gelethod. The resultant nanostructured V2O5 films exhibited switch-

ble wettability from superhydrophilicity to superhydrophobicityy UV irradiation and dark storage, respectively (Fig. 5). Theetting transition can be attributed to cooperation between thehotosensitivity of V2O5 and the surface roughness of its nano-tructure, which has submicron- to micron-scale apertures. Anmportant and interesting property used in the fields of drug deliv-ry, sensors, and separation is pH-responsive wettability. Chent al. (2010b) fabricated pH-responsive substrates by chemicalodification of a rough gold surface. The rough gold surfaceas used as a nonplanar substrate by applying LBL assembly

nd electrochemical deposition. After chemisorption of 2-(11-ercaptoundecanamido)benzoic acid, the rough surface exhibited

H-responsive wettability from superhydrophobicity to superhy-rophilicity. The use of pH-responsive coating on gold thread canrovide larger supporting forces at low pH than at high pH due toydrophobicity at high pH, making it possible to control the motionf gold threads in water.

To extend the application of stimuli-responsive surfaces,any different synthesis methods have been developed to con-

truct dual- or multiple-responsive materials (Jin, Yang, Li, Liu, Jiang, 2012; Xin & Hao, 2010). For example, Xia et al.

2007) fabricated multi-stimuli-responsive surfaces that exhibitedeversible switching between superhydrophilicity and super-ydrophobicity in response to glucose, temperature, and pH.he multi-stimuli responsive surfaces were constructed using alock copolymer coating containing pH/glucose- and temperature-ensitive blocks. Tan, Cao, Yang, Wang, & Sun (2011) preparedolystyrene/polyaniline (PS/PANI) composite microspheres resem-ling sea urchin core-shell structures using seeded emulsionolymerization. These sea urchin-like PS/PANI composites con-ained radial arrays of nanofibers and could switch between

uperhydrophilicity and superhydrophobicity in response to eitherlectrical potential or pH. In this growing field, the design andonstruction of robust surfaces with multi-stimuli-responsiveettability is an exciting direction. However, mechanical stability

S. Yang et al. / Particuology 11 (2013) 361– 370 367

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Self-healing is a unique function of living organisms thathas inspired a large number of studies focusing on the design

Fig. 6. Microencapsulated healing agents can be embedded in structural composite

ig. 5. Photoresponsive wettability of the nanostructured V2O5 substrate: (left) watark storage and (right) SEM image of an individual rose-like spherical V2O5 nanos

s a crucial problem for practical applications of responsive surfacesith special wettability.

. Self-healing surfaces

Self-healing is an inherent characteristic of living organismsBrochu, Craig, & Reichert, 2011). Biomaterials can maintain theiruperhydrophobicity by regenerating their epicuticular wax layerfter receiving damage. This mechanism of self-healing is widelyound in nature, but can also be used in the design of new typesf man-made self-healing materials. For example, by incorporat-ng a microencapsulated healing agent and a catalytic chemicalrigger within an epoxy matrix, a special self-healing functionan be achieved (White et al., 2001). When a crack approaches,his structural polymeric material releases the healing agent. Poly-

erization of the healing agent is triggered by contact withn embedded catalyst, thereby bonding the crack faces (Fig. 6).nspired by the self-healing superhydrophobicity of living plants,

any different approaches have been developed to construct self-ealing superhydrophobic or superamphiphobic materials (Ionov

Synytska, 2012; Li, Li, & Sun, 2010; Liu, Wang, Yu, Zhou, & Xue,012; Puretskiy, Stoychev, Synytska, & Ionov, 2012; Wu, Meure, &olomon, 2008; Yin, Xue, Chen, & Fan, 2009).

LBL assembly is a substrate-independent method for the fab-ication of various types of coatings with well-tailored chemicalompositions and architectures. Li et al. (2010) fabricated porousnd flexible coatings with micro- and nano-scaled hierarchicaltructures using LBL assembly of polyelectrolyte complexes com-osed of poly(allylamine hydrochloride) and sulfonated poly(etherther ketone) with poly(acrylic acid). By incorporating heal-ng agents of reacted fluoroalkylsilane in the layered polymericoatings, the materials exhibited self-healing superhydrophobic-ty (Fig. 7). When the top layer of fluoroalkyl chains decomposes orhe coatings are scratched, the healing agents migrate to the surfaceo restore superhydrophobicity. This self-healing process can beepeated many times without decreasing the superhydrophobicity.

Using mesoporous silica as a reservoir for hydrophobicolecules (octadecylamine, ODA), Liu et al. (2012b) prepared

elf-healing hydrophobic surfaces without using fluoro-containingompounds. After damage, ODA can migrate to the surface andromote self-healing (Fig. 8). A polydopamine (Pdop) film waseposited onto ODA-loaded silica for controlled release, chem-

cal bonding of ODA, and improvement of its compatibility as bulk additive in resin. This allowed the coatings to self-healheir superhydrophobicity in a wet or moist environment much

aster than under a dry environment. Wang, Liu, Zhou, andiu (2011) fabricated a self-healing superamphiphobic surfacen anodized alumina by filling intrinsic pores with low surfacenergy perfluorooctyl acid. In this case, the nanopores acted as

mirhf

plet profiles, (middle) reversible wettability transitions following UV exposure andre (Lim et al., 2007).

anoreserviors. After damaging the superamphiphobicity with2 plasma treatment, the damaged surface automatically healedithin 48 h at room temperature by releasing perfluorooctyl

cid from the nanopores onto the top surface. Recently, durable,elf-healing superamphiphobic surfaces were fabricated from aomogeneous mixture of fluorinated-decyl polyhedral oligomericilsesquioxane and hydrolyzed fluorinated alkyl silane using a dip-oating approach (Wang et al., 2011b). These coatings exhibitedemarkable superhydrophobic and superoleophobic self-healingroperties and excellent resistance to UV light, acid exposure,

atrices containing catalysts that polymerize the healing agents. (a) A crack formsn the matrix wherever damage occurs. (b) The crack ruptures the microcapsules,eleasing the healing agent into the crack plane through capillary action. (c) Theealing agent contacts the catalyst, triggering polymerization that bonds the crack

aces, closing the crack (White et al., 2001).

368 S. Yang et al. / Particuology 11 (2013) 361– 370

Fig. 7. (a) Top-view SEM image of the scratched coating. (b) Enlarged SEM image of the scratches in (a) (marked with an arrow). (c) and (d) Wetting characterization of thescratched coating before (c) and after (d) self-healing (Li et al., 2010).

F ecylamt l., 201

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7

idbmmwitw

a

dnc

rs2etpflls

A

ig. 8. The working principle of self-healing of superhydrophobic coatings. The octado complete the self-healing process after damage by O2 plasma treatment (Liu et a

nd construction of self-healing superhydrophobic surfaces. Theseunctional surfaces can recover their structure and special wett-bility when damaged. Therefore, endowing superhydrophobic oruperoleophobic surfaces with self-healing should be an interest-ng research field for smart materials.

. Conclusions and outlook

Bio-inspired special wettability is an interesting research fieldn surface science. In this review, we highlighted the recentevelopments in bio-inspired special wetting surfaces preparedy nanoparticle assembly. Nanoparticle assembly is an effectiveethod for constructing functional multiscale structures of variousorphologies, making it possible to fabricate bio-inspired surfacesith special wettability. In the past decade, a large number of bio-

nspired functional materials with special wettability fabricated

hrough nanoparticle assembly have emerged. In the near future,e think the following research directions should be addressed.

The control and the manipulation of nanoparticle assemblyre crucial for achieving functional nanostructures. Therefore, a

rB

ine on the outermost polydopamine layer can be replenished from silica reservoirs2b).

eeper understanding of the organizational mechanism underlyinganoscale assembly will guide the rational design and reproducibleonstruction of many desirable multiscale structures.

In nature, the inherent multiscale structure of biological mate-ials is not unifunctional, but is instead multifunctional (i.e.,tructures possess more than one function) (Liu and Jiang, 2011b,011c). Creating multifunctional materials should be a goal for sci-ntists, and learning from nature is an effective avenue for pursuinghis goal. For example, inspired by the liquid-repellent Nepenthesitcher, slippery surfaces with multifunctional integration wereabricated by infiltrating low-surface-energy porous solids withubricating liquids (Wong et al., 2011), demonstrating exceptionaliquid- and ice-repellency, repeatable self-healing, extreme pres-ure stability, and enhanced optical transparency.

cknowledgments

We appreciate the financial supports from the National Natu-al Science Foundation of China (21273016, 21001013), Nationalasic Research Program of China (2013CB933003, 2010CB934700),

ology

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rogram for New Century Excellent Talents in University, Beijingatural Science Foundation (2122035), and the Key Research Pro-ram of the Chinese Academy of Sciences (KJZD-EW-M01).

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