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Applied Surface Science 256 (2010) 6736–6742

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

uperhydrophobic cotton fabric fabricated by electrostatic assembly of silicaanoparticles and its remarkable buoyancy

an Zhao, Yanwei Tang, Xungai Wang, Tong Lin ∗

entre for Material and Fibre Innovation, Deakin University, Geelong, VIC 3217, Australia

r t i c l e i n f o

rticle history:eceived 4 February 2010eceived in revised form 22 April 2010ccepted 22 April 2010vailable online 29 April 2010

a b s t r a c t

Highly hydrophilic cotton fabrics were rendered superhydrophobic via electrostatic layer-by-layerassembly of polyelectrolyte/silica nanoparticle multilayers on cotton fibers, followed with a fluoroalkyl-silane treatment. The surface morphology of the silica nanoparticle-coated fibers, which results in thevariety of the hydrophobicity, can be tailored by controlling the multilayer number. Although with the

◦

eywords:otton fabricsuperhydrophobicayer-by-layer assemblyelf-cleaning

static contact angle larger than 150 , in the case of 1 or 3 multilayers, the fabrics showed sticky propertywith a high contact angle hysteresis (>45◦). For the cotton fabrics assembled with 5 multilayers or more,slippery superhydrophobicity with a contact angle hysteresis lower than 10◦ was achieved. The buoyancyof the superhydrophobic fabric was examined by using a miniature boat made with the fabric. The super-hydrophobic fabric boat exhibited a remarkable loading capacity; for a boat with a volume of 8.0 cm3, themaximum loading was 11.6 or 12.2 g when the boat weight is included. Moreover, the superhydrophobic

ason

ater-repellency cotton fabric showed a re

. Introduction

In recent years, superhydrophobic surfaces have attracted greatttention for their wide applications in water-repellency, self-leaning, anti-sticking and anti-fouling [1–4]. Surface wettabilitys governed by both the chemical composition and the geomet-ic structure. In nature, the unusual superhydrophobicity of lotuseaves, with static water contact angles larger than 150◦ and slidingngles less than 10◦, is known to originate from the combinationf micro- and nano-scale hierarchical structures and low surfacenergy materials on the surface [5]. For fabric surfaces, they havehe natural micrometer-scale roughness coming from the fibershemselves and the woven structure. Inspired by lotus leaves,esearchers have aimed to generate secondary nano-scale struc-ures by incorporating carbon nanotubes [6,7], gold particles [8],ilica particles [9–18], ZnO nanorods [19], or copper crystallites [20]nto the micrometer-scale fibers to fabricate superhydrophobicabrics. Among them, however, contact angle hysteresis (defineds the difference between the advancing and the receding contactngles) was rarely mentioned [7,20], and some works have consid-

red superhydrophobic fabrics purely on the basis of the criterionf static contact angle larger than 150◦, which is not sufficient touarantee a low sliding angle for self-cleaning behavior.

∗ Corresponding author. Tel.: +61 3 5227 1245.E-mail address: tong.lin@deakin.edu.au (T. Lin).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.04.082

able durability to withstand at least 30 machine washing cycles.© 2010 Elsevier B.V. All rights reserved.

Electrostatic layer-by-layer (LbL) assembly is a versatile tech-nique based on alternative adsorption of oppositely chargedpolyelectrolytes, inorganic nanoparticles, macromolecules or evensupramolecular systems on charged substrates to build up multi-layered composite films in a controlled manner [21]. Since the LbLassembly technique is independent of the size and topography ofsubstrates, and uniform multilayers can be formed on substrateswith different spatial structures, charged fibers can be used as sub-strates to conduct the electrostatic assembly [22]. Compared withpreviously reported methods like dip-coating [16] and padding[15] used to generate nanostructures on fibers, the LbL assemblytechnique has the advantage of being able to tailor the surfacemorphology of the nanostructures by controlling the assemblycycles, and also the advantage of good durability because of theelectrostatic interactions between the negatively charged silicananoparticles and the polycations. Here it should be mentioned thatfor superhydrophobic fabric, its durability against washing remainsa great challenge; Daoud et al. [15] reported the static contact angleof cotton fabric decreased from 141◦ to 105◦ after 10 wash cycles,and Gao et al. [23] also reported the dramatic decrease in the staticcontact angle of superhydrophobic cotton fabric from 155◦ to about105◦ after 10 wash cycles. In this study, we report on the fabricationof superhydrophobic cotton fabrics by electrostatic LbL assembly

of silica nanoparticles and polycations on cotton fibers and sub-sequent treatment with fluoroalkylsilane. The surface morphologyof the silica nanoparticle-assembled fibers can be tailored by theassembly cycles, which makes it available to study the effect ofthe surface morphology on the static contact angle and the contact

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tion voltage of 200 kV. Fourier transform infrared (FTIR) spectrawere recorded on a Bruker VERTEX 70 instrument using atten-

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Y. Zhao et al. / Applied Surfa

ngle hysteresis. The buoyancy of the superhydrophobic cotton fab-ic was investigated by studying the loading capacity of a miniatureoat made with the fabric. In addition, we also tested the durabilityf the superhydrophobicity after washing.

. Experimental

.1. Materials and chemicals

Plain-weave cotton fabrics were used in this study. The fab-ics were cleaned with acetone before use. TetraethylorthosilicateTEOS), ammonium hydroxide (28% in water), and poly(allylamineydrochloride) (PAH, Mw ∼ 56,000) were purchased from Sigma-ldrich. Poly(acrylic acid) (PAA, Mw ∼ 50,000, 25% aqueousolution) was obtained from Polysciences. Fluoroalkylsilane (FAS,ridecafluorooctyltriethoxysilane, CF3(CF2)5(CH2)2Si(OCH2CH3)3,ynasylan F8261) was supplied by Degussa. All chemicals weresed as received without further purification.

.2. Preparation of silica nanoparticles

Silica nanoparticles were synthesized according to the Stöberethod [24]. Briefly, 1.5 mL of TEOS was added dropwise to a solu-

ion containing 50 mL of ethanol, 1.0 mL of deionized water, and.0 mL of ammonium hydroxide (28%) under magnetic stirring.

◦

fter the mixture was stirred at 40 C for 4 h, an additional 1.0 mLf TEOS was added and the reaction was allowed to continue fornother 12 h with stirring. The obtained silica nanoparticles wereurified by centrifugation (10,000 rpm, 10 min) and re-dispersed inater three times.

ig. 1. (a) TEM image and (b) size distribution of the as-synthesized silica nanopar-icles.

nce 256 (2010) 6736–6742 6737

2.3. Preparation of superhydrophobic cotton fabrics

To obtain charged substrates for electrostatic assembly, thecotton fabrics were first treated with an aqueous solution contain-ing 2.0 wt% PAA and 1.0 wt% NaH2PO2 at 85 ◦C for 30 min. Afterextensive rinsing with deionized water to remove the unreactedchemicals, the fabrics were oven-dried at 100 ◦C for 20 min. Thetreated fabric was then immersed separately into PAH (1.0 wt%)and PAA (1.0 wt%) solutions to form a PAH/PAA bilayer on the fab-ric, after which nanoparticle-containing multilayers (PAH/SiO2)n

(n = 1, 3, 5, 7) were assembled by alternately immersing the treatedfabric into PAH (1.0 wt%) solution and a solution containing 1.0 wt%SiO2 nanoparticles. The pH values of PAH and PAA solutions wereadjusted with NaOH to about 7.5 and 7, respectively. For eachlayer, the immersion time was 5 min, followed by washing threetimes with deionized water. Finally, the treated cotton fabrics weredipped in 2.0 wt% FAS in hexane for 1 h, and subsequently dried at100 ◦C for 30 min.

2.4. Characterizations

Transmission electron microscopy (TEM) was performed on aJEOL JEM-2100 transmission electron microscope at an accelera-

uated total reflectance (ATR) mode with a resolution of 4 cmaccumulating 32 scans. X-ray photoelectron spectroscopy (XPS)measurements were made using a Kratos Axis Ultra system with a

Fig. 2. ATR–FTIR spectra of (a) untreated cotton fabric and (b) PAA-grafted cottonfabric, and cotton fabrics assembled with (PAH/SiO2)n multilayers: (c) n = 1, (d) n = 3,(e) n = 5, and (f) n = 7.

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738 Y. Zhao et al. / Applied Surfa

onochromatized Al K� radiation at 1486.6 eV as the X-ray source.hermogravimetric analysis (TGA) was conducted on a NetzschTA 409 PC thermal analyzer at a heating rate of 10 ◦C/min in aitrogen atmosphere. The surface morphologies of the samplesere examined using a Leica S440 scanning electron microscope

SEM) operated at an acceleration voltage of 10.0 kV. Contactngles were measured using a CAM101 video camera based con-act angle measurement system (KSV Instruments Ltd., Finland).

or static contact angle measurements, 8-�L water drops werelaced on the fabric surface and still images were recorded.o measure the advancing and receding contact angles, a per-endicular syringe needle was maintained in contact with the

Fig. 3. SEM images of untreated cotton fabric (a and b) and cotton fabrics assem

nce 256 (2010) 6736–6742

water drop and the substrate was slowly moved horizontally.The change of droplet shape due to the shift of the substratewas recorded by the video camera at a speed of 30 frames persecond. At the instant when the drop was about to move, thecontact angles at the advancing and receding contact lines weretaken as the advancing and receding contact angles, respectively[25,26]. Washing fastness or durability was evaluated in referenceto standard test method for fabric coating (AATCC Test Method

61-2006 test No. 2A). This standard wash procedure is equivalentto five cycles of home machine launderings. For convenience, weuse the equivalent number of home machine launderings in thispaper.

bled with (PAH/SiO2)n multilayers: (c) n = 1, (d) n = 3, (e) n = 5, and (f) n = 7.

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fabric was partially immersed in water, the wicking of waterled to the complete wetting of the whole fabric (the left-handpanel of Fig. 6c). When the cotton fabrics were just modified with(PAH/SiO2)n multilayers, the fabrics turned hydrophobic, with thewater contact angles in the range of 120–130◦. Further treatment

Y. Zhao et al. / Applied Surfa

. Results and discussion

Fig. 1 shows a typical TEM image and the size distribution of theegatively charged silica nanoparticles synthesized by the base-atalyzed hydrolysis and condensation of TEOS (Stöber method24]). The average particle size calculated based on TEM imagesas 68 nm with a standard deviation of 5 nm. Cotton fibers con-

ain abundant hydroxyl groups, making the fiber surface highlyydrophilic. Although cotton fibers are known to be slightlyharged negatively because of the ionization of some hydroxylroups [27], PAA was grafted onto cotton fibers through a NaH2PO2-atalyzed esterification reaction [28] to make the cotton fibersharged enough for electrostatic assembly. The ATR–FTIR spectra,hown in Fig. 2a and b, indicate the appearance of a carboxyl stretchand at 1720 cm−1 after the reaction, confirming that the PAA haseen grafted successfully onto the cotton fibers.

The as-grafted cotton fabrics were used as substrates to con-uct electrostatic assembly of silica nanoparticle multilayers. Fig. 2hows the ATR–FTIR spectra of cotton fabrics assembled withPAH/SiO2)n multilayers. After the assembly of 5 layers, the absorp-ion band at 3340 cm−1, corresponding to the hydroxyl groupsf cellulose, became less evident, and the absorption bands at54 and 792 cm−1, attributed to Si–OH stretching and Si–O–Siymmetric stretching vibrations, appeared. Although the bandf Si–O–Si asymmetric stretching vibrations at 1130–1000 cm−1

verlaps with that of the cellulose C–O bonds, a peak located at088 cm−1 can be seen clearly after the PAH/SiO2 assembly. Theseesults confirmed the presence of silica nanoparticles on cottonabrics. However, for the fabric assembled with 1 or 3 layers ofAH/SiO2, the spectra were similar to that of the untreated pristineotton fabric, which is due to the incomplete coverage of the silicaanoparticles on the fiber surface, as evidenced by the followingEM observation.

Fig. 3 shows the SEM images of the cotton samples. For thentreated pristine cotton fabric, the fabric weave structure andative striations along the fiber can be clearly observed (Fig. 3a and). When the surface was assembled with 1 or 3 layers of PAH/SiO2,ilica nanoparticles were found to partially cover the fiber sur-ace at random (Fig. 3c and d). This can be used to explain thebove FTIR analysis. Five layers of PAH/SiO2 assembly resulted inomogeneous coverage of silica nanoparticles on the cotton fibersFig. 3e). When the fabric was coated with 7 layers of PAH/SiO2,

ore nanoparticles were assembled and they aggregated on theotton fibers (Fig. 3f).

XPS measurement was used to examine the chemical composi-ion of the coated fabric surface. For the untreated pristine cottonabric, only peaks corresponding to C and O were observed, ashown in Fig. 4a. When the fabric was assembled with (PAH/SiO2)5ultilayers, two more distinctive peaks appeared at 153 and

02 eV, which are attributed to Si 2s and Si 2p, respectively. Inddition, the N 1s originating from the PAH layers was detectedt 398 eV. This measurement confirmed the assembly of positivelyharged PAH and negatively charged silica nanoparticles on the cot-on fibers. After being modified with FAS, new peaks with bindingnergies of 833, 687 and 291 eV appeared, corresponding to F KLL,1s, and C 1s of FAS, respectively (Fig. 4c).

Fig. 5 shows the TGA curves of the fabrics. The thermalecomposition mainly occurred in a narrow temperature range of00–370 ◦C and the cotton fabrics assembled with silica nanopar-icles showed slightly higher thermal stability compared to thentreated fabric. When the untreated cotton fabric was heated

o 600 ◦C at a rate of 10 ◦C/min, only 14% of the original weightemained. When the silica nanoparticles were assembled to theber surface, the same heating condition resulted in 20% remain-

ng weight. The remaining weight percentage was further increasedo 24% after the coated fabric was modified with FAS. These results

Fig. 4. XPS spectra of untreated cotton fabric (a) and cotton fabric assembled with(PAH/SiO2)5 multilayers (b) before and (c) after modification with FAS.

confirmed the incorporation of silica nanoparticles and FAS ontothe cotton fibers.

As mentioned above, the pristine cotton fabrics are hydrophilicand can be completely wetted by water because of the abun-dant surface hydroxyl groups (Fig. 6a). When the untreated cotton

Fig. 5. TGA curves of untreated cotton fabric (a) and cotton fabric assembled with(PAH/SiO2)5 multilayers (b) before and (c) after modification with FAS.

6740 Y. Zhao et al. / Applied Surface Science 256 (2010) 6736–6742

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ig. 6. Photographs of water drops on (a) untreated cotton fabric and (b) superhyduperhydrophobic (right) cotton fabrics being immersed in water.

f the (PAH/SiO2)n assembled fabric with FAS rendered the fabricuperhydrophobic. A typical photograph of a spherical water dropormed on the surface of the modified cotton fabric is shown inig. 6b.

When the superhydrophobic fabric was immersed in water,he fabric depressed the water surface and an obvious dimpleas observed, and the fabric surface appeared mirror-like (the

ight-hand panel of Fig. 6c). This interesting optical property wasttributed to the plastron layer formed by the trapped air betweenhe water and the superhydrophobic fabric [29–31]. This also sug-ests that the wetting state was dominated by the Cassie–Baxterodel [32], in which the liquid does not completely fill up the

pace between protrusions and there are air pockets trapped athe solid–liquid interface. For a heterogeneous interface composedf two fractions (one with area fraction f1 and contact angle �1 andhe other with f2 and �2, f1 + f2 = 1), the apparent contact angle �CBs given by cos �CB = f1 cos �1 + f2 cos �2. When the interface consistsf a solid–liquid fraction (f1 = f, �1 = �e) and a liquid–vapor fractionf2 = 1 − f, cos �2 = −1), the apparent contact angle is calculated asos �CB = f cos �e + f − 1, where f is the projected area fraction of theolid surface wetted by water and �e is the equilibrium contactngle on the flat surface. Actually, for the wetted area, the rough-ess factor r, defined as the ratio of the actual area of the solidurface to the projected area on the horizontal plane, also con-ributes to the increase of contact angle. This effect can be described

y another model as developed by Wenzel [33], which assumes thathe liquid fills up the space between protrusions, and the appar-nt contact angle �W is given by cos �W = r cos �e. Therefore, theassie–Baxter equation can be modified as cos �CB = rf f cos �e + f − 134,35], where rf is the ratio of the actual area to the projected area

bic cotton fabric assembled with (PAH/SiO2)5 multilayers. (c) Untreated (left) and

of the solid surface that is wetted by the liquid. This equation sug-gests that for an area where the roughness is beyond a certain level,air is trapped below the drop; and for the wetted area where theroughness is below this level, the Wenzel model can still be applied,i.e., both the Wenzel and Cassie–Baxter states may coexist [36].

Although both the Wenzel and Cassie–Baxter wetting states leadto an increased contact angle for hydrophobic surfaces, their maindifference lies in the magnitude of the contact angle hysteresis. Dueto the intimate contact at the solid–liquid interface, a water dropin the Wenzel state has a higher contact angle hysteresis and willadhere very efficiently to the substrate in spite of a large static con-tact angle, which is referred to as “sticky” superhydrophobicity. Inour case, although the static contact angles for the treated cottonfabrics were all in the range between 151◦ and 157◦, the contactangle hysteresis varied with the number of PAH/SiO2 layer assem-bled (Fig. 7). For the cotton fabrics assembled with 1 or 3 layersof PAH/SiO2, the contact angle hysteresis was larger than 45◦, andthe fabric surface is sticky. However, for the fabric treated with 5or 7 layers of PAH/SiO2, the contact angle hysteresis decreased toless than 10◦, leading to a “slippery” superhydrophobic surface. Thesignificant decrease in contact angle hysteresis can be attributed tothe reduced contribution from the fully wetted Wenzel state.

The force required to move the water drops on the sur-face is dependent on the contact angle hysteresis [37], F ≈ � lv(cos �R − cos �A) W, where � lv is the interfacial tension of the water

at the water–air interface, W is the width of the wetted area perpen-dicular to the movement direction, and �A and �R are the advancingand receding contact angle, respectively. According to this equa-tion, the values of F normalized with respect to W can be calculated,and the results are shown in Fig. 7. For the cotton fabrics assem-

Y. Zhao et al. / Applied Surface Science 256 (2010) 6736–6742 6741

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angle hysteresis of the cotton fabric assembled with (PAH/SiO2)5multilayers still remained lower than 10◦ after 10 cycles of homelaundering, and the static contact angle remained above 150◦ evenafter 30 cycles (Fig. 9).

ig. 7. (a) Static contact angles, (b) advancing/receding contact angles, and (c) con-act angle hysteresis (�) and the force (�) required to move water drops on theuperhydrophobic fabrics assembled with (PAH/SiO2)n multilayers.

led with 1 and 3 layers of PAH/SiO2, the force was 59.3 and9.1 dyn/cm, respectively. When the number of PAH/SiO2 layersas increased to 5 and 7, the force decreased dramatically to 3.1 and

.3 dyn/cm, respectively. This suggests that it requires only aboutne-thirteenth of the force, 3.1 dyn/cm vs. 39.1 dyn/cm, to movewater drop on the fabric assembled with 5 layers of PAH/SiO2

ompared to that with 3 layers of PAH/SiO2. It should be pointedut that for the fabric assembled with 5 or 7 layers of PAH/SiO2,he sliding angles were hard to measure because the water dropsolled easily on the fabric surface when the fabric was just gentlyoved horizontally.In an attempt to investigate the buoyancy of the superhy-

rophobic cotton fabrics, we made a miniature boat by using theuperhydrophobic fabric assembled with (PAH/SiO2)5 multilayers,nd loaded the boat with heavy rubber (density 1.7 g cm−3). Theolume of the boat was 8.0 cm3. According to Archimedes’ principle,he loading capacity of this boat on water surface should be 8.0 g.owever, we found that the maximum loading capacity for thisoat was 11.6 or 12.2 g if the boat weight is included. This suggestshat besides the buoyancy force associated with the boat volume of.0 cm3, there is an extra buoyancy force to support the extra load-

ng of 4.2 g. Fig. 8 shows the floating behavior of a superhydrophobicabric boat with a maximum loading. As can be seen, the boat stilloated even when its upper edges were below the water surface,hich can be ascribed to the extremely low surface energy of the

abric. Therefore, one contribution to the extra buoyancy force ishe surface tension force related to the upper perimeter of the boat.nother contribution to the extra buoyancy force comes from thelastron effect [38,39]. The trapped air film surrounding the fabric

uter surface provides additional displaced volume of water, andhus extra buoyancy force.

For practical applications, the durability of the superhydropho-ic surface is important. In our study, the affinity between thePAH/SiO2)5 multilayers and the cotton fiber was tested by immers-

Fig. 8. Photographs of a loaded superhydrophobic fabric boat floating on watersurface: (a) top view; (b) side view.

ing the fabric into a sonication bath containing ethanol, which cancompletely wet the superhydrophobic fabric. After continuous son-ication for 1 h, no significant changes in the contact angles werefound. In addition, the wash durability test revealed that the contact

Fig. 9. Static contact angle and contact angle hysteresis of cotton fabric assembledwith (PAH/SiO2)5 multilayers as a function of the number of wash cycles.

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. Conclusions

Superhydrophobic cotton fabrics have been prepared by thelectrostatic self-assembly of silica nanoparticles and subsequenturface hydrophobic treatment. The surface morphology of theesulting cotton fibers can be adjusted by the number of theAH/SiO2 layers assembled. Although the criterion of static con-act angle larger than 150◦ can be easily achieved with only onessembly cycle, the contact angle hysteresis varied considerablyepending on the assembly cycles; the fabrics showed sticky prop-rty with a contact angle hysteresis higher than 45◦ for 1 or 3ssembly cycles, while slippery superhydrophobicity with a con-act angle hysteresis lower than 10◦ was obtained for the cottonabrics assembled with 5 multilayers or more. A superhydropho-ic fabric boat of 8.0 cm3 in volume exhibited a remarkable loadingapacity with a maximum loading of 11.6 or 12.2 g when the boateight is included. Moreover, such cotton fabrics had a reasonableurability to withstand at least 30 machine washing cycles.

cknowledgements

We wish to acknowledge the funding support from Deakinniversity under the Alfred Deakin Postdoctoral Fellowship and

he Central Research Grant schemes. We also thank Dr Adriennehandler-Temple at La Trobe University for assistance in the XPSeasurement.

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