Toward Superlyophobic Surfaces

Weihua Ming a,b,∗, Boxun Leng b,c, Rik Hoefnagels b, Di Wu b, Gijsbertus de With b and

Zhengzhong Shao c

a Nanostructured Polymers Research Center, Materials Science Program, University of NewHampshire, Durham, NH 03824, USA

b Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology,P.O. Box 513, 5600 MB Eindhoven, The Netherlands

c Key Laboratory of Molecular Engineering of Polymers of Ministry of Education,Advanced Materials Laboratory, Department of Macromolecular Science, Fudan University,

Shanghai 200433, China

AbstractWe report on our recent endeavor in developing superlyophobic surfaces, on which both water and hexade-cane demonstrate contact angles greater than 150◦ and, even more importantly, roll-off angles for 20-µldroplets of less than 10◦. Our superlyophobic surfaces are based on a multilength-scale structure, eitherfrom a raspberry-like topography or from particle-containing woven fabrics. The multilength-scale rough-ness plays a major role in rendering the surfaces superoleophobic (the liquid droplets are in the Cassiewetting state), especially in terms of obtaining low hexadecane roll-off angles. In addition, surface perfluo-rination has proven to be essential in achieving superoleophobicity.

KeywordsSuperlyophobicity, superhydrophobicity, superoleophobicity, multilength-scale roughness, surface perfluo-rination

1. Introduction

There has been substantial interest in developing superhydrophobic surfaces in bothacademia and industry due to their potentially wide applications, including self-cleaning property with respect to water. In Nature, there are many elegant examplesof perfectly designed surface wettability. For instance, the leaves of the sacred lotusflower, known as “rising out of muddy water, untainted”, demonstrate self-cleaningproperty, which is due to a combination of a proper surface chemistry and a pe-culiar topographic feature based on dual-scale roughness: the coarse-scale roughstructure is about 10–20 µm in size whereas the finer structure on top of the coarse

* To whom correspondence should be addressed. E-mail: W.Ming@unh.edu

Contact Angle, Wettability and Adhesion, Vol. 6

Koninklijke Brill NV, Leiden, 2009

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structure is in the range of a few hundred nanometers [1–3]. There are many otherinteresting superhydrophobic surfaces, such as legs of the water strider [4] and thegecko’s feet [5]. Surface roughness at a dual- or multi-length scale has shown to bevery effective in generating this surprising nonwetting behavior, especially for ob-taining low water roll-off angles, which has inspired many biomimetic approachesto obtain artificial superhydrophobic surfaces [6–14]. On the other hand, the ma-jority of the reported superhydrophobic surfaces are not super oil-repellent; it ishighly desirable for superhydrophobic surfaces to be also oil repellent to maintaintheir superhydrophobicity. For instance, in an industrial or household environmenta superhydrophobic surface with poor oil repellency can be easily contaminatedby oily substances, which, in turn, will compromise its surface superhydropho-bicity. Therefore, superlyophobic (or superhygrophobic [15]) surfaces combiningsuperhydrophobic and superoleophobic properties are desirable for practical appli-cations [16].

Despite extensive investigations on superhydrophobic surfaces, studies on super-oleophobic surfaces with high repellency against liquids with a low surface tension(<35 mN/m) have been rather limited so far. Oil repellency has been examinedon various perfluorinated, superhydrophobic surfaces [17–27], with reported staticcontact angles (CAs) for benzene, hexadecane or rapeseed oil in the range of 135–160◦. No receding contact angles or roll-off angles for oil droplets were reportedon most of the studied surfaces [17–26], so it is difficult to judge whether thesesurfaces are truly superoleophobic. For instance, despite a high static CA (160◦)for rapeseed oil on a superhydrophobic surface made of surface-fluorinated carbonnanotubes [24], the rapeseed oil droplet remained pinned to the surface (indicatingvery low receding CA) when the sample was tilted, underlying the importance ofachieving high receding CAs (thus, low contact angle hysteresis and low roll-offangles). Only those surfaces with high CAs (>150◦) and low roll-off angles for oildroplets can be regarded as truly superoleophobic surfaces.

Very recently, truly superoleophobic surfaces have been achieved on the basis ofmicrohoodoo [16, 28] and nanonail [29] structures, as exemplified by low contactangle hysteresis for probe liquids of low surface tension (<30 mN/m) like octaneand ethanol. In both cases, the key to obtaining true superoleophobicity is the re-entrant surface structure, thus ensuring the entrapment of air beneath the top solidsurface and preventing the transition from the Cassie–Baxter state to the Wenzelstate [16, 18, 28, 29]. However, the fabrication of these superoleophobic surfacesinvolves lithography and etching steps, which may limit their practical applications.

In this contribution, we will summarize our recent efforts in developing su-perlyophobic (both superhydrophobic and superoleophobic) surfaces, includingraspberry-like surfaces and particle-containing textiles, both containing dual- ormultilength-scale surface structures.

Toward Superlyophobic Surfaces 193

2. Lyophobicity on Raspberry-Like Surfaces

2.1. Superhydrophobic Surfaces from Raspberry-Like Particles

Inspired by the lotus leaf structure, we prepared epoxy-based polydimethylsilox-ane (PDMS)-surface-modified, superhydrophobic films with dual-scale hierarchi-cal structure originating from well-defined raspberry-like particles, as shown inScheme 1 [6]. First, a conventional cross-linked film based on epoxy-amine systemwas prepared with unreacted epoxy groups available for further surface grafting.Second, amino-functionalized raspberry-like silica particles (core particles: 700 nmin diameter; shell particles: 70 nm) were then chemically deposited onto the epoxyfilms to generate a dual-scale surface roughness. Finally, a layer of monoepoxy-end-capped PDMS was grafted onto the raspberry-like particles to render the filmsurface hydrophobic.

The advancing water contact angle (CA) on a smooth epoxy-based film (surfacemodified with PDMS) was 107 ± 2◦, with a CA hysteresis (CAH) of about 40◦.When only large silica particles (700 nm) or only small particles (70 nm) weredeposited (surface also modified with PDMS) on the epoxy-amine film, the wateradvancing CA increased to 150◦, but at the same time the CAH also increased sig-nificantly to ∼60◦. In a sharp contrast, on the superhydrophobic films containingraspberry-like particles (Fig. 1b), the advancing CA for water was about 165◦ andthe roll-off angle of a 10-µl water droplet was <3◦ (Fig. 2) [6]. The superhydropho-bic surface demonstrated self-cleaning property with respect to water, similar to thelotus leaf.

We used atomic force microscopy (AFM) to examine the surface topographyof the superhydrophobic films. As shown in Fig. 1b, the topographic image of thesuperhydrophobic film clearly shows a dual-scale structure: the micrometer-levelstructure can be ascribed to the large core particles (700 nm) of the raspberry-like particles, while on each of these microparticles there is a finer structure ata sub-micrometer level due to the 70-nm shell nanoparticles [6]. Obviously, thetopographic feature of the raspberry-like particles was completely preserved in thesuperhydrophobic film. The dual-scale surface structure mimics that of a lotus leaf.

Scheme 1. Preparation of superhydrophobic films based on raspberry-like particles [6].

194 W. Ming et al.

(a) (b) (c)

Figure 1. (a) A raspberry fruit with a naturally occurring dual-scale structure and (b) the surfacetopography (AFM) of our nature-inspired, superhydrophobic films from raspberry-like particles [6],which to some extent resembles (c) the surface morphology (SEM) of the lotus leaf.

Figure 2. Roll-off of a 10-µl water droplet on our superhydrophobic film as the film was tilted fromthe horizontal position to 1.8◦.

(a) (b)

Scheme 2. Schematic illustration of the air/water interfacial area on a polymer surface covered by particles:(a) no particle embedment; and (b) particle partially embedded into the polymer matrix.

2.2. Mechanically Robust Raspberry-Like Surfaces from Layer-by-Layer ParticleDeposition

For the films we discussed in Section 2.1, an obvious drawback is their poor me-chanical robustness, due to the fact that there would be only point contacts betweenthe raspberry-like particles and the polymer films (despite the presence of covalentbonding between them). To improve mechanical robustness of the superhydropho-bic films, we used the layer-by-layer particle deposition approach and obtainedfilms with particles partially embedded into the films (Scheme 2b), in addition tothe covalent bonding between the particles and the polymer matrix. The partiallyembedded particles would demonstrate similar effect on the surface wettability on

Toward Superlyophobic Surfaces 195

the basis of the following rationale: there is no difference in terms of air/water con-tact area fraction between the case (a) and the case (b) in Scheme 2 as long aswater does not have direct contact with the polymer surface. In other words, whatis important to the liquid repellency of a structured surface is not the volume of theentrapped air, but the air/liquid interfacial area. (The entrapped air volume, on theother hand, may have impact on the stability of the superhydrophobic state, whichwill not be discussed here.)

The layer-by-layer particle deposition approach we have adopted is as follows.First, a layer of a partially pre-cured epoxy was prepared (the epoxy we used herewas Epikote 1004 from Shell Chemical, which is a solid at room temperature).Then, a single layer of epoxy-modified silica microparticles (∼800 nm in diameter)was spin-coated on top of the epoxy film. After that, the film was fully cured at an el-evated temperature, leading to completely cross-linked epoxy films with micropar-ticles partially embedded. Next, amine-modified silica nanoparticles (∼50 nm)were deposited onto the monolayer of microparticles. The particle-covered filmwas then treated with a SiCl4 toluene solution, which resulted in cross-linkingbetween silica micro- and nanoparticles and also among nanoparticles to ensuremechanical robustness of the films. In the end, the films were chemically modifiedwith 1H ,1H ,2H ,2H -perfluorodecyltrichlorosilane (RfSi) in toluene (4 v%) at 0◦Cfor 3 h.

As shown in Fig. 3, when the epoxy film was fully cured before the 800-nmparticle deposition, silica particles were completely on top of the polymer coating(Fig. 3d), and there was no particle embedment. On the other hand, if the epoxywas not pre-cured at all before the particle deposition, silica particles became com-pletely embedded into the polymer matrix (image not shown), and the film surfacewas quite smooth and would not have a significant effect on the surface wettability.Neither of these two films was suitable to develop robust superhydrophobic sur-faces. When the epoxy conversion during the pre-curing step was 44%, despite thatthe particles protruded out (Fig. 3a), the individual particles appeared to be com-pletely covered by the epoxy film, and the surface roughness may not be sufficientto lead to superhydrophobicity. In contract, at epoxy conversions of 57% and 65%during the pre-curing step, about 40% (Fig. 3b) and 20% (Fig. 3c), respectively, ofeach individual particle was embedded into the polymer matrix. The morphologyof these two films resembled the scenario we depicted in Scheme 2b.

We used the pull-off test [30]1 to examine the mechanical stability of the filmswith particles 40% and 20% partially embedded in the polymeric matrix, and found

1 In brief, a sample was first sputter-coated with a 10-nm thick gold layer and then glued to a pull-offstud (stainless steel, d = 8 mm) by an epoxy glue (cured for 24 h at room temperature), followed by cuttingaround the stud. The as-prepared sample was then clamped in a TesT 810 tensile instrument and connected tothe load cell via a long cable to make sure the force application angle was 90◦ at any moment. Subsequently,the clamped sample was moved downwards at a constant speed until fracture occurred. The tests wereperformed in air at room temperature using a tensile machine crosshead speed of 1 mm/min.

196 W. Ming et al.

(a) (b)

(c) (d)

Figure 3. SEM images of epoxy films with silica microparticles embedded into the films to variousdegrees. The epoxy films were pre-cured at different epoxy conversions: (a) 44%, (b) 57%, (c) 65%,and (d) full epoxy conversion, before the particle deposition, which was followed by complete curingat 100◦C.

that with the 40% partial embedment none of particles could be pulled out, indicat-ing the film was mechanically robust. In comparison, about 2% of the particles wereindeed pulled out of the film with the 20% partial embedment. On the film with par-ticles 40% partially embedded, we deposited a second layer of silica nanoparticles(50 nm), followed by SiCl4 and RfSi treatments. The SEM images of the film inFig. 4 demonstrate evidently a dual-scale surface roughness for the film, which issimilar to the morphology obtained directly from raspberry-like particles and mim-ics the lotus leaf structure. The mechanical robustness of this superhydrophobicsurface was also confirmed by the pull-off test. The water advancing CA on thefilm was 156 ± 0.5◦ with a CA hysteresis of about 4◦; both values were close to theCA data for the dual-scale surfaces described in Section 2.1.

2.3. Oleophobicity on Raspberry-Like Surfaces

To examine the oleophobicity of the dual-scale structured surfaces, we used mix-tures of water and ethanol as probe liquids. Water and ethanol are completelymiscible at all fractions, allowing us to tune surface tension of the mixture from22.1 mN/m for pure ethanol, at 20◦C, to 72.8 mN/m for pure water, as shown inFig. 5.

Toward Superlyophobic Surfaces 197

(a) (b)

Figure 4. SEM images of a superhydrophobic film made from the layer-by-layer particle depositionapproach: (a) side view and (b) top view. The insert shows the photo of a 10-µl water droplet on thefilm.

Figure 5. Surface tension of water/ethanol mixtures as a function of ethanol volume fraction at 20◦C.

We first examined the contact angles of the water/ethanol mixtures on PDMS-based superhydrophobic surfaces. As shown in Fig. 6, although the advancing CAwas greater than 140◦ for the interrogating liquids with a surface tension larger than35 mN/m, a small CAH (<5◦) was observed only for those interrogating liquidswith a surface tension larger than 60 mN/m. Most oleophilic liquids have a surfacetension less than 40 mN/m, thus the PDMS-based superhydrophobic surfaces arenot really oleophobic, particularly as judged from the standpoint of contact anglehysteresis.

Instead of the PDMS-modified surface, we then examined perfluoroalkyl-modified superhydrophobic surfaces. We used RfSi to modify the raspberry-likesurfaces that contained surface amino groups. The advancing CA for pure water onthis film was 168◦ with a CAH as low as 2◦ (Fig. 7). For the water/ethanol mixtureswith surface tension greater than 35 mN/m, the CA was above 160◦ and the CAHwas less than 10◦, as shown in Fig. 7, indicating that the surface-perfluorinatedsuperhydrophobic surfaces may also be highly oleophobic. The film was then sub-

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Figure 6. Contact angles and CAH values of water/ethanol mixtures on PDMS-based superhydropho-bic surfaces. The lines serve as a guide to the eye only.

Figure 7. Contact angles and CAH values of water/ethanol mixtures on perfluoroalkyl-based super-hydrophobic surfaces. The lines serve as a guide to the eye only.

jected to interrogation by hexadecane (a hydrophobic liquid with a surface tensionof 27.5 mN/m at 20◦C) and sunflower oil (surface tension: 33.0 mN/m at 20◦C).The contact angle for a 5-µl sunflower oil droplet was 132◦ (Fig. 8b). The statichexadecane CA was 125◦ for a 5-µl droplet (Fig. 8a), significantly greater than that(typically 80◦) on smooth perfluoroalkyl-modified surfaces (for instance [31, 32]).However, the roll-off angles for both hexadecane and sunflower oil on the superhy-drophobic surface appeared to be very high (>60◦). Therefore, we can only deemthe surface oleophobic, not yet superoleophobic.

Toward Superlyophobic Surfaces 199

(a) (b)

Figure 8. Images of 5-µl droplets of (a) hexadecane and (b) sunflower oil on the surface-fluorinatedsuperhydrophobic raspberry-like surface.

3. Superlyophobic Textiles

The intrinsic roughness of woven textiles can be considered as ‘local surface cur-vature’ (analogous to a re-entrant structure) [16, 28, 29]. Combining this localcurvature with silica particles, it is feasible to generate dual- and multilength-scalesurface structures, possibly leading to superlyophobic surfaces.

3.1. Superhydrophobic Textiles

We have prepared superhydrophobic cotton textiles by covalently introducing alayer of silica particles onto the fiber surface and subsequent surface hydrophobi-zation [33]. Our particle-based approach is schematically shown in Scheme 3:silica microparticles were first covalently bonded to the cotton fibers via in situStöber reaction [34] due to the abundant hydroxyl groups in cellulose; aminogroups were then introduced to the particle surface via the reaction with 3-aminopropyltriethoxysilane (APS); and finally monoepoxy-functionalized PDMSwas used to react with the amino groups to hydrophobize the fiber surface. Afterthe modification, normally hydrophilic cotton was turned superhydrophobic, whichexhibited a static water CA of 155◦ for a 10-µl droplet. The roll-off angle of waterdroplets depended on the droplet volume, ranging from 7◦ for a droplet of 50 µl to15◦ for a 10-µl droplet. It should be noted that due to the stick-out of fibers from thecotton fabric, the measurement of contact angles was often not straightforward be-cause of the difficulty in determining the baseline of the water droplet. Also because

Scheme 3. Procedure to make a superhydrophobic cotton with PDMS modification.

200 W. Ming et al.

of the protruding fibers, it was difficult to obtain accurate values for advancing andreceding water contact angles, therefore only static CAs were reported. The rela-tively high roll-off angles may be partially due to the protruding fibers in the cottonfabric, which have some elasticity and thus can exert forces on the water droplet[35].

We also extended this modification strategy to synthetic textiles such as polyesterfabrics. To render the polyester fabric surface reactive, a UV/ozone treatment wasperformed for 30 min to generate surface hydroxyl groups on the polyester beforethe incorporation of silica particles and PDMS. There was no visible macroscopicchange after the modification steps. However, the UV/ozone treatment should notlast too long; otherwise, too much oxidation and degradation would lead to yellow-ing of the fabric. As shown in Fig. 9, the silica particles were uniformly distributedalong the polyester fiber surface, and a dual-scale structure was obtained after theparticle incorporation. The polyester fabric was also turned superhydrophobic: thestatic and roll-off angles for 10-µl water droplets were 149 ± 3◦ and 12 ± 1◦, re-spectively. The polyester fabric showed water CA data similar to the cotton fabric,but the roll-off angle was relatively smaller, probably due to the absence of the pro-truding fibers in the cotton fabric (the different woven structures may also lead tothe different surface wettability).

3.2. Superoleophobic Textiles

It is known that PDMS is hydrophobic but not oleophobic. The PDMS-modifiedfabric can indeed be completely wetted by hexadecane and sunflower oil, whichhave surface tensions of 27.5 and 33.0 mN/m at 20◦C, respectively. To render a tex-tile sample oleophobic, the PDMS modification is not sufficient. Therefore, a per-fluoralkylsilane (1H ,1H ,2H ,2H -perfluorodecyltrichlorosilane, RfSi) was used forthe surface modification to render the fabric oleophobic.

The modification procedure is illustrated in Scheme 4a: immediately after the co-valent incorporation of microparticles onto the fiber surface, perfluorooctyl groupswere grafted onto the particle surface (also to the fiber surface via the reaction with

(a) (b)

Figure 9. SEM images of particle-covered polyester fabric: (a) low magnification and (b) high mag-nification.

Toward Superlyophobic Surfaces 201

Scheme 4. Procedures to make superoleophobic cottons with surface perfluorination: (a) with a monolayerof particles and (b) with a dual-layer of particles.

(a) (b)

Figure 10. SEM images for cotton fabrics containing (a) a single layer of silica microparticles and(b) dual layers of silica micro-/nano-particles (800 nm/160 nm). The inserts show the images of 10-µl(a) and 5-µl (b) hexadecane droplets on the two samples after being perfluorinated. The white line in(a) is the droplet baseline from which the contact angle was determined.

the remaining hydroxyl groups in cellulose). A typical SEM image of the modifiedcotton fabric is shown in Fig. 10a. On the perfluorooctyl-modified cotton fabric,the static CA and the roll-off angle for a 15-µl sunflower oil droplet were 140 ± 2◦and 24 ± 2◦, respectively [33]. When hexadecane was used as the probe liquid, itsstatic CA was about 137◦ for a 10-µl droplet (insert in Fig. 10a), which is signif-icantly greater than that on smooth perfluoroalkyl-modified surfaces. The roll-offangle for the same droplet was 34 ± 5◦. Obviously, the surface-fluorinated cottonfabric was turned highly oleophobic. However, the sample cannot be deemed trulysuperoleophobic, since the roll-off angle for hexadecane was still too large.

We further introduced a raspberry-like dual-scale structure onto the woven cottonfibers, leading to a triple-size surface structure [36]. As shown in Scheme 4b, sil-ica microparticles (diameter: ∼800 nm) were first in situ generated and covalently

202 W. Ming et al.

bonded to the cotton fibers. After treatment with APS and hydrochloric acid, thesurface charge was turned positive due to the protonation of amino groups. Nega-tively charged silica nanoparticles (∼160 nm) were then electrostatically adsorbedonto the fiber surface, leading to a triple-scale structure: raspberry-like particlesbonded to the woven fabric (Fig. 10b). The obtained rough structure was stabilizedby SiCl4 cross-linking, followed by surface modification with RfSi. It was observedthat the silica nanoparticles also covered part of the fiber surface not covered by mi-croparticles. This may be considered as beneficial for the roughened surface, similarto the lotus leaf on which nanostructured protrusions cover both micropapillae andthe lower part of the leaf [37].

The addition of nanoparticles did not change much the static CA (all above 150◦)for water droplets, compared to the sample with only microparticles. However, theroll-off angles decreased substantially; for instance, for a 20-µl water droplet, theroll-off angle decreased from 12 ± 4◦ for the sample with only microparticles to5 ± 1◦ for the sample also containing nanoparticles. Our results clearly indicatethat the samples with additional silica nanoparticles have demonstrated better waterrepellency than those with only microparticles [33, 35].

For C16H34 droplets, the incorporation of silica nanoparticles to the cotton fab-rics led to much higher static CA: 152 ± 2◦ for 5-µl droplets. The increase was alsoapparent in the insert images in Fig. 10. More significantly, the roll-off angles de-creased dramatically when nanoparticles were incorporated onto the fiber surface:for 20-µl droplets, roll-off angles as low as 9◦ were observed. The roll-off anglesdemonstrated strong dependency on the droplet volume: roll-off angles for 10-µland 5-µl droplets were about 15◦ and 30◦, respectively, likely owing to the elasticnature of the cotton fiber as previously discussed. With the high hexadecane andwater repellency, we can conclude that these samples are both superhydrophobicand superoleophobic. We examined how much our modified cotton fabrics “hated”both water and hexadecane: the sample was first submerged into these two liquids;when the sample was lifted out of these two liquids, the liquid contact line retreatedvery rapidly from the top edge of the modified cotton, and the samples were notwetted by either liquid at all.

Two distinct models, the Wenzel [38] and Cassie–Baxter [39] models, have beenextensively used to explain the roughness effect on the contact angles of liquidson a roughened surface. The Wenzel model describes the wetting regime in whichthe liquid penetrates into the roughened surface (intimate contact between the liq-uid and the solid surface always exists), which usually leads to high contact anglehysteresis. In contrast, in the Cassie wetting regime there is air trapped betweenthe solid surface and liquid, which results in much smaller hysteresis. When a liq-uid droplet sits on a liquid-repellent cotton textile surface, the wetting behavior canbe described by the Cassie–Baxter equation [39], cos θCB = fls cos θ0 − flv, whereθCB is the observed CA on a rough surface, θ0 is the intrinsic CA on the corre-sponding smooth surface (θ0 is 120◦ for water and 80◦ for C16H34, respectively,on a perfluoroalkyl surface), fls is the liquid/solid contact area divided by the pro-

Toward Superlyophobic Surfaces 203

jected area, and flv is the liquid/vapor contact area divided by the projected area.This equation has been modified to account for the local surface roughness on thewetted area, as follows [33, 40, 41]: cos θCB = rff cos θ0 + f − 1, where f is thefraction of the projected area of the solid surface wetted by water (flv = 1 − f )and rf is the surface roughness factor of the wetted area (rf � 1). By introduc-ing a layer of microparticles onto the fiber surface, rf can indeed be increasedfor a PDMS-modified cotton fiber, leading to superhydrophobic cotton [33]. Withthe surface perfluoroalkyl modification, the cotton fabric was turned to be highlyoleophobic [33], as demonstrated by a C16H34 CA of 137◦, but not superoleopho-bic. It has become obvious that by incorporating a second layer of nanoparticlesonto the microparticle-covered fiber, rf can be further increased to an even higherlevel, which allows the wetting by C16H34 to be in the Cassie regime, as clearlymanifested by the high static CA and the small roll-off angle for C16H34 [36].The creation of a triple-length-scale roughness (the woven fiber, microparticle, andnanoparticle) has proven to be essential to achieving superoleophobicity.

4. Conclusions

In summary, we have successfully obtained superlyophobic surfaces on the basis ofmultilength-scale structures, such as a dual-/triple-scale raspberry-like surface andcotton textiles with a dual-scale nano/microparticle structure incorporated onto thewoven fabrics. Our superlyophobic surfaces were completely nonwettable by bothwater and hexadecane, which showed both high contact angles and low roll-offangles. Mechanically robust surfaces have been successfully obtained via a layer-by-layer particle deposition approach. Superhydrophobic surfaces can be easilyachieved by using a dual-scale structure, in combination with the surface hydropho-bization with PDMS. To achieve superoleophobicity, the presence of a multilength(>2)-scale surface roughness has proven to be essential for this particle-based ap-proach, especially in terms of low roll-off angles. Our particle-based approach isnot limited to the use of silica particles; our approach can be easily extended tousing functional polymeric microparticles and nanoparticles, which can be readilysynthesized by dispersion and emulsion polymerization, respectively, as buildingblocks to prepare multilength-scale structured surfaces.

Acknowledgements

Part of the work was financially sponsored by DSM Research. We thank ChinaScholarship Council for supporting BL’s stay at Eindhoven.

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