Ultra-repellency of Al surfaces: design and evaluation

Y. Zhu, Y. M. Hu , L. Ma, H.-Y. Nie, W. M. Lau

� American Coatings Association 2018

Abstract Aluminum (Al) surfaces with ultra-repel-lency as well as desirable robustness were designed andfabricated. With photolithographic patterning of athick SU-8 layer and sputtering of a thin Al film, re-entrant micro-pillar textured Al surfaces were pre-pared. After derivatization with perfluoroalkyl phos-phoric acid (FPA), the textured Al surfaces showedultra-repellency for a wide variety of liquids. Thecontact angles (CAs) of deionized (DI) water, hexade-cane and dodecane were larger than 150�, and those ofmethanol and ethanol were larger than 100�. Thesliding angles (SAs) of DI water, hexadecane anddodecane were 5�, 10�, and 10�, respectively, showingexcellent superamphiphobicity. The SAs of methanoland ethanol were in the range of 20�–30�. Therobustness of the ultra-repellent Al surface was eval-uated by three parameters: robust height (H*), robustangle (T*) and robust factor (A*). For the DI waterprobing, the values of the parameters are H* » 403,

T* » 119 and A* » 92, respectively, indicative of adesirable robustness. We clarified that only re-entrantstructures can support composite liquid–solid–vaporinterfaces when the corresponding Young’s CAs aresmaller than 90�, and the function of the nanometerstructures of the hierarchical textures which werewidely adopted to fabricate superamphiphobic surfacesis to help construct re-entrant structures. FPA deriva-tization is effective in lowering the surface energy of Alsurfaces, combining with re-entrant textures to providea simple and high throughput approach to ultra-repellency for a wide variety of liquids.

Keywords Ultra-repellency, Al surface, Re-entranttexture, PFA, Robustness

Introduction

Ultra liquid-repellency with contact angles (CAs) ofliquid droplets placed on solid surfaces being largerthan 150� for water and oils (superamphiphobicity),and 90� for alcohols, together with the sliding angles(SAs) being as small as possible, has generatedextensive interests.1–9 Aluminum (Al) and its alloyshave been extensively studied due to their wideapplications in architecture, transmission lines, elec-tronic elements, and so forth.10–15 Many studies havebeen carried out to make Al surfaces superhydropho-bic.16–23 However, the surface tensions of oils andalcohols are in the range of 20–50 mN/m, lower thanthat of water.24,25 In general, Young’s CAs (defined asthe CA on an ideal surface, hY) of oils and alcohols aresmaller than 90� on Al surfaces even after beingmodified by materials having ultra-low surface ten-sions. We clarified that, from the surface structurepoint of view, ultra-repellency can only be achieved byappropriate re-entrant surface texture, and the hierar-chical micro/nanometer structure which has been

Electronic supplementary material The online version of thisarticle (https://doi.org/10.1007/s11998-017-0012-9) contains sup-plementary material, which is available to authorized users.

Y. ZhuKunming University of Science and Technology, Kunming650093, China

Y. M. Hu (&)College of Engineering, Dali University, Dali 671003, Chinae-mail: yongmaohu@163.com

L. Ma, H.-Y.NieSurface Science Western, The University of WesternOntario, 999 Collip Circle, London, ON N6G 0J3, Canada

W. M. LauCenter for Green Innovation, School of Mathematics andPhysics, University of Science and Technology Beijing,Beijing 100083, China

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commonly used in designing ultra-repellency is one ofthe re-entrant performances. Therefore, in order toachieve ultra-repellent Al surfaces, a two-step ap-proach is typically required: (1) creation of re-entrantsurface textures and (2) modification of the re-entranttextured surfaces with low surface tension materials.

Re-entrant textures can be constructed by theprocesses of electrospinning,26,27 anisotropic etch-ing,26,27 micrometer carbon sphere deposition,28 car-bon sphere template deposition,29 photolithography,and so on. It is worth mentioning that approaches ofthe electrochemical process,30 nanoparticle load-ing,31,32 chemical corrosion33 to superoleophobicity ofAl surfaces have been effectively demonstrated. Weconsider that the function of nanometer structures inthe textures mentioned above is to help the transfor-mation of common microstructures to re-entrantstructures, or, in other words, all of them have built-in re-entrant surface textures. In order to study theimpact of surface structure on the wetting properties,anisotropic etching is a candidate process because ofthe accurate size control and good reproducibility.However, the instruments required are often expen-sive, increasing the overall cost. Hence, an accuratesurface pattern process of photolithography has advan-tages.

In the present work, we designed and fabricated re-entrant Al surfaces through a simple approach. Re-entrant micro-pillar-patterned SU-8 surfaces were firstprepared, followed by magnetron-sputtering of approx-imate 70 nm of Al film on top. SU-8 resist is an epoxy-based negative photo-resist which has been widely usedto fabricate thick patterns with smooth walls. Patternsmade by SU-8 are strong, stiff and chemically stable.Since the thickness of the Al film was too small toinfluence the morphology of the micro-pillar pattern,the ultimate sample surfaces could be regarded as re-entrant micro-pillar-textured Al surfaces.

In the surface modification strategy, fluoroalkylsi-lane,22,34 Teflon,35 perfluorooctyltriethoxysilane36 andother molecules have been used to lower the Al surfaceenergy. However, the adhesion between Al and thesemolecules is limited because there are no chemicalbonds formed between the Al substrate and theorganic molecules. We have demonstrated that alkylphosphoric acids, such as dodecylphosphonic acid andoctadecylphosphonic acid (OPA), can effectively con-struct bidentate P–O–Al covalent bonds via a conden-sation reaction between the hydroxyl of Al and thephosphoric acid headgroups.6,7 The covalent bondsbetween Al and the organophosphonic acid head-groups dramatically improved the adhesion, ensuringexcellent durability of the composite system.

While alkyl phosphoric acids can effectively modifyAl surfaces, the surface energy of densely packedmethylene chains and the terminal –CH3 groups is toohigh to repel oils and alcohols. However, analysisbased on a Zisman plot shows that the perfluoroalkylphosphoric acid (FPA, CF3(CF2)13(CH2)2PO(OH)2;see Fig. 1a) was able to further lower the surface

energy. The critical tension of such a smooth Alsurface was lowered to 17.73 mN/m, rendering FPA agood candidate for Al surface modification towardultra-repellency.

Based on the idea mentioned above, ultra-repel-lency of Al surfaces was demonstrated via re-entrantmicro-pillar texture and FPA modification. These Alsurfaces showed an excellent repellency against a widevariety of polar and nonpolar liquids, including water,hexadecane, dodecane, methanol, and ethanol. Therobustness of these ultra-repellent surfaces has beendiscussed and evaluated using failure analysis devel-oped by Tuteja and coworkers.26

Experimental

The photolithography process of �200 lm SU-8 pat-terns is given as follows: spin-coated on an Si wafer at aspeed of 3000 rpm for 20 s, prebaking at 95�C for 1 h,near-UV (400 nm) contacting lithography, postbakingat 95�C for 30 min and development for 20 min in 2-acetoxy-1-methoxypropane (PGMEA, C6H12O3). Theconstruction of the ultimate re-entrant micro-pillartexturedAl surfaces has beendepicted elsewhere.6,7,18,19

The schematic diagram is shown in Fig. 1b.Crystalline FPA powders (purchased from Specific

Polymers, France) were preheated to 100�C prior touse to eliminate moisture. A 1-mM FPA solution inethanol:tetrahydrofuran (THF) (1:1 ratio) was used tomodify the Al surfaces. In the derivatization step,samples with re-entrant textured Al surfaces wereimmersed in the FPA solution for 2–3 s, followed byethanol and DI water rinsing and a final N2 streamdrying.

Al surfaces were characterized with CA measure-ments, scanning electron microscopy (SEM) and X-rayphotoelectron spectroscopy (XPS). Readers are re-ferred to the literature,6,7 for the detailed procedures.

Results and discussion

Surface morphology

In the micrometer scale, SU-8, Al/SU-8 and FPA/Al/SU-8 surfaces showed no obvious difference. Forsimplicity, Fig. 2 only shows (a) images of the FPA/Al/SU-8 surface and (b) the dimensions of the pillars.The FPA/Al/SU-8 surface exhibited a re-entranttapered micro-pillar (RTMP) texture with top radius(R), height (H) and inter-pillar gap (2D) of 5.1, 19.6,and 14.2 lm, respectively. The feature angle of thetextured surface was estimated to be 87.8�. Theinterface of Al/SU-8 was identified and the thicknessof the Al layer was 69 nm. The 69-nm Al layer fullycovered the SU-8 surface, ensuring that the finalproduct was FPA-modified RTMP-textured Al sur-faces.

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Surface chemistries

Figure 3a shows the XPS spectra of the Al/SU-8 andFPA/Al/SU-8 surfaces. The spectrum of Al/SU-8agrees well with that of blank Al.18 In the spectrum

of FPA/Al/SU-8, the peaks of phosphorus (P) andfluorine (F) are clearly seen in the binding energyrange of 175–200 and 680–695 eV, respectively.The presence of P and F confirmed the FPAderivatization.

HH

Si

SU-8(a) (b)

Al/SU-8/SiFPA

OPOHHO

FF

F

FF

FF

FF

FF

FF

FF

FF

FF

FF

FF

FF

FFFF H

H

Fig. 1: (a) Molecular structure of FPA used as the modifier in this work and (b) schematic diagram of the fabrication of re-entrant micro-pillar patterned Al surfaces. Re-entrant micro-pillars were first patterned on the SU-8 photoresist surface byphotolithography followed by sputtering deposition of Al film

(a) (b)

60 µm

69 n

m

20 µm

10.2 µm

8.7 µm

14.2 µm

19.6

µm

= 87.8

Fig. 2: SEM images of (a) top view and (b) profile of the FPA derivatized Al surfaces. The surfaces exhibit a re-entranttapered micro-pillar texture with top radius (R), height (H), distance between two adjacent micro-pillars (2D) and featureangle of 5.1 lm, 19.6 lm, 14.2 lm, and 87.8�, respectively

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The high-resolution C1s core level spectrum rangingfrom 280 to 296 eV is shown in Fig. 3b. Five compo-nents of �CF3 (291.13 eV), –CF2 (289.00 eV), –CH2–CF2 (287.41 eV), C–C (283.20 eV) and C–H (281.92)were observed.37 The concentration of –CF3 estimatedfrom the spectra is 6.38%, well consistent with thetheoretical value (6.25%) obtained from the molecularstructure of FPA. The successful derivatization of theultralow surface energy end group of –CF3 (about6.7 mJ/m2) on the Al surface was the key to the ultra-repellency.

Multiple curve-fitting solutions were conductedwhen analyzing the O1s core level lines in Fig. 3c.According to references,38–41 a reasonable scheme wasadopted, in which O1s was de-convoluted into twopeaks: a stronger peak at 529.10 eV with FWHM of1.93 eV represents the main contribution of P-O–Albonds; another peak at 530.31 eV, with FWHM of1.58 eV is attributed to P = O bonds. The presence ofP = O bonds suggests the bonding configuration of P-O-Al is either mono- or bidentate, rather than triden-tate. The O1s line in this spectrum did not show thehighest binding energy component at �538 eV, whichis typically observed for organophosphonic acid at-tached on silicon oxide surfaces through hydrogenbonds or for bulk and multilayers of OPA molecules(which has the same headgroup as FPA) on siliconoxide surfaces.41 The lack of this component hereindicates the FPA layer derivatization on Al surfaces

was monolayer. The weight of the integrated intensityof P–O–Al (and –OH) in total O1s intensity was 73.7%,which is higher than the theoretical value of 66.7% forthe FPA molecule.

Figure 3d shows an Al metal peak at 69.45 eV, anAl2O3 peak at 69.84 eV, and an Al–O bond peak at72.27 eV. The Al–O bond peak corresponds to the P-O-Al component at 529.10 eV in the O1s peak. Theresults agree well with the reported peak separationof �2.8 eV.39,42

The bonding energy of the core level peaks of P2s,and F1s were found at 188.80 eV and 686.20 eV,respectively. These results are consistent with thosereported by Sarkar and Paynter,43 suggesting the lowsurface energy FPA film was successfully assembled onthe Al surfaces.

Liquid repellency

We adopted five kinds of liquids to probe the wetta-bility of the FPA-derivatized Al surfaces. They weredeionized water (DI water, clv = 72.1 mN/m), dode-cane (clv = 25.4 mN/m), hexadecane (clv = 27.5 mN/m), methanol (clv = 22.5 mN/m) and ethanol (clv =22.39 mN/m). The CAs of the five liquids on thederivatized Al surfaces are shown in Fig. 4. Thederivatized Al surfaces show excellent liquid repel-lency with static CAs of DI water, dodecane and

O

OP

534 532 530 528 5260.0

3.0k

6.0k

9.0k O1s

= OH

Inte

nsity

(CP

S)

Binding energy (eV)75.0 72.5 70.0

0.0

500.0

1.0k

1.5k

2.0k

Al2p

P–O–Al

Al2O

3

Al2p

Inte

nsity

(CP

S)

Binding nergy (eV)

296 294 292 290 288 286 284 282 280

1.0k

2.0k

3.0k

4.0k

5.0k

C HC C

CH2

CF2

CF2

Inte

nsity

(CP

S)

Binding energy (eV)

CF3

C1s

1000 800 600 400 200 0

P2s F

2s

Al2p

Al2s

C1s

O1s

F1s

(KLL)

(KLL)

Blank Al FPA/Al

Inte

nsity

(arb

. uni

ts)

Binding energy (eV)

(a)

(c)

(b)

(d)

F

–

– –– –

e

–P–O–Al

Fig. 3: Survey XPS spectra of blank Al and PFA/Al surfaces (a); (b)–(d) high-resolution XPS spectra of C1s, O1s, and Al2p,respectively

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hexadecane being larger than 150�. By comparison, theCAs of ethanol and methanol were much smaller, at110� and 100�, respectively.

The sliding angles (SAs) of DI water, dodecane andhexadecane droplets were below 10�, showing anexcellent superamphiphobicity. The SAs of ethanoland alcohols were in the range of 20�–30�. Althoughthe SAs of alcohols were larger than the defined super-repellency requirement of SA < 10�, the as-preparedAl surfaces showed ultra-repellency of a wide varietyof liquids.

The impact of the surface texture on liquidrepellency

In order to clarify the impact of the surface texture onliquid wettability, we begin the discussion with twowidely used wetting regimes, the Wenzel and Cassie–Baxter models, and the free energy of the contactedliquid–solid system. When a liquid droplet is placed ona textured surface, it will stay in either a fully wettedWenzel state44 or a solid–liquid–vapor compositecontact Cassie–Baxter state.45 In the Wenzel state,the apparent CA, h, of a droplet is given by cos h =r cos hY, where r is the surface roughness ratio definedas the actual surface area divided by the projectedsurface area, hence r ‡ 1. Usually, the full liquid–solidcontact leads to high CA hysteresis (CAH) which isdefined as the difference between the advancing andreceding CAs. As a consequence, when liquid dropletsmove through the solid surfaces, they are actually notreally rolling off the textured surface.44,45 In contrast, acomposite interface facilitates both non-wetting (highapparent CA, h > 90�) and easy droplet rolling-off (lowCAH), because of the small liquid–solid contactarea.6–9,46,47 In this state, the apparent CA is given by

the Cassie–Baxter equation cos h ¼ rf f cos hY þ f�1,26,48,49 where rf and f are the roughness ratio andthe projected fraction of the wetted area, respectively.

The liquid (droplet)–solid system tends to stay on astate with the minimum overall free energy which is afunction of the apparent CA, h, and the ambientenvironment. The dimensionless overall free energy,G*, of a solid–liquid–vapor composite contact systemcan be written as48:

G� ¼ G

clvp1=3 3Vð Þ2=3¼ F�2=3 hð Þ½2� 2 cos h� fð Þ sin2 h�

ð1Þ

where clv is the liquid–vapor interface tension, and V isthe total volume of the droplet, and

F hð Þ � ½2� 3 cos hþ cos3 h� ð2Þ

U fð Þ � rf f cos hY þ f � 1 ð3Þ

Equation (1) is established on assumptions that50,51

(1) the equilibrium shape of the droplet is spherical, andall its distortions are limited to the contact region; (2) theliquid–vapor contact area at the composite interface isquasi planar, thus volume of liquid inside the roughnessgrooves is negligible; (3) the projected solid–liquid areais approximately equal to the base area of the sphericaldroplet; and (4) the surveyed total area of the solidsurface is a constant and does not affect the minimiza-tion of the free energy, hence, it is taken as zero.

Marmur48 minimized the overall free energy andshowed that, for hydrophobic surfaces (hY > 90s), CA(h) is determined by: (1) the Cassie–Baxter equation inthe case of f „ 0 and (2) p in the case of f = 0. In

addition, the sign of d2 rf f� ��

df 2 can be used as acriterion of whether the system is in the Wenzel stateor the Cassie–Baxter state.

In order to achieve ultra-repellent surfaces, oils andalcohols with a surface tension smaller than 50 mN/mare often preferred, since, for these liquids, hY isgenerally smaller than 90� and common rough struc-tures cannot fulfill the requirement.

Figure 5 shows a comparison between two differenttextured surfaces with the same solid surface energy.When hY\90�\u (u is the feature angle of the localgeometry), as shown in Fig. 5a, the net traction, F,originating from the surface tension of the liquid at theliquid–vapor interface is downward, promoting theimbibition of the liquid into the solid texture. Acomposite liquid–solid interface cannot be sustainedand a fully-wetted contact is often the result in thiscase. On the other hand, if u � hY\90�, as shown inFig. 5b, F is directed upward and a composite solid–liquid–air interface can be supported. Hence, only re-entrant textured surfaces can support composite liq-uid–solid–vapor interfaces. It is worth noting thateffective surface textures, constructed by electrochem-

DI water HexadecaneDodecane Methanol Ethanol0

30

60

90

120

150

180S

tatic

con

tact

ang

le ()

Liquids

Fig. 4: The CAs of DI water (clv ¼ 72:1 mN/m), hexadecane(clv ¼ 27:5 mN/m), dodecane (clv ¼ 25:4 mN/m), methanol(clv ¼ 22:95 mN/m), and ethanol (clv = 22.31 mN/m), respec-tively

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ical processes,30 nanoparticle loading,31,32 and chemicalcorrosion33 leading to superoleophobicity of Al sur-faces, have been reported. On the other hand, straight(not re-entrant) micro-columns with Al surfaces con-structed at the same time16,17 could not repel oils evenwith FPA modification. We consider that, essentially,the function of the nanostructures in those cases was tohelp the transformation of the common microstruc-tures to re-entrant structures.

Requirements of a composite interface can also beobtained by minimizing the free energy depicted inequation (1). For FPA-derivatized Al surface texture,the requirements are (1) d rf f

� ��df ¼ � cos�1 u ¼

� cos�1 hY , or hY¼u; and (2) In addition,

d2 rf f� ��

df 2 ¼ 0. In the case of hY > 90s (e.g., wateron flat FPA/Al surface, hY = 120s), the predictedapparent CA h = 180s. Actually, in the present work,the apparent CA of water is 168�. hY (<90�) are 73.5�and 66.5� for hexadecane and dodecane, respectively,and are too small to be detected for ethanol andmethanol. They are all smaller than u and do not fulfillthe relationship of hY¼u. However, the system indeedsupports a stable composite interface. The differencesbetween the experimental observations and the theo-retical predictions are believed to result from threereasons. First, the Al surfaces prepared via sputteringwere not perfectly smooth and had nanometer-scaleroughness.18,19 As a consequence, according to theWenzel regime, the hY values measured were larger (inphobic cases) or smaller (in philic cases) than thetheoretical values. Second, the calculation in Figs. 5aand 5b is based on the assumption that the local CA ofthe composite interface is hY, However, the actual CAwill be larger than hY because of the gravity of thedroplets. Finally, the liquid–vapor contact area at thecomposite interface is actually a sagging surface ratherthan planar (see Fig. 5b).

Evaluation of the as-prepared surface robustness

According to Tuteja and coworkers,26,27 the robustnessof textured liquid repellent surfaces can be evaluated

by three parameters: robustness height (H*), robust-ness angle (T*) and robustness factor (A*). They arerelated through an equation 1=A� ¼ 1=H� þ 1=T�.

Both robustness parameters of H* and T* quantifythe sagging and distortion of the liquid–vapor interfaceas a consequence of the pressure difference across theinterface. Such a pressure difference could arise fromthe application of external pressure, the momentum ofa liquid droplet released from a height, or the Laplacepressure within the droplet.52,53 Considering the dia-grams in Fig. 5b, the failure of the composite regime isexpected to result from the local sagging of the liquid–vapor interface. The parameter, H*, provides a dimen-sionless measurement of the pressure, PH, required forthe sagging height, h, of the liquid–vapor interface toreach the maximum pore depth, H. H� ¼ PH=Pprf,

where Pref ¼ 2clv�‘cap is defined as the characteristic

pressure. This is determined by a balance between thesurface forces and the body forces acting on the fluidinterface. In other words, it is close to the minimumpressure difference across the composite solid–liquid-vapor interface for millimeter-scale droplets or largerpuddles on extremely non-wetting, textured surfaces.

‘cap ¼ffiffiffiffiffiffiffiffiffiffiffiffifficlv=qg

pis the capillary length of the fluid, q the

liquid density, and g the acceleration of gravity. For there-entrant texture shown in Fig. 5b, H* takes the form:

H� ¼ PH

Pref¼ D� H‘cap

Dð4Þ

where D� ¼ 2pR4ðDþRÞ2�pR2

is defined as the feature size ofthe system. In the case of water probing of the presenttextured FPA/Al surfaces, H* » 403.

High values of the robustness height H* representthe formation of a robust composite interface. How-ever, a composite interface with H* 1 can stilltransit to a fully wetted one because a shift in the localcontact angle could happen due to the sagging of theliquid–vapor interface. On any textured surface, thelocal liquid–vapor interface makes an angle with thesolid surface. The liquid–vapor interface becomesmore severely distorted with the increase of appliedpressure. This distortion causes the liquid–vapor inter-

H

2D

Substrate

Rsag

R

(b) (c)

Substrate

(a)

F

F

dh (R+D)d(rf f)

(R+D)d f

h

–

Fig. 5: (a, b) Schematic diagrams of two different solid surfaces having the same surface energy and the same hY, butdifferent geometric angles (u), and (c) top view of re-entrant micro-pillar pattern

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face to advance downward to a higher value ofh ¼ uþ dh. Any additional pressure will aggravatethe distortion. The local vapor–liquid interface canreach the bottom of the re-entrant structure, resultingin a fully wetted interface. In other words, thecomposite interface transforms into a fully wettedone when the sagging angle reaches dh ¼ h� u.Therefore, we can evaluate the robustness pressure(Ph) required to force a sagging angle of dh ¼ h� u.Hence, T* takes the form:

T� ¼ Ph

Pref¼ D� sinðhY � uÞ‘cap ð5Þ

for water probing in the presence of the Al surface,T* » 119. It should be mentioned that for hexadecane,dodecane, ethanol and methanol, Equation (5) cannotbe used for an effective evaluation because the valuesof hY are smaller than u. We consider the reasons arethe same as mentioned in the section on ‘‘The impactof the surface texture on liquid repellency.’’

Any external pressure will cause a simultaneousincrease in both the sagging height (h) and the saggingangle (dh). A composite interface will be fully wettedthrough a combination of the two mechanisms dis-cussed above. Thus, a composite robustness factor, A*,should be used to evaluate the robustness of compositeinterfaces. A* increases with the robustness parametersH* and T*. Large values of A* ( 1) imply a robustcomposite interface, corresponding to a high energybarrier between the metastable composite interfaceand a globally equilibrated, fully wetted one. Smallvalues of A* (<1) imply that the composite interfacecannot maintain its stability against small perturbationsuch as a small increase of the pressure differenceacross the liquid–vapor interface.

For the micro-pillar texture, features of R, D, H andu are defined by the photolithography of SU-8, and thedesign factors are weakly coupled. Surfaces with both ahigh apparent contact angle and a highly robustcomposite interface can be achieved simultaneously.The robustness factor, A*, of the FPA-derivatized Alsurfaces presented in this work was calculated as 92,which is comparable to the values reported by Tutejaet al.,26 indicating a good robustness.

Conclusions

Re-entrant micro-pillar-textured Al surfaces were fab-ricated by photolithography of SU-8 and sputtering ofAl film. After FPA derivatization, the textured Alsurfaces showed ultra-repellency against a wide varietyof liquids. The CAs of DI water, hexadecane anddodecane were larger than 150�, and those of methanoland ethanol were above 100�. The SAs of DI water,hexadecane and dodecane droplets were below 10�,showing a desirable superamphiphobicity. The SAs ofthe alcohols were in the range of 20�–30�.

The robustness height, angle and factors of the ultra-repellent Al surfaces were found to be 403, 119, and 92,respectively, suggesting a good robustness. We believethat optimizing the design parameters of R, D and uwill further improve the robustness factor A*.

This work has demonstrated that FPA derivatiza-tion is effective in lowering Al surface energy. Thecombination of re-entrant texture and FPA derivati-zation provides a simple and high throughput approachto ultra-repellence against a wide variety of liquids.

Acknowledgments This study was supported by theNational Natural Science Foundation of China (NSFC)(Grant Numbers 11564002 and 11764003), the ScientificResearch Fund Project of Yunnan Province EducationDepartment of China (Grant Number 2015Z165), theScientific Research Fund of Dali University (GrantNumber KYBS201301) and the analysis and testingfoundation of Kunming University of Science andTechnology (Grant Number 2016T20110028). Theauthors thank Dr. Kar Man Leung for the lithographyand sputtering of the substrates.

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