This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 New J. Chem., 2014, 38, 1011--1018 | 1011

Cite this: NewJ.Chem., 2014,

38, 1011

A facile approach for preparing biomimeticpolymer macroporous structures with petal orlotus effects†

Shan Peng and Wenli Deng*

In this paper, three kinds of superhydrophobic polymethylmethacrylate (PMMA) macroporous membranes

with controlled adhesion were fabricated by a simple hierarchical alumina template wetting method. This

alumina template was obtained through a convenient one-step anodization method, achieving a

hierarchical structure combining the upper macroporous structure and lower nanopore topography. These

final PMMA products perfectly replicate the original macroporous structure of the template. By simply

controlling the reaction temperature, we can obtain the different PMMA surfaces with diverse topographies.

The testing results confirm that the PMMA samples not only could achieve superhydrophobicity after

the post-modification treatment but also present huge differences in adhesive abilities. The shallow bowl-

shape structure presents a slippery property, which has a low sliding angle (SA) of 31; the deep bowl-shape

morphology possesses a SA of 301, presenting a little sticky performance; the deep honey-comb texture

possesses a large SA, which can hold a 10 mL water droplet even upside down, totally showing strong sticky

water adhesion. Noticeably, the as-prepared superhydrophobic PMMA samples have remarkable resistivity

to acid/alkali, various organic solvents and a long-time period. Moreover, the approach in this work can

also be applied to any other soluble polymers. It is believed that our work provide a convenient and

promising method to prepare polymer superhydrophobic surfaces with controlled adhesion, which has

been rarely reported up to now.

1. Introduction

In nature, many plants and insects with special structures andwettabilities have been paid much attention by researchers formany years.1 The term ‘‘lotus effect’’ has been used to describesurfaces with a water contact angle (WCA) higher than 1501 anda sliding angle lower than 101. Another superhydrophobic surfacewith high water adhesion has also aroused great interest in recentyears. Some plants like rose petals can not only keep super-hydrophobicity with a WCA as high as 152.41, but also possessthe ability to suspend water droplets, which get pinned to thesurface firmly.2 This phenomenon is called the ‘‘petal effect’’,presenting both a high water contact angle and a high contactangle hysteresis (CAH, the difference between the advancing

and receding angles). Previous reports have proposed that theadhesion difference of the two kinds of plants is attributed to thesizes of the microstructures on the surfaces.2a,3 Since superhy-drophobic surfaces have an extensive range of applications inmany important fields including self-cleaning,4 oil-repellent sur-faces,5 low-dragging coatings,6 anti-reflection,7 oil/water separa-tion8 and no-loss microdroplet transportation,9 numerous studieshave reported the preparation of superhydrophobic surfaces withsticky or slippery water adhesion through using many kinds ofmethods such as electrospinning,10 anodic oxidation,11 electro-deposition,4,12 chemical etching,13 and hydrothermal methods.14

However, owing to the complex and fickle environment outside,single adhesion is hard to fulfill the demands of practicalapplications, thus researchers recently have developed manymethods to prepare superhydrophobic surfaces with tunableadhesion.3,15 It is well known that a low surface energy combinedwith high surface roughness could result in superhydrophobiceffects and the water adhesion on the superhydrophobic surfacescould also be regulated by regulating the chemical compositionsand surface structures. For example, by adjusting the nitro cellulosedosage concentrations, Lai’s group fabricated superhydrophobicsponge-like TiO2 films with controllable adhesion.15b Wang et al.prepared cauliflower-like silica nanospheres with tunable adhesion

College of Materials Science and Engineering, South China University of

Technology, Wushan Road, Tianhe District, Guangzhou 510640, P. R. China.

E-mail: wldeng@scut.edu.cn; Tel: +86-020-22236708

† Electronic supplementary information (ESI) available: Fig. S1: the cross-sectional SEM images of samples A–C. Fig. S2: the static water contact anglesof diverse PMMA macroporous samples before modification. Fig. S3: the enlargedSEM images of samples A–C. Fig. S4: the static water contact angles of the slipperysuperhydrophobic PMMA sample before and after 6 months. See DOI: 10.1039/c3nj01156a

Received (in Montpellier, France)25th September 2013,Accepted 16th December 2013

DOI: 10.1039/c3nj01156a

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through regulating chemical compositions.15f Cheng et al. realizedthe controlled adhesion on the superhydrophobic Cu(OH)2

substrates by controlling the reaction time and the chain lengthof the thiol.15e Li et al. used the chemical etching method toprepare the superhydrophobic CuO surfaces with tunable adhe-sion by controlling the etching time.15d However, these reportsmainly focus on realizing controllable adhesion on the metal orinorganic substrates, few reports pay much attention to achievethis property on the superhydrophobic polymer surface.15c,16 Infact, although many studies have successfully fabricated super-hydrophobic polymer surfaces,15c,17 it is still not easy to use thesimple ways to achieve superhydrophobicity since the surfaceroughness at the micro/nanoscale cannot be readily achieved onpolymer surfaces by self-assembly, to say nothing of the controllableadhesion. Moreover, most of studies have concentrated on studyingthe effects of the chemical modification on the adhesion and fewon the factors concerning the surface structure. The mechanismthat the topographies create great effects on adhesion is not wellestablished up to now. Actually, the relationship between themorphology of the solid surface and adhesion is very useful andcan be regarded as a rule to produce the micro/nano-scale struc-tures in order to obtain artificial superhydrophobic surfaces withsticky or slippery water adhesion. Thus it is of great significanceto develop more simple methods to prepare superhydrophobicpolymer surfaces with regulated adhesion and investigate moreinformation about the relationship between the surface topographyand the water adhesion.

Ordered macroporous materials have been attracting muchattention because of their outstanding properties such as largesurface and high porosity, which make them suitable for wideapplications in various industrial fields.18 Nowadays, there aremany synthesis methods to prepare the macroporous materials,such as polymer microspheres,19 and the breath figures method.20

Compared with these methods, the template wetting method isdefinitely a highly effective and well-established approach toproduce functional materials.21 The morphology of the templatedecides the resultant structure of the product. Inorganic oxidesuch as anodic alumina oxide (AAO) has always been consideredas the perfect template due to its regular pore arrangementin hexagonal patterns and it can also be removed easily.22

One-dimensional materials can be easily prepared through repli-cating their pore structures inversely.23 However, the normal AAOtemplate is constricted by its applications in fabricating nanoscalematerials and it is very difficult and rigorous to enlarge its porediameter from the nanoscale to the microscale.24

In this paper, we prepare three kinds of superhydrophobicmacroporous PMMA surfaces with controllable adhesion througha template wetting method. The template used in this work is thehierarchical alumina membrane (HAM), which is easily fabricatedby the one-step anodization method. This hierarchical structureconsisted of the macroporous structure on the top layer and thenormal AAO nanopores positioned underlayer. It is demonstratedthat the resultant PMMA surfaces replicate the macroporousstructures of the HAM template successfully. By simply adjustingthe wetting temperature, we can obtain the PMMA surfaces withdiverse morphologies. With the subsequent modification, we find

that these samples can all achieve superhydrophobicity, especiallythey present huge morphology-dependent adhesion. Moreover,the as-prepared PMMA samples show stable superhydrophobicityover a wide pH range, organic solvents and a long-term timeperiod. We believe that this paper provides meaningful inspira-tion to the relationship between the surface morphology andwater adhesion. This method can also be appropriate for anykind of soluble polymer, which has wide application values inthe future.

2. Experimental

The methods to prepare the HAM template in this work aresimple and cost-effective. Several pieces of (50 mm � 10 mm,0.22 mm thickness) low-purity aluminum slices (composition:Al 97.15%, O 2.16 wt%, Si 0.13 wt% and Fe 0.56%) were cleanedultrasonically with acetone, ethanol and deionized water in asequence to get rid of purities and grease. Then the cleanedaluminum foil was used as a working anode without polishing, anda platinum (Pt) electrode was employed as a cathode. The anodiza-tion was conducted in 0.3 M oxalic acid (C2H2O4) at 60 V at the airtemperature (about 30 1C). After the one-step anodization wasprocessed only for 8 h, the HAM template can be obtained. Thenit was rinsed with deionized water and dried under N2 before use.

We used the HAM as the template and polymethylmetha-crylate (PMMA) as a polymer. At first, PMMA powder (Mw =350.000 g mol�1) was dissolved in CH2Cl2 to prepare 20 wt%PMMA–CH2Cl2 solution. Then a droplet of PMMA solutionwas quickly placed on the template. Through controlling thewetting temperature, the various PMMA topographies wereobtained. The concrete procedures and parameters are as follows:(A) Sample A: a droplet of 20 wt% PMMA–CH2Cl2 solution wasbrought into close contact with the HAM template, the specimenwas then placed at room temperature for 5 h. Finally, sample Awith the shallow bowl-shape macrostructure was obtained afterremoving the HAM substrate by using 3 M NaOH solution.(B) Sample B: once the PMMA solution touched the HAM template,sample B was heated at 80 1C for 5 minutes immediately, then itwas cooled down to room temperature quickly. The next stepsare the same as sample A. Finally sample B with the deep bowl-shape structure could be obtained. (C) Sample C: the onlydifferent parameter with sample B employed in this methodis the heating temperature, we heated sample C at a highertemperature of 150 1C, the next treatments are the same assample B. Sample C with the deep honey-comb structure couldform after the template was removed. Finally, the three kindsof PMMA samples were modified by dipping into 1H, 1H, 2H,2H-perfluorodecyltriethoxysilane (PDES) ethanol solution for2 h followed by drying at 60 1C for 2 h and therefore the super-hydrophobic PMMA surfaces were achieved.

The water contact angle (WCA) and slide angle (SA) measure-ments were carried out on an OCA35 (DataPhysics, Germany)equipped with a video camera and a titling stage. Sliding angleswere measured by slowly tilting the sample stage until the waterdroplet started moving. The static WCA and SA values were the

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average of seven measurements by using 3–10 mL water drops.Scanning electron microscopy (SEM) images were obtained on afield emission scanning electron microscope (FESEM, NOVANANOSEM 430). The X-ray photoelectron spectrum (XPS) wasrecorded on a Kratos AXis Ultra X-ray photoelectron spectroscope.

The chemical stability of the as-prepared superhydrophobicPMMA samples was evaluated by dropping the droplets withdiverse pH values onto the surface of the specimens and theircorresponding WCA and SA were subsequently investigated using asimilar method as described above. The resistance towards varioussolvent solutions was determined as follows: after the superhydro-phobic samples were immersed into the various solvents (includingtoluene, ethanol, acetone, hexane and tetrahydrofuran) for 10 min,the samples were dried in an oven at 60 1C for 10 min undervacuum before evaluation. And finally, we can investigate theirWCA and SA values after the treatment. In addition, the hot waterused in this work was around 50 1C.

3. Results and discussion

The preparation process of the HAM template is rapid andconvenient. We use the aluminum plate with the purity of97.17% as the raw material and upon anodization in 0.3 Moxalic acid for 8 h under the environmental conditions, thehierarchical alumina membrane template was obtained. Thedual structures of the HAM template are confirmed by SEMimages. As shown in Fig. 1a, an obvious porous morphology canbe seen from the top surface layer. The enlarged SEM image ofone hole indicates that its diameter is about 10 mm (the inset ofFig. 1a). When a hole is magnified, we find that there are largequantities of honey-comb AAO nanopores with the diameter ofabout 100 nm positioned at the bottom (Fig. 1b). During theone-step anodization process of aluminum, the normal AAOnanopores form on the aluminum substrate in the first stage,however, with the progress of anodization, the sidewalls ofnanopores and the joints of the three hexagonal cells at the topposition are easily etched gradually due to the higher acidgradient.25 When the nanowalls become thinner and thinner,it would lead to the formation of nanowires eventually. After thesetop nanopores have been totally etched away, large-area nanowirearrays are created on the top surface and the large groove-structurecan be produced after an appropriate self-assembly process.

In addition, the long anodizing time definitely results in a largequantity of heat, which cannot be released quickly in time.This would make the volume of the nanopores expand moreeffectively, leading to the destruction more drastically. Whenthe nanopores of the upper region completely transform intonanowires, the remanent nanopores at the lower region aremaintained, and a hierarchical structure is constructed on thealuminum substrate.

The template wetting method is very effective and reproducible.It relies on capillary forces to draw polymer solutions or polymermelts into the nanochannels of the nanohole materials. Thetemplate is simply used as a scaffold and the final polymerduplicates the structure of the template. In our current work,HAM and PMMA are used as the inorganic template and polymercomponents, respectively. Through controlling the reactiontemperature, we can obtain the PMMA membranes with diversemorphologies. Fig. 2 illustrates the mechanism of the procedure toprepare the PMMA macroporous structures. It is well known thatthe polymer melts or solutions with low surface energy tend to wetthe porous templates such as AAO or silicon.26 The polymericsolution wets the inner walls of the porous template and formsa wetting layer.27 Considering the special hierarchical structureof the HAM template (Fig. 2a), the PMMA–CH2Cl2 solutionwould wet the walls of both the top layer of large grooves andthe bottom layer of nanopores of the template. However, due tothe capillary action of the internal nanopores at the bottom,CH2Cl2 quickly flow into the bottom of the large grooves andthen the nanopores because of its benign fluidity. Most of theCH2Cl2 is gathered at the bottom of the template and form thesolvent centre (Fig. 2b), which lead to the separation processbetween the solvent and the PMMA polymer layer. The polymerinfiltrates along the walls to reach a certain height and thesecond wetting makes the wetting rise to the same or lowerheight.27a The polymer solution deposits along the walls of theporous structure layer by layer and thus making the filmthicker. The wetting procedure could be carried on repeatedlyuntil the solvent completely evaporated. As a result, the PMMApolymer could replicate the hierarchical structure of the HAM

Fig. 1 (a) Low-magnification SEM image of the HAM template. The insetshows the enlarged SEM images of one hole of the template. (b) High-magnification SEM images of the bottom region of the template.

Fig. 2 Illustration of the mechanism of the preparation process of theas-prepared PMMA macroporous structures.

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template. However, when the sample is treated by heating, theheating induces further separation and greatly influencesthe fluidity of the PMMA layer and the evaporation speedof the solvent centre, which further creates huge effects onthe morphologies of the resultant PMMA sample. When thetemperature is high enough, the solvent evaporates drasticallyand every large groove on the top layer of the template becomesa foaming centre. The upward solvent vapour forms instantlyand rushes towards the surface quickly (Fig. 2c), causing theformed structure on the top surface to expand. After the HAMtemplate is removed by 3 M NaOH solution, the target PMMAmacroporous films are finally obtained (Fig. 2d).

The SEM images of the final PMMA products (samples A, B,and C) are shown in Fig. 3. Sample A was not heated afterthe PMMA–CH2Cl2 solution comes in contact with the HAMtemplate and then we placed it under the ambient conditionsfor 5 h. When the PMMA was solidified, we can obtain the PMMAmembrane with shallow bowl-shape morphology (Fig. 3a). It canbe observed that the diameter of one bowl was about 5–10 mm,which is in agreement with the dimension of the HAM template.It is demonstrated that the heating treatment affects the structureof the final PMMA product. As can be seen from Fig. 3c, whensample B was heated at 80 1C for 5 minutes immediately after thePMMA solution was deposited on the HAM surface, a largeamount of deep bowl-shape holes with the diameter of about5–10 mm finally formed in this case. When sample C was heatedat a higher temperature of 150 1C, we can obtain the deep honey-comb shape structure (Fig. 3e). From the magnifying SEMimages of one hole (Fig. 3b, d and f), a change in the hole depthfrom shallow to deep can be observed clearly. The mechanism of

the morphology differences of the three samples can be easilyunderstood from the schematic shown in Fig. 2. In the case ofsample A, when the polymeric solution wetted the HAM templateat room temperature, the solution infiltrated along the walls ofboth the upper large grooves and the lower nanopores of thetemplate. On one hand, CH2Cl2 quickly flowed into the bottom,resulting in the low solvent concentration in the top layer andhigh solvent concentration in the bottom layer. Because of thelack of the solvent on the top surface, the polymer layer theretended to solidify and thus the fluidity becomes worse. On theother hand, the molecular weight of the PMMA polymer used inthis work was relatively high and sample A was not treated byheating. These factors would further hinder the mobility of thepolymer. Because the polymer fluidity on the top layer of sampleA was very slow, its ability to penetrate along the walls of thelarge grooves also decreased and the height which the PMMAfilm layer could reach was limited, therefore resulting in theformation of the shallow bowl shape morphology. Meanwhile,those polymer solutions which wetted the nanopores at thebottom would form the PMMA nanowires. Once the templatewas removed, those nanowires would aggregate and lean on eachother closely (Fig. S1a, ESI†). From the above reasons, we couldeasily understand the structure of sample A. However, when thesample was treated by heating, the situation became different.Sample B was treated by heating immediately at 80 1C after thepolymer solution was deposited on the surface of the HAMtemplate. On one hand, it is understood that the heatingtreatment could make the fluidity of the polymer solution easierand thus the polymer could creep along the walls of the largegrooves toplayer with a longer height than sample A. On theother hand, the heat reached the bottom of the template rapidlybased on the principle of heat transfer, making the solventvolatilize more rapidly and every groove became a spume centre,which rushed from the bottom to the surface. The impulsiveforce of CH2Cl2 vapour breaks the cohesive force in the bottomlayer of PMMA nanowire arrays to some degree and thus leadingto the interspace among the PMMA bunches (Fig. S1b, ESI†).These reasons could be used to explain the formation of the deepbowl shape structure of sample B. In the case of sample C, theheating temperature was enhanced to 150 1C, which was higherthan the glass transition temperature of PMMA. The polymerfluidity in this case became much easier than samples A and B,therefore the polymer could wet the template more easily andcould reach a much greater height of the walls of the large poreson the top layer of the template. The solvent CH2Cl2 evaporatedmore drastically under the higher treating temperature andformed the CH2Cl2 vapour, which would rush towards thesurface immediately. The PMMA deposited in grooves wasexpanded and penetrated by the CH2Cl2 vapour that escapedfrom the surface, finally resulting in the formation of thevesicular-porous morphology of sample C. During the heatingprocess, the drastic foaming phenomenon could be easilyobserved and after being heated for 5 min, the sample wascooled to room temperature quickly. The foamed PMMA was leftno time to collapse and the deep honey comb structure ofsample C was kept. The macroporous morphology and the large

Fig. 3 (a and b) Low and high-magnification SEM images of sample A withthe shallow bowl-shaped structure. (c and d) Low and high-magnificationSEM images of sample B with the deep bowl shaped morphology. (e and f)Low and high-magnification SEM images of sample C with the deephoney-comb structure.

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porous depth of sample C could also be easily observed from itsside-view SEM images (Fig. S1c, ESI†).

Fig. 4a shows the full XPS spectrum of the PMMA membraneafter modification. The extra element compositions of F can beseen clearly. The strong F signal at 688.88 eV (Fig. 4b) furtherindicates that a stable monolayer of PDES has been self-assembled on the PMMA surface successfully.

We next investigate the wettability and water adhesion of thethree samples. The original flat PMMA membrane presents ahydrophobic state (901), while the WCAs of samples A–C beforemodification are 1081, 881, 671, respectively (Fig. S2, ESI†),however, after modification with the PDES, all the three samplescan achieve WCAs more than 1501, which show superhydrophobicproperties. Since the discovery of superhydrophobic petal leaveswith strong water adhesion, a wettability study often accompaniesthe investigation of water adhesion. In this work, we also did aseries of experiments to figure out the water adhesion of eachPMMA sample. To our surprise, we find that the three samplesexhibit very different adhesive abilities. Sample A can achieve aWCA of 1581 after modification and it presents a slipperystate with a SA of 31 (Fig. 5a and b). In the case of sample B, wefind that it can also achieve superhydrophobicity with a WCA

of 1551 (Fig. 5c) and presents a little sticky property with a SAof 301 (Fig. 5d). Sample C also presents superhydrophobicitywith a WCA of 1521 (Fig. 5e). However, it is very interesting tofind that sample C shows strong water adhesion and even cansupport an inverted 10 mL water droplet without any movement(Fig. 5f). Generally, two models are frequently used to explain thesuperhydrophobic surfaces with slippery or sticky property: theCassie–Baxter model28 and the Cassie-impregnating model.2a

In the Cassie–Baxter model, there are large quantities of air-pockets in the rough surface and the water droplet cannotpenetrate into the rough structure and therefore roll off easily.While in the Cassie-impregnating model, the water droplet canpartially infiltrate into the microstructure of the surface, allowingthe droplet to pin on the surface while it still keeps superhydro-phobicity. The adhesion behaviour of the water drop is mainlyrelated to the CA hysteresis and the continuous three-phasecontact line. The more discontinuous contact line should havesmaller energy barrier and thus the water droplet on the surfacecan roll off easily. On the other hand, the more continuous three-phase contact line should definitely arouse effective CA hysteresisand surface adhesion. In our experiments, since the chemicalcompositions of the three samples are the same, therefore thestructure differences are considered as the main reason for thesehuge adhesion differences. Sample A with the shallow bowl-shapetopography possesses large WCA and low SA, which can be explainedby the Cassie-Baxter model. Large amounts of air pockets existin the shallow macropores on the surface, which can greatlyreduce the contact area between the droplet and the substrate

Fig. 4 (a) XPS spectra of the as-prepared PMMA macroporous structuresbefore and after modification with PDES. (b) The high-magnification Fregion of the surface after modification.

Fig. 5 Wettability and adhesion performance of the as-prepared PMMAsamples. (a and b) Sample A possesses a WCA and SA of 1581 and 31,respectively. (c and d) A 3 mL water droplet in contact and leaving the surfaceof sample B, presenting a WCA and SA of 1551 and 301, respectively. (e and f)A 10 mL water droplet was positioned on the surface of sample C with thetilting angles of 01 and 1801, showing strong sticky water adhesion. (g) TheCassie–Baxter model. (h) The Cassie-impregnating model.

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and therefore making the water droplet move off the surfaceeasily (Fig. 5g). In addition, the PMMA is an original hydro-phobic material, thus the relative more contact area (comparedwith samples B and C) between the droplet and the substrate onthe surface of sample A would increase the slide possibility.Sample C with the deep honeycomb macroporous structure canachieve superhydrophobicity and strong water adhesion. Onone hand, the deeper porous structures of sample C would resultin stronger capillary force, which can force the water droplet toenter into the large hole structures. This phenomenon is in accordwith the Cassie-impregnating model: a partial water dropletimpregnates into the macroporous structure and is caught bythe surface (Fig. 5h), therefore allowing the water droplet to stick tothe surface. In addition, based on the enlarged SEM images ofsamples A–C (Fig. S3, ESI†), the larger porosity of the honey-comb wall structure of sample C even creates much strongercapillary forces, which result in effective water adhesion. On theother hand, according to the explanation about the high wateradhesion of the rose petals proposed by Feng et al.,2a the largerhole diameter of sample C is also an important reason for itsstrong water adhesion. From the above reasons, we can easilyunderstand the petal effects of the deep honey-comb structure.The deep bowl-shape of sample B also has a large WCA similarto sample A. However, its deeper hole structure could partiallygenerate more capillary force and therefore arouse a little stickywater adhesion, presenting a SA of 301.

When a 5 mL water droplet falls toward the slippery PMMAsurface (sample A), a quick obvious rebound phenomenon canbe observed. As indicated by Fig. 6, as the droplet impacts thesurface (Fig. 6a), it rebounds elastically with a certain velocity(Fig. 6b). Then it hits the surface again (Fig. 6c) and reboundsagain, which presents more deformation and rebounds higherthan the first time (Fig. 6d). It can be seen that the dropletbounces some times before coming to rest and finally rolls offthe PMMA surface (Fig. 6e and f). There is almost no trace ofwater left on the surface, which further demonstrates thefascinating non-wetting performance of the slippery sample A.

The superhydrophobic sample C with strong water adhesion inthis work can also be used as a ‘‘mechanical hand’’ to transfersmall water droplets without any loss from a low adhesive surfaceto a hydrophilic one. As shown in Fig. 7, a 5 mL water droplet wasfirst placed on the slippery superhydrophobic PMMA surface.Then the sticky superhydrophobic PMMA sample with the deephoney-comb structure was made to contact with the water

droplet (Fig. 7a) and the water droplet was observed to becompletely transferred to the sticky surface quickly (Fig. 7b andc). Finally, the water droplet was easily released by contactingthe hydrophilic silicon surface (Fig. 7d). The whole procedurewas finished in a short time.

Due to these remarkable performances of the PMMA, wethen investigate the stability of the as-prepared superhydro-phobic PMMA surfaces. Fig. 8a shows the plot of pH values andWCA, SA values of the slippery PMMA surface (sample A). Fromthese two curves, we can clearly learn that the changes in thepH value almost do not cause any effects on the WCA and SAvalues on this surface, which demonstrate that this slipperysurface also shows a stable slippery superhydrophobicity in thispH range. In the case of the sticky superhydrophobic PMMAsurface (sample C), we also find that there is almost no obviouschange in the WCA values over the wide pH range from 1 to 12,presenting a wonderful superhydrophobic stability and it stillshows a strong adhesive ability during these pH range scope.Fig. 8b shows the WCAs of sample A after it was treated with thedifferent solvents such as ethanol, acetone, and n-hexane. Fromthe graph, we observe that the WCAs almost do not change and

Fig. 6 Sequence of a 5 mL water droplet impacting the slippery as-prepared PMMA surface. (a–d) The liquid falls from a certain height,approaches and bounces the surface, and finally rolls across the surface(e, f). The snapshots are obtained by using a high-speed camera.

Fig. 7 Transferring process of a 5 mL water droplet from the slipperysample A to the hydrophilic silicon surface by using the sticky sample C:(a) Placing a 5 mL water droplet on the slippery sample A. (b) Contacting thewater droplet with the sticky sample C surface in this work. (c) Transferringthe 5 mL water droplet to the sample C surface successfully. (d) Releasingthe water droplet to the hydrophilic surface.

Fig. 8 (a) WCAs and the corresponding SAs of sample A at different pHvalues. (b) The WCAs of sample A after it was treated by diverse solvents.The insets show the shapes of the water droplets.

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can still be kept more than 1551 no matter what kind of solvent itwas immersed in, which further demonstrates the stable super-hydrophobicity of the as-prepared PMMA surface. We believe thatthe persistent superhydrophobicity of our as-prepared sample byimmersion in various solvents is mainly ascribed to its macropor-ous structure. It is well understood that the porous structure couldbe maintained more easily compared to the protruding structure(which easily gets flat when interrupted by the external stimula-tion). When the sample C was placed under the air conditions for6 months, it would not lose its superhydrophobicity and could stillmaintain a WCA of 1531 (Fig. S4, ESI†). Actually, the ability of thesuperhydrophobic surface to resist the harsh environment is largelyrelated to the property of the polymer, thus if we change to anothersoluble polymer which shows inherent high-performance such asgood mechanical strength, high thermal stability, high chemicaland wear resistance, then more superhydrophobic polymer sur-faces with more excellent performances can be obtained by usingthe methods discussed in our paper.

4. Conclusion

In summary, a facile HAM template wetting method was developedto fabricate three kinds of superhydrophobic PMMA surfaces withcontrolled water adhesion. The HAM template was easily obtainedthrough a one-step anodization method. The PMMA samples withdiverse topographies were prepared by simply regulating the reac-tion temperature. The three PMMA samples presented a largemorphology-dependent adhesion phenomenon: the shallow bowl-shape morphology achieves the ‘‘lotus effect’’ with a small SA of 31,the deep bowl-shape structure possesses a medium SA of 301 andthe deep honey-comb structure totally shows a ‘‘petal effect’’ withstrong sticky water adhesion. Moreover, our as-prepared PMMAsamples present stable superhydrophobic properties under theharsh conditions such as acid/alkali, various organic solvents anda long-term time period, which may find their significant applica-tions in many harsh fields. Furthermore, considering the simplestrategy in this work, our method can also be appropriate for anyother soluble polymers and we also believe that this work will behelpful for providing important inspirations in synthesizing super-hydrophobic surfaces with controlled adhesion in the polymer area.

Acknowledgements

Financial support from the National Natural Science Foundation ofChina (21103053, 91023002, and 51073059), the National Programon Key Basic Research Project (2012CB932900 and 2009CB930604)and the Cooperation Project in Industry, Education and Researchof Guangdong Province and Ministry of Education of China (No.2011B090400376) is gratefully acknowledged.

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