Super-Hydrophobic PDMS Surface with Ultra-Low Adhesive Force

Communication DOI: 10.1002/marc.200500458 1805Summary: Rough polydimethylsiloxane (PDMS) surfacecontaining micro-, submicro- and nano-composite structureswas fabricated using a facile one-step laser etching method.Such surface shows a super-hydrophobic character withcontact angle higher than 1608 and sliding angle lower than58, i.e. self-cleaning effect like lotus leaf. The wettabilities ofthe rough PDMS surfaces can be tunable by simply controllingthe size of etched microstructures. The adhesive forcebetween etched PDMS surface and water droplet is evaluated,and the structure effect is deduced by comparing it with thoseown a single nano- or micro-scale structures. This superhydrophobicPDMS surface can be widely applied to manyareas such as liquid transportation without loss, and micropump(creating pushing-force) needless micro-fluidicdevices.Etched PDMS surface containing micro-, submicro-, andnano-composite structures shows a self-cleaning effect withwater CA as high as 1628 and SA lower than 58.Super-Hydrophobic PDMS Surface with Ultra-LowAdhesive Force aMeihua Jin, 1,2,3 Xinjian Feng, 2 Jinming Xi, 2 Jin Zhai, 2 Kilwon Cho, 3 Lin Feng,* 1,2 Lei Jiang* 21 Department of Chemistry, Tsinghua University, Beijing 100084, P. R. ChinaE-mail: fl@mail.tsinghua.edu.cn2 Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. ChinaE-mail: jianglei@iccas.ac.cn3 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, KoreaReceived: July 1, 2005; Revised: August 11, 2005; Accepted: September 20, 2005; DOI: 10.1002/marc.200500458Keywords: adhesive force; micro-submicro-nano composite structures; polydimethylsiloxane; roughness; super-hydrophobicIntroductionThe wettability of a solid surface is an important property ofmaterials, which depends on both the surface chemicalcomposition and the surface geometrical microstructures.[1] In recent years, natural super-hydrophobicity ofsolid surfaces has attracted much interest because of theirimportance in fundamental research, practical applications,and the inspired mimetic attempts. [2,3] The origin of theself-cleaning property of lotus leaves has been revealed tobe a cooperative effect of micro- and nano-scale structureson their surfaces. [4] In general, there are two criteria toevaluate self-cleaning: a super-hydrophobic surface with avery high water contact angle (CA > 1508) and a very lowsliding angle (SA < 108), in which SA can also be expresseda: Supporting information for this article is available at thebottom of the article’s abstract page, which can be accessedfrom the journal’s homepage at http://www.mrc-journal.de, orfrom the author.in terms of the difference between advancing and recedingCA, i.e., hysteresis. Generally, the former criterion has beenthoroughly studied in the past 60 years [1–4] while the latterhas only been discussed so far theoretically or experimentallyto a very limited extent, [5,6] due to the absence of directexperimental measurements of it.Polydimethylsiloxane (PDMS), as a typical elastomericmaterial, has been widely used in soft-lithography. [7]The water repellence, strong resistance, and low-densitysurface properties also make it an attractive material toreplace porcelain and glass in housing and high-voltageoutdoor insulator. Recently, it has also been introduced intomicro-fluidic devices as channels, and many excellentresults have been achieved. [8] However, the intrinsichydrophobicity of the flat PDMS surface with the low-CA(about 1008–1108) and high-SA (>908) restricts its wideapplications to some extent. [9]In this communication, we demonstrate a facile one-steplaser-etching method to fabricate rough PDMS surfaceMacromol. Rapid Commun. 2005, 26, 1805–1809ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1806 M. Jin et al.containing micro-, submicro-, and nano-composite structures.Importantly, such surface is super-hydrophobic withCA higher than 1608 and SA lower than 58. The wettabilitiesof the rough PDMS surfaces are tunable by simplycontrolling the size of etched microstructures. The adhesiveforce between etched PDMS surface and water droplet canbe evaluated, and the structure effect can be deduced bycomparing it with those of a single nano- or micro-scalestructures. The super-hydrophobic PDMS surface can bewidely applied to many areas such as liquid transportationwithout loss, micro-pump (creating pushing-force) needlessmicro-fluidic devices, and so on.Experimental PartSynthesisSmooth PDMS SurfacePolydimethylsiloxane elastomer kits (Sylgard 184) werepurchased from Dow Corning (Midland, MI). A prepolymerof PDMS was poured onto the clean glass plate and solidified at60 8C for 10 h; thus, the smooth PDMS film can be formed withthe thickness of 2 mm. The area of 1.5 1.5 cm 2 was used foreach experiment.Micro-, Submicro-, and Nano-CompositeStructured PDMS SurfaceThe schematic illustration of the laser-etching process on thesurface of smooth PDMS film was shown in Figure S1(Supporting Information). The pulse laser used to generatemicrogrooves on PDMS smooth surface was an Nd:YAG laser(New Wave Research QuikLaze TM , USA). The wavelengthand repetition rate of the laser pulse were 532 nm and 20 Hz,respectively, as well as the energy was 5 J cm 2 . PDMSsurfaces with different microgrooves in width and depth can beeasily obtained by adjusting the size of the rectangular laserspot and the size of the laser facula.Nanostructured PDMS SurfaceA prepolymer of PDMS toluene solution was spin-coated on apolycarbonate (PC) nanopillars surface prepared using themethod we reported before [10] to replicate its nanostructures.After being completely solidified at 60 8C for 6 h, PDMSnanopillars film was formed and peeled off from PC surface.The schematic illustration of replicating process on PCnanopillars to form nanostructured PDMS surface was shownin Figure S2 (Supporting Information). The average diameterand the height of the PDMS nanopillars thus prepared are ca. 72and 77 nm, respectively, while the surface roughness is ca.120 nm.Microstructured PDMS SurfaceLaser-etching method was used to fabricate the micropillarstructure on silicon surface at first. Then using the etchedsilicon surface to replace the PC nanopillars the process wasrepeated as shown in Figure S2 (Supporting Information), thatis, the PDMS toluene solution was spin-coated on siliconand solidified at 60 8C for 6 h; thus, PDMS surface withmicropillars was formed. The average diameter and the heightof the pillars thus prepared are ca. 25 and 0.5 mm, respectively,while the surface roughness is ca. 228 nm.CharacterizationThe scanning electron microscopy (SEM) images wereobtained on a JSM-6700F scanning electron microscope(JEOL, Japan) at 3.0 kV. Prior to the measurements, the sampleswere coated with a thin gold film by means of a vacuumsputter to improve electrical conductivity. The CAs and SAswere measured on a CA system (OCA20, Dataphysics,Germany) at ambient temperature. The average CA and SAvalues were obtained by measuring five different positions ofthe same sample. The average roughness can be calculatedfrom the topographical images by atomic force microscopy(AFM) (SPI 3800N, Seiko, Japan).Results and DiscussionAs reported, etching or photolithography is an effectivemethod to fabricate rough surfaces. [2c,5b] Herein, the laseretchingmachine was used to create the rough PDMSsurface in a region of about 1 1cm 2 . Figure 1(a) shows atypical SEM image of an etched PDMS surface, which iscomposed of a regular array of convexes with ca. 25 mminwidth, 10 mm in depth, and 2 1 mm in interdistance. Themagnified image of Figure 1(a) shows that there are manyparticles with the size of 0.5–3.2 mm on the top surface ofeach convex, which can enhance the surface roughness[Figure 1(b)]. To make a difference, we called these particlesas ‘‘submicro structures’’ compared with theconvexes in microscale (we called microconvexes). Furthermagnified SEM image of a single PDMS microconvex[Figure 1(c)] shows that many irregular nanoparticles withthe average diameter of 109 nm distributed randomlyon the surface, some of which accumulated together toform one submicro block. These ‘‘micro-submicro-nanocompositestructures’’ are generated by the irradiation oflaser. The microconvexes are formed when the laser pulse isapplied, while the sputtering and the composition changingof PDMS during the laser-etching process may generate the‘‘submicro- and nano-structures.’’ [9] The width and depth ofthe microgrooves for rough PDMS surface can be easilycontrolled by adjusting the size of the rectangular laser spotand the size of the laser facula; thus, PDMS surfaces withdifferent convex width (W) of about 10, 18, 25, 36, and50 mm were obtained. Figure 1(d) (left) is a magnified SEMimage of a single microconvex with a width of about 50 mm,and that of a flat PDMS surface without laser treating isshown in Figure 1(d) (right) as comparison.The surface wettability was evaluated with a water CAsystem. For a flat PDMS surface, it exhibited a water CA ofMacromol. Rapid Commun. 2005, 26, 1805–1809 www.mrc-journal.de ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Super-Hydrophobic PDMS Surface with Ultra-Low Adhesive Force 1807Figure 1. (a) Typical SEM images of the laser-etched PDMS surface with the convex widthof about 25 mm, showing the regular arrays of microconvexes; (b) magnified image of (a),showing the submicro structures on each convex; (c) high-resolution image of a singleconvex of (b), showing the nanoparticles composed of each submicro block; (d) highresolutionimage of a single convex with width of about 50 mm (left) and a flat PDMSsurface (right).about 1138, while for the etched PDMS surface with microsubmicro-nano-compositestructures (W ¼ 25 mm), it is ashigh as 1628 Figure 2(a)]. More important, the SA of thisrough surface is ultra-low, i.e., less than 58 [Figure 2(b)].These results indicated that the etched PDMS surface showsa typical self-cleaning property. In order to evaluate therelationship between the convex widths and the surfacewettabilities, PDMS surfaces with different size of microconvexwere selected and compared as shown in Figure 2(c).As a result, with the increase of the convex width, the valuesof CA decrease slowly at first; while when it is larger than36 mm, the CA sharply decreases from 1608 (W ¼ 25 mm) to1358 (W ¼ 50 mm). As to that of the SA, it increases slowlyat first and increases abruptly when the width of the convexis larger than 50 mm. On the flat PDMS surface, the waterdroplet does not slide down even when the substrate was 908tilted or turned upside down, showing a large hysteresis. Inthis case, surface roughness, which is the ratio of the actualarea of a rough surface to the geometric projected are, iseffectively enhanced by the ‘‘micro-submicro-nano structures.’’The equation established by Cassie and Baxter asfollows can describe the CA on a composite surface (y r ), [1b]cos y r ¼ f 1 cos y f 2 ð1Þhere y r and y are the CA on PDMS surface with rough andflat structures, respectively; f 1 and f 2 are the fractionalinterface areas of PDMS surface and air in the troughsbetween particles, respectively (i.e., f 1 þ f 2 ¼ 1). It is easy todeduce from this equation that increasing the value of f 2 ,i.e., the larger fraction of air on the surface, will lead to theincrease of y r . Accordingly, the air trapping in the roughsurface can significantly decrease the contact area betweenwater and the solid surface of PDMS to show high-waterCA. On the other hand, the discontinuous triple phase(solid, liquid, and air) contact line [5,6] of the rough PDMSsurface can lead to the small SA. This sliding behavior ofliquid on the solid surface can be attributed to the adhesiveeffect between them.Recently, we developed a new method to evaluate theadhesive force between a super-hydrophobic surface andwater by using a high-sensitivity micro-electromechanicalbalance system (Dataphysics DCAT 11, Germany). [11]Herein, the adhesive forces of PDMS surfaces with differentsurface roughness were measured in this way. For comparison,PDMS surfaces with a single nano- and microscalestructures were also fabricated, whose SEM imagesare shown in Figure 3. Figure 4 (inset) recorded a typicalforce-distance curve during the measuring process fromMacromol. Rapid Commun. 2005, 26, 1805–1809 www.mrc-journal.de ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1808 M. Jin et al.Figure 2. (a) Photograph of a water droplet on the rough PDMS surface with a convex width ofabout 25 mm. The water CA is about 1628; (b) sliding behavior of a water droplet on thesame rough PDMS surface shown in Figure 1(a). The SA is less than 58; (c) therelationships between the widths of microconvexes and the CA (left)/SA (right),respectively.when the water droplet contacted with the PDMS surface(W ¼ 25 mm as the example) to the time when it was takenaway by a metal ring. An optical microscope lens and aCCD cameral system are also used to take photographs atone frame per second. A water droplet of 5 mg wascontacted with a metal ring first, and the force of the balancesystem was initialized to be zero. The ring moves downwarduntil the water droplet contacts with the substrate.Then it moves upward; the force increases gradually until itreaches its maximum. The shape of the water dropletchanges from spherical to elliptical. When the ring movesup further, the contact force reduces to zero sharply, and theshape of the water droplet changes from elliptical tospherical again. The force that water suffered from canalso be regarded as the counterforce of the attractiveforce caused by the rough substrate. As a result, thelargest force of PDMS surface with micro-submicro-nanocompositestructures is merely about 6.5 mN, and appears atthe time just before the water droplet leaves the surface. Theforce to take the water droplet away from the substrates withdifferent surface roughness was measured and shown inFigure 4. The PDMS surface with micro-submicro-nanostructures, which is the roughest surface here, shows theultra-low adhesive force between water and the PDMSsurface. While for that with a single nano- and micro-scalestructures, the adhesive force increases greatly because ofthe decrease of the surface roughness. As to the smoothPDMS surface, the maximum adhesive force of 180 mN isobtained. Therefore, the rough PDMS surface fabricated byetching approach exhibit ultra-low adhesive force withwater.ConclusionWe have successfully created a super-hydrophobic PDMSsurface with ultra-low water adhesive force by a simple onesteplaser-cutting method, which can also be extended toFigure 3. SEM images of PDMS surfaces: (a) nano-scale structures of PDMS surface;(b) micro-scale structures of PDMS surface.Macromol. Rapid Commun. 2005, 26, 1805–1809 www.mrc-journal.de ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Super-Hydrophobic PDMS Surface with Ultra-Low Adhesive Force 1809Figure 4. Chart showing the relationship between the adhesive forces of PDMS surfaces totheir surface roughness. Inserts are the force-distance curves recorded before and after thewater droplet contacts with the laser-etched PDMS surface with a convex width of about25 mm, where the photographs indicate shapes of the water droplets taken at differentstages during the force measuring process.fabricate other polymers. The special micro-submicro-nanostructures significantly enhance the surface roughness andlead to the unusual self-cleaning property, which can beused in micro-fluidic channels with diminished resistance.Such excellent property of the PDMS surface can alsogreatly improve its performances and be widely used.Acknowledgements: The authors thank the financial support ofthis work by NSFC (No. (90306011 and No. 20125102), theNational 863 Project (2003AA302780), and National 973 Project(2003CB716902).[1] [1a] R. N. Wenzel, Ind. Eng. Chem. 1936, 28, 988; [1b]A. B. D. Cassie, S. Baxter, Trans. Faraday Soc. 1944, 40,546; [1c] S. Herminghaus, Europhys. Lett. 2000, 52, 165.[2] [2a] T. Onda, S. Shibuichi, N. Satoh, K. Tsujii, Langmuir1996, 12, 2125; [2b] A. Lafuma, D. Quéré, Nat. Mater. 2003,2, 457; [2c] J. Bico, C. Marzolin, D. Quéré, Europhys. Lett.1999, 47, 220; [2d] J. Genzer, K. Efimenko, Science 2000,290, 2130; [2e] H. Y. Erbil, A. L. Demirel, Y. Avc, O. Mert,Science 2003, 299, 1377; [2f] L. Zhai, F. C. Cebeci,R. E. Cohen, M. F. Rubner, Nano Lett. 2004, 4, 1349; [2g]X. Zhang, F. Shi, X. Yu, H. Liu, Y. Fu, Z. Wang, L. Jiang,X. Li, J. Am. Chem. Soc. 2004, 126, 3064; [2h] X. Lu,C. Zhang, Y. Han, Macromol. Rapid Commun. 2004, 25,1606.[3] [3a] A. R. Parker, C. R. Lawrence, Nature 2001, 414, 33; [3b]P. Aussillous, D. Quéré, Nature 2001, 411, 924; [3c] S.Herminghuas, A. Otten, Langmuir 2004, 20, 2405; [3d] N. A.Patankar, Langmuir 2004, 20, 8209; [3e] A. Marmur,Langmuir 2004, 20, 3517; [3f] Q. Xie, J. Xu, L. Feng, L.Jiang, W. Tang, X. Luo, C. C. Han, Adv. Mater. 2004, 16, 302.[4] [4a] L. Feng, S. H. Li, Y. S. Li, H. J. Li, L. J. Zhang, J. Zhai,Y.L.Song,B.Q.Liu,L.Jiang,D.B.Zhu,Adv. Mater. 2002, 14,1857; [4b] R. Blossey, Nat. Mater. 2003, 2, 301; [4c] J. Han,D. Lee, C. Ryu, K. Cho, J. Am. Chem. Soc. 2004, 126, 4796.[5] [5a] M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T.Watanabe, Langmuir 2000, 16, 5754; [5b] D. Öner,T.J.McCarthy, Langmuir 2000, 16, 7777; [5c] Z. Ysohimitsu, A.Nakajima, T. Watanabe, K. Hashimoto, Langmuir 2002, 18,5818.[6] [6a] A. Marmur, Langmuir 2003, 19, 8343; [6b] D. Quéré,Langmuir 1998, 14, 2213.[7] [7a] Y. Xia, J. A. Rogers, K. E. Paul, G. M. Whitesides,Chem. Rev. 1999, 99, 1823; [7b] H. W. Wu, T. W. Odom, D. T.Chiu, G. M. Whitesides, J. Am. Chem. Soc. 2003, 125, 554.[8] [8a] M. A. Unger, H. P. Chou, T. Thorsen, A. Scheer, S. R.Quake, Science 2000, 288, 113; [8b] A. R. Wheeler, W. R.Throndset, R. J. Whelan, A. M. Leach, R. N. Zare, Y. H. Liao,K. Farrell, I. D. Manger, A. Daridon, Anal. Chem. 2003, 75,3581.[9] [9a] M. T. Khorasani, H. Mirzadeh, P. G. Sammes, Radiat.Phys. Chem. 1996, 47, 881; [9b] M. T. Khorasani, H.Mirzadeh, J. Appl. Polym. Sci. 2004, 91, 2042.[10] C. Guo, L. Feng, J. Zhai, G. Wang, Y. Song, L. Jiang, D. Zhu,Chem. Phys. Chem. 2004, 5, 750.[11] [11a] M. Jin, X. Feng, L. Feng, T. Sun, J. Zhai, T. Li, L. Jiang,Adv. Mater. 2005, 17, 1977; [11b] R. Daw, Nature 2005, 436,471.Macromol. Rapid Commun. 2005, 26, 1805–1809 www.mrc-journal.de ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim