7196 DOI: 10.1021/la900732h Langmuir 2009, 25(13), 7196–7199Published on Web 05/27/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Wet Self-Cleaning of Biologically Inspired Elastomer Mushroom ShapedMicrofibrillar Adhesives

Seok Kim, Eugene Cheung, and Metin Sitti*

Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

Received March 1, 2009. Revised Manuscript Received May 18, 2009

We report that hydrophilic polyurethane mushroom shaped microfiber arrays possess wet self-cleaning ability usingthe lotus effect as biologically inspired synthetic fibrillar adhesives. In comparison with a flat surface made of the samepolyurethane, the fiber array exhibited almost 100%wet self-cleaning without any degradation of adhesive strength.Weattribute this cleaning ability to the mushroom shaped tip ending geometry of the fiber array, which causes the fiberarray to be apparently hydrophobic even though the fiber material is hydrophilic. These results suggest that tip endingshape is one of the significant design parameters for developing contamination-resistant polymer fibrillar adhesives.

The adhesion ability of fibrillar structures in nature hasmotivated many researchers to study the principles behind thisadhesion and develop synthetic fibrillar adhesives inspired bythese biological fibrillar structures.1-23 In addition to the adhe-sion ability, self-cleaning ability is also one of the attractiveproperties of the biological fibrillar structures, which has beenstudied for gecko foot hairs by Hansen and Autumn.24 Theyshowed that geckoswere able to clean their dirt-contaminated feetby stepping on clean dry surfaces. This cleaning by contacting onclean surfaces can be defined as contact self-cleaning. The leavesof the lotus plant exhibit another naturally occurring example of

self-cleaning.25,26 Micro/nanoscale roughness as observed onthese leaves can make a surface (super)hydrophobic, and dirtparticles on them can then be removed by water droplets, whichcapture the dirt particles as they roll off the surface. This waterdroplet based surface cleaning is known as the lotus effect. Severalresearchers have demonstrated these two types of self-cleaningwith synthetic fibrillar adhesives which were made of photoresist,parylene, carbon nanotubes (CNTs), and polypropylene.27-30

Particularly, Lee and Fearing demonstrated contact self-cleaningon hard polymer (Young’s modulus > 1.5 GPa) nanofiberarrays,29 whereas Sethi et al. showed wet self-cleaning on CNTsusing water droplets.30 In contrast with self-cleaning studies withhardmaterial fiber arrays, there is a lack ofworks on self-cleaningof soft elastomer fibrillar adhesives. In this work, we demonstratethat mushroom shaped microfiber arrays (re-entrant texturedsurface31) made of a hydrophilic soft elastomer are also able topossess wet self-cleaning using the lotus effect as biologicallyinspired fibrillar adhesives. It is worth noting that simple cylind-rical structures with hydrophobic materials can also show super-hydrophobicity and wet self-cleaning.32-34

Elastomer mushroom shaped microfiber arrays with a 300 μmthick backing layer were fabricated with polyurethane (BJBEnterprises, ST-1060) using methods that we demonstratedpreviously.12 After a negative silicon template for the fiber arraywas fabricated using deep reactive ion etch and the notchingeffect, the liquid precursor mixture of the polyurethane waspoured into the template. The polyurethane precursor was curedat room temperature for 1 day, and the fiber arraywas released byremoving the silicon template with XeF2. It is worth noting thatthe mushroom shaped tips of the fibers were exposed to XeF2

*To whom correspondence should be addressed. E-mail: sitti@cmu.edu.(1) Autumn, K.; Liang, Y.; Hsieh, T.; Zesch, W.; Chan, W.-P.; Kenny, T.;

Fearing, R.; Full, R. J. Nature 2000, 405, 681.(2) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.;

Sponberg, S.; Kenny, T.; Fearing, R.; Israelachvili, J. N.; Full, R. J. Proc. Natl.Acad. Sci. U.S.A. 2002, 99, 12252.(3) Arzt, E.; Gorb, S.; Spolenak, R. Proc. Natl. Acad. Sci. U.S.A. 2003, 100,

10603.(4) Persson, B. N. J. J. Chem. Phys. 2003, 118, 7614.(5) Huber, G.; Mantz, H.; Spolenak, R.; Mecke, K.; Jacobs, K.; Gorb, S. N.;

Arzt, E. Proc. Natl. Acad. Sci. U.S.A. 2005, 102(45), 16293.(6) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov,

A. A.; Shapoval, S. Y. Nat. Mater. 2003, 2, 461.(7) Sitti, M.; Fearing, R. S. J. Adhes. Sci. Technol. 2003, 17(8), 1055.(8) Zhao, Y.; Tong, T.; Dezeit, L.; Kashani, A.; Meyyapan, M.; Majumdar, A.

J. Vac. Sci. Technol., B 2006, 24, 331.(9) Crosby, A. J.; Hageman, M.; Duncan, A. Langmuir 2005, 21, 11738.(10) Gorb, S.; Varenberg, M.; Peressadko, A.; Tuma, J. J. R. Soc. Interface

2007, 4, 271.(11) Jeong, H. E.; Lee, S. H.; Kim, P.; Suh, K. Y. Nano Lett. 2006, 6, 1508.(12) Kim, S.; Sitti, M. Appl. Phys. Lett. 2006, 89, 261911.(13) Majidi, C.; Groff, R. E.; Autumn, K.; Baek, S.; Bush, B.; Gravish, N.;

Maboudian, R.;Maeno, Y.; Schubert, B.;Wilkinson,M.; Fearing, R. S.Phys. Rev.Lett. 2006, 97, 076103.(14) Aksak, B.; Murphy, M. P.; Sitti, M. Langmuir 2007, 23, 3322.(15) Kim, S.; Aksak, B.; Sitti, M. Appl. Phys. Lett. 2007, 91, 221913.(16) Santos, D.; Spenko, M.; Parness, A.; Kim, S.; Cutkosky, M. J. Adhes. Sci.

Technol. 2007, 21, 1317.(17) Murphy, M.; Aksak, B.; Sitti, M. J. Adhes. Sci. Technol. 2007, 21, 1281.(18) Reddy, S.; Arzt, E.; del Campo, A. Adv. Mater. 2007, 19, 3833.(19) Northen,M. T.; Greiner, C.; Arzt, E.; Turner, K. L.Adv.Mater. 2008, 20, 1.(20) Xie, T.; Xiao, X. Chem. Mater. 2008, 20, 2866.(21) Kim, S.; Sitti, M.; Hui, C. Y.; Long, R.; Jagota, A. Appl. Phys. Lett. 2007,

91, 161905.(22) Long, R.; Hui, C.-Y.; Kim, S.; Sitti, M. J. Appl. Phys. 2008, 104, 044301.(23) Glassmaker, N. J.; Jagota, A.; Hui, C.-Y.; Noderer, W. L.; Chaudhury, M.

K. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10786.(24) Hansen, W. R.; Autumn, K. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 385.(25) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1.(26) Solga, A.; Cerman, Z.; Striffer, B. F.; Spaeth, M.; Barthlott, W. Bioinspira-

tion Biomimetics 2007, 2, S216.

(27) Tsai, Y.-C.; Shih, P.-J.; Lin, T.-H.; Shih, W.-P. Proceedings of the IEEEInternational Conference on Nano/Micro Engineered and Molecular Systems,Zhuhai, China, 2006.

(28) Kustandi, T. S.; Samper, V. D.; Yi, D. K.; Ng, W. S.; Neuzil, P.; Sun, W.Adv. Funct. Mater. 2007, 17, 2211.

(29) Lee, J.; Fearing, R. S. Langmuir 2008, 24, 10587.(30) Sethi, S.; Ge, L.; Ci, L.; Ajayan, P. M.; Dhinojwala, A. Nano Lett. 2008, 8,

822.(31) Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. Proc.

Natl. Acad. Sci. U.S.A. 2008, 105, 18200.(32) Davies, J.; Haq, S.; Hawke, T.; Sargent, J. P. Int. J. Adhes. Adhes. 2009, 29,

380.(33) Zhang, X.; Kong, B.; Tsui, O. K. C.; Yang, X. J. Chem. Phys. 2007, 127,

014703.(34) Jung, Y. C.; Bhushan, B. Langmuir 2008, 24, 6262.

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DOI: 10.1021/la900732h 7197Langmuir 2009, 25(13), 7196–7199

Kim et al. Letter

during the fiber array releasing step, resulting in a change of tipsurface material properties by surface fluorination.35 A flatelastomer surface sample with 300 μm thickness was also pre-pared by exposing the surface to XeF2 for the same amount oftime used to release the fiber array to ensure a fair comparisonbetween the flat surface and the fiber array. A scanning electronmicroscopy (SEM) image of the fabricated fiber array in Figure 1shows every single fiber has a mushroom shaped (re-entrantshape31) tip ending although the sidewall of the tip is locallyrough due to the notching effect.12

To characterize wet self-cleaning using the lotus effect on theflat surface and the fiber array, the static contact angle andcontact angle hysteresis (difference between advancing and reced-ing contact angles36) of the two samples weremeasured by placinga deionized (DI)water droplet on the samples. The static apparentcontact angle (θapp) of a water droplet on the fiber array was 128�,and the static equilibrium contact angle (θ) of a water droplet onthe flat surface was 82�. Contact angle hysteresis was 87� for thefiber array and 97.5� for the flat surface. Due to the contact angleand contact angle hysteresis differences between the two samples,a 60 μL water droplet was necessary to free flow on the flatsurface, whereas a 20 μLwater droplet was enough to free roll offthe fiber array when the samples were tilted to a 90� angle. Ascontaminating elements, 5-50 μm diameter silica sphericalparticles (Duke Scientific Corporation, Spherical GlassMaterials(Dry)) were used. If these particles are transferred to the fiberarray by pressing hard onto a surface, very small particles(especially the ones with a diameter smaller than the fiber stemdiameter) could be embedded inside the fiber array, which coulddegrade the fiber array adhesion repeatability and magnitude.Therefore, we gently dropped the particles by gravity on the flatsurface and the fiber array, and we selected the smallest particlediameter as 5 μm tominimize particle embedding during pressingonto a given substrate during adhesion testing. Then, 60 and20 μL water droplets were placed on flat and fiber array samples,respectively. The two samples were tilted to a 90� angle to causethe water droplets tomove across the samples forwashing. Beforeand after this washing, the number of particles in a predefinedarea (a 0.9 mmby 0.7 mm rectangle) was counted through optical

microscope images and their wet self-cleaning abilities werecompared. In addition, pull-off forces of the fiber array weremeasured before contamination and after contamination andwashing to inspect any possible adhesion degradation by wetself-cleaning. For pull-off force measurements, we useda glass hemisphere instead of a flat glass surface as the test contactsurface to minimize alignment errors during the experiments.12

A 6 mm diameter smooth glass hemisphere (ISP Optics, QU-HS-6) attached to a load cell (Transducer Techniques, GSO-25)was moved vertically by a motorized stage (Newport, MFA-CC)with 100 nm resolution. The hemisphere was pushed into thesample surface with a preload force and retracted at a slow speedof 1 μm/s. During retracting, the maximum tensile force wasmeasured as the pull-off force.

Figure 2 shows that, after washing, the particles still remainedon the flat surface and tended to congregate whereas all particleswere removed by washing the fiber array. It was observed that afree-flowingwater droplet left tiny droplets on the flat surface andthen the tiny droplets wet particles locally and congregated themduring drying. This observation is consistent with the fact that theflat polyurethane surface is natively hydrophilic (θ < 90�).However, due to apparent hydrophobicity (θapp > 90�), a waterdroplet rolled off the fiber array was able to capture the particlesduring rolling in the same way as the lotus effect. These experi-ments were repeated six times at different locations, and thenumber of cleaned particles from the two samples was counted ineach experiment. Pull-off forces for the fiber array were alsomeasured six times at different locations during these experi-ments. Wet self-cleaning ability with percentage bars for twosamples andpull-off forces of the fiber array for different preloadsare illustrated in Figure 3. In contrast to the flat surface, the fiberarray exhibited almost 100% wet self-cleaning (Figure 3a). Dur-ing the measurements, we observed negligible adhesion from thecontaminated fiber array. To simply quantify it, we conductedthree additional pull-off force measurements with the contami-nated fiber array for a 20 mN preload. The results showed 0.12,0.07, and 1.58 mN pull-off forces.

Figure 4a, b shows profile views of water droplets on the flatsurface and the fiber array, respectively. The fiber array isapparently hydrophobic (θapp = 128�), while the flat surface is

Figure 1. SEM image of a polyurethanemushroomshapedmicro-fiber array with approximately 5 μm stem diameter, 9 μm tipdiameter, and 20 μm length. Figure 2. Optical microscopic images of surfaces contaminated

with silica particles (a,c) and surfaces washed with DI water (b,d)for a flat surface (a,b) and a fiber array (c,d), respectively. (Scalebars: 200 μm.)

(35) Hougham, G.; Cassidy, P. E.; Johns, K.; Davidson, T. Fluoropolymers 1:Synthesis; Kluwer Academic/Plenum Publishers: New York, 1999; Chapter 15.(36) Tuteja, A.; Choi, W.; McKinley, G. H.; Cohen, R. E.; Rubner, M. F.MRS

Bull. 2008, 33, 752.

7198 DOI: 10.1021/la900732h Langmuir 2009, 25(13), 7196–7199

Letter Kim et al.

hydrophilic (θ = 82�). Studies show that a predefined roughsurface with gradually decreasing surface energy or increasingequilibrium contact angle (θ), which ismeasured on a flat surface,creates a transition between two contact interfaces.31,36-38 One isa fully wet interface that conforms to theWenzelmodel,39 and theother is a solid-air composite interface that conforms to theCassie model.40 The large contact area between the liquid andsolid in the Wenzel state leads to high contact angle hysteresis,whichdoes not allowwater droplets to readily roll off of the roughsurface. By contrast, a composite interface of the Cassie statefacilitates both nonwetting (high apparent contact angle) as wellas easy water droplet roll-off because of the small total contactarea between the water droplet and the solid substrate.31 Thesemodels predict the apparent contact angle (θapp) of a waterdroplet on the rough surface for a particular θ value. Once thetopography of a rough surface is defined, a critical contact angle(θc) for the transition can be obtained by equating theWenzel andCassie equations.When θ is above θc, the composite interface haslower overall free energy and the composite interface is in a stablestate.When θ is below θc, the fully wet interface is in a stable state.Based on the topography of our microfiber array, the lines of thetwo models were drawn to predict a stable interface between thewater droplet and the fiber array for a particular θ of the flatsurface. The two lines are illustrated in Figure 4c simultaneouslywith the critical contact angle (θc = 101�) and a point determinedby the cosines of θ and θapp, which are measured on two samples.Since θ is smaller than θc, the interface between the water dropletand the fiber array is predicted to be stable as a fully wet interface.Interestingly, θapp is 128�. This observation suggests that thewater droplet on the fiber array is not in a stable state but rather ina metastable state31,36-38 of the composite interface on the fiberarray. This metastable composite solid-liquid-air interface isconsistent with the Cassie model and corresponds to a localminimum in the free energy, even though the fully wettedWenzelstate corresponds to a globalminimum in the free energy.31 It wasreported that this metastable state is possible when a rough

surface is a re-entrant textured surface; that is, the surfacetopography cannot be described by a simple univalued functionz = h(x,y), and a vector projected normal to the x-y planeintersects the texture more than once.37 Since every fiber hasa mushroom shaped tip, ultimately the fiber array can be are-entrant textured surface.

To demonstrate this approach qualitatively, we fabricated twoother polyurethane (ST-1060) fiber arrays using methodsdescribed by Aksak et al.,14,17 which are called type 1 and type2 herein. Type 1 is a 50 μm diameter, 100 μm length, and 120 μmcenter-to-center spacing cylindrical fiber array in a square pack-ing arrangement. Type 2 has the same dimensions as type 1 otherthan mushroom tip endings 100 μm in diameter (Figure 4d,e).When a 20 μLDIwater droplet was deposited onto both samples,a droplet on type 1 was easily wet into surface unless it was placedon the fiber array very carefully whereas a droplet was very stablydeposited on type 2. Although Jung and Bhushan previouslyshowed that this difference in stability could be attributed to theenlarged tip area, their structures were natively hydrophobic.34 InFigure 4d,e, it is shown that the net traction at solid-liquid-airinterfaces can depend on the fiber tip shape, particularly when thefiber material is hydrophilic (θ < 90 o). In a simple cylindricalfiber array (type 1) case, the downward net traction leads to a fullywetted interface (Figure 4d). The edge of the fiber tip serves to pinthe water interface, allowing for apparent superhydrophobicity,but any disturbance that causes the water to push past the edge ofthe fiber onto the sides results in wetting as observed in theexperiments.However, in amushroom shaped fiber array (type 2)case, the upward net traction can support sagged water unless thesolid-liquid interface reaches the cylinder surface under the tip(Figure 4e). Thus, stablewater droplet deposition on type 2 can be

Figure 3. Percentage of particles removed from a flat surface anda microfiber array for wet self-cleaning experiments (a), andthe fiber array pull-off forces before contamination and after wetself-cleaning (b).

Figure 4. Optical microscopic profile views of DI water dropletson a flat surface (a) and a fiber array (b), and a diagram ofWenzeland Cassie models of the fiber array (c). Schematic diagramsillustrating possible liquid-air interfaces on two different fiberarrays having the same equilibrium contact angle (d, e).

(37) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge,G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618.(38) Quere, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14, 1109.(39) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.(40) Cassie, A. S. D.; Baxter, S. Trans. Faraday. Soc. 1944, 40, 546.

DOI: 10.1021/la900732h 7199Langmuir 2009, 25(13), 7196–7199

Kim et al. Letter

attributed to not only the enlarged tip area but also the re-entrantshape31 of the mushroom tips. Consequently, even though thefiber material is natively hydrophilic, our microfiber array withmushroom shaped tip endings can be apparently hydrophobicenough to possess stable wet self-cleaning.

In summary, elastomer microfiber arrays with mushroomshaped tip endings exhibit wet self-cleaning seen in lotus leavesin nature, while providing dry adhesion. Almost 100% of5-50 μm diameter silica particles on the fiber array were cleaned

using the lotus effect even though the material is natively hydro-philic. Moreover, contaminated and then wet self-cleaned fiberarrays exhibited almost 100% adhesion recovery. This remark-able wet self-cleaning ability results from fibrillar structuring withmushroom shaped tip endings, which causes the switch from ahydrophilic flat surface to an apparently hydrophobic solid-aircomposite surface. Thus, it is demonstrated that tip ending shapedesign of fiber arrays is one of the important parameters fordeveloping contamination-resistant fibrillar adhesives.