Patterned Superhydrophobic Surfaces: Toward a Synthetic Mimic of

NANO LETTERS

Patterned Superhydrophobic Surfaces: Toward a Synthetic Mimic of the Namib Desert Beetle

2006 Vol. 6, No. 6 1213-1217

Lei Zhai,‡ Michael C. Berg,† Fevzi C¸ . Cebeci, Yushan Kim,‡ John M. Milwid,‡ Michael F. Rubner,*,‡ and Robert E. Cohen*,† Departments of Chemical Engineering and Materials Science and Engineering, and the Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received March 22, 2006; Revised Manuscript Received April 19, 2006

ABSTRACT The present study demonstrates a surface structure that mimics the water harvesting wing surface of the Namib Desert beetle. Hydrophilic patterns on superhydrophobic surfaces were created with water/2-propanol solutions of a polyelectrolyte to produce surfaces with extreme hydrophobic contrast. Selective deposition of multilayer films onto the hydrophilic patterns introduces different properties to the area including superhydrophilicity. Potential applications of such surfaces include water harvesting surfaces, controlled drug release coatings, open-air microchannel devices, and lab-on-chip devices.

In areas of limited water such as the Namib Desert, nature has developed elegant schemes for harvesting water from the atmosphere. For example, the Stenocara beetle in the Namib Desert uses the hydrophilic/superhydrophobic patterned surface of its wings to collect drinking water from fog-laden wind. In a foggy dawn, the Stenocara beetle tilts its body forward into the wind to capture small water droplets in the fog. After these small water droplets coalesce into bigger droplets, they roll down into the beetle’s mouth, providing the beetle with a fresh morning drink. Parker and co-workers found that the structure of the beetle’s back enabled this unique water collecting ability.1 The Stenocara beetle’s back consists of an array of surface bumps decorated on top with hydrophilic spots (about 100 µm in diameter) on a superhydrophobic background. The small water droplets in the morning fog accumulate on the hydrophilic area and coalesce. When the weight of a growing droplet is sufficient to overcome the binding forces of the hydrophilic region, it detaches and rolls down the superhydrophobic surface to the beetle’s mouth. Besides this amazing water capturing application, surfaces patterned with different wetting properties (hydrophobic vs hydrophilic) have applications in microfluidic channels2 and the rapid evaluation of complex bioactivities.3-7 Patterned surfaces with dissimilar wetting properties have been achieved using techniques such as microcontact print* To whom correspondence should be addressed. E-mail: [email protected] (R.E.C.); [email protected] (M.F.R.). † Department of Chemical Engineering. ‡ Department of Materials Science and Engineering. 10.1021/nl060644q CCC: $33.50 Published on Web 05/02/2006

© 2006 American Chemical Society

ing,8,9 chemical vapor deposition,10 and photolithography.11-13 However, these approaches often involve complicated procedures to introduce functional groups to the patterned areas or are limited by the range of functional groups that can be introduced to the patterned areas. Therefore, a simple and more effective route to generate arrays of patterns with different wetting properties and chemical functionalities is highly desirable. Furthermore, surfaces with extreme wetting properties such as superhydrophilic patterns on a superhydrophobic surface offer new possibilities in the fabrication of novel devices such as planar microcanals (open-air microfluidic channels).14 Open-air microfluidic channels offer advantages such as the facile handling of small amount of liquids, the possibility of massive parallel processing, direct accessibility, and ease of cleaning.14-16 The availability of patterned surfaces with superhydrophobic and superhydrophilic regions would greatly enhance the utility and function of such devices and move us beyond nature’s impressive accomplishment with the Namib beetle. Currently, there is only one report of a patterned superhydrophobic/superhydrophilic surface with limited capability for surface functionalization.11 A superhydrophobic surface has a water droplet advancing contact angle of 150° or higher. In addition, the contact angle hysteresis is very low (receding contact angles are within 5° of advancing angles).17,18 Many surfaces in nature, including those of the lotus leaf and duck wings, derive their superhydrophobic characteristics from a multilength scale surface texture over-coated with hydrophobic materials.19,20

These structures have inspired scientists to generate superhydrophobic surfaces using a variety of methods.21-34 In contrast, a superhydrophilic surface typically exhibits water droplet advancing contact angles less than 5°,35-38 and in some cases, the time for complete wetting by small droplets of water has been observed to be less than 0.5 s.38 A number of approaches have been explored to generate superhydrophilic surfaces oftentimes for applications in self-cleaning.35-38 We now report a simple method to create hydrophilic or superhydrophilic patterns on an otherwise superhydrophobic surface. This is accomplished by selectively delivering polyelectrolytes to the surface in a mixed water/2-propanol solvent. This approach can be used to create patterned surfaces that mimic the water harvesting structure of the Stenocara beetle’s back. In addition, specific functional groups or layer-by-layer assembled polyelectrolyte multilayers can be selectively and easily introduced onto the patterned areas. We have previously reported the fabrication of stable superhydrophobic coatings using polyelectrolyte multilayer films (advancing contact angles as high as 172° with receding contact angles of about 169°).32 In our superhydrophobic coatings, rough microporous poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) microstructures decorated with PAH/silica nanoparticles (PAH/SiO2) are covered with a hydrophobic network of semi-fluorosilane molecules (designated as PAA/PAH/silica nanoparticle/semi-fluorosilane). With this structure in mind, we reasoned that if we could deliver polyelectrolytes through the semi-fluorosilane network, then they would form electrostatic bonds with the underlying PAH or the silica nanoparticles. At the same time, part of the charged polymer chains would remain on the superhydrophobic surface, changing the local wetting properties of the surface. To test this strategy, we created a hydrophilic domain on a superhydrophobic surface by using poly(fluorescein isothiocyanate allylamine hydrochloride) (FITC-PAH). The superhydrophobic surface is not wettable to a 1% FITC-PAH aqueous solution. However, Mohammadi et al. reported that aqueous solutions of various surfactants could wet a superhydrophobic surface since the surfactant reduced the surface tension of the solution.39 Soeno and co-workers also reported the wetting of their superhydrophobic surfaces with a water/ ethanol mixture.40 In our case, FITC-PAH was dissolved into a mixture of water with a low surface tension organic solvent, 2-propanol (the surface tension of 2-propanol is 21.7 mJ/m2 while the surface tension of water is 72.2 mJ/m2).41 Figure 1 shows the variation of the contact angle of a water/2propanol drop on a superhydrophobic surface with varying 2-propanol concentration. The surface tension decreases with increasing 2-propanol concentration, leading to a decrease of contact angles. Since a wetting solution with minimum spreading on a superhydrophobic surface is required in our patterning scheme, we used a water/2-propanol (60/40 v/v) mixture (contact angle ) 77°) as the solvent to make our 1% polyelectrolyte solutions. To pattern a hydrophilic region on the superhydrophobic surface, a pattern of microdrops of 1% FITC-PAH solution 1214

Figure 1. Contact angles of water/2-propanol mixtures on a (PAA/ PAH/silica nanoparticle/semi-fluorosilane) superhydrophobic surface. Water/2-propanol droplet size: 6 µL.

was micropipeted onto the surface. After the solvent evaporated, the patterned surface was rinsed with deionized water to remove any loosely bound polyelectrolytes. The fluorescence intensity of the fluorescein isothiocyanate was analyzed to monitor the density of FITC-PAH on the surface. The fluorescence intensity of the patterned domains decreased after the rinse step, suggesting the removal of loosely bound polyelectrolyte. However, since some FITC-PAH still stayed on the superhydrophobic surface, the contact angle and contact angle hysteresis of the patterned domains changed significantly (advancing contact angle ) 149°, receding contact angle ) 66°), generating a hydrophobic contrast between the patterned area and its surroundings. X-ray photoelectron spectroscopy (XPS) was used to examine the relative atomic composition of the superhydrophobic surface and the hydrophilic region. After patterning, the atomic concentration of nitrogen in the patterned area increased from 1.0% to 2.6% and the atomic concentration of carbon increased from 24.0% to 26.6%. These results indicate that FITC-PAH molecules anchored on the superhydrophobic surface are responsible for the altered wetting characteristics of the patterned domains. A structure mimicking the Stenocara beetle’s back was built by depositing an array of hydrophilic spots with size of 750 µm onto a superhydrophobic surface by using a poly(acrylic acid) (PAA) water/2-propanol solution. The advancing water contact angle in the patterned region was 144°, whereas the receding contact angle was 12°. These hydrophilic patterns mimic the wax-free areas on the Stenocara beetle’s back, which are known to collect small water droplets from the fog. Figure 2 demonstrates the behavior of small water droplets on a PAA patterned superhydrophobic surface. Spraying a mist of water onto the surface leads to small water droplets (∼250 µm) that do not wet the superhydrophobic surface and form nearly perfect spheres. Most of the droplets bounced and rolled on the superhydrophobic regions and eventually stuck to the patterned hydrophilic regions where large water droplets were formed. This process mimics the water-capturing capabilities of the Stenocara beetle’s back and can be used for capturing very small drops of water and converting them into larger drops. Nano Lett., Vol. 6, No. 6, 2006

Figure 2. (a) Small water droplets sprayed on a (PAA/PAH/silica nanoparticle/semi-fluorosilane) superhydrophobic surface with an array of hydrophilic domains patterned with a 1% PAA water/2propanol solution (scale bar ) 5 mm). (b) Sprayed small water droplets accumulate on the patterned hydrophilic area shown in (a) (scale bar ) 750 µm).

In addition to polyelectrolytes, we investigated the ability of charged small molecules to bind to a superhydrophobic surface and to change the surface properties. A superhydrophobic surface was patterned with a 0.1% 2-propanol solution of methylene blue (a positively charged dye) and a 0.1% 2-propanol rose bengal (a negatively charged dye) solution. The patterned areas became hydrophilic after the dye was deposited on the surface. However, unlike the area patterned with polyelectrolytes, these areas regained their superhydrophobic properties after the dyes were washed away with water. This is attributed to the fact that charged small molecules do not have the long polymer chains needed to bind electrostatically with the underlying polyelectrolytes or silica nanoparticles. Some amount of the dye, however, was conveyed into the semi-fluorosilane network by the 2-propanol and became trapped inside the polyelectrolyte multilayer structure, as indicated by the color that remained in the patterned area. Rinsing with water does not redissolve the trapped dye because the superhydrophobic surface prevents water from penetrating into the structure. However, if the surface is rendered hydrophilic by using a PAA water/ 2-propanol solution, then these dyes can be released from the hydrophilic regions upon immersion into water. This approach can be employed to load chemicals through the superhydrophobic surface into the underlying multilayer structure. The porous polyelectrolyte multilayer structure then behaves as a reservoir for these chemicals which can be released to an aqueous medium through subsequently created hydrophilic portals, potentially leading to controlled drug release applications. When a patterned superhydrophobic surface is exposed to an aqueous solution of a chemical, the hydrophobic contrast between the hydrophilic domains and the superhydrophobic background directs molecules to the hydrophilic domains. This phenomenon was tested by assembling polyelectrolyte multilayers selectively onto the hydrophilic patNano Lett., Vol. 6, No. 6, 2006

Figure 3. Selective deposition of polyelectrolyte multilayers onto an array of hydrophilic domains patterned on a (PAA/PAH/silica nanoparticle/semi-fluorosilane) superhydrophobic surface. Top: fluorescence intensity as a function of the number of deposited layers of (FITC-PAH/PAA) multilayers (R2 ) 0.966). Bottom: fluorescence micrograph of (FITC-PAH/PAA) multilayers built onto patterned superhydrophobic multilayer films (80 layers) (scale bar ) 500 µm).

terns by using the layer-by-layer technique. In this process, a patterned superhydrophobic film was completely submerged in both the polymer and rinse baths. The nonwetting superhydrophobic background acted as a resist to multilayer buildup. The construction of PAH/PAA multilayers was monitored by incorporating 0.01% FITC-PAH into the PAH solution. Figure 3 shows a linear increase in fluorescence intensity as more layers were built onto the patterned regions along with a micrograph of patterned dots of fluorescently labeled multilayers. The selective deposition of multilayers onto the patterned surface was confirmed by the large fluorescence intensity contrast between the patterned regions and the superhydrophobic background. It is important to note that the surface was subjected to more than 240 sequential exposures to polycation, polyanion, and water solutions. A superhydrophobic surface containing hydrophilic domains with 40 assembled bilayers of PAH/PAA (advancing contact angle ) 71°, receding contact angle ) 25°) was examined by using atomic force microscopy (AFM). In the AFM images of the superhydrophobic regions (parts A and B of Figure 4), 20 nm diameter silica nanoparticles can be clearly observed. However, they are no longer visible in the AFM images of the hydrophilic domains over-coated with polyelectrolyte multilayers (parts C and D of Figure 4). This observation was made on both the tops of the hills and the bottoms of the valleys of the microtextured hydrophilic regions. The decrease of RMS surface roughness from 54 to 40 nm after the multilayer deposition further supports the idea that polyelectrolytes can be selectively assembled onto the patterned area. The fabrication of a surface with patterns of extreme wetting properties was performed by building superhydro1215

Figure 4. AFM images of a (PAA/PAH/silica nanoparticle/semifluorosilane) superhydrophobic surface (A height, B phase) and a hydrophilic domain selectively over-coated with 80 assembled layers of (PAA/PAH) (C height, D phase) (size of each image ) 1 µm).

philic canals on a superhydrophobic background. To create superhydrophilic canals, superhydrophilic mutlilayers need to be deposited selectively onto hydrophilic stripes previously patterned on a superhydrophobic surface. We have recently reported the fabrication of stable superhydrophilic coatings (water contact angle drops below 5° in less than 0.3 s) by the layer-by-layer assembly of PAH and silica nanoparticles.38 In the present work, we have exploited the hydrophobic contrast between hydrophilic areas and the superhydrophobic background to direct the deposition of PAH and the silica nanoparticles selectively onto the patterned hydrophilic regions; in this way, superhydrophilic coatings were built exclusively on the patterned area using the previously reported layer-by-layer deposition scheme. The water wetting behavior of microcanals with various numbers of PAH/SiO2 bilayers was examined by using a video contact angle instrument. In a microcanal with four bilayers of PAH/SiO2 (Figure 5A), it took more than 10 s for a water droplet to spread 1 cm along the 750 µm wide microcanal. The observation of a small bulge at t ) 0.21 s in Figure 5A indicates that the capillary force of the coating cannot distribute the water fast enough to make the canal superhydrophilic. However, in a microcanal with 14 bilayers of PAH/SiO2 (Figure 5B), water spread completely along a 6 cm long microcanal in 2 s (see Supporting Information for a video of this effect). This observation is consistent with our previous report that a sufficient number of PAH/SiO2 bilayers is needed to create stable superhydrophilic behavior.38 As water is added to a superhydrophilic microcanal, the water level increases with a continuously increasing contact angle at the edge of the canal. When the contact angle exceeds 90°, stability is lost and a bulge forms in the channel. The bulge disappears when the contact angle falls below 90°. Our observation is in good agreement with the work of Gau and co-workers who reported that stable semicylinder 1216

Figure 5. 750 µm wide canals built on a patterned superhydrophobic surface: (A) A water droplet (1.5 mm diameter) spreads along a hydrophilic canal comprised of 4 bilayers of PAH/SiO2; (B) A water droplet (1.5 mm diameter) spreads along a superhydrophilic canal comprised of 14 bilayers of PAH/SiO2.

structures of water in a microcanal required a contact angle at the edge below 90°.14 Gau et al. also reported the construction of liquid bridges between adjacent canals through the coalescence of bulges. These liquid bridges can be used to mix the solutions in the canals.14 However, without a superhydrophobic background, the liquid bridge can wet the area between two canals and contaminate the surface. In our system, since the canals were constructed on a superhydrophobic surface, the bridge can be broken by simply withdrawing solution from the canals without leaving any solution outside of the canals. Patterns with extreme wetting properties can generate densely packed small reaction sites for the rapid evaluation of complex biomolecular interactions.3-5 In these applications, the patterned area should have a consistent and uniform morphology and have no interactions between adjacent sites. The high wetting contrast between the patterned area and the background as well as the cytophobicity and protein adsorption resistance of a superhydrophobic background make our patterned superhydrophobic surfaces good candidates for such applications. Figure 6 shows an array of circular hydrophilic spots with diameter of 750 ( 30 µm on a superhydrophobic surface. Solution of 2 µL of various UVexcitable fluorescent dyes [AMCA (7-amino-4-methylcoumarin-3-acetic acid, blue), rhodamine (red), and fluorescein (green)] were deposited onto the individual spots. Through the use of the same technique, a specific functionalizing reagent could be delivered to a spot, providing a simple approach to explore cell viability, adhesion, and response to different reagents. In conclusion, we have created hydrophilic patterns on superhydrophobic surfaces with water harvesting characteristics similar to the Namib Desert beetle. Various properties Nano Lett., Vol. 6, No. 6, 2006

Figure 6. Patterned superhydrophobic surface with hydrophilic domains spotted with various dye-containing solutions [AMCA (7amino-4-methylcoumarin-3-acetic acid, blue), rhodamine (red), and fluorescein (green)] (scale bar ) 10 mm).

and chemical functionalities can be introduced to the patterned area by selective delivery of chemicals or multilayer films onto the area, providing a general approach to generate patterns with desired properties including superhydrophilicity. These patterns can be created in a number of ways compatible with current technologies including inkjet printing, micropipeting, and microcontact printing (results to be published). Acknowledgment. This work was supported in part by the DARPA BOSS Program and the MRSEC Program of the National Science Foundation under Award Number DMR 02-13282. We also made use of the Shared CMSE Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR 02-13282. F.C. acknowledges the Istanbul Technical University President Office Grant to Support Long Term Research Activities Abroad For Researchers. J.M. acknowledges the MIT/MRSEC/MPC REU program. Supporting Information Available: Detailed information on fabricating a superhydrophobic surface, patterning a superhydrophobic surface, building superhydrophilic microcanals on a superhydrophobic surface, and a video of water traveling in a superhydrophilic canal. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Parker, A. R.; Lawrence, C. R. Nature 2001, 414, 33. (2) Lam, P.; Wynne, K. J.; Wnek, G. E. Langmuir 2002, 18, 948. (3) Orner, B. P.; Derda, R.; Lewis, R. L.; Thomson, J. A.; Kiessling, L. L. J. Am. Chem. Soc. 2004, 126, 10808. (4) Ito, Y.; Nogawa, M. Biomaterials 2003, 24, 3021. (5) Niemeyer, C. M.; Blohm, D. Angew. Chem., Int. Ed. 1999, 38, 2865.

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