Biomimetic Ultrathin Whitening by Capillary-Force-Induced Random Clustering of Hydrogel Micropillar Arrays Dinesh Chandra, † Shu Yang,* ,† Andre A. Soshinsky, ‡ and Robert J. Gambogi § Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104, Johnson and Johnson Consumer Products, 185 Tabor Road, Morris Plains, New Jersey 07950, and Johnson and Johnson Consumer Products, 199 Grandview Road, Skillman, New Jersey 08558 ABSTRACT Capillary-force-induced collapse of high-aspect-ratio microstructures has often been considered a failure mechanism in device fabrication. Here, we study capillary-force-induced clustering behavior of highly ordered hydrogel micropillar arrays from 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) and explore their utility as ultrathin whitening layers (less than 9 μm thick). When exposed to water, followed by drying in an air stream, the micropillars were softened, bent, and randomly clustered together because of competition between the capillary force and elastic restoring force of the pillars. By varying the relative composition of the water-swellable PHEMA and glassy PMMA, we modulated the elastic modulus of the pillars in the wet state spanning over 3 orders of magnitude. By minimizing the sum of the capillary meniscus interaction energy and the elastic bending energy of the pillars for a cluster, we estimated the average cluster size as a function of the elastic modulus of the pillars, which agreed well with the experimental observation. The randomly clustered micropillar arrays appeared white in color because of random light scattering from the clusters, similar to the observation in the white beetles, whose scales consist of a few micrometer-thick random networks of microfilaments. KEYWORDS: micropillar • biomimetic • ultrathin whitening • capillary force • clustering INTRODUCTION H igh-aspect-ratio micropillar arrays play an important role in a wide range of applications, including superhydrophobic surfaces and tunable wetting (1-3), force sensing and actuation (4, 5), dry adhesives (6), tissue engineering (4, 5), and filtration and separation (7). However, because of their small effective stiffness, high- aspect-ratio structures tend to deform under external forces. Recently, such structural collapse has been harnessed to induce the spontaneous formation of helicity for particle trapping (8). Previously, we have reported the fabrication of stable high-aspect-ratio (height-to-diameter ratio up to 20) epoxy micropillar arrays in air (9). However, many applica- tions for tall micropillar arrays require them to be used in a liquid environment, where the combination of a decreased elastic modulus of the pillars due to swelling and the capillary force during drying could adversely cause them to collapse (10-12). On the other hand, the capillary force has been utilized, for example, to form carbon nanotube foams (13) and to self-assemble superstructures (8, 10, 14). One im- portant question is whether we can predict the clustering behavior of high-aspect-ratio pillars due to capillary force, which will be important to the utilization of a capillary-driven self-assembly process. Here, we report both theoretical and experimental studies of the clustering behavior of hydrogel micropillar arrays with well-defined geometries induced by capillary force and their utility as biomimetic ultrathin whitening layers. By varying the composition of the water-swellable 2-hydroxyethyl- methacrylate (PHEMA) component versus the glassy, non- swellable methyl methacrylate (PMMA) as the micropillar material, we systematically modulated the effective elastic modulus of the micropillars from poly(2-hydoxylmethacry- late-co-methyl methacrylate) (PHEMA-co-PMMA) in the wet state over 3 orders of magnitude. The ability to tune the elastic modulus of the micropillars over a wide range allowed us to experimentally study the effect of the elastic modulus on clustering. Clustering of macroscopic fibers when with- drawn from a liquid bath has been studied previously in terms of the balance between the elastic bending energy and capillary energy (15, 16). It is important to point out that in these studies the capillary energy due to clustering originated from a reduction in the liquid-vapor surface area along a part of the fiber length withdrawn from the liquid. However, in the case of micropillar clustering during liquid evapora- tion, the relevant capillary energy is the interaction energy of the liquid menisci (17) surrounding the micropillars, while the latter are still immersed in the liquid except at the tip (Figure 3a-ii). Few have systematically investigated micro- pillar clustering by considering the meniscus interaction energy. Here, by minimizing the sum of the capillary me- niscus interaction energy and the elastic bending energy of * To whom correspondence should be addressed. E-mail: shuyang@ seas.upenn.edu. Received for review April 11, 2009 and accepted June 29, 2009 † University of Pennsylvania. ‡ Johnson and Johnson Consumer Products, 185 Tabor Road. § Johnson and Johnson Consumer Products, 199 Grandview Road. DOI: 10.1021/am900253z © 2009 American Chemical Society ARTICLE 1698 VOL. 1 • NO. 8 • 1698–1704 • 2009 www.acsami.org Published on Web 07/10/2009 Downloaded by UNIV OF PENN on August 26, 2009 | http://pubs.acs.org Publication Date (Web): July 10, 2009 | doi: 10.1021/am900253z