Biomimetic ultrathin whitening by capillary-force-induced random clustering of hydrogel micropillar arrays

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 mum 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.