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.

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Plasma polymerized hydrocarbons made from ethane and methane were produced under different reactor conditions and probed by solid‐state carbon‐13 nuclear magnetic resonance (13C‐NMR) with cross‐polarization and magic‐angle sample spinning. NMR experiments provided structural information about the plasma polymers. The conditions of low power, high hydrocarbon gas flow rate, and no added hydrogen gas appeared to give the highest amount of nonprotonated sp3 hybridized carbons in the films for the reactor design used. The use of methane or ethane as reactor gas did not affect plasma polymer structure significantly.

Composite hydrogel films are prepared by photocrosslinking of partially polymerized poly(2‐hydroxyethyl methacrylate‐co‐methyl methacrylate) prepolymer solution dispersed with aragonite precipitated calcium carbonate (PCC) particles. The effects of PCC aspect ratio (AR, 4.2–6.6) and volume fraction (ϕPCC) are studied with respect to the prepolymer viscosity, the maximum packing fraction (ϕm), and optical properties. ϕm obtained from Kreigher–Dougherty fitting is found to decrease with increasing AR of PCC particles, from ϕm = 0.33 (AR = 4.2) to ϕm = 0.21 (AR = 6.6). Scanning electron microscopy images revealed that the particles are uniformly dispersed and randomly orientated in the polymer network when ϕPCC < ϕm. Optically transparent neat hydrogel films become opaque with increasing particle concentration and AR because of random light scattering from the PCC particles. A whiteness up to 80 is obtained from 100 μm thick composite hydrogel film in the wet state with PCC AR = 6.6 and ϕPCC = 0.19 (near ϕm). POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers