Janus microspheres for a highly flexible and impregnable water-repelling interface.

As part of the evolutionary drive to survive and reproduce, some organisms have developed unique surface morphologies. For example, mosquitoes have anti-fog compound eyes that are decorated with small boss arrays, and some desert beetles have patterned wings that enable them to collect water droplets from the atmosphere. Geckos can climb vertical walls and even hang upside down from the ceiling because of spatula arrays on their footpads. Many researchers have sought to mimic such natural surface morphologies to develop useful materials. Artificial superhydrophobic surfaces, which are a representative group of biomimetic materials, have great potential in a wide range of industrial applications owing to their self-cleaning, antifogging, and anti-biofouling properties. Natural waterrepelling objects, such as sundews and lotus leaves, butterfly wings, and duck feathers, have inspired researchers to explore various morphologies, ranging from disordered to highly textured surfaces, in efforts to prepare superhydrophobic surfaces on solid films. However, most studies on superhydrophobic materials performed to date have focused on methods for preparing flat solid surfaces in an inexpensive and simple manner. Herein, we have sought to develop superhydrophobic materials that do not require flat substrates. Taking inspiration from superhydrophobic small objects, such as the scales of butterflies or moths and the legs of water striders, we fabricated and investigated superhydrophobic microspheres with a complex surface morphology in conjunction with hydrophobic surface moieties. The high mobility of the superhydrophobic microspheres gives rise to unique interfacial properties that cannot be achieved using conventional superhydrophobic materials of solid film type. To create the complex surface morphology on the microspheres, we employed emulsion droplets (a Pickering emulsion) decorated with silica particles as a template. Droplets of the photocurable resin ethoxylated trimethylolpropane triacrylate (ETPTA) containing silica particles were generated, and microspheres with silica particle arrays on their surfaces were obtained after photopolymerization of the droplet phase. Through subsequent selective removal of the silica particles by a wet-etching process, we obtained microspheres with surfaces covered with cavity arrays. Further to creating the complex morphology, we incorporated a hydrophobic moiety on the surface to achieve superhydrophobicity, which was achieved simply by applying reactive ion etching (RIE) with sulfur hexafluoride. These procedures are summarized in Figure 1a.