Microwrinkles: shape-tunability and applications.

Recently, spontaneously formed microwrinkle patterns on hard coating-capped elastomer surfaces have attracted the attention of both the scientific and applied research communities, because of their simple fabrication process and practical potential in diverse applications. The periodicity and statistical orientation of the microwrinkle stripes can be controlled by applying uniaxial or isotropic compressive strain. Thus, microwrinkles have been applied to cell patterning, optical gratings, pattern templates for further patterning, surfaces with anisotropic wetting properties, surfaces with unique adhesion properties, and metrology of ultrathin film properties. Our group has focused on tuning the structure of microwrinkles by exerting additional strain. This is almost impossible for micropatterns fabricated on a hard Si wafer; therefore, our technique is based on a soft substrate and the non-linear response of the system to external strain. The dynamic shape-tunability of the micropatterns shows potential for new applications, in which switching states of a system could be induced by a change in the physical boundary conditions, namely, the shape of microwrinkles. This feature article summarizes our laboratory's recent work on controlling the stripe pattern of microwrinkles and the application of shape-tunable microwrinkles to liquid manipulation and liquid crystal alignment. For liquid manipulation, the microgrooves of the microwrinkles are used as an open channel capillary, in which the tunable groove depth controls the capillary action of the liquid in the grooves. Further changes in the direction of the microgrooves, which are filled with liquid, can induce the division of the liquids into small droplets. Such methods for shaping liquids are made possible by only macroscopic control of the strain applied to the sample. The alignment of a nematic liquid crystal was also investigated. Nematic liquid crystals can be aligned by anisotropic microgrooves; therefore, we have demonstrated that microwrinkles can be used for this purpose. In addition, because microwrinkles are shape-tunable, the liquid crystal alignment could be repeatedly switched. Our research demonstrates that shape-tunable microwrinkles provide new physical boundary conditions that can control the states of the bounded material, for example: the shape of liquid droplets and the alignment of a nematic director. We expect that many other systems that interact with the tunable boundary will lead to the discovery of new phenomena and technologies.