Harnessing the instabilities of soft matter: Dynamically tuning of wetting, assembly and pattern transformation in polymer microstructures

In this dissertation, we have investigated the fabrication, mechanical instability and applications of two kinds of polymer micro/nano-structures: high-aspect-ratio (HAR) polymer pillar arrays, and periodic porous elastomer membranes. For HAR polymer pillar arrays, we demonstrated the fabrication of high-aspect-ratio (up to 18) polymer micropillars with different shapes and dimensions by replica molding. Capillary force lithography (CFL) is also demonstrated as a simple and flexible method to fabricate microstructures with controlled aspect ratios. Meanwhile, by introducing conventional photoresist SU-8, CFL is successfully coupled with photolithography and used to create hierarchical 2D or 3D structures, which greatly expand the capability of current capillary force lithography. The mechanical stability of HAR structures with varied materials and different aspect ratio, density and shape were also studied and the results show that the adhesive forces from environment are the major cause of structure collapsing. When HAR polymer pillars are subjected to different solvents treatment, both capillary force and solvent swelling need to be considered to completely understand the structure instability. On HAR micropillar array, thermoresponsive polymer brushes, poly ( N-isopropylacrylamide) (PNIPAAm), were selectively grafted at different locations for dynamically tuning surface wetting or pattern assembly. When the temperature changed from 40°C to 20°, depending on the location of polymer brushes, different wetting transitions, either from a composite solid/air state (Cassie state) to a composite solid/liquid state (Hemi-wicking state) or a transition between two Cassie states were observed. Meanwhile, the dynamically tuning of water contact angle enables us to control capillary drying force and thus harness pattern collapse to create superlattice micropatterns. For periodic porous elastomer membrane, a novel pattern transformation effect is discovered due to the mechanical instability of membrane under solvent swelling. To harness this elastic instability, we convectively assemble nanoparticles onto a swollen membrane and capture this dynamic pattern transformation process. By using the nanoparticle film that imprinted with deformed pattern as a master mold, the complex pattern formed by elastic deformation can be transferred into other materials through replica molding, capillary imprinting & selective etching. The unique 3D morphology of the elastic deformation pattern allows us to perform gradient etching, allowing more flexibility to control pattern morphology