Center for Functional Nanomaterials Seminar

Center for Functional Nanomaterials Seminar Monday, February 23, 2015 10:00 a.m. Conference Room B Capillary instability of periodic polymer thin-film structures Zheng Zhang Department of Mechanical Engineering University of Colorado at Boulder In this talk, I will present the simultaneous capillary instability among periodic polymer lithographic structures. The first system is straight polymer stripes arranged in parallel with equal spacing and suspended atop an immiscible medium. Polystyrene (PS) and poly (methyl methacrylate) (PMMA) were used because of their immiscibility and well-characterized physical properties. When annealed at a temperature above the glass transition temperature of both component polymers, the stripes undulated and ruptured via capillary instability. We found that the PS-to-PMMA viscosity ratio and substrate confinement to be key factors that influenced the instability. The second system that I will show is suspended concentric PS rings. The rings ruptured upon annealing, with three possible types of phase correlation between neighboring rings. In the case of weak substrate confinement, the rings ruptured independently when they were sparsely distanced, but via an out-of-phase mode when packed closer. If the substrate confinement was strong, the rings would rupture via an in-phase mode, resulting in radially aligned droplets. The concentric ring geometry caused a competition between the phase correlation of neighboring rings and the kinetically favorable wavelength, yielding an intriguing, recursive surface pattern. This frustrated pattern formation behavior was accounted for by a scaling analysis. Our experiments can serve as a basis for correlated capillary instability among curved objects, which can be a powerful tool for creating unique surface patterns. Host: Charles