Mechanics of micro-structured soft materials and their applications in adhesion and wetting

Mechanical instability and large deformation are pervasive occurrences on stressed soft material surface patterns, which are normally detrimental to device performance. In this dissertation we show that these phenomena can be harnessed on structured polymeric thin films for surface patterning, strong dry adhesion and guided wettability. On dyed SU-8 photoresist films with graded depth-wise crosslinking density, we study the swelling-induced wrinkling. We demonstrate that isotropic surface wrinkles can be aligned with low aspect ratio 1-D channel-type pre-patterns. By varying the pitch and height ratios, defined as the pre-pattern pitch and height of the channels to the wrinkle wavelength and amplitude, respectively, we construct a morphological diagram of the confined wrinkles. For pitch ratios much larger than 1, the wrinkle morphology is predominantly isotropic. As pitch ratio decreases to ~ 1, the wrinkles arrange to out-of-phase 1-D bumps along the mountain regions of the channels. For pitch ratio much smaller than 1, the wrinkles evolve from in-phase perpendicular (to the channels) wrinkles, coexisting perpendicular wrinkles and localized patterns back to isotropic wrinkles in the order of decreasing height ratios. In a separate material system, we utilize the buckling of high aspect ratio shape memory polymer (SMP) pillars to develop a strong interlocking dry adhesive. We engage the two identical pillar arrays together above the glass transition temperature of SMP (80 °C), where the SMP modulus drops by 3 orders of magnitude, leading to mutual buckling and deformation of the pillars when interlocked. Our finite element analysis and comparison of the calculated adhesion versus experimental data suggest that the adhesion force originates primarily from the pillar interweaving and secondarily from pillar indentation. The resultant pillar-to-pillar adhesion forces in normal (~ 54 N/cm2) and shear (~ 72 N/cm2) direction are found much larger than the pillar-to-flat (~ 12 N/cm2 in normal and ~ 15 N/cm2 in shear) and flat-to-flat contacts (~ 7 N/cm 2 in normal and ~ 16 N/cm2 in shear). We further tune the adhesion anisotropy, designated by the ratio of shear to normal adhesion, by changing the pillar spacing. In spite of the strong adhesion, we show that the engaged adhesive can be easily separate on demand by heating to 80 °C. Using similar SMP pillars, we design a reconfigurable surface to control surface wettability. Specifically, we report that the water droplets convert from low adhesion Cassie state on the original (or recovered) straight pillars to fully pinned Wenzel state on deformed pillars. Potentially, surface with both straight and deformed pillars can be utilized as a reprogrammable water collecting surface. Employing the deformed pillars, we present an advanced patterning method based on the deformed SMP pillars to transfer nanoparticle assemblies from donor substrates onto selected locations of pillars. Finally, we show a new approach to precisely control the tilting angle of the SMP pillars by coating the deformed pillars with a layer of metal, which hinders the full recovery of SMP pillars. On the composite surface, we observe strong anisotropic liquid spreading behavior, where the liquid propagates predominantly in the opposite direction of the pillar tilting