In this dissertation we studied the mechanical instability and large deformation of structured soft material to tune surface properties. On a low aspect ratio channel-type surface structure, we investigated the alignment of wrinkle morphology in the presence of these surface patterns on a photoresist with depth-wise gradient crosslinking density. Using high aspect ratio shape memory polymer (SMP) pillar arrays, we designed a dry adhesive based on mechanical interlocking if buckled pillars to gain strong jointing force whose magnitude can be tuned by geometry and temperature. We fine-tuned the deformation/recovery behaviors of SMP pillars as a reconfigurable surface for manipulation of wettability and its anisotropy, as well as sticky fingers to grab nanoparticles for fabrication of hierarchical structures.
We started our investigation from the size effect of low aspect ratio 1-D patterns on the thin film wrinkling instability. We developed a new wrinkling system based on dyed SU-8 photoresist, which formed a gradient crosslinking density UV exposure. By combining thermoplasticity and photopatternability of SU-8, we investigated physical confinement of wrinkling using the 1-D pre-patterns. The size ratio of the pre-patterns to the intrinsic wrinkles (rp: pre-pattern pitch to wrinkle wavelength; rh: pre-pattern height to wrinkle amplitude) was carefully chosen so that both rp and rh lied in the order of ~ 0.1-10. At rp ~ 10, it was found that the wrinkles remained anisotropic but confined, primary on the mountain region of the SU-8 pre-pattern. By lowering rp to ~ 1, the
wrinkles were fully confined to form 1-D bumps with out-of-phase alignment to the neighboring mountain regions. At the smallest rp ~ 0.1, the wrinkle morphology was found highly sensitive to rh. For rh on the order of ~ 1, the wrinkle aligned perpendicularly to the pre-patterns, showing a long-range and in-phase order. Decreasing
rh to ~ 0.1 led to dual-orientational alignment, with one set of wrinkling still grow perpendicularly to the pre-patterns, the other set of high amplitude localized fold form in parallel to the pre-patterns. Further decreasing rh recovers the isotropic wrinkles. We constructed a morphological diagram based on these findings, which should pave the way to fully control wrinkles via design of the surface pre-patterns.
Separately, we studied the instability and large deformation of high aspect ratio pillar arrays. In particular, we designed a new dry adhesive, whose adhesion force originated from interlocking of mutually buckled pillar arrays (hexagonal array, 1µm diameter, 1 µm spacing and aspect ratio 4) under a load. We used thermally activated SMP, whose Young’s modulus could be greatly reduced by three orders of magnitude from room temperature to 80 °C, above its glass transition temperature. The tunable mechanical strength endowed low buckling threshold and easy adhesive engagement at 80 °C but high stiffness and high adhesion force after engagement by cooling the pillars back to the room temperature. Microscopically, we found that the pillars mainly interweaved with each other with some contribution to the adhesion strength from pillar indentation to each other. The latter is similar to the gecko-like fibrillar contact, whose effective adhesion is shown experimentally and theoretically to be lower than the interweaving state. A Moiré pattern was revealed as a result of periodic contact and misalignment between two pillar arrays. The total pillar-to-pillar adhesion force reached
~ 54 N/cm2 and ~ 72 N/cm2 in normal and shear direction, respectively, much higher than the pillar-to-flat (~ 12 N/cm2 in normal and ~ 15 N/cm2 in shear) and flat-to-flat (~ 7 N/cm2 in normal and ~ 16 N/cm2 in shear) counterparts. We further showed that the adhesion anisotropy ψ, defined as the ratio of shear to normal adhesion force, can be tuned by changing the pillar spacing, going from ~ 1.3 dramatically to ~ 5.4 as the spacing doubled. Despite of the strong adhesive force obtained at room temperature after engagement, the adhesive could be effortlessly separated on demand by reheating back to 80 °C.
Under the applied preload with complex pillar-pillar interaction and prolonged heating under deformation, the SMP pillars could not be recovered to the original shapes. However, the SMP pillars were fully recoverable under uniform shearing and collapsing. We utilized this behavior to create a reconfigurable surface for wettability control and advanced patterning. Specifically, the original/recovered pillars exhibited Cassie wetting state with a high droplet contact angle and low sliding angle. The fully deformed surfaces, on the other hand, displayed Wenzel wetting state, where the droplet was fully pinned with anisotropic droplet shape at large pillar spacing. We demonstrated that such high wetting contrast can be employed for liquid collection. We also applied the original or deformed pillars to pick up nanoparticles on to a selected location of pillars (top vs. the side walls) as a facile method to fabricate hierarchical structures. Moreover, we showed that it was possible to precisely control SMP deformation by depositing a thin metal layer onto the deformed structures, limiting the SMP pillars from full recovery. This allowed us to access SMP pillars at different tilting angle with high precision. More intriguingly,
the tilted composite structure possessed a unique unidirectional liquid propagation opposite to the pillar tilting direction.