PROGRAMMABLE WRINKLE PATTERNING ON MICROPARTICLES

Abstract

Wrinkles can be defined as sinusoidal topography with ridge and valley structures, and they commonly exist in various organisms like human skins. Many scientists have studied to understand the fundamental principles of the natural wrinkling phenomenon in various material systems. Moreover, engineers have also paid attention to these spontaneously generated wrinkle patterns found in nature, even with complex structures in micro/nano scale, because it is hard to fabricate them with conventional lithography technologies. Therefore, various bottom-up patterning methods based on the mechanical instability have been developed as alternatives to top-down patterning approaches. To utilize wrinkling as patterning purposes, appropriate control mechanisms are required in the fabrication processes due to the random nature of it. Although numerous patterning technologies with controllability have been developed by pre-patterning the substrates or films, engineering the stress states, and others, it was elusive to achieve both the flexible pattern design (e.g., precise control of in individual ridge to any geometry) and the high-throughput production of heterogeneously patterned structures, simultaneously. In this dissertation, a new wrinkle patterning platform based on the microparticle substrate is presented, which is able to realize them and thus to extend utility of the wrinkle patterns. For this purpose, polymeric microparticles coated with silica film were utilized for the unit structure, because the parameters to program the resulting wrinkle patterns (e.g., elastic modulus, film thickness, and geometry of the microparticle) could be dynamically tuned in each microparticle during the fabrication processes. By shrinking the homogeneously or heterogeneously programmed silica-coated microparticles, a few thousands of wrinkled microstructures could be constructed in a single fabrication process. First, the random wrinkle patterns were generated on plane, disk-type microparticles, and they were utilized as unclonable codes analogous to human fingerprint for anti-counterfeiting purposes. Using conventional fingerprint identification algorithms, the authentication system of these artificial fingerprints was demonstrated, and the uniqueness, individuality, and durability of them were verified. This application was the first functionalization of random wrinkle patterns. Next, several control techniques were applied to tune the degree of the pattern randomness or the directionality of ridges in low-level. Further, an elaborate wrinkle control mechanism was developed by pre-patterning the ridge guiding structures consisting of small grooves on the surface of the polymeric microparticles. This slightly modified patterning method allowed the self-organization of microstructures with precise control of the individual ridge orientation over the randomness. Not only the anisotropic, orthogonal, and hexagonal ridge patterns, but also the letter-shaped ridge patterns were realized. Although this dissertation focused on the polymeric microparticles covered by silica, the presented programmable wrinkle patterning concept could be also applied to other materials or substrates systems. It is expected that this patterning technology and the resulting structures could be utilized for various purposes other than the presented applications, including those for useful experimental platforms in studying mechanical instability.