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Open Access Publications from the University of California

Self-Assembly, Pre-Assembly, and Configurational Reactions of Shape-Designed Colloids in Fluctuating Brownian Monolayers

  • Author(s): Wang, Po-Yuan
  • Advisor(s): Dunn, Bruce S
  • Mason, Thomas G
  • et al.

Self-assembly is a promising method for creating new materials. When building blocks are slowly crowded to induce self-assembly in the presence of Brownian excitations, their shapes play important roles in determining the densities at which these building blocks jam and the type in the resulting structures. In this thesis, we describe how to design and lithographically fabricate microscale annular sector particles (ASPs) and disperse them in a solution containing surfactant and depletion agent to form a stable two-dimensional (2D) system. After slowly osmotically compressing these ASPs in 2D using a gravitational force, we observe that a high percentage of lock-and-key dimers exists in the high density region. We determine the area fraction of these ASPs, thereby obtaining the system’s 2D osmotic equation of state and the dimerization equilibrium constant K. We find that K increases exponentially with this 2D osmotic pressure.

Beyond achiral C-shapes, we have been inspired by dimer crystals of chiral proteins, so we have created systems of lithographically fabricated proteomimetic colloids to explore how monolayers of Brownian colloids self-assemble during slow crowding. We modify these ASPs by adding a circular head to only one terminal edge and force the dimerization pathway to select only one chirality through steric suppression. Time-lapse video microscopy reveals that enantiopure dimers crystallize and grow through a kinetic mechanism that we have identified and called tautomerization translocation reactions, which lead to the expulsion of monomer defects from crystallites. We also show that the types and structures of dimer crystals can be tuned by shaping or "lithographically mutating" the location and size of the head on the chiral monomers.

Self-assembling multi-scale hierarchical superstructures that have few defects through slow crowding is an extremely difficult challenge. To overcome this, we utilize photolithography to create pre-assembled complex superstructures that are defect-free. Moreover, we demonstrate that 2D liquid quasi-crystals can be created by the combination of photolithography and chemical release processes. We have precisely pre-assembled Penrose kite-and-dart tiles at high density into a pentagonal quasi-crystalline pattern on the designed mask, and release these particles into a specially optimized solution-dispersion while maintaining the monolayer. We use high resolution time-lapse optical microscopy to study the equilibrium dynamic behavior of fluctuating Brownian kite-and-dart tiles, motifs, and superstructures long after they have been released from the substrate. Moreover, by removing a confining wall, we reveal the melting of this fluctuating quasi-crystal through a pentatic liquid quasi-crystalline regime.

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