We design annular sector particles (ASPs), which have a 270-degree opening angle (i.e. idealized C-shapes), as monomer building blocks and mass-produce millions of them using photolithography. We make an aqueous dispersion of microscale ASPs and a nanoscale depletion agent, load this dispersion into a rectangular cuvette, and slightly tilt this cuvette to form a two-dimensional (2D) gravitational column. These Brownian ASPs, which experience hard interactions and roughness-controlled depletion attractions, are osmotically compressed to high densities slowly. After three months, near the top of column, the ASPs predominantly appear as individual monomers and form a low-density fluid-like region. However, below this surface region, at a higher applied 2D gravitational osmotic pressure (Π2D), ASPs at higher densities begin to form a large population of interpenetrating, lock-and-key dimers, which co-exist with monomers in a reaction zone. Below this reaction zone, ASPs in the bottom of column form a disordered, jammed racemic mixture of chiral (+) and (-) dimers in which a few isolated monomers are seen. We measure the particle area fractions of monomers and dimers as a function of depth from microscope images and calculate the ASPs system's 2D osmotic equation of state. From these area fractions in the reaction zone, we determine the equilibrium constant K of the dimerization reaction using the law of mass action. Overall, K grows exponentially with Π2D, leading to a high degree of dimerization in the bottom of the column.

Colloidal dimerization of hard annular sector particles

251st National Meeting (2016)

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