Colloidal particles with heterogeneous surfaces offer rich possibilities for controlled self-assembly. We have developed a method for preparing micrometer-sized polystyrene spheres with circular flat spots of controlled radius and location. The flats are created by settling the particles onto a flat glass substrate and then raising the temperature above the glass-transition temperature of the polymer for a controlled time (t). The polymer particle spreads on the glass such that the radius of the flat grows with time. We present a scaling theory for the hydrodynamics of the flattening process, finding that the radius of the flat grows as t(1/3). The model is in good agreement with our experimental observations of the flat radius versus spreading time as well as with previous studies in the literature for sintering polymer spheres.

Chains of micrometer-size colloidal particles have been self-assembled that are flexible, mechanically stable, and observable in optical microscopy. The chains sometimes have more than 30 particles, and we call them "polloidal chains". A key aspect of the work is the careful modeling of the interparticle forces between partially flattened polystyrene spheres. This modeling helped us to identify a narrow window of system conditions that produce interparticle physical bonds with a bond energy greater than 15kT, as well as a gap of fluid between particles that enables freely rotating bonds and flexible chains. The formation of the chains is well-modeled using linear condensation growth from classical polymer theory, suggesting that the chains might be used experimentally as large-scale, relatively slow moving models for polymer chains.

Bottom-up fabrication methods are used to assemble strong yet flexible colloidal doublets. Part of a spherical particle is flattened, increasing the effective interaction area with another particle having a flat region. In the presence of a moderate ionic strength, the flat region on one particle will preferentially "bond" to a flat region on another particle in a deep (≥10 kT) secondary energy minimum. No external field is applied during the assembly process. Under the right conditions, the flat-flat bonding strength is ≥10× that of a sphere-sphere interaction. Not only can flat-flat bonds be quite strong, but they are expected to remain freely rotatable and flexible, with negligible energy barriers for rotation because particles reside in a deep secondary energy minimum with a ~20-30 nm layer of fluid between the ~1 μm radius particles. We present a controlled technique to flatten the particles at room temperature, the modeling of the interparticle forces for flattened spheres, and the experimental data for the self-assembly of flat-flat doublets.