Directed Assembly of colloidal rods, spheres and their mixtures

Abstract

The directed assembly of colloidal rods, spheres and their mixtures in external fields such as gravity, electric fields and shear flow was investigated. Using confocal microscopy we determined the experimental phase diagram of a binary mixture of colloidal silica rods and spheres that form a binary smectic (Sm2) phase from sedimentation-diffusion equilibria. In this way we were able to directly determine thermodynamic quantities such as osmotic pressure, local and global order parameters. The results were compared with results from computer simulations of hard spheres and hard rods bulk. Moreover, we were able to follow the sedimentation dynamics of binary mixtures of silica rods and spheres over time. We investigated two systems: in the first system silica spheres were smaller than the rod diameter, this mixture forms a Sm2-phase; in the second system the spheres were larger than the rod diameter. In the first system the Sm2-phase nucleated from a nematic layer at the bottom of the sample, once the pressure and volume fraction of particles was high enough. In the second system a transition occurred from a homogenous to a heterogeneous mixture with small clusters of rods. This structure did not evolve in time and appeared to be kinetically trapped. Next, we investigated the electrophoresis of silica particles using confocal microscopy. We showed that using crosscorrelation for image analysis; we decreased the measurement time of electrophoresis experiments for silica rods in the limit of thin double layers. At intermediate double layers, silica rods showed an electrophoretic mobility that was dependent on the orientation of the rod with the applied electric field. We measured this orientation-dependent mobility versus the relative double layer thickness. As the relative size of the double layer increased, the anisotropy in the electrophoretic mobility of the rods increased. However, as the double layer became thicker than the rod diameter, the trend reversed; the anisotropy in mobility of the rods decreased. We investigated the directed assembly of colloidal rods and spheres into thin films using shear alignment. We used spin coating to obtain thin films of colloidal rods, with the silica rods radially aligned along the direction of flow. Moreover, using doctor blade coating (DBC) crystalline colloidal films composed of silica spheres were obtained. The process of DBC was visualized in real-time with a confocal microscope. We established that, with the DBC setup used, capillary forces determined the properties of the obtained colloidal crystalline film. Moreover, we made the colloidal films permanent by polymerizing the continuous phase using UV-light. To conclude, the optical properties of supraparticles composed of CdSe/multishell nanocrystals were investigated. These supraparticles did not suffer from non-radiative recombination, but showed stable and bright photoluminescence. We observed bright emission from multiexciton states at high excitation powers, while supraparticles excited with two-photon excitation did not show emission from multiexciton states. Furthermore, these supraparticles exhibited shape-dependent optical properties. Supraparticles, deposited on a surface, showed the onset of whispering gallery modes in microscope images. Moreover, supraparticles that retained their spherical shape exhibited sharp Mie modes, while multiexciton emission was absent.