Fluorescent colloidal silica rods - synthesis and phase behavior

Abstract

Although the experimental study of spherical colloids has been extensive, similar studies on rod-like particles are rare because suitable model systems are scarce. To fulfill this need, we present the synthesis of monodisperse rod-like silica colloids with tunable dimensions. Rods were produced with diameters of 200 nm and larger and lengths up to 10 µm, which resulted in aspect ratios ranging from 1 to 25. The growth mechanism of these rods involves emulsion droplets of water in pentanol, inside which silica condensation takes place. Since the silica nucleus is attached to the water/pentanol interface, the supply of reactants to the nucleus is anisotropic, causing it to grow on one side only, which results in rod formation. The rods were made suitable for quantitative real-space studies by confocal laser scanning microscopy. Several methods of fluorescent labeling are presented that resulted in constant fluorescence levels, gradients from one rod-end to the other, and even patterns of two colors. Single particle imaging was achieved by creating core-shell rods that had a fluorescent core and a non-fluorescent shell. Alternatively, the rods could be dispersed in a solvent with a low dielectric constant to induce micron-sized double layers. To enable quantitative measurements, a tracking algorithm was developed that identifies the rods' positions and orientations. The newly developed model system was used to study the phase behavior of rods. By combining real-space confocal laser scanning microscopy and small angle X-ray scattering methods, a phase diagram depending on concentration and aspect ratio was constructed, which shows good qualitative agreement with simulation results in literature. This phase diagram includes nematic and smectic phases for the higher aspect ratios. Also, the effect of external fields (electric fields, shear and templates) on the phase behavior was studied. In an electric field, rods aligned themselves with the applied field due to an induced dipole moment. The polarization of the rods was determined and compared with a theoretical model for the polarization of ellipsoids. In more concentrated dispersions, external electric fields were used to influence the phase behavior of the rods. Para-nematic phases were induced this way, as well as well-aligned smectic phases with a decreased number of defects. At high field strengths, a new crystal structure was found. Steady shear flow was used to create nematic, columnar and smectic liquid crystal phases which were aligned with their director in the flow direction. The application of oscillatory shear led to the formation of smectic-like domains in a sample that was previously in a shear aligned nematic state. Finally, smectic liquid crystals were aligned successfully by templates consisting of trenches of various widths and spacings. The area with well aligned rods was restricted to the template area with sharp boundaries. For short rods, which do not form smectics spontaneously, templates with trench widths smaller than the rod length were used to induce the formation of an aligned smectic-B phase. Larger trench widths did induce more order than was measured in sediments on a flat wall, but less than induced by the narrow trenches.