Towards Directional Colloidal Interactions

Colloids are particles with a size on the scale of microns in at least one dimension. The central theme of this thesis is the synthesis of model colloids with anisotropic interactions - often called `patchy' colloids, as well as the search for new ways to assemble such colloids. Methods to build non-close-packed structures from isotropic colloids do exist (e.g. using binary crystals), and also anisotropic colloids can give rise to interesting crystal structures. An advantage of patchy colloids is that additional features such as a varying surface charge or hydrophobicity throughout the system can be introduced. We presented various new patchy model systems and various new assembly routes. Firstly, we presented a colloidal `patchy' model system of silica particles with fluorescently-labeled titania protrusions. The dye labeling rendered the particles available for studies by confocal microscopy, and we present preliminary results of the distribution of number of patches per particle obtained in this way. We also show that the titania protrusions can be converted to crystalline titania (of the anatase polymorph) which may be interesting for catalysis and active matter. Secondly, we prepared a system of lollipop-shaped particles by condensation of a silane coupling agent (3-methacryloxypropyltrimethoxysilane, MPS) onto the flat end of nail-shaped silica particles. We show that these particles not only can be aligned in an electric field, but in a direct field also orient with all the MPS lobes to the same side. We also show that these particles can be dispersed in low-polar solvents and obtain long-range order, and that the vectorial orientation can also be achieved in low-polar solvents. On the fundamental side, this offers the possibility for a new type of ‘oriented’ crystal. For practical applications it would be interesting to incorporate other functional groups into the MPS lobe. Thirdly, we study a popular method to self-assemble patchy particles in more detail, namely the use of the depletion interaction. We experimentally show that it is possible in a mixture of rough and smooth silica spheres, to selectively immobilize the smooth silica particles. These results were compared to numerical simulations of the inter-particle potentials. Finally, we studied the self-assembly of bowl-shaped colloids and spherical particles in an electric field, where we find that the particles form a composite structure which is a result of the energy landscape around the bowls in the electric field. The composite particles were formed using either silica or polystyrene spheres. We show that the composite particles can be purified using viscosity step-gradient centrifugation, which points at an additional advantage of our assembly route: the possibility to produce new type of patchy particles.