Chiral and Active Colloids

Chirality is a geometric property of objects the mirror image of which cannot be brought to coincide with the original. Whereas some properties, such as evaporation temperature and colour, of left- and right-handed molecules are identical, other chemical, biological and optical properties are not. The absence of mirror symmetry leads to captivating effects such as the coupling of translational and rotational degrees of freedom and chiral liquid crystal formation. Unfortunately, it is not feasible to visualize the effects of chirality on the molecular level. Therefore, we investigated preparation routes for chiral colloids which, just as molecules, display thermal diffusion but with the crucial advantage that they can be imaged with optical microscopy.
Two methods were employed to synthesize chiral colloids from isotropic spheres. The first method relies on the confinement of colloidal spheres in hollow microtubes. Microtubes self-assemble from sugar and surfactant molecules as characterized in situ with high resolution small-angle x-ray scattering spanning more than three orders of magnitude of spatial scales. The tubular microstructures consist of equally spaced curved bilayers forming a collection of concentric hollow cylinders with a pore diameter of roughly one micrometre. By adding colloidal spheres to the microtubes, colloid-in-tube assemblies are obtained. Depending on the colloid-to-tube diameter ratio, various structures are formed. Upon removing the microtubes these structures fall apart. Therefore, photo-responsive polystyrene colloids were developed that can photo-crosslink triggered by UV-light. These coumarin-modified spheres form covalent bonds holding the spheres in place even after dissolution of the microtubes. Aqueous suspensions with bulk quantities of a library of ordered structures, including helical sphere chains, have eventually become available. The colloidal equivalents of chiral molecules were used to study their Brownian dynamics.
The second preparation route for chiral colloids combines dipolar hard spheres and depletion interaction. Superparamagnetic silica spheres were grafted with polymer hairs and end-functionalized with coumarin molecules. In an external magnetic field, these spheres align while forming linear sphere chains. Upon UV-irradiation, permanent magnetisable sphere chains in bulk are obtained. When a magnetic field is applied in the presence of depletion polymer, the linear sphere chains wind around each other transforming into helical chains. The number of neighbours is maximized in a helical configuration compared to a linear one, lowering the energy of the system. The application of a homogeneous field appeared to be crucialleadingtothedevelopmentofaHelmholtzcube.
The final Part of this Thesis describes the realization of cubic microswimmers. Superball-shaped colloids were coated with platinum that catalyses the decomposition of hydrogen peroxide into water and oxygen propelling the colloids via self-diffusiophoresis. The speed of the active particles increases with both fuel concentration and temperature. At short times the particles undergo directed motion, whereas at longer times a random walk with an enhanced diffusion coefficient is observed.