Kolloide in externen elektrischen und magnetischen Feldern

Colloids play an important role in research. They are used both as test objects and as investigation tools in many different fields, such as optics, micro- and nanoelectronics, microfluidics, automobile and petrol industry etc. In fundamental research, colloidal suspensions serve as models for atomic and molecular systems. In the current work, I investigated suspensions of solid colloids (also called particles) in external electric and magnetic fields.

External fields modify the interactions of colloids among themselves and with the solvent and are therefore a classical approach to study non-equilibrium behavior. In the present thesis, I studied non-equilibrium effects of high electric field gradients on the motion of PMMA particles in a density-matched solvent in an electric bottle. Due to the inhomogeneous distribution of the dielectrophoretic force (DEP-force) a zone depleted of colloids formed in the region of the electrode edge shortly after switching on the electric voltage. Later the front of migrating colloids developed into a wave-like form. This instability resembles the Rayleigh-Taylor-Instability. It does not depend on frequency but on field strength.

Whereas the interplay between dielectric force and hydrodynamics described above happens on the scale of 100 µm, the electric bottle also enables to gradually increase the concentration of colloids to a value sufficient for crystallization. I investigated the local rheological and dynamic propertied of coexisting fluid and crystalline phases of PMMA colloid suspensions by dragging magnetic colloids through these two phases using magnetic fields.

In the crystalline phase a minimum force ("pinning-force") is necessary to induce motion of magnetic colloids which depends on the interaction between colloids within the crystal. According to my experiments, the pinning-force depends on colloid concentration and colloidal interactions. Due to their strong interactions, magnetic particles may aggregate and form chains even in dilute solutions (later called trains). Magnetic colloid trains in colloidal suspensions rheologically behave as cylinders.

I also studied the dynamic properties of crystals of PMMA colloids in two dimensions by repeatedly rotating magnetic colloid trains. Crystal lattices far from the colloid trains only elastically deformed while the magnetic colloid trains were oscillating. The influence of the colloid trains on the motion of surrounding colloids exponentially decreases with distance according to the Lindemann parameter. The crystal melted when a colloid train was pulled through a crystalline matrix. According to time- and space resolved laser scanning confocal microscopy, the colloids took less than 2 minutes to recrystallize.

These results show that colloidal probe of complex shape not only give information on microrheological properties, but are also suitable for studying defects dynamics.

Wall effects and confinement become increasingly important in colloidal suspensions in microfluidic applications. To address the influence of these aspects, I studied colloid motion and distribution in micro-channels under the influence of electric DC fields. Micro-channels were fabricated using optical lithography and the process was optimized. Both the migration velocity and the packing density of colloids depend on channel shape. It turned out that in electric fields colloids in microchannels are driven by electroosmosis rather than electrophoresis. In contrast, gravity driven colloid motion depends on the local packing density of the colloids and on the slope (tilting angle) of the channel. At sufficient packing density I also observed separation of colloid and solvent streams. All of these phenomena may be relevant in processes carried out in microfluidic channels, and the quantitative results presented here may contribute to optimization.