Manipulating colloids with charges and electric fields

Abstract

This thesis presents the results of experimental investigations on a variety of colloidal suspensions. Colloidal particles are at least a hundred times larger than atoms or molecules, but suspended in a liquid they display the same phase behavior, including fluid and crystalline phases. Due to their relatively large size, colloids are much easier to investigate and manipulate, though. This makes them excellent condensed matter model systems. With this in mind, we studied micrometer-sized perspex (‘PMMA’) spheres, labeled with a fluorescent dye for high-resolution confocal microscopy imaging, and suspended in a low-polar mixture of the organic solvents cyclohexyl bromide and cis-decalin. This system offered us the flexibility to change the interactions between the particles from ‘hard-sphere-like’ to long-ranged repulsive (between like-charged particles), long-ranged attractive (between oppositely charged particles) and dipolar (in an electric field). We investigated the phase behavior of our suspensions as a function of the particle concentration, the ionic strength of the solvent and the particles’ charges. In this way, we obtained new insight in the freezing and melting behavior of like-charged and oppositely charged colloids. Interestingly, we found that the latter can readily form large crystals, thus defying the common belief that plus-minus interactions inevitably lead to aggregation. Moreover, we demonstrated that these systems can serve as a reliable model system for classical ionic matter (‘salts’), and that opposite-charge interactions can greatly facilitate the self-assembly of new structures with special properties for applications. On a slightly different note, we also studied electrostatic effects in mixtures of the cyclohexyl bromide solvent and water, both with and without colloidal particles present. This provided new insight in the stabilization mechanisms of oil-water emulsions and gave us control over the self-assembly of various useful colloidal structures. Besides modifying the particle charge, we employed the sensitivity of colloids to ‘external fields’ to manipulate the structure and dynamics of our suspensions. In particular, we used an electric field, in which the particles acquired a dipole moment. The induced dipole-dipole interactions gave rise to uniquely different crystalline and non-crystalline structures, due to their anisotropic nature. We explored the phase behavior as a function of the particle concentration, the electric field strength and the field geometry, and showed how one can rapidly switch from one structure to another. The latter is particularly interesting for applications. Finally, we also studied much weaker, inhomogeneous electric fields. In this case, the dipole moment of the particles was too small to change the phase behavior, but large enough to induce dielectrophoretic motion, driving the particles to the areas with the lowest field strength. We demonstrated how this can be used to manipulate the local particle concentration inside a sealed sample, on a time scale of minutes-weeks. The combination with real-time confocal microscopy allowed us to follow all particle rearrangements during the densification. Such controlled compression is of interest to colloidal model studies and the fabrication of high-quality crystals for applications. After all, for all suspensions the particle concentration is one of the most important factors determining the behavior.