Structure and Properties of Charged Colloidal Systems

Abstract

This dissertation explores the changes in structure of colloidal systems on the introduction of repulsive interactions. Colloidal gels are well understood when all particle interactions are attractive, but their structure is fundamentally changed when repulsive interactions compete with those attractive interactions, as in the case of a binary gel of oppositely charged particles. Similarly, colloidal crystals are well understood when interactions are approximately hard-sphere, but again, the structure and material properties change when a long-range repulsion is introduced, giving a colloidal `Wigner' crystal. My research quantitatively investigates these effects in experimental model systems. I use confocal microscopy to directly image in three dimensions suspensions of micron-scale colloidal particles which are monodisperse, index- and density-matched, fluorescent, and electrostatically charged. I use standard image-processing techniques to obtain the precise location of each particle in the imaging volume in order to analyze both global and local structure. In the case of the binary gel, I observe gelation of oppositely charged particles, controlled by varying the total particle volume fraction, the interaction strength, and the mixing ratio of the two particle species. I find that contrary to commonly studied purely attractive gels, in which weakly quenched gels are more compact and less tenuous, particles in these binary gels form fewer contacts and the gels become more tenuous as we approach the gel line, and the average attractive bond number emerges as a critical parameter for gelation. This suggests that a different mechanism governs gel formation and structure in binary gels, in which attractive and repulsive interactions compete. In the case of the long-range-repulsive colloidal `Wigner' crystals, I find a body-centered-cubic crystalline phase at particle volume fractions near 15%, in contrast to the face-centered-cubic crystalline phase found at volume fractions above 50% for hard spheres. The soft interactions in these repulsive crystals permit large fluctuations, with typical particle displacements up to 20% of the nearest-neighbor spacing. I determine the three independent crystalline elastic constants, and find that the crystals are very compliant (c ~ 5-40mPa), and strongly anisotropic at all volume fractions studied. I also observe a sharp interface between the fluid and crystalline phases.