In this thesis we investigate the behavior of aqueous colloidal suspensions in response to externally applied electric fields, at two very different length scales. At the nanometer length scale, we elucidate the mechanisms for liquid-phase nanoparticle growth and aggregation via in situ scanning transmission electron microscopy (STEM). We then investigate the aggregation behavior of micron-scale colloidal particles near an electrode in an external AC electric field, with an emphasis on understanding the role of electrohydrodynamic (EHD) flows.;Our work with nanoparticle growth in STEM yielded three key results. First, to establish the limitations and common artifacts for in situ liquid STEM, we first performed a broad survey of the artifacts observed for this new technique. We provided further experimental evidence that these interactions are primarily a result of secondary electron formation in the fluid and windows. Second, we then observed nucleation and growth of silver nanoparticles in fluid and showed experimentally that there is a threshold electron dose below which observable nucleation does not occur, which we explained in terms of the supersaturation condition for nucleation. We found that single silver nanoparticle growth rates and morphologies are dependent on the STEM beam current, and reconciled the different growth modes and morphologies with reaction and diffusion limited growth. Finally, we showed that while the mean growth rate of ensembles of silver nanoparticles was consistent with Ostwald ripening, the particle size distribution and direct observations of numerous aggregation events strongly suggested that Ostwald ripening was not occurring. We demonstrated instead that Smoluchowski coagulation kinetics quantitatively captures both the mean growth rate and the particle size distribution, and found good agreement between the analytical model and experiments.;The second major thrust of our work, involving micron-scale colloidal particles in AC electric fields, also yielded three key results. First, we showed experimentally that the electrolyte dependent aggregation rate of colloidal particles in AC electric fields was consistent with an EHD scaling model for a single particle near an electrode. We balance the attractive EHD fluid flow with repulsive induced dipole-dipole forces, and found that particle separation is the result of a greatly reduced attractive EHD flow, which is overtaken by induced dipolar forces. Second, we showed that a counterintuitive colloidal crystal phase transition from random closed packed (RCP) to hexagonally close packed (HCP) occurred when changing the applied electric field from high to low frequency. We demonstrated experimentally that the colloidal particles were positioned higher above the electrode at low frequencies, causing them to be more diffusive and consequently disrupt the order of the colloidal crystal. Finally, we showed a surprising bifurcation in colloidal particle height above an electrode in a low frequency AC electric field. The particle height bifurcation was random with respect to individual particles, and the number of particles that moved into the upper layer of the bifurcation was dependent on the applied voltage and frequency. The results suggested the existence of both a secondary and a tertiary minimum in the potential energy with respect to particle separation from the electrode. The thesis concludes with suggestions for future work with in situ nanoparticle growth and EHD colloidal aggregation in electric fields.
展开▼
