Structural transformations in colloidal gelation with controlled particle interactions

The effects of attractive and repulsive interactions between colloidal particles, and applied electric fields on the behavior of gelled charged colloids were studied using confocal microscopy. Sterically stabilized poly(methyl methacrylate) colloids were synthesized in the lab and dispersed in a density and refractive index matched organic solvent mixture. Attractive interparticle interactions were generated by the addition of non-adsorbing polymer to induce the depletion effect, and repulsive interactions were tuned by the addition of charge-reversing salt. Samples at various concentrations of polymer and salt were observed by confocal microscopy, and the various structures seen along the gel transition in the phase diagram were described using quantitative image processing. The pair correlation function and particle contact number were used as measures of short-range structure, and number density fluctuations within a sample as measures of long-range structure.; The change in the microscopic structure of the gel was observed as the attractive interactions between particles were manipulated. As the attraction was increased, two transitions were traversed. The first transition corresponded to a formation of the gel network, and an increase in the heterogeneity of the sample, while the second transition corresponded to the formation of a different, more homogeneous, gel network with a characteristic length scale.; To study the effects of repulsive interactions upon the structure of colloidal gelation, a charge-reversing salt was added to colloidal dispersions to change the magnitude, distribution, and sign of the charge. Structures were produced which were similar to those produced when only the attractive interactions were varied. Additional structures were discovered which were not accessible with the manipulation of attractive interactions alone.; To study the effects of externally applied forces on gels, a glass microfluidic channel was fabricated and a depletion gel of charged colloids inside the channel was broken by an applied electric field. The breakage was studied by confocal microscopy and showed a decrease in number density fluctuations for regions of the gel which had high number density fluctuations. Little change was observed in the number density fluctuations for regions of the gel which were initially relatively homogeneous. The change in the gel structure was permanent.