Why do LikeCharged Macroions Attract and how do Solids Flow?
Abstract
This thesis develops an improved understanding of the interactions between charged macroions in solution and their behavior under shear. A new local density functional theory (LDFT) for macroion interactions is presented which incorporates correlations between counterions. This theory is first applied to the canonical double layer problem: parallel charged plates. Results for the double layer agree well with more accurate and timeconsuming calculations. Counterintuitive attractive interactions between likecharged plates are shown to result from nonlocal effects, and simple criteria for attraction are given. The LDFT and Monte Carlo (MC) simulations are then used to study interactions between charged spherical macroions which are commonly described by the simple DerjaguinLandauVerweyOverbeek (DLVO) potential. The WignerSeitz sphere approximation is tested by comparing simulations with hard sphere and periodic boundary conditions. The spherical approximation works well even at high volume fractions and counterion densities. While the DLVO potential for the actual macroion charge gives a poor description of interactions, the MC results can be reproduced if a constant effective charge is used. A DLVO potential with an appropriate effective charge is then used to calculate the nonequilibrium phase diagram of sheared macroion suspensions. In equilibrium, interactions between macroions are strong enough to produce fcc or bcc crystalline order at low added salt concentrations. These crystals are 10^{12} times softer than conventional matter. Increasing the shear rate initially disorders the crystalline phases, and then reorders them into new structures. Crystals near their equilibrium melting point are actually shearmelted and then recrystallized. Crystals far from their equilibrium melting point never melt. The phase diagrams for bcc and fcc equilibrium crystalline structures can be scaled to yield universal phase transition lines. Plots of the normalized shear stress vs. shear rate also collapse onto universal fcc, bcc and fluid branches. The flow mechanism in sheared solid phases is shown to be planes sliding over planes.
 Publication:

Ph.D. Thesis
 Pub Date:
 1992
 Bibcode:
 1992PhDT.......147S
 Keywords:

 MACROIONS;
 Physics: General; Physics: Condensed Matter; Chemistry: Physical