Particle force light scattering: A technique for measuring interparticle forces

Colloidal stability is governed by interparticle forces, but for nanocolloids, both theory and experimental data are scarce. This thesis aims to develop a new experimental technique for measuring (nano)colloidal forces. The first part of my thesis will extend our current technique, video microscopy differential electrophoresis (VMDE), to enable measurements on linear colloidal triplets. By using a third particle as a "handle", forces can be measured between particles with identical average zeta potentials (i.e., surface charge densities). Semi-analytical solutions to the electrokinetic equations for three spheres were developed, with results similar to those for two spheres, thus enabling the straightforward interpretation of experiments. Results show that, while nearly-touching particles aggregate irreversibly in the absence of dispersants, the addition of trace quantities of the polyelectrolyte sodium polystyrene sulfonate (NaPSS) enables the particles to form triplets that break with forces of O(10 pN).; Secondly, my thesis will introduce a new technique, particle force light scattering (PFLS), for circumventing the second limiting factor. This technique involves the use of differential electrophoresis in a small-angle light scattering system to measure forces between aqueous (nano)colloids. Results were obtained for functionalized polystyrene latex (PSL) particles 80--5000 nm in diameter. In solutions containing 1--100 muM concentrations of NaPSS, aggregates broke with forces ranging from 0.1--100 pN in magnitude.; Further optimization of PFLS has resulted from the modeling of a limitation that exists for both PFLS and VMDE. This limitation is the competition between Brownian motion and electrophoresis for the breaking of (nano)colloidal aggregates. Two separate (but related) Brownian dynamics simulations were developed. The first applies to VMDE where we study only one or two aggregates at a time. The results of these simulations indicate a lower size limit of O(1 mum) for VMDE and the experimental conditions listed above.; The second simulation applies to PFLS where we study 104--10 10 aggregates together using light scattering. Although the simulation yields reliable results based on our assumptions, it failed to capture all of the essential physics needed to appropriately describe the PFLS problem. (Abstract shortened by UMI.)...