Direct measurement of colloidal interactions with holographic optical tweezers

This thesis describes experimental studies of colloidal interactions. After a brief introduction to some essential aspects of colloidal physics in Chapter 1, and to optical tweezers and digital video microscopy in Chapter 2, we present in Chapter 3 an analysis of the hydrodynamic interactions among colloidal particles. We propose a simple model for the hydrodynamic interactions among sufficiently separated colloidal particles, and prove its validity analyzing the vibrations of a one dimensional lattice of colloidal particles held by holographic optical tweezers. This model predicts a crossover from highly overdamped lattice vibrations, to propagating modes for systems having a sufficient number of particles.;In Chapter 4 we study the equilibrium structure of highly deionized colloidal dispersions sedimented on a bounding surface. The particles in these systems develop an identical charge, and interact with an effective electrostatic interaction predicted to be purely repulsive. Several previous experiments, however, report a long ranged attractive component in the effective potential, but only for sufficiently low ionic concentrations. We present evidence for a qualitative dependence of the effective interparticle forces on the bounding surface's chemistry, with the attraction disappearing altogether when the bounding surface is coated with a thin layer of gold. We interpret this as evidence of the influence that ionizable groups on the glass surface have on the interactions among sedimented colloidal particles.;We conclude by introducing in Chapter 5 a new method to measure directly the effective interaction among two colloidal particles. This method, based on the analysis of the diffusion of pairs of colloids along a line tweezer, is both local and fast, and it does not require a separate calibration for the line potential. It is strictly valid even in presence of light induced colloidal interactions, which have been a concern in previous experiments. We test the validity of our analysis using both simulations and experiments.