An optical technique for measuring force between a colloidal particle and a flat surface

The behavior of colloidal particles in solution is greatly affected by the interaction the particles have with their surroundings. This may take the form of particle-particle interactions or the interaction of a single particle with a nearby solid wall. These interactions are characterized by the forces that are generated as a function of the separation distance between two surfaces. Fundamental phenomena such as particle deposition, solution rheology, and even microbial adhesion primarily depend on the magnitude and range of these fundamental forces as the particles move through the fluid.; Several experimental techniques can measure these small forces directly. However, there is no existing technique for measuring forces on particles having diameters on the order of 1 μm or less. This size range is especially important for studies of bacterial or viral adhesion mechanisms where the nominal diameter can be much smaller than 1 μm.; This dissertation describes a novel technique for measuring the static and dynamic forces that arise between a single colloidal particle and a flat plate. A single-beam gradient optical trap is used as a sensitive force transducer and evanescent wave light scattering is used to determine the particle position within the trap. The static force is measured by observing the equilibrium position of the particle within the trap, while the dynamic force is measured from the relaxation time of the particle fluctuations near the equilibrium position. Each force contribution is measured as a function of the particle-surface separation distance by moving the particle toward the surface in nanometer-sized increments. Absolute separation distances are determined by curve fitting the viscous force data to hydrodynamic theory in regions where the static force is negligible.; Measurements of static force agree well with classical Derjaguin-Landau-Verwey-Overbeek theory over the entire range of separation distances. Measured dynamic force agrees well with hydrodynamic theory until there is appreciable overlap of the electrical double layers at close separations. This departure may be due to a coupling of hydrodynamic and electrical phenomena that greatly enhances the viscous drag.