Hydrodynamic interactions couple the Brownian motion of all colloidal particles, yet the fundamental equations of hydrodynamics defy exact solution for all but the simplest colloidal systems. We measure Brownian motion in a variety of few-body colloidal systems using blinking optical tweezers and interpret our data in light of the predictions of an approximate theory, Stokeslet analysis. Stokeslet analysis describes the weak hydrodynamic coupling of arbitrary numbers of colloidal spheres in free and confined geometries. This theory assumes that the separations between the spheres are much greater than their radii and models their hydrodynamic interactions as those between point-like particles. We find that Stokeslet analysis' predictions are consistent with our measurements, and that it accounts for many-body interactions ignored by the linear superposition of hydrodynamic drag forces.; While Stokeslet analysis can be scaled to analyze larger many-body systems, conventional optical tweezer techniques cannot manipulate more than a few particles. Therefore, we have invented a simple, robust and inexpensive technique to create multiple optical traps from a single laser beam using computer generated holograms. These holographic optical tweezer arrays can trap thousands of particles in arbitrary configurations. In addition to enabling direct measurements of many-body interactions, holographic optical tweezer arrays can be applied to study structural phase transitions in colloidal systems, to assemble microscopically textured materials, and to sort and manipulate biological materials.
展开▼
