Optical tweezer based study of the motion of a sphere in an oscillatory boundary layer

Drag forces on a single polystyrene sphere in the vicinity of an oscillatory plate have been measured using an optical tweezer. The phase of the sphere is found to be a sensitive probe of the dynamics. The evolution of the phase from an inertia-coupled regime to a velocity-coupled regime is explored. The frequency dependence of the response is found to be characteristic of a damped oscillator with an effective inertia which is orders of magnitude greater than that of the bare sphere.