Motion of a sphere in an oscillatory boundary layer: an optical tweezer based study

TL;DR

This study uses optical tweezers to measure drag forces on a sphere near an oscillating boundary, revealing phase dynamics and a frequency response akin to a damped oscillator with effective inertia.

Contribution

It introduces a novel optical tweezer method to analyze sphere motion in oscillatory boundary layers, highlighting phase behavior and inertial effects.

Findings

Phase is a sensitive indicator of sphere dynamics

Transition from inertial to velocity coupling observed

Frequency response resembles a damped oscillator with large effective inertia

Abstract

The drag forces acting on a single polystyrene sphere in the vicinity of an oscillating glass plate have been measured using an optical tweezer. The phase of the sphere is found to be a sensitive probe of the dynamics of the sphere. The evolution of the phase from an inertially-coupled regime to a purely velocity-coupled regime is explored. Moreover, the frequency dependent response is found to be characteristic of a damped oscillator with an effective inertia which is several orders of magnitude greater than that of the particle.