  The motion of submerged magnetic microspheres rolling at a glass-water
interface has been studied using magnetic rotation and optical tweezers
combined with bright-field microscopy particle tracking techniques. Individual
microspheres of varying surface roughness were magnetically rotated both in and
out of an optical trap to induce rolling, along either plain glass cover slides
or glass cover slides functionalized with polyethylene glycol. It has been
observed that the manipulated microspheres exhibited nonlinear dynamic
rolling-while-slipping motion characterized by two motional regimes: At low
rotational frequencies, the speed of microspheres free-rolling along the
surface increased proportionately with magnetic rotation rate; however, a
further increase in the rotation frequency beyond a certain threshold revealed
a sharp transition to a motion in which the microspheres slipped with respect
to the external magnetic field resulting in decreased rolling speeds. The
effects of surface-microsphere interactions on the position of this threshold
frequency are posed and investigated. Similar experiments with microspheres
rolling while slipping in an optical trap showed congruent results.
