Slipping friction of an optically and magnetically manipulated microsphere rolling at a glass-water interface

TL;DR

This study investigates the nonlinear slipping and rolling behavior of magnetic microspheres at a glass-water interface, revealing a transition from rolling to slipping at a critical magnetic rotation frequency.

Contribution

It introduces combined magnetic and optical manipulation techniques to analyze microsphere rolling dynamics and identifies the threshold for slipping behavior.

Findings

Microspheres exhibit nonlinear rolling-while-slipping motion.

A sharp transition from rolling to slipping occurs at a specific rotation frequency.

Surface interactions influence the slipping threshold.

Abstract

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.