Air-coupled excitation of rocking motion of individual microspheres on surfaces

Abstract

In current study, it is demonstrated that the rocking motion of a single microsphere can be excited by means of impulsive acoustic fields generated by air-coupled transduction and the transient response of the particle can be detected optically. Such observation indicates the existence of rolling moment resistance of particle-surface adhesion bonds. A set of experiments were conducted for measuring the rocking resonance frequency of a microsphere on a flat substrate. The waveforms corresponding to the transient out-of-plane displacement of the rocking particle are used to extract the work of adhesion of the bond between the particle and substrate. Some of the frequency responses of the particles exhibited small shifts in their peak frequencies, which are away from their resonance frequencies, when compared to those of the base motion. These shifts are attributed to the viscoelastic damping effect due to capillary water meniscus formation in the adhesion bond of the particle-substrate system.