Capillary effects on a particle rolling on a plane surface in the presence of a thin liquid film

An experimental study has been performed of the effects of a liquid film on a particle rolling on a planar surface using a combination of laser-induced fluorescence and particle-image velocimetry. Contact angle hysteresis leads to asymmetry of the liquid meniscus, resulting in a difference in contact angle between the front and rear sections of the meniscus relative to the rolling particle. This asymmetry results in a capillary torque that resists the rolling motion of the particle. The particle rolling motion also induces a viscous transport of fluid from the front to the rear of the particle, which acts to shift the location of the contact point. The laser-induced fluorescence method is used to characterize the meniscus asymmetry and the resulting change in contact angle on the two sides of the particle. Particle-image velocimetry in various horizontal and vertical cross-sectional planes is used to examine the flow trajectories and velocity magnitude within the meniscus in the presence of rolling. All experiments are conducted at small capillary number, so that the meniscus is approximately circular in shape.