Pre-rolling leaning of microparticles

At nano/micro-scales, the resistance of a particle to rolling is a critical factor in many applications and biological phenomena as it affects particle adhesion, motion and flow as well as particle manipulation (e.g. picking and placing of microparticles). In present work, a non-contact non-invasive experimental method is detailed and applied to determine the pre-rolling critical leaning angle (CLA) of single microparticles. Transient Rayleigh surface acoustic waves (SAW) are utilized as the excitation mechanism and interferometry and image processing as detection/monitoring and analysis techniques for capturing the micro/nano-scale dynamics of the microparticles. The CLA values for a set of 30 PSL (polystyrene latex) microparticles with a diameter of 14.9±0.6μm on a soda-lime glass substrate are reported. The CLA of the studied particles are determined to be between 0.9 and 7.8°. It is also observed that during a SAW field burst cycle, microparticles could change their drifting (rolling and/or sliding) directions, speeds and accelerations. The trajectory of a particle is often found to be non-linear. This nonlinear behavior is attributed to the inhomogeneity of the surface properties of the particles and the substrate as well as possible electric charge density, chemistry variations and potential contamination on surfaces. Moreover, the effect of electric charge (developed due to the triboelectric effect) on the particle drifting motion is investigated. It is found that the percentage of drifting particles is not only a function of the amplitude of the excitation field but also a function of possible electrostatic charges developed on the particles due to the drifting motion on the substrate. In addition to their potential uses in particle manipulation, removal and adhesion characterization, the reported results could be utilized in numerical simulations of microparticle motion and deposition. [Display omitted] •An experimental technique for determining the critical leaning angle of a microparticle.•Currently there exists no analytical expression for predicting critical leaning angles.•The presented measurement approach is based on surface acoustic wave excitation.•The key advantage of the current approach is its non-contact and non-invasive nature.