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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 ± 0.6 μm 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. [ABSTRACT FROM AUTHOR]

Toner microparticles are coated with silica nanoparticles for modulating their adhesiveness to optimize two competing requirements in the electrophotographic process: low adhesion for facilitating particle transport and high adhesion with deposited substrates prior to high temperature fusing. In addition to the adhesion effect of nanoparticle coating, nanoparticle-coated toner microparticles are subjected to various thermal fields during utilization. Understanding the effect of temperature on particle adhesion properties has the potential to improve energy efficiency, process predictability and print resolution. In current work, the effect of surface temperature on the adhesion bond between single nanoparticle-coated toner microparticles and a silicon substrate is investigated in a non-contact/non-invasive manner by monitoring the rocking dynamics of acoustically excited single microparticles on a silicon wafer surface. In the reported spectral experiments, at a range of substrate temperature levels from 15 to 30 °C, the work-of-adhesion values of the surface-particle bond are evaluated from the acquired rocking resonance frequencies of the single toner microparticles. Rayleigh Surface Acoustic Waves (SAW) are employed to generate highly controllable surface motion to excite the vibrational modes of deposited microparticles, through which the out-of-plane transient responses of their apexes are acquired using a laser vibrometer. In addition to surface temperature, the effect of coating nanoparticle spatial distribution on the adhesion properties of the microparticle is observed and explained by proposing possible contact configurations, depending upon how silica nanoparticles are accumulated in and around the particle-surface contact zone. Finally, it is observed and reported that the relative humidity (RH) can play a significant role in microparticles rocking motion dynamics and adhesion. [ABSTRACT FROM AUTHOR]

Electrostatic charge significantly affects the adhesion of microparticles. Presently the nature of surface charge distribution on a single microparticle and the effect of electrostatic charge as a contributor to its adhesion are not well understood. As a result of the strong discrepancies between the experimental adhesion measurements data and theoretical predictions over the years, some questions regarding adhesion force contributors in charged microparticles still remain unresolved. One reason for the current state of knowledge is difficulties associated with accurate measurement of particle charge. In current work, a non-contact and non-destructive opto-ultrasonic method is presented and employed to predict the equivalent bulk charge on a single charged micro-scale toner particle and to provide insight into the nature of charge distribution on its surface. From the vibrational spectral response of the particle to the ultrasonic excitation of the substrate, irregular shifting patterns of the vibrational (in-plane rocking) resonance frequencies of the particle are observed for the applied levels of substrate surface voltage (0–1500 V), implying non-uniform (patch-charges) surface charge distribution on the microparticle. For predicting the equivalent bulk charge on a single patch-charged toner particle from this resonance frequency shift, a mathematical model is developed and employed. In conclusion, it is demonstrated that, in a non-invasive/non-contact manner, the total charge on a single microparticle can be predicted using the presented experimental approach, and evidence for the non-uniform charge distribution on a single particle is observed and reported. [ABSTRACT FROM AUTHOR]