Microparticle detachment from surfaces exposed to turbulent air flow: controlled experiments and modeling

This work presents the results of experiments conducted to characterize the detachment of microparticles from surfaces exposed to turbulent air during accelerated free-stream flow. Smooth glass plates used as substrates are scanned with an atomic force microscope to determine their roughness-height distributions. Microparticles of different sizes, materials and shapes (mostly microspheres) are deposited as sparse monolayers onto the substrates under controlled clean and dry conditions. The microparticles attach to the substrate in a condition of static equilibrium due to adhesion and reside completely within the viscous sublayer as the flow is accelerated. Microvideographic observations of individual microparticle detachment show that detachment occurs primarily as rolling motion along the surface and not as lift-off. Detachment is not necessarily followed by entrainment in the flow. Results are presented as detachment fractions as function of time. The experimental results reveal that detachment is governed by a balance of the moments of aerodynamic drag and rough-surface pull-off forces. This is substantiated using a recently developed attachment theory that takes into account surface roughness to determine the pull-off force of microparticles. The sensitivity of the free-stream threshold velocity for detachment to five factors contained in the experiments and the model is analyzed. Results indicate that the surface energy of adhesion and the microsphere radius have the most influence on the threshold velocity for detachment.