Effects of temporal flow acceleration on the detachment of microparticles from surfaces

The detachment of microparticles from surfaces subjected to various temporal accelerations of air was studied experimentally. Stainless microspheres, approximately 70 μ m in diameter, deposited as a sparse monolayer on smooth glass substrates, were subjected to temporal accelerations ranging from 0.01 to 2.0 m/s 2. Higher accelerations up to 23 m/s 2 were investigated without particles on the surface to further characterize the temporally evolving flow. Microvideographic images of the particles on the surfaces were acquired. These were used to determine the detachment fraction of the particles versus the free-stream flow velocity at different temporal flow accelerations. For the relatively slower flow accelerations, approximately 0.3 m/s 2 or less, microparticle detachment was independent of acceleration to within the experimental uncertainty. For the relatively more rapid accelerations, from approximately 0.3 m/s 2 to at least 2.0 m/s 2, the flow velocity required to detach one half of the microparticles increased with acceleration. Near-wall velocity measurements supported that rapid temporal acceleration delayed the onset of turbulence, thereby affecting the boundary layer characteristics, causing a decline of turbulent bursting in the wall-layer, and, hence, suppressing the detachment process.