Controlled Wind Tunnel Experiments for Particle Bounceoff and Resuspension

The dynamics of particle rebound and resuspension were examined by using uranine particles, polymer microspheres, spores, and pollen in wind tunnel experiments. Particle diameters were 5–42 μm. Results show that both the fraction of rebound and the resuspension rate are strongly dependent on the free stream velocity, particle size, and relative humidity. The effects of relative humidity are more significant at lower windspeeds; a greater relative humidity appears to change the shape of the distribution of adhesion force, mainly affecting the lower range but not greatly affecting the upper end of the distribution. Resuspension rates decrease with time, essentially defining two regimes. The first regime lasts for < 1 min; after this time, the most easily resuspended particles have been removed, leaving only particles with much smaller resuspension rates for the second regime. At a windspeed of 6 m / s, the upper 20% of the distribution of turbulent fluctuations is responsible for ～ 65% of the particle resuspension. Once resuspended, the particles have trajectories which depend on characteristics of the turbulent airflow and not on the initial velocity of release from the surface. Overall, the data show that resuspension is more sensitive to the type of particle than to the type of surface; particle shape and composition may be more important than particle size.