Effect of relative humidity on particle bounce in inertial impactors*

The role of relative humidity on particle bounce in an inertial impactor was investigated. Inertial impactors are devices used to obtain particle size distributions by passing particle-laden air through a nozzle and collecting particles on a flat surface placed directly below the nozzle. In this research monodisperse hygroscopic particles were impacted on a flat surface of a single stage inertial impactor while varying the relative humidity of the flow. The results show that a clear ring pattern occurs when the relative humidity is high. When the relative humidity is low secondary deposits beyond the ring occur. It was hypothesized that this behavior is due to the complex interactions between (i) the increase in repulsion between the hygroscopic particles and the hydrophobic surface with relative humidity, (ii) multiple bounces along the particle trajectory, and (iii) kinetic energy loss at each particle bounce. Particle trajectory simulations were performed where the effect of particle/surface interactions was quantified via the Hamaker constant (A) and the loss of kinetic energy via the coefficient of restitution (e). Values of (A, e) were found that forced the simulations to agree with the experimental results to a reasonable degree. The possible role of other effects is discussed including the Magnus effect and Saffman lift forces.