The effect of relative humidity on deposition pattern in inertial impactors: The role of particle elasticity and surface attraction

Abstract

The role of relative humidity on particle bounce in an inertial impactor was investigated through a program of experiments and particle trajectory simulations. Inertial impactors are devices used to obtain particle size distributions by passing particle-laden air through a nozzle and collecting particles having sufficient inertia on a flat surface placed directly below the nozzle. Herein monodisperse hygroscopic particles impacted the flat hydrophobic surface of a single stage inertial impactor while varying the relative humidity of the flow. The results show that a circular deposition pattern occurs when the relative humidity is high. When the relative humidity is low secondary deposits beyond the circular deposit also occur. Particle trajectory simulations were performed where particle/surface interactions were quantified via the Hamaker constant (A) and the loss of kinetic energy via the coefficient of restitution (e). In a first-of-its-kind approach, (A, e) were iteratively adjusted until prominent features of the particle deposits in the experiments and simulations agreed, thereby providing values of (A,e) for each relative humidity. The results show that the observed deposition behavior is due to (i) the increase with relative humidity of the repulsion between the hygroscopic particles and the hydrophobic surface, (ii) multiple bounces along the particle trajectory, and (iii) kinetic energy loss at each particle bounce.