Particle resuspension from surfaces

Abstract

A series of experiments were performed in order to investigate how fine particles are resuspended from a surface once they are adhered to the surface through van der Waals interactions. Model spherical particles were generated, deposited on well- characterized substrates, and exposed to various types of forces. Impulsive forces were applied to the particles using impacting particles and impinging shock waves, and steady shearing forces were applied by impinging gas jets and a laminar channel flow. In each case, the threshold conditions to resuspension were determined by optically monitoring particle removal from the substrate. Particles of various size and material properties were considered. Resuspension thresholds were reproducible and unambiguous in the model experiments. The threshold to resuspension of the model spheres exposed to impinging gas jets was evidenced by abrupt changes in removal efficiency at specific locations. An analytical method to solve the flow field produced by an impinging jet was developed in order to calculate the shear stress experienced by the particles at the threshold locations. An inviscid model of normal jet impingement provided the free stream conditions for the ensuing boundary layer analysis. Calculated shear stress distributions using the model agree with previous measurements. Theoretical descriptions of the resuspension mechanisms were inferred from the observed resuspension thresholds. Such an undertaking involves the combination of existing equilibrium adhesion models and the kinetics of aerodynamic or particle collisional interactions. Resuspension models based on equilibrium adhesion theories did not accurately describe the observed size dependence of threshold applied shear stress, or the effect on resuspension of force duration. A kinetic model was developed to qualitatively describe these observed trends. The particle removal techniques were applied to a study of particle sampling from explosive fingerprint transfer deposits and fingerprint simulations. A crude sampling system consisting of an impinging gas jet and a suction tube/filter assembly was calibrated with model composite spheres made from a mixture of polystyrene and 2,4,6-trinitrotoluene. Observed removal and collection efficiencies were related to the results of the model experiments. It was found that calibrated sampling systems could be used effectively to test the accuracy of fingerprint simulations.