THRESHOLD FOR RESUSPENSION BY PARTICLE IMPACTION

The threshold velocity for the resuspension of 8.6-mum diameter ammonium fluorescein particles from a Tedlar surface by impaction of 3-mum ammonium fluorescein particles has been measured to be 9.3 +/- 4.3 m / s. The resuspension rate, observed with an optical particle counter, has a wide variability. Image analysis of the particle patterns on the surface suggests that particle resuspension takes place at a radius somewhat smaller than that of the nozzle of the impactor used in the experiments. Exposure of some test surfaces to laboratory air for several days prior to the measurements did not change the results significantly. A theoretical model has been shown to predict a threshold velocity within a factor of 2 of the experiment. The major uncertainty involves the value of the surface energy parameter, a difficulty encountered previously in a study of particle bounce. The model is based on the transfer of vertical momentum to the struck particle which is subsequently compressed against the surface. Elastic rebound then supplies the lift-off force to overcome the surface adhesion force. An alternative calculation based on inceptive rotational motion induced by off-center impacts predicts a threshold almost two orders of magnitude lower than observed.