Breakup of Latex Doublets by Impaction

The breakup by impaction of a simple agglomerate consisting of two identical spherical particles has been studied. The agglomerates were doublets produced by nebulizing suspensions of latex particles. The doublet concentrations before and after impaction were determined from the particle size spectra obtained with an optical particle counter. Preliminary experiments showed that the latex doublets did not break up in the acceleration nozzle of the impactor. The doublet fraction (DF) is defined as the doublet/singlet concentration ratio after impaction to that before impaction. The DF of 2.99-μm Dow latex measured at √Stk = 1.62 decreased by about a factor of 2 as the relative humidity (RH) was varied from 8% to 85%. The remaining experiments were made at low RH with seven different surfactant-free suspensions of polystyrene latex stabilized by the addition of charged surface functional groups. All of the data, for particle sizes ranging from 1.62 to 4.07 μm and impact velocities from 10 to 80 m/s, could be fitted by the expression, DF = 18.2 v−0.58 D−1.60, where the impact velocity v is in m/s and the particle diameter D is in μm. The van der Waals adhesion energy is calculated to be less than 1/3000 of the kinetic energy needed to break up half of the doublets. The repulsive electrostatic energy of the surface functional groups is estimated to be about 1.25 times larger than the van der Waals adhesion energy. It is concluded that the doublets are probably bound by bridges of residue left after droplet evaporation.