Transient bipolar charging of a coagulating nanometer aerosol

The transient charging process of a nanometer aerosol undergoing Brownian coagulation is studied theoretically and experimentally. The numerical solution of the population balance equations, which include charging and coagulation terms but not diffusion losses, is in reasonable agreement with the experimental results. Due to coagulation, for each particle size there exists an optimum residence time at which the output concentration of charged particles is a maximum. The optimum residence time decreases as the particle size decreases, an obvious consequence of Brownian coagulation. Furthermore, the experimentally measured optimum residence times for the smallest particles are about one order of magnitude lower than the time required to attain the charging equilibrium state, so that the maximum attainable concentration of charged nanometer particles is considerably smaller than theoretically expected. These results are important to the design of equipment aimed to generate or measure nanometer particles in concentrations as high as possible.