Current amplification in an electrostatic trap by soot dendrite growth and fragmentation: Application to soot sensors

Abstract This paper examines the surprisingly large electrical current, nanoamps instead of picoamps, observed when sampling soot aerosol through an electrostatic trap. Aside from its fundamental interest, this phenomenon has potential practical application to motor vehicle on-board diagnostics of particulate matter emissions. The effect originates from the collection of electrically charged particles naturally present in flame produced soot onto the opposing polarity electrodes, but the measured current is ~10 3 times larger than carried by the soot. This amplified current is proportional to the flux of soot particles in steady state, but voltage and flow perturbations substantially magnify the effect. The amplification is not immediate; rather it follows a quiescent period, inversely proportional to soot concentration, which can last hundreds of seconds before climbing rapidly to tens of nanoamps. Concomitantly, large highly charged particles appear at the trap exit, of order 10 μm and carrying tens to hundreds of charges. In situ microscopy reveals that this coincides with the growth of dendritic structures aligned with the electric field, which collapse when it is turned off. As charged soot particles flow through the trap these structures exhibit vigorous apparent motion and detachment from one electrode and deposition onto the other. We present a kinetic model to explain this trap behavior. It is based on the assumption that the dendrites grow to a critical height at which the electric field force exceeds the binding force and the dendrites fragment. These highly charged fragments then deposit on the opposite electrode propagating a chain reaction of dendrite growth, fragmentation, and charge transport that produces the observed current amplification.