AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Transmission of Charged Nanoparticles through an Adverse Axial Electric Field and Its Improvement
RUNLONG CAI, Jingkun Jiang, Tsinghua University, China
Abstract Number: 170 Working Group: Instrumentation and Methods
Abstract In aerosol studies, it is often encountered that charged nanoparticles or ions are transported against an adverse electric field. For instance, when classifying them according to their electrical mobility using a differential mobility analyzer (DMA), charged particles usually have to migrate against the adverse electric field in the sample outlet (or inlet in some cases). An analytical solution for the transmission efficiency of non-diffusive particles through the adverse electric field was proposed by Tammet (2015), which agrees well with experimentally measured transmission efficiencies. For sub-10 nm particles, diffusional losses are usually non-negligible when estimating the transmission efficiency through a tubing. In this study, we proposed an analytical model for estimating the transmission efficiency of charged nanoparticles (and ions) through an adverse axial electric field. Particle losses due to diffusion and electrostatic migration are both considered. Particle concentration profile at the cross-section of the tubing exit is calculated by solving the Graetz problem when neglecting particle electrical migration, then the effect electrostatic losses is coupled to this concentration profile using the method proposed by Tammet (2015). The transmission efficiency is shown to be a function of only two dimensionless parameters that characterize particle diffusion and electrostatic migration, respectively. The proposed analytical model was applied for an electrical mobility filter and the sample outlet of a differential mobility analyzer. The transmission efficiency estimated using the analytical model agrees well with those simulated using a Monte Carlo method and experimentally measured transmission efficiencies for sub-6 nm particles. For the tested conditions, the mean absolute difference between the transmission efficiencies estimated using the analytical model and the simulated efficiencies is 0.8%.
The transmission efficiency of nanoparticles through a miniature cylindrical DMA was improved using this analytical model. A DMA sample outlet made of electrostatic dissipative tubing was used to reduce the adverse electric field intensity. Although the flow field and particle diffusion in a real DMA sample outlet are usually complex, we found that the length of the electrostatic dissipative tubing corresponding to relatively high transmission efficiency in typical conditions can be practically determined using a rough criterion: the length is recommended to be longer than a threshold value such that the ratio of particle electrostatic migration velocity to average air flow velocity is smaller than 0.2. Compared to the Delrin insulator which was conventionally used at the DMA sample outlet, the new electrostatic dissipative sample outlet increased the penetration efficiency of ~1.5 nm ions by 30-50% in a typical operating condition.