Aerosol classification by dielectrophoresis: a theoretical study on spherical particles

Abstract

The possibilities and limitations using dielectrophoresis (DEP) for the dry classification of spherical aerosol particles was evaluated at low concentrations in a theoretical study. For an instrument with the geometry of concentric cylinders (similar to cylindrical DMA), the dependencies of target particle diameter d * P , resolution, and yield of the DEP classification on residence time, applied electric field strength, and pressure of the carrier gas were investigated. Further, the diffusion influence on the classification was considered. It was found that d * P scales with the mean gas flow velocity u gas , classifier length L, and electric field strength E as d * P ∝ (u gas /L) 0.5 E −1. The resolution of the classification depends on the particle diameter and scales proportionally to d * P 1.3. It is constrained by the flow ratio β (i.e., sheath gas to aerosol flow), electrode diameters, and applied electric field strength. The classification yield increases with the ratio of the width of the extended outlet slit s e to the diffusion induced broadening σ z. As expected, resolution and yield exhibit opposite dependencies on s e /σ z. Our simulations show that DEP classification can principally cover a highly interesting particle size range from 100 nm to 10 µm while being directly particle size-selective and particle charge independent. The dry classification of particles with diameters below 10 µm is key to obtain high-value powders with narrow size distributions. These fine particles find application in a variety of high-value products such as coatings 1 , printing products 2 , porous functional structures 3 , particle-reinforced polymers 4 and electrodes in electro-chemical energy storage 5. Many established classification technologies suffer from insufficient selectivity or throughput in this size range. For instance sieving, which is a common tool in industrial separation, comes only close to this size range when performed as wet technique 6. When starting from a dry powder, wet sieving could lead to agglomeration and material property changes and would additionally require an uneconomical drying step following the classification. Deflection wheel classifiers exhibit a significant loss in sharpness of cut and throughput in this particle size range 7. Electrophoretic mobility classification relies on the defined charging of the particles, and thus particle-size-dependent charge probability distributions. Hence, particle size is not directly measured but merely the electrophoretic mobility distribution of the particles. A standard inversion procedure exists to back-calculate size distribution from electrophoretic mobility measurements. Nevertheless, exact particle size classification is not possible with this technique because particles of different size may have the same electrophoretic mobility due to their charge. A comprehensive survey concerning charging of airborne particles is given for example by Friedlander 8 , chapter 2. In this manuscript we outline the behaviour of a particle-charge-independent...

A review of dielectrophoretic separation and classification of non‐biological particles

“…(B) Sketch of a particle size classifier based on DEP as envisioned by our group. After prior calibration, target particles can be selected by tuning applied field strength and will exit through the sample outlet (solid trajectory), nontarget particles will impact the inner electrode above or below the outlet slit [156].…”

“…Motivated by the Baron separator, we theoretically investigated the possibility to classify spherical airborne particles smaller than 10 µm using DEP [156]. Such fine particles find application in coatings, printing products, particle‐reinforced polymers, or porous functional structures.…”