Measurement and dynamic simulation of particle trajectories in an electrodynamic balance: Characterization of particle drag force coefficient/mass ratios

Abstract

Complementary measurement and simulation methods are described that enable rapid characterization of drag force coefficient/mass (Cd/m) ratios of individual particles in the 20 to 200 μm size range. Individual particles are suspended in a charge trap known as an electro-dynamic balance (EDB). A step change is applied to the EDB end cap voltage to stimulate a dynamic response of the particle from an initial steady state. The resulting transient response is measured by means of a high speed, two-dimensional diode array and imaging system which provides an analog output indicating particle position along the EDB center axis at frame rates of 6200 per second. A particle dynamic model (PDM) is developed to simulate trajectories of particles in the EDB. The PDM is a force balance simulation which accounts for field forces, gravitational forces, and drag forces acting on a particle. Simulations are performed using particle Cd/m ratio as a fit parameter to match model outputs with measurements. Data are presented for polystyrene and glass spheres showing that Cd/m ratios can be determined with an uncertainty less than ±5 percent. When applied to characterize spheres, particle densities, diameters, and mass can be accurately determined. The methods described are an integral part of a powerful new system that enables detailed characterization of mass, density, external surface area, and volume of irregular shaped particles in the 20 to 200 μm size range.