(90e) Simulation of Electrohydrodynamic Atomization for Enhanced Particle Collection Efficiency in An Encapsulation Chamber

Electrohydrodynamic atomization (EHDA) is a promising method for the fabrication of micro- and nano-sized particles with narrow size distribution and better morphologies than a few conventional methods of particle fabrication such as spray drying. Further enhancement in monodispersity and morphology of particles can also be achieved using an enclosed shuttle chamber with controlled solvent evaporation rate during the fabrication process. The objective of this study is to simulate the fluid and particle dynamics in the EHDA chamber, thereby providing a means of predicting collection efficiency prior to experimental verification.

The first part of the simulations focuses on identifying the effects of the charge-to-mass ratio of particles on their collection efficiency. This is necessary because usage of the chamber restricts measurements of the actual charge-to-mass ratio. In the second part of the study, the Poisson equation is solved for the prediction of electrical force due to charge repulsion in the simulation of particle trajectories. For particles moving in a fluid, buoyancy, gravitation and drag are the three major mechanical forces that influence their trajectories. Two dominant classes of entities contribute to the determination of electrical potential profiles in the EHDA chamber. The first type of objects is the stationary surfaces which are charged conductors. These comprise of the nozzle, ring and collection plate. The electrical potential due to such entities can be predicted by the Laplace equation with the appropriate boundary conditions. The second type of entities is the moving charged particles. With the addition of such entities, the Poisson equation is solved for the overall electrical potential profiles. All the forces acting on a particle are combined to determine its acceleration and trajectory based on the Lagrangian approach.

Since organic solvent has occupied more than 90% of volume of each droplet at the outlet of the EHDA nozzle, organic solvent is evaporated from the droplets during the flight process. This evaporation results in the shrinkage of the droplets during the flight, decreasing their weight and size and finally converting them into particles. Therefore, droplet shrinkage has a considerable effect on droplet motion and collection in the chamber. A set of dimensionless partial differential equations are solved to investigate the effect of the evaporation of a liquid droplet accompanying a nitrogen stream in the EHDA process. A better geometry for EHDA shuttle chamber with enhanced particle collection efficiency can be designed using the results of this simulation study.

Key words: Electrohydrodynamic atomization; particle collection efficiency; particle trajectory; simulation.