Combined effects of electrophoresis and thermophoresis on particle deposition onto flat surfaces

Abstract

Particle deposition on a flat surface was studied experimentally and numerically under special consideration of thermal and electrostatic fields. In particular a new detection technique was developed to detect and count deposited submicron particles on a wafer. With this technique it was possible to perform deposition experiments with particles of diameter as small as 0.08μm, a size which is usually measured with a scanning electron microscope. The experimental study consisted of generating a monodisperse fluorescent polystyrene latex aerosol, transporting the aerosol through a vertical flow channel, depositing the monodisperse aerosol on a flat horizontal surface with a wafer in its centre, heated to a desired temperature and connected to an electrical potential. The deposited particles on the wafer were detected and counted with a fluorescent light microscope connected to a video camera and an image processing unit. Experimental data of particle deposition were obtained for a flat horizontal surface with a free stream velocity of 30 cm s −1. The temperature difference between the surface and the airflow was 0–10 K, the electrostatic field strength was 0–100 V cm −1 and the particle diameters ranged from 0.08 to 3 μm. For the first time, well-defined experiments were done which considered the combined effect of thermal and electrostatic fields on particle deposition. The experimental data were compared with the results of one dimensional numerical model calculations that took into account coupled deposition mechanisms due to convection, diffusion, sedimentation, thermophoresis and electrophoresis. The comparison showed good agreement between the measured particle deposition velocity and the numerical results.