The effects of electrostatic forces on the thermophoretic suppression of particle diffusional deposition onto hot surfaces

Abstract

By raising the temperature of a surface above that of the ambient fluid, a temperature gradient that results in a thermophoretic force that repels particles from the surface is created. It has been recommended that this effect be used to reduce the contamination of product surfaces during manufacturing. It has been shown that even modest levels of electrostatic interactions between particles and surfaces can overwhelm the thermophoretic repulsion available by raising the temperature of the surface 10% above that of the ambient fluid. We present an analysis for axisymmetric, viscous stagnation-point flow, extending the work of Friedlander et al. (1988, J. Colloid Interface Sci.125, 351), and show the temperature differences that must be achieved to succeed in preventing particle deposition due to diffusion, gravitation, and electrostatic attraction. Simple algebraic relationships are established that allow for a ready determination of the temperature differences needed to suppress particle diffusional deposition onto surfaces under the action of uniform external forces, such as gravitational and coulombic forces, and a spatially dependent external force, which for this study we have chosen to be the electrostatic image force. These relationships are further verified through approximate analytical and "exact" numerical solutions also given here. Various results of practical interest are discussed. For example, for a flow intensity of 6.67 s-1 in air, an electric field as small as 100 V/cm makes it unlikely that particles with a minimal charge distribution (Boltzmann) will be kept from deposition on a surface by raising the surface temperature 10 or 20% above the ambient temperature. This emphasizes the importance of reducing the electric charge levels of product surfaces during manufacturing. © 1990.