Electrostatic Charge Limits for Small Particles

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Objectives To clarify some limitations involved in the electrostatic charging of small solid particles and liquid droplets commonly used in industrial applications. Statement of Problem Electrostatic forces are widely used in many industrial applications that involve the movement and deposition of small particles. The particles are typically in the size range from submicron to the order of one hundred microns. The effectiveness of these forces is directly dependent upon the magnitude of charge that can be applied to the particles. Since electrostatic charge resides on the surface, it is commonly assumed that the total charge is proportional to surface area giving rise to the prediction that the charge to mass ratio (Q/M) is proportional to 1/r where r is radius. However common practice has shown that there are a number of situations where this is not true. In particular, non spherical solid particles cannot simply be assumed to be equivalent to those of a spherical particle of the same effective area. Particle shape and surface roughness can dramatically affect the charge. In the case of liquid droplets, although the particles are usually spherical, the values of Q/M have been reported to be inversely proportional to radius to the power x, where x may range from 1 to 2. Expected Significance It is believed that this work will result in a clearer understanding of the effect of shape, size and surface roughness of solid particles used in various industrial processes enabling optimization of the deposition and adhesion forces. In the case of liquid droplets, the study of the effect of the mode of droplet formation and the difference between the induction and corona charging cases should lead to further understanding of applications where both are present such as in a commercial liquid paint spraying system.