| The unpredictable behavior of granular materials is one of the largest stumbling blocks on the way to satisfactory design and control of many manufacturing processes. There are many factors which contribute to the complex behavior of granular materials, however, the effect of electrostatic forces is both one of the least studied aspects of granular materials as well as one of the most important. Electrostatics can cause agglomeration of otherwise free flowing powders, the adhesion of grains to equipment surfaces, and particles of differing composition or size to segregate. In addition, electrostatic discharges can ignite dangerous dust explosions. With a better understanding of the effects of electrostatics, as well as the processes at work during the charging of particulate flows, some of these issues can begin to be addressed. To these ends, we examine the charge and agglomeration of grains and investigate how flows of particles charge, and in turn, how they are affected by those charges.;In this work, we show that the behavior of flows of uncharged grains through chutes can be approximated by simple models, but when charging takes place the behaviors change dramatically, replacing fluid-like flow instabilities with significant agglomeration, making the flow much more difficult to model. To understand and predict these transitions from relatively simple flow to a more complex/agglomerated state requires that the distribution of charge amongst the particles, as well as the effect of large electric fields, produced by either charged surfaces or particles, be investigated.;It was found that highly nonuniform distributions of charge could be generated by flowing particles. This distribution can result in significantly different forces acting on charged and uncharged grains and suggests that many of the problems commonly encountered during powder handling may be due to a small fraction of the particles.;Lastly, significant agglomeration was observed when particles were subjected to nonuniform electric fields. Nonuniform electric fields, which are produced as particles charge, cause polarization of the particles and attraction between grains. The resulting forces acting on the particles (referred to as dielectrophoretic forces) were large enough to produce both large agglomerates of powder, as well as adhesion to equipment surfaces. These forces may also be utilized to control particle behavior as this adhesion depends on the presence of an easily controlled external electric field. |
