Effects of electrostatics on granular dynamics

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Powders and grains are used by the thousands of tons to process polymers, catalysts, pharmaceuticals, and building materials. However, when powders and grains are electrically charged, they stick to surfaces and collect into aggregates. This leads to severe industrial problems, for example, producing large variations in the active ingredient concentration in pharmaceuticals. Likewise, sticking of thick, tenacious layers of polymers within processing equipment can force manufacturers to shut down operations and jack-hammer the stuck layers from the equipment walls. On the positive side, some technologies such as electrospraying and photocopying rely on the sticking of charged particles. Despite the profound problems and significant opportunities associated with granular charging and sticking, the topic is remarkably poorly understood. In previous work, the principal investigator has shown that particle aggregates are affected by patterns of charge, rather than total charge, on granular surfaces. The research in this award will develop new methods to measure charge patterns that correlate to sticking behaviors. The research team will perform both computer simulations and laboratory tests to confirm the connection between these new measurements and the formation of electrostatic sticking behaviors. New methods to control sticking in industrial operations will also be investigated.The research will consist of three specific aims. First, investigators will develop and validate a method of measuring dipole moments of free particles in microgravity experiments designed to evaluate electrostatic aggregation of free particles in vacuum and under microgravity -- i.e. under nearly ideal conditions. These experiments are being performed by collaborators through separate funding. Second, the experiments produce aggregates of 50 micron particles whose individual charges can be individually evaluated: the principal investigator and his team will use this information as input to simulations of forces acting on the aggregates to establish the magnitudes of net charge and dipole moments on each particle required to hold the aggregates together. The outcome at this point will be direct, computer-guided experimental evaluations of relative strengths of dipole and net charge influences on aggregate cohesion. Third, the researchers will perform independent experiments to evaluate the net charge and dipole moment on individual particles involved in controlled collisions. This will permit them to both establish how large dipole components are likely to become as a result of collisional dynamics, and to determine whether these magnitudes agree with the microgravity experiments. Having completed these aims, the investigators will for the first time have access to experimentally validated methods for obtaining dipole moments of free particles. The researchers will have applied the methods to the description of purely electrostatic forces acting both on aggregates ("aggregation"), and between aggregates and surfaces ("sticking"). This analysis will permit the reliable evaluation of the influence of electrostatics on sticking in a unique set of experiments and simulations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.