Pound for pound, common wheat flour contains nearly twice the explosive power of TNT. To harness this power requires only that flour particles be suspended and exposed to a spark. Consequently, industries expend significant efforts to control static charging in powders. Despite these precautions, static discharges regularly cause fatal dust explosions. Less dramatically, electrostatic charging of particles is widely applied industrially, for example in conventional printing5, electrocoating6, and filtration, as well as in more modern applications such as crystal self-assembly. Electrification is ubiquitous in natural granular systems as well, where it influences Aeolian sand transport9 and geological sedimentation patterning. Surprisingly, after centuries of study, some of the most basic properties of electrification of solids defy explanation. As one brief example, anyone who has received a shock after scuffing their shoes on a nylon rug in wintertime can attest that insulators, which lack free charge carriers, transfer charge more readily than conductors. This effect has been extensively studied and proven to be independent of the ability of conductors to carry charges to ground.
Intellectual Merit: Nowhere is the importance of granular electrification more pressing than in pharmaceutical development. Probably the best known example of this is in the aerosol delivery of pharmaceuticals, used both for pulmonary therapeutics (as in asthma) and to deliver drugs that either will not survive in the digestive tract (e.g. insulin) or are otherwise unsuitable for other dosage forms (e.g. agents with low water solubility, or macromolecules that are too large to diffuse transdermally). Aerosolization depends critically on control of particle charging to prevent aggregation and to promote delivery to the deep lung. Less widely known, it has recently been demonstrated that many of the flow and mixing complications that plague granular processing are directly attributable to electrostatic charging. Such complications include spontaneous segregation of mechanically indistinguishable materials, and aggregation of grains into complex clusters that can alternatively impede or augment a granular flow depending on subtle experimental details. Until recently, no major research program has dealt with these basic, yet practically important, problems: industrially, they are addressed on a trouble-shooting basis where little is learned for the long term, and fundamentally, their root causes are ignored. The objective of this proposal is to advance the understanding of granular behaviors in the presence of static charging. A major part of this work will be to produce experimentally validated computational tools. This will improve existing industrial granular processes that are notorious for poorly understood mixing and flow difficulties, and will produce new insights into very basic, yet deceptively complex, granular behaviors. The methods that will be used to achieve this objective combine focused experiments in generally applicable geometries side-by-side with the development of direct computational simulations of flow, mixing, segregation, and aggregation of charged grains.
Broader Impacts: One does not need to look hard to find broad implications of this research. On the industrial side, the implications are clearcut: in the pharmaceutical industry, beyond aerosol applications, entire production plants and product lines are not infrequently shut down due to failures to control granular flow and mixing, and better understanding of granular electrostatics will improve the reliability of these processes. Further afield, there is a longstanding, unresolved paradox in geophysics that grains in sandstorms generate large charge gradients although they have little beyond other sand grains to rub against. Despite both field and laboratory investigations, the sources of these gradients remain elusive as they depend on poorly understood dynamics between charged particles. Mechanisms have also recently been proposed to link granular charging to Martian landforms. Granular electrostatics also will impact new technologies, including nanocomposites under development for applications such as high energy Lithium batteries, and novel semiconductor devices. These applications involve the preparation, processing and mixing of grains with highly charged crystal habits, and will benefit from the improved understanding of both the basic and the applied sciences of charged particle behaviors that this proposal will bring. The research in this proposal will be integrated with educational and outreach initiatives including graduate, undergraduate and high school research training in particle technology. In addition, the Rutgers PI's will continue to target the recruitment of women and minorities.
Pound for pound, common wheat flour contains twice the explosive power of TNT. To harness this power requires only that flour particles be suspended in air and exposed to a spark. Consequently, industries expend significant efforts to control static charging in powders. Despite these precautions, static discharges regularly cause fatal dust explosions. Less dramatically, electrostatic charging of particles is widely applied industrially, for example in printing, coating, and filtration. Electrification is important in natural systems as well, where it influences dust storms and sedimentation. Surprisingly, despite centuries of study, some of the most basic properties of electrification of solids defy explanation. As one brief example, anyone who has received a shock after scuffing their shoes on a nylon rug in wintertime can attest that insulators, which lack free charge carriers, transfer charge more readily than conductors. This effect has been extensively studied and proven to be independent of the ability of conductors to carry charges to ground. Until recently, no major research program has dealt with these basic, yet practically important, problems: industrially, they are addressed on a trouble-shooting basis where little is learned for the long term, and fundamentally, their root causes are ignored. In this work, we have investigated the root causes of granular charging, and we have revealed new and unanticipated behaviors associated with these causes. Specifically, 1) We have demonstrated for the first time that absolutely identical materials, rubbed completely symmetrically, develop strong charges, and so existing models for contact charging are needed. 2) We have provided a new theoretical framework for the generation of charge on powders and grains (including identical materials as described in 1, above), and we have confirmed through experiments and simulations that this framework does successfully predict the quantity of charge imparted on these materials. 3) We have shown that static charge on powders can produce greater difficulties in powder processing (e.g. during the manufacture of pharmaceuticals) than other effects, and so we have confirmed that the use of commercial static eliminators can improve powder flow more than traditional additives. 4) We have discovered that when powdered materials slip (as occurs during industrial processing as well as in geological events such as earthquakes and landslides) they produce strong and reproducible voltage signals. Moreover, these signals can precede the slip event. This final result suggests that future technologies may be able to both predict future slip events and detect incipient failures in powdered materials. If in the future this should prove to be the case, the prediction of slip events would be of clear importance for earthquake forecasting, and the detection of failures could be significant for the testing of powder-based materials such as high precision ceramics.
