Mechanistic Investigation of Bipolar charging in granular materials: Experiments and multi-scale models.
Raj Mukherjeea, Vipul Guptaa , Prasad Perib , Vinit Sharmac, Bodhisattwa Chaudhuri a,c
aDepartment of Pharmaceutical Sciences, University of Connecticut, Storrs, CT, 06269, USA.
bMerck & Co., Inc, West Point, PA, 19486,USA.
cInstitute of Material Sciences (IMS), University of Connecticut, Storrs, CT, 06269, USA.
Tribocharging attributes to the mechanism of static electrification between two dissimilar material surfaces when brought into contact and separated, triggering multiple impediments like segregation, agglomeration, material loss and severe explosions. Bipolar charging is a special case of triboelectrification observed between the surfaces of the same material, acquiring opposite polarities based on particle size differences. The intricacies and complications of the polarity induced by bipolar charging is often suppressed by the dominant particle wall interactions in the premise of the net final charge of the system, undermining the charge mitigation approaches in process manufacturing and incapacitating the complete alleviation of its enmeshed disadvantages. Therefore, a thorough study has been undertaken to understand the effects of various material properties (work function and particle size) and operating conditions on tribocharging and procurement of opposite charges in similar materials. Surface characterization for two different sizes of lactose and MCC were done by multiple analytical techniques including, X-ray photoelectron spectroscopy, Energy dispersive X-ray spectroscopy, Interferometer-profilometer studies, Scanning electron microscopy, Dynamic vapor sorption studies and Thermo-gravimetric analysis. Analytical methods confirmed the differences in surface roughness, surface impurity and surface moisture contents between different particle sizes of the same material. The MCC particles showed differences in moisture content at different RH conditions, however the same was not evident in Lactose monohydrate. Humidity controlled hopper-chute experiments were performed to measure the specific charge of two excipients against both metal (Aluminum) and insulator (PVC) at different concentrations. In both MCC & Lactose, irrespective of particle size differences, the net specific charges of the systems were positive or negative based on the interacting chute wall material (PVC or Al). The effects of bi-polarity in similar materials were further studied with glass beads in a specialized assembly made up of glass, using the concept of Brazil nut effect to segregate particles based on size differences under controlled vibration. The trends obtained from the collected samples indicated differences in polarity between two different sized glass beads. A multi-scale model was developed using quantum (DFT) models and bulk-scale (DEM) models to study the electrostatics behavior in the granular media. Semi-empirical methods (MOPAC) and DFT were used to calculate the work functions of Al, PVC, Glass, lactose and MCC and to determine the changes in work function due to moisture adsorption. In-silico bulk scale models, to predict the particle dynamics and charge transfer (particle-particle and particle-wall) in the hopper-chute systems were established by Discrete Element Modeling (DEM). The computational model validated the magnitude of frequency and amplitude implemented to obtain the Brazil nut effect for the specialized assembly in the experimental set up and confirmed specific regimes of segregation and mixing, observed during the experiments. Depending on the moisture content per unit surface area, particle of the same material was found to acquire positive or negative charges during contact electrification. The charge profiles of lactose, MCC and the glass beads obtained from DEM calculations were similar to the experiments and followed the work function differentials obtained from DFT and semi-empirical calculations. DEM calculations tracked the trajectory and charge profiles of individual particles and confirmed bipolar charging in MCC binary systems in hopper-chutes and the glass beads in the specialized assembly; small particles charged negatively and large particles charged positively. Lactose monohydrate did not show any bipolar charging.