(99a) Discrete Element Modeling to Predict Triboelectrification in Pharmaceutical Powders

Authors: 
Mukherjee, R., UConn
Chaudhuri, B., University of Connecticut
Triboelectrification is referred to the process of charge transfer between two different material surfaces when they are brought into contact and separated [1], causing the material surfaces to charge either positively or negatively depending on the material properties and the mechanisms of charge transfer. Charging due to triboelectrification is a common phenomenon and over the years, number of operations like milling, blending, pneumatic transport, and silo discharge, have all been impacted due to the numerous hazards posed by tribocharging [2]. The pharmaceutical powders are insulators with high resistivity and slow relaxation times [3]. The probable mechanisms of charge transfer in insulators may include electron transfer, ion transfer, bond dissociation, chemical changes, and material or mass transfer [4]. A Discrete Element Model (DEM) has been developed to predict triboelectric behavior in pharmaceutical powders to consider the inter-particle and particle-wall charge transfer and interactions based on the condenser model proposed by Matsusaka, Ghadiri [5]. The model considers charge accumulation based on electron transfer mediated by the work function differences between the materials. Work function is defined as the minimum energy or thermodynamic work required to remove an electron from a solid surface, and have been calculated using semi-empirical and DFT (Density Functional Theory) techniques. Amongst the multiple physio-chemical factors effecting triboelectrification, particle sizes play a major role to mediate charge transfer, both between identical and different material surfaces. The study also probes in to the probable mechanism that might actuate opposite polarities between two different size fractions of the same material based on differential specific moisture content [6].

The charge transfer patterns of two different sizes of Micro crystalline cellulose (MCC) has been analyzed using a simple unidirectional hopper-chute geometry against two different material surfaces (Aluminum and PVC). Experiments similar to DEM simulations are also performed to validate the model. The triboelectric series obtained from the quantum scale calculation puts the work function in the order Al<MCC (with moisture) < MCC (no moisture) <PVC [6]. Alteration of work functions due to the variation of moisture content are accomplished by introducing water monolayers to MCC slabs in the course of the quantum calculations. Both size fractions of MCC particles charged positively against PVC surfaces, while charged negatively against Aluminum in the DEM simulations and in experiments. Various surface and bulk analytical techniques have also been implemented to analyze and co-relate the effects of size fractions on triboelelectrification. No significant bi polarity between the different size fractions of MCC could be confirmed in the hopper chute experiments. However, based on the specific moisture content of the individual particles, the larger MCC particles are found to have increased number of moisture molecules compared to the smaller size fractions. The DEM model is simulated accordingly, suggesting the larger particles to charge positively due to increased number of moisture molecules in coming in contact with the smaller counterparts, which charge negatively. The polarities of the net charge of the system were significantly dependent on the chute wall materials due to dominant particle wall interactions. The final charge of the system from DEM based simulations were in accordance to the charge profiles obtained in the hopper chute experiments. In order to further validate the inter-particle charge transfer in the experiments, two different size fractions of the MCC particles are introduced in a vibrating cylinder at different concentration, and the net charge of the system tend to alter the charging patterns (magnitude and polarity) based on the concentration of large or small particles. DEM simulations performed on vibrated cylinders confirm the experimental findings.

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5. Matsusaka, S., M. Ghadiri, and H. Masuda, Electrification of an elastic sphere by repeated impacts on a metal plate. Journal of Physics D: Applied Physics, 2000. 33(18): p. 2311.

6. Mukherjee, R., et al., Effects of particle size on the triboelectrification phenomenon in pharmaceutical excipients: experiments and multi-scale modeling. Asian Journal of Pharmaceutical Sciences, 2016.