Influence of Trough Material on Triboelectric Charging and Wall Friction of Microcrystalline Cellulose Powders During Shear Testing

Triboelectric charging can significantly affect the flow behavior of insulating powders on wall surfaces. Microcrystalline cellulose (MCC), a widely used pharmaceutical excipient, is particularly prone to charge accumulation during particle-to-wall contact. In this study, wall friction shear tests were used to examine how trough material affects triboelectric charging and wall friction of MCC under quasi-static flow conditions. Three trough materials were evaluated including stainless steel (metallic), polylactic acid (PLA), and polytetrafluoroethylene (PTFE) by using both standard and layer-by-layer preparation methods. The generated electrostatic charge was measured using a Faraday cup, and the corresponding wall yield loci over a PTFE wall coupon were determined from shear tests. Charge measured after shearing for the standard preparation method revealed that PTFE trough produced charge magnitudes ~16–21 times greater than stainless steel trough (64.5 nC vs. 3.1 nC for MCC 102; 59.7 nC vs. 3.8 nC for MCC 203), while PLA trough produced an intermediate and particularly powder-dependent response, including a polarity reversal for MCC 203 (−38.9 nC vs. +3.8 nC on stainless steel trough). This charging contributed to a direct increase in wall adhesion, with τad increasing by 61.6% (MCC 102) and 41.8% (MCC 203) on PTFE relative to stainless steel. The layer-by-layer method amplified charging further for both conductive and insulating troughs. Similarly, adhesion trends were generally consistent across layering preparation methods, with PTFE trough again showing the largest increase relative to stainless steel (40.5% for MCC 102; 55.6% for MCC 203), while PLA trough showed a more variable response, including a slight decrease in adhesion to MCC 102 (−11.7%). PTFE and PLA troughs produced substantially higher charges than the metallic trough, resulting in higher shear stress across the wall yield loci of two MCC powder sizes. The main effect of insulating materials was a non-negligible increase in adhesion. Consequently, the experimental characterization of wall friction via shear testing should be performed with a trough material with an electrical conductivity similar to that of the investigated wall coupon to ensure accurate process replication.