Stick–Slip Behavior of Ethenzamide-Based Binary Powder Mixtures Evaluated Using the Shear Cell Method: φCSL as a Robust State Parameter

Stick–slip (SS) behavior often disrupts the smooth flow of pharmaceutical powders during transfer and can impair product uniformity. In this study, binary powder systems consisting of ethenzamide and pharmaceutical excipients were investigated using a constant-volume shear cell method to clarify the factors governing SS behavior in mixed powder formulations. Eight pharmaceutical excipients with distinct particle properties were selected to analyze the effects on SS behavior as a function of the mixing ratio. X-ray computed tomography (XRCT) was used to characterize interparticle void structures in excipient powder beds. The SS amplitude and interval increased with increasing consolidation load. The addition of excipients progressively suppressed the SS behavior depending on their void characteristics. Although limited to discrete mixing ratios and static XRCT, excipients with a higher fraction of interparticle voids of moderate size (5.6–23 µm) tended to suppress SS behavior at lower mixing ratios. Correlation analyses revealed strong positive correlations between the SS amplitude and static resistance parameters, such as the angle of internal friction and cohesion, whereas correlations with steady-state flow parameters were weak when data across different consolidation loads were analyzed together. At a fixed consolidation load, SS amplitude showed a clearer correlation with the dynamic friction angle (φCSL), indicating that φCSL reflects a flow-related property of the powder bed at the critical state and shows relatively robust behavior against variations in consolidation conditions.