Effective friction in adhesive mixtures intended for inhalation: Simulation of oblique impact of adhesive units

Abstract

Oblique impact between adhesive units was studied using the Discrete Element Method (DEM) as a means to assess the effective friction coefficient and its dependence on particle shape, surface coverage ratio (SCR; 0.5, 0.75 and 1) and impact angle ( 1 5 ∘ − 7 5 ∘ ) in a low handling velocity regime (0.6 – 1.6 m/s). Adhesive units were created in silico from a spherical carrier particle (100 μm diameter) and monodisperse fine (drug) particles (3 μm large) of a spherical, triangular bipyramidal or tetrahedral shape. The particle interaction was modeled using the Hertz-Mindlin contact model with JKR adhesion (surface energy 0.03 J/m 2 ). A total of over 300 simulations were performed and the effective friction coefficient was extracted from the normal and tangential forces experienced by the carrier particles. The adhered fines resulted in a significantly reduced friction coefficient in comparison to the bare carriers but no additional reduction was observed with increasing SCR, suggesting a saturation already at an SCR of 0.5. The effective friction coefficient was independent of the impact velocity for relative velocities over 1 m/s. The impact angle had a significant effect on the friction coefficient, with head-on type collisions (15°) resulting in the lowest and grazing-type collisions (75°) the highest values. This was especially pronounced for the spherical fines whereas the triangular bipyramidal and especially the tetrahedral fines resulted in a more modest dependence on impact angle but overall considerably higher friction coefficients. These results could be used to better understand and formulate adhesive mixtures. • Adhesive units with different fine shapes were formed in-silico. • Binary collisions with different impact angle to study effective friction. • Effective friction is signifcantly lower compared to bare carrier particles. • Tetrahedral shape with angles towards grazing impact showed highest friction.