In order to perform the drug delivery via lung, tracking the trajectories of fine inhaled particles in the acinar airways is of high importance. The causes of irreversibility in the motion of fine particles (0.1–1 micron) and chaotic flow deep in the acinar region of lung has been always under investigation. In this study we demonstrate the importance of geometric hystersis and asynchrony of lung deformation on the issue. We adapted a 2D axisymmetric geometry of alveolated duct from recent relative works and deformed it in a way that some hystersis would appear in a respiration period. The overall deformation of duct was corresponding to the transpulmonary pressure of lung reported in physiological books and the Re number was matched with the actual flow in acinar airways. The geometric hystersis had the same amount as the measured hystersis in rat lungs available in relative literature. We performed a lagrangian particle tracking for different cases with and without geometric hystersis. Results confirm that as soon as some asynchrony appears in wall motion the trajectories of particles become extremely irreversible and hence we propose that the geometric hystersis can be a principal reason for observed irreversibility in acinar airways.

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