Lung deposition behavior for straight versus curved aerosol fibers is known to be different based on existing experimental data. However, our understanding of actual fiber dynamics in the respiratory system remains far from being complete. In particular, it is not clear how fiber shape influences particle motion in the lungs. This article presents the results of direct numerical simulations in a linear shear flow of the rotational dynamics of high aspect ratio fibers with complex shapes such as elliptic rods, torus segments, and helices. Our findings show that as expected the rotational behavior of complex fibers is different from that observed for straight symmetrical particles. In particular, we observe secondary rotation for our particles, which is perpendicular to the shear plane (a plane with the unit normal parallel to the local vorticity) as well as more frequent flipping (for helical particles) than observed for straight fibers with the same lengths and diameters. In view of our results, it can be suggested that respiratory tract deposition for particles with complex shapes will exhibit enhanced interception compared with the deposition of particles with simpler shapes.

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Results indicate that externally forcing local alignment of high aspect ratio particles can increase local deposition considerably.

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In our article Roshchenko et al. (2011) we study the influence of different aspects of particle geometries on particle rotational behavior in a linear shear flow, which locally approximates the flow deep in the lungs. Particularly, we were interested in certain geometries and mechanical properties that are representative of common fiber shapes found in industrial/indoor aerosols. Our goals, the scope of our research, and its outcomes were described in the introduction and discussion sections of our article.The statements in the Letter to the Editor that question the relation and value of our results in the study of particle deposition in the lungs are not correct. In particular, before our publication appeared, there had not been any quantitative studies demonstrating the kind of results that we present, and we have been able to characterize certain behaviors of fibers that are not self-evident. First, we found that in a 2-D flow, real particles may gain significant inclination (we call it the outof-plane angle) to the plane with unit normal parallel to the local vorticity in a relatively short time. In the context of lung deposition, this finding means that a particle, while traveling through airways, has enough time to develop (or to increase) the out-of-plane angle even without secondary flow expected on top of the linear translation in axial direction. Second, our study is the first to report that helical particles exhibit more frequent flipping in ordered periodic rotation (angle φ Figure 8, Table 2 and Discussion in Roshchenko et al. [2011]), which may lead to increased interception for particles close to the walls or alter their sedimentation velocity closer to the duct axis.