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1 Physiology Program, Harvard School of Public Health, Boston, Massachusetts 02115; and 2 Department of Chemistry and Chemical Engineering, Kanazawa University, Kanazawa, Japan
Mixing associated
with "stretch-and-fold" convective flow patterns has recently
been demonstrated to play a potentially important role in aerosol
transport and deposition deep in the lung (J. P. Butler and A. Tsuda.
J. Appl. Physiol. 83: 800-809,
1997), but the origin of this potent mechanism is not well
characterized. In this study we hypothesized that even a small degree
of asynchrony in otherwise reversible alveolar wall motion is
sufficient to cause flow irreversibility and
stretch-and-fold convective mixing. We tested this
hypothesis using a large-scale acinar model consisting of a
T-shaped junction of three short, straight, square ducts. The model was filled with silicone oil, and alveolar wall motion was
simulated by pistons in two of the ducts. The pistons were driven to
generate a low-Reynolds-number cyclic flow with a small amount of
asynchrony in boundary motion adjusted to match the degree of geometric
(as distinguished from pressure-volume) hysteresis found in rabbit
lungs (H. Miki, J. P. Butler, R. A. Rogers, and J. Lehr.
J. Appl. Physiol. 75: 1630-1636,
1993). Tracer dye was introduced into the system, and its motion was
monitored. The results showed that even a slight asynchrony in boundary
motion leads to flow irreversibility with complicated swirling tracer patterns. Importantly, the kinematic irreversibility resulted in
stretching of the tracer with narrowing of the separation between adjacent tracer lines, and when the cycle-by-cycle narrowing of lateral
distance reached the slowly growing diffusion distance of the tracer,
mixing abruptly took place. This coupling of evolving convective flow
patterns with diffusion is the essence of the stretch-and-fold
mechanism. We conclude that even a small degree of boundary asynchrony
can give rise to stretch-and-fold convective mixing, thereby leading to
transport and deposition of fine and ultrafine aerosol particles deep
in the lung.
geometric hysteresis; fluid dynamics; aerosol; Stokes flow; chaotic mixing
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