Effects of gravity and surface tension on steady microbubble propagation inasymmetric bifurcating airways

Mechanical ventilation is nowadays a well-developed, safe, and necessary strategy foracute respiratory distress syndrome patients to survive. However, the propagation ofmicrobubbles in airway bifurcations during mechanical ventilation makes the existing lunginjury more severe. In this paper, finite element and direct interface tracking techniqueswere utilized to simulate steady microbubble propagation in a two-dimensional asymmetricbifurcating airway filled with a viscous fluid. Inertial effects were neglected, and thenumerical solution of Stokes’s equations was used to investigate how gravity and surfacetension defined by a Bond (Bo) number and capillary (Ca) number influence the magnitudesof pressure gradients, shear stresses, and shear stress gradients on the bifurcatingdaughter airway wall. It is found that increasing Bo significantly influenced both thebubble shape and hydrodynamic stresses, where Bo ≥ 0.25 results in a significant increasein bubble elevation and pressure gradient in the upper daughter wall. Although for both Boand Ca, the magnitude of the pressure gradient is always much larger in the upper daughterairway wall, Ca has a great role in amplifying the magnitude of the pressure gradient. Inconclusion, both gravity and surface tension play a key role in the steady microbubblepropagation and hydrodynamic stresses in the bifurcating airways.