Animations illustrating capillary instability

Airway closure example

Here a thin film coats the inner surface of a cylindrical tube with circular cross section. The red shaded area is the film phase and the white shaded area is the core, which in this example, is passive. The lower panel shows how the minimum core radius (Rmin for short) changes with time: the smaller it gets the thicker the film becomes. A liquid plug occurs soon after Rmin reaches approximately 0.4.

Saturation of capillary instability

The next animation shows an example of saturation of capillary instability as a result of an oscillatory stress. Initially the surface-tension driven instability causes the liquid film (shaded in red) to thicken. Once the film becomes sufficiently thick a bulge forms. One can see this bulge translate from one side of the domain to the other, losing fluid an depositing a layer behind it. Ahead of it, there is a capillary wave, and film thinning, so that less fluid enters the bulge than leaves. For high frequencies, there is little time during which the core flow is weak, and the bulge does not grow sufficiently to cause a liquid plug to form.