This question follows the lead of a previous question (What kind of fluid is sand?), where basic properties of sand were discussed.

A comment to that same question linked to a videoclip where sand was made behave as a fluid by blowing some air through it from below.

My understanding is that the ripples and all the fluid-like features are consequence of a large amount of air being trapped underneath the grains of sand and flowing upwards so to make grains float.

Moreover I would explain the fact that air can percolate almost homogeneously through the sand by observing that the percolation will tend to follow layers of equal pressure in the sand, so that air will end up dispersing through al the width of the sand box before finally escaping from the sand. I am not very convinced by this, so any comment and clarification is appreciated.

The real question is different though: in the video it is shown that at some points, when the airflow increases sufficiently, the sand begins bubbling and air escapes through large bubbles.

The presence of bubbles means that there is a surface tension between the "liquid sand" and air.

I can understand ripples and vortexes since they are present in pure air too, but I would not associate surface tension to neither of sand and air.

So how can a mixture of gas and solid (air and sand) exhibit surface tension?

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    $\begingroup$ I say that bubbles do not necessarily mean existence of a surface tension. Moving sand is one of the nastiest problem in physics. We should not look too much at it, have fun and feel strong with simple black holes instead. $\endgroup$ – patta May 13 '19 at 15:41
  • $\begingroup$ Found this video, bubbles from the side: youtube.com/watch?v=FcNuxk8vDu8 $\endgroup$ – patta May 13 '19 at 16:13
  • $\begingroup$ :) BHs till the big crunch! These bubbles stirred my interest because the possibility to introduce some sort of effective surface tension could have analogues in very far areas of physics.. $\endgroup$ – AoZora May 13 '19 at 16:18
  • $\begingroup$ "instability of fluidized beds" give a lot of papers that try to explain when bubbling occur. I tried to figure out a model myself but no success. Effective surface tension, that will be fun, you may get it for some regimes $\endgroup$ – patta May 14 '19 at 9:00
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    $\begingroup$ Yeah I looked at some of those papers, like Sundaresan INSTABILITIES IN FLUIDIZED BEDS. Annual Review of Fluid Mechanics (2003), but the studies are very quantitative and I cannot understand very well how they build this model of fluidized beds. I am thinking that maybe bubbles in this case are just the result of the isotropy of pressure in their content and (approximately) in their surrounding. $\endgroup$ – AoZora May 14 '19 at 9:54

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