My guess is that it is about eddies in the airstream. Places where the sand is denser block from the wind resistance from the sand behind. So the sand behind “catches up” because it doesn’t have to fight air resistance. The sand in front of it is moving air out of the way and leaving a low pressure region in its wake. In fact, eddies could even create a downward airflow behind a sand ball, that tends to compress it and give it coherence.

I’ve no idea if it’s still there, but the Science Museum in London used to have a tub of sand fluidised in this way for kids to play with. It was very cool.

When air gets between the sand grains, they effectively act like a fluid, so you get effects like in the second video (the impact of the ball drives air into the surrounding sand, temporarily turning it into a fluid. You even get “surface tension” effects, I would suppose because of the pressure gradients (the trapped air has an easier time escaping at the surface of the sand).

Still, it’s a lovely demonstration of how subtle and wonderful nature is.

In the first few milliseconds this would be a small effect, as there would only be minor differences between the air pressure in one region and another, but as time went on you’d see more and more sand being forced into smaller regions, forming “droplets”.

That’s a completely different mechanism from the way water or mercury forms droplets though, which has to do with cohesion and surface tension.

Anyway, thanks for highlighting an area of physics that often gets too little attention, and too few interested students.