Question Fill sponge in unity 2d

Guys I wish I could fill a rectangular container like a sponge.
For now I'm doing this:
- I create a rectangular container
_I apply colliders to the edges
_I create very small physical balls
_with a collision event I prevent the balls that enter the container from leaving.
Of course I'm using physics to do this.
What I would like, however, is to give the idea of expansion (see image) which of course I can't use physics. Can anyone advise me how I should go about it?

 

You could try disabling gravity and rely on the automatic penetration-resolving of the physics engine. Maybe add a weak, random velocity. Also make sure to have some dampening.

 

Thanks for the advice, actually I've already done some tests of this type but I didn't get the desired result. Too weak penetration mixes the particles too much but if I increase it a bit many can no longer penetrate in sponge

 

Any other ideas?

 

would you know any video clip or gif that would show similar effect?
(just to get some idea how it should look)

 

I don't actually find much, but for example look here: (at 0:48)
I would like something that spreads like a stain

or also:
https://www.youtube.com/shorts/I8w9hpL6-S0

 

that looks more like flood fill, could try that..

but maybe needs effect like in those reaction/diffusion models,
so that it doesn't spread so evenly (like floodfill).

https://github.com/keijiro/RDSystem