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I'm using the new pressure settings for cloth sim in Blender 2.9 and I'm able to use a moving object with collision on it to squeeze a spherical shape with cloth and internal pressure applied to it. It's fairly convincing except that if you view the deformation from the side, it squeezes realistically at the point of pressure, but the overall volume is decreasing. On a real balloon the sides would inflate outwards around the point of pressure. Similar to the way this toy inflates in areas that aren't being restricted by pressure: https://www.youtube.com/watch?v=fA7_78PouHw

I'm trying to do a visual demonstration of this effect for a presentation, is there a way to tweak the settings to achieve this using physics, or barring that, a way that I could convincingly fake it? My best guess at the latter option would be to bake the physics animation, then try to add volume to the sides at the point of it being squeezed in the animation. But I'm not sure if this approach would be worth pursuing, would it be possible to do it with shape keys and animate that?

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    $\begingroup$ "the overall volume is decreasing" - if you apply pressure on a container, you increase the pressure outside the container, and therefore the pressure inside the container should equalize, which, for the same mass of the gas inside, should result in smaller volume. "On a real balloon the sides would inflate outwards around the point of pressure" - I get your point and this is the result I'm getting: i.imgur.com/Gor5qPs.gif - if you add some screenshots or upload your file to blend-exchange.com you will make it easier to help you. $\endgroup$ – Markus von Broady Mar 15 at 18:35
  • $\begingroup$ I think what OP is talking about is that the balloon doesn't stretch in unrestricted areas when squeezed. Think about squeezing a stress ball, how portions of it give and balloon out. In your simulation, it is more like squeezing a canvas bag rather than a balloon. $\endgroup$ – Michael Mar 15 at 20:37
  • $\begingroup$ @Michael Well put with the canvas bag. In order to simulate a rubber balloon instead lets decrease the cloth's resistance to stretching ("tension"): i.imgur.com/jHJX5af.gif But I'm not sure that's the issue, best to wait for Mr falldeaf. $\endgroup$ – Markus von Broady Mar 15 at 21:03
  • $\begingroup$ @Michael is exactly right! Sorry for the ambiguity. I essentially need the balloon to expand out in areas where it's not being restricted by external pressure. This is a very strange toy, but I think a good example of the phenomenon I'm hoping to replicate: youtube.com/watch?v=fA7_78PouHw My current example looks almost exactly like your gif, where there doesn't appear to be any lateral stretching and expanding of the object. $\endgroup$ – falldeaf Mar 16 at 17:52
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I found a solution that gave the more dramatic expansion that I was hoping for. First, as @Markus von Broady mentioned, lowering the cloth modifiers tension, but in addition I added pressure to the simulation when an object with collision put pressure on the system. So I added key-frames to the 'Pressure' setting that coincided with the system getting more pressure from an external force.

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