Hi guys

I am trying to use Vellum system to animate a mattress made of a few air-capsules. The envelope of each capsule is very thin, so the mass-weight of the material is very low. Inside the capsule is air pressure, which can be changed.

So I have used CLOTH and PRESSURE constraint to set up the attributes of the object. The stretch and bend stiffness of the cloth constraint was set up very high and the pressure stiffness as well - let's say the maximum to keep the sim stable. So I would expect, that the pressure and the stiffness constraints of the envelope (the mass of the envelope was set up very low) will keep the shape. But it deforms the shape as you can see on the pics. After decreasing the constraints the shape should step by step deform. But the kind of deformation doesn't seem to be correct. When I have a balloon full of air under some pressure, it means the pressure is same for every inch of the surface and the weight of the envelope is so low, that gravity doesn't affect the deformation so much. When decreasing pressure in the balloon the envelope starts to shrink, but the changes of the shape should be at the beginning smooth over whole surface - the envelope shouldn't fall on the floor and there deform to the flat shape and keep a kind of inflated shape on the top. The top and bottom part should have more or less the same shape since gravity has low affect on such light envelope.

Does anyone know how to simulate correctly deflating balloon? Many thanks for any advice.