Postoffice
Hi there!
You can think of materials like layers in Photoshop, so the base material is the bottom layer. The layers above it take precedence, so there should be a material higher up the 'stack' than the base material with non-zero pressure/surface tension values.
The idea is that when you come to do simulations, you have to 'ramp' these values in. To do that, there are keyable multipliers on the zTissue node. (the reason for this is that zMaterial attrs are, at the time of writing, not keyable)
So on the zTissue for the fat, key surfaceTensionEnvelope and pressureEnvelope to 0 on the first frame of simulation, and set another keyframe at a value of 1, 10 frames after that.
This gives the collisions a chance to resolve. Without setting the keyframes, pressure and surface tension are 'full force' from the beginning, and you may see the fat go right through the muscles because collisions get missed.
To see results quickly, just disable the epidermis_cloth and check that the fat is performing as expected. You can turn the epidermis back on later once you're ready to start seeing higher level wrinkling etc.
Hope that helps,
Andy