Help Raising stickiness in collision material makes particles spastic but not sticky

[josemendez]

I'm trying to make fluid behave as much like a thick, stiff paste as possible. I though I could achieve that using collision material settings, but it looks like the friction in stickiness parameters only have a very limited effect on the fluid.

As the name implies, collision materials only affect collision behavior. They have zero influence on internal fluid dynamics, just like Unity's physic materials have zero effect on internal rigidbody dynamics (you can't make a rigidbody rotate slowly by adjusting its physics material, for instance. You'd have to set its angular damping instead.)


What you want to do is increase the fluid's viscosity.

You can get thick, viscous fluid that slips off surfaces by using high viscosity and low stickiness/friction.
Or, you can get thin, runny fluid that sticks to surfaces easily. Stickiness, friction and viscosity are completely independent from each other.


Note that very high viscosity values will cause fluid to be unstable (jitter, exploding/flying particles). This is because large viscosity values require stepping the simulation in very small increments (in other words, a very small timestep) to maintain stability.

Note this is not some arbitrary Obi limitation/bug. Viscosity is extremely costly to simulate, high viscosity values (clay-like) cannot be simulated in realtime. Just for comparison, simulating just a few seconds worth of toothpaste-like behavior takes >10 hours on a state of the art simulator like RealFlow:
https://www.youtube.com/watch?v=oa43MoGSrRI