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This tool allows grains to wake up and go to sleep based on the motion of the simulation around it. It is similar to the Awake By Geometry shelf tool, but instead of activating particles strictly by being near an object, this tool setups up the simulation so that particles can be awakened by the velocity of the particles around them. Typically you would want to use this tool whenever you want to optimize your simulation, especially if only a small part of the simulation will be active at any given time. This is especially true if you've already used Awake By Geometry.
Using Auto-Sleep Particles
The Velocity Threshold parameter on the POP Awaken node will control how easily other particles are awaked by other fast moving particles.
The Wakeup Scale parameter on the POP Awaken node will control how large the wake up radius is around the particles being tagged by the Velocity Threshold.
Turn on the Start Asleep checkbox on the Sleeping tab of the POP Solver to ensure all the particles in your simulation start out sleeping. This is useful if you don’t want to let your sand settle. However, this can cause a drooping effect when sand is activated and settles.