## Particle Physics operator

The Particle Physics operator allows you to assign inter-particle collision and cohesion forces.

### Types Rollout

• Enable particle collisions: enables inter-particle collision forces.

• Enable particle attractions: enables inter-particle cohesion forces.

### Collide Rollout

• Radius type: the particle property from which to derive the collision radius.

• Radius: the absolute collision radius value.

• Multiplier: the multiplier to apply to non-absolute radius values.

• Tolerance: a multiplier applied to each particle’s radius value, that is used for neighbor searches.

#### Collision Force

• Stiffness: the strength of the collision resolution force.

• Friction %: the amount of inter-particle friction that will be applied.

• Mass z-bias: the amount of artificial mass adjustment to make on stacked particles.

• Stiffness affects inertia: controls whether the stiffness value of collisions will affect inertia of colliding particles.
##### Stiffness Solver
• Simple: collisions are solved in a straight-forward manner. Their stiffness is mostly dependent on solver step count.

• Constrained: collisions are solved in a way that makes their stiffness somewhat independent of solver step count, at a small performance cost. This setting has no effect on collisions whose stiffness value is 1.

### Attract rollout

• Absolute radius: the attract radius of each particle will be set to a specific value.

• Radius: the specific attract radius value.

• Shape radius: the attract radius of each particle will be set to each particle’s shape mesh radius.

• Scale radius: the attract radius of each particle will be set to each particle’s maximum scale dimension.

• Multiplier: the multiplier to apply to shape/scale radius values.

#### Attract Force

• Strength: the strength of the attraction force.

• Friction %: the amount of inter-particle friction that will be applied.

• Stiffness affects inertia: controls whether the stiffness value of attraction will affect inertia of attracted particles.

### Simulation Rollout

#### Integrator

The integrator is what converts collision/attraction data and assigns it to particle velocities.

• Integrate into bind solver: controls whether all collision/attraction calculations will be controlled by the Particle Bind Solver, which executes after all operators have been evaluated.
• OpenCL acceleration: controls whether the particle neighbor search will be conducted on the GPU.

• Max neighbors: controls the maximum number of results the neighbor search algorithm will return for a single particle.

• Accelerator: controls which search acceleration structure will be used.
• Substeps: the number of solver steps to perform.

• Affect pos/velocity/both: controls which particle properties the solver will affect.

• Affect axis X/Y/Z: controls which axis of each particle property will be affected.

#### Particle interaction

• Process only this event: particle physics forces will only be computed between particles in this event.

• Affect only this event: particle physics forces will be computed between all relevant particles in the flow, but only particles in the current event will be affected by the forces.

• Ignore binding neighbors: if two particles are connecting by a binding, particle physics forces will not affect them.

• Ignore starting penetrations: if two particles are intersecting when they both first enter the event, they will be set to ignore each other.

• Stop ignoring after separation: if two particles that are ignoring each other (due to starting penetrations) separate, they will be set to no longer ignore each other.

##### Clustering

Cluster settings within the “ignore starting penetration” parameters allow you to define which clusters of particles to ignore starting penetrations with.

• Enable clustering: controls whether ignored starting penetrations will be determined by cluster values.

• Channel: the particle data channel to get cluster values from.

• Cluster if equal: particle pairs with starting penetrations must have equal cluster values in order to be ignored.

• Cluster if different: particle pairs with starting penetrations must have different cluster values in order to be ignored.

• Simulation groups: controls which particle simulation groups will determine which particles will interact.

#### Display

• Draw collision shapes: draws spheres representing particle collision radii.

• Draw collision tolerance shapes: draws spheres representing particle collision tolerance radii.

• Draw attraction shapes: draws a sphere around each particle, representing its attraction radius.

• Draw neighbors: draws a connection between all interacting sets of particles.

#### Uniqueness

• Seed: the seed value for all varied parameters.