The new additions to the Blender particle system allow particles to interact in two ways - deflection and force fields.
Force fields
This allows you to set any Object as a particle “attractor”. Particles will then be attracted (or repelled) based on the center of the Object.
You can set Ipo keys for the parameters too – so you can vary the strength of the effect with time. Of course you can also just animate the position of the force-field Object.
Deflection
Allows you to set any Mesh Object as a particle “deflector”, and particles will then bounce off the surface of the Mesh. Make sure that the normals of the surface are facing towards the particles for correct deflection.
You can control how high the particles bounce (“surface damping”), add a random element to the bounce and define the percentage of particles which pass through the mesh.
You can set IPO keys for the deflection parameters - so for example you can vary the amount of damping on a surface with time.
Deflector Meshes can be animated, but vertex keys are not supported. The results with moving deflectors are variable – slow moving meshes, moving away from the direction of motion of the particles work better.
Particles can “leak” through the Mesh if the deflector moves much though and also if the mesh is complicated.
The particle interaction settings are controlled via a new "Particle Interaction" panel which is shown in the new “Effects” tab when Objects are selected.
After changing any parameters, you will need to go back to the particle effect and click "Recalc All". Since particle calculation can now take some time (especially with gravity), the progress of the calculation is shown in the console. (Note; a button to recalculate particle system will be added in the new "Particle Interaction" Panel as well.
Since Blender Particle paths are calculated using key positions, you'll have to make sure sufficient keys are available to show the collisions or force effects with detail. More keys means longer calculation times and usage of more memory (100k particles, 50 keys = 150 MB)
The "Deflection" and "Force Field" buttons switch the corresponding effects on or off for all particle systems in the scene. Deflection now only works with Meshes. Force fields are available for all Object types, and currently only a point-based (spherical fall-off) field is implemented.
Both options take Blender layers into account. So you can move a force field to an iinvisible layer (like # 20) and make sure the Particle emittor has that layer set as well.
Strength
The strength of the force field effect.
Fall-off
How the force diminishes with distance.
Surface damping
Controls the way particles "bounce" off a surface.
Random damping
Adds a random element to the bounce.
Permeability
Percentage of particles passing through the mesh.
To insert Ipo keyframes for any of the above parameters, hover the mouse cursor in the Effects window and press I.
The Ipo curves can be edited in the "Object" Ipo types in the Ipo window. The curves are at the bottom of the list, below the "Col" curves.
Controls the strength of the force field. It depends on the fall-off setting though, so some tweaking may be needed to get the effect you are looking for.
Changes how the force changes with distance from the Object center. A value of 2.0 is equivalent to "real" gravity.
This parameter controls how much the particles rebound from the mesh when they collide.
Varies the actual surface damping randomnly upwards from the current surface damping value - e.g. if surface damping = 0.8 and random damping = 0.5, the actual surface damping is varied between 0.8 and 0.9
Controls the percentage of particles which pass through the mesh. 0.0 = no particles pass through, 1.0 = all particles pass through
