Particle Physics
Make particles solid balls that bump into each other, roll, pile up and come to rest: sand, grains, gravel, beads and ball pits.
Particles bump into each other as balls (and into those of other events with this operator), pile up on a ground from Collide, and push and are pushed by rigid bodies.
- Add it
- EventAdd OperatorPhysicsParticle Physics
- Sends on
- Nothing: it has no wire.
- Influence
- No: a value can’t weigh it.
When to use it
Whenever particles should take up room: sand pouring into a heap, gravel, cereal, beads, a ball pit, snow that piles. Without it, particles pass through each other.
It works across events: particles bump into those of every other event with Particle Physics too. They push rigid bodies and are pushed by them.
Set it up
Particles need a floor to pile on: add a Collide with Ground (or objects) to the event.
Settings
As the flow editor’s sidebar shows them. Changing any of them marks the bake old; those marked Keyframable can also change through a bake.
How heavy for its size, in kg/m³: water 1,000, wood about 600, sand about 1,600, stone about 2,500, steel 7,850. A heavy body pushes light ones aside, and particles push rigid bodies by it.
Grains at rest hold this hard before they slide (Coulomb's static friction, against the push they take from the grains under them), so heaps keep their slope instead of slowly spreading; Friction acts once they slide. Usually above Friction. 0 for the old, simpler grip. Not with binds or soft bodies in the scene.
With Static Friction, grains roll, and a rolling grain is held back by this (rolling resistance), so heaps stand steeper; Friction still sets most of the slope. About 0.2 for dry sand, more for rough or angular grains.
Particles touching others and slower than this stop, so piles stand. 0 never stops them.
Times overlaps and contacts are pushed apart, and binds pulled back, each substep. More holds piles and stacks up and binds tighter; slower.
Hold the particles out of this event's Collide objects while they push apart, as the ground holds them, so heaps stand on objects instead of slumping off or through them. They don't bounce off the objects. Not with binds or soft bodies in the scene.
How hard grains hold to those just apart, in g (9.81 m/s²): damp sand stands in steep heaps. 0 for none.
Radius and what you see
Each particle bumps as a ball of Radius, times its scale. The sphere it shows as is the Display panel's Sphere Radius: set it to the same, so what you see is what bumps. With shapes, keep the Radius about their size.
Steeper piles
Friction is what lets a heap stand steep; with little of it grains slide into a flat puddle. Rest Below stops particles that touch others and move slower than this, so piles stand still instead of creeping. Set it to 0 and they never stop.
Static Friction is how hard grains at rest hold before they start to slide, against how hard the grains under them push back. A heap with it keeps its slope where one with Friction alone slowly spreads, and a stream poured on top builds a cone instead of cratering it. Friction takes over once a grain slides, so Static Friction is usually above it: 0.8 against 0.5 for dry sand. 0 leaves the older, simpler grip, so old scenes bake as before. It does nothing in a scene with binds or soft bodies.
With Static Friction, grains also roll, and Rolling is how hard they resist it: at 0 a grain rolls freely off a heap's slope, so heaps come out flatter; 0.2 (the default) is round sand; raise it for angular grains, gravel or crushed rock, which lock together and stand steeper. Friction still sets most of the slope.
Grains with Static Friction look after a few things themselves:
- Substeps. They take more than the bake asks for while they fall fast (so a grain never jumps into a heap), and enough that a heap at rest doesn't sag. A pour bakes slower than the same grains without it.
- Settling. A grain that has stayed slow and touching for half a second goes to sleep where it is, until something moves it again, so a finished heap stands perfectly still.
- A stream. Grains born in a box or a volume leave it at least fast enough to carry the birth rate out, so a busy spout pours a tight stream instead of a spray.
Rounds is how many times a step overlaps are pushed apart. A tall pile sags with too few; raise it (and the bake's substeps) for deep heaps and fast, small grains. More is slower.
Works with
- Collide: its ground is their floor, and its objects their walls.
- Rigid Body in another event: grains push bodies by their Density, and bodies push them.
- An event takes one Particle Physics, and can't also hold a Rigid Body, Liquid, Follow Curve or Goal.
Problems you may see
event: operator: an event takes only one
Errors stop a bake; warnings don’t. Every message is on the Messages page.
Related
- Particles that pile up: sand and grains
- The Sand starter, which bumps without a flow
Surface Force
Pull particles onto an object's surface, or a set distance off it, from inside or out: they hold there, ride it as it moves, and slide along it.
Particle Force
Particles pull each other within a reach, or push apart with a negative strength: clumping goo, blobs, swarms that cluster or spread evenly.