How to keep particles moving around inside an invisible box I'm trying to create a kind of mystical shadow thing, made from black particles of varying size. Essentially, I'm using a shape (I've tried it with several things: A cube, a custom shape I made, a cylinder, a sphere, etc) as the outline, and trying to have a bunch of particles surrounding the edge of the shape, or else filling in the inside without going out, or much out.

I can get it to work for a still image by just having the start and end frames set to 1, but for an animation, how would I get them to constantly move around (not just bounce off the walls like when I tried having an invisible collision shape) inside the volume of the object?
I think "brownian motion" is probably what you're looking for.



The setup for this example includes two cubes: one as the emitter, and one (slightly larger) as a collision object, so that the particles don't wander too far.  These particles are emitted from the volume and I gave them a very slight starting velocity from the normals, but most of the motion comes from the Brownian force  under "Physics".  It is important to give the particles a small amount of drag and/or damp, otherwise the Brownian motion will keep adding up and the particles will increase in velocity over time.