# Particle speed and density

I am using a Particle system to simulate falling dust / debris. The particle start and end fairly quickly. They should look like a short burst of falling dust.

I cant figure out how to get the particles to fall at slightly different speeds.

The particles should be falling downwards with very little Brownian motion.

The fact that the particles are falling at the same speed makes them look odd / too uniform (almost like a static image of some particles which is moving down rather than individual particles.

Also the particles end at a set frame so I get a uniform trailing edge (as below).

A bit of air drag helps a bit but not much.

Question 1. how to vary the downward speed of the particles?

Question2. Can the density of particles be animated so I can brake up the distribution?

Thanks Oliver

• Other than the number of particles, start and end alomost all of the prameters are keyframable, so you can very them over time, including gravity, random, and lifetime.
– user1853
Commented Sep 5, 2014 at 20:10
• How about adding a noise texture to the emitter and use Time(default) in the Influence panel? Commented Sep 5, 2014 at 22:53
• you could add a second particle system that starts towards the end of the first to subtlety fill in that empty space. Commented Sep 6, 2014 at 1:08
• Commented Sep 6, 2014 at 3:36
• Use texture for particle system which influences on Time and Lifetime. Depending on amount of textures and their influence you can achieve a lot of effects. Commented Apr 21, 2016 at 8:39

Yesterday I found out how to control the particle's speed. You can do this from "Timestep"!

You can find it in the "Particles" tab in the "Properties" window. Then go to "Physics" menu. Over there is the "Timestep" slide, which by default is on "0.040" which is quite fast. So if you want slower the particles you can change this to "0.005" or even for very slow motion you can set it to "0.0005".

And most of all!!! You can keyframe the particles "Timestep", so you can do nice and cool animations. I have made a quick demo in which you can see that. It's here: Particles speed test in Blender .

P.S. I want to apologize about my bad english (it's not perfect).

Gravity would be the biggest factor that determines the speed of particles falling. You can change the amount that gravity effects the particles and you can also increasing the damping to slow down their movement.

The biggest thing that would get the effect you want would be to use more than one particle system. As an extreme, you could add 100 particle systems to an emitter and have 100 emitters in a scene. Each of these particle systems can have different gravity and damp settings so will be moving at different speeds.

With a different number of particles from each emitter you can have 80% of your dust settle in 3 seconds, another 10% in 5 seconds and the last 10% take 10 seconds to settle.

This also helps with the "all ending on the same frame". With multiple particle systems you can have the emitters with higher particle counts finish on frame 200 then have an emitter sending two particles a frame finish on frame 250, with some varying amounts being emitted in between. With multiple emitters or broken geometry emitters you can have these particles emitted in different places instead of being concentrated to one small area.

You can also use multiple layers to your advantage, with an emitter on a different layer you can have a force field effect the one emitter's particles so they move differently to give a separate appearance.

This may be overkill for just a small puff of dust, but for a good looking effect I would use smoke particles.

You can read more about the smoke simulator here. The documentation is kind of old and uses BI, so here is here is a video on using it in cycles.

• Cool. yes I have used some smoke for some of my stuff. But for this I wanted slightly larger more defined debris particles. Commented Sep 5, 2014 at 19:37
• I have and image example using rigid body large chunks, particle dust and smoke fine dust, but I cant post it here. Commented Sep 5, 2014 at 19:42