From there, Richard walks you step-by-step through creating the Matrix Code Effect, as featured in the popular Matrix films, and how to customize it for your own title sequences. You'll also learn how to use image sequences to create run cycles and animated particles. You'll learn cool techniques you can use in your productions, like the ever-popular revealing of a logo from a string of randomized letters, bouncing animated text off a wall with real physics properties, and blowing up selected letters in a line of moving text. Richard dives in at the deep end to show you how to create unique particle effects like snow and fountains, before taking you into Particle Playground's incredible particle text effects. Our Particle Playground expert Richard Lainhart fills you in on the basics of how particle systems work, and how they differ from After Effects' standard keyframing techniques.
Yet Particle Playground is a wonderful environment for all kinds of effects (particularly text effects) that are difficult or impossible to achieve with other tools. I'm sure someone else will chime in with another way.The Adobe After Effects Particle Playground plugin is a powerful environment for particle systems & other eye-catching special effects, but few After Effects users even know it's there! And what's more, many of those who do find its interface mysterious and intimidating. Hopefully this is what you're looking for. If you want a gradual stop then advance however many frames you want before setting the speed effect key frame to 0 Advance 1 frame and set the speed effect to 0. Set a second key frame just before the frame where you want the particles to stop and keep the same speed. Add the speed effect to the new composite and key frame the first frame at it's default. Or another option that's a little easier, create a new composite shot and bring in the composite shot containing the particle simulator. This wouldn't work if you intend on moving the camera around though as the content of the PNG frame wouldn't be in 3D space. I don't know if the particle emitter can freeze particles in one place but you could, at the point where you want them to stop, export one frame and bring the PNG file back in at the exact point where they stop. That way you can alter almost all properties of particles over their individual lifetime. If you want them to stop individually after they have lived for a certain amount of time, you have to adjust the speed curve in the lifetime panel (you might have to make that panel visible by clicking the 9-squares-grid icon next to the undo/redo-buttons and selecting it).You could also use a more complicated system utilizing mobile emitters and killing them off with a keyframed force, but for just freezing the effect this is probably a little bit overkill. Using linear or smooth keyframes you can also gradually make them stop. Using constant keyframes, having it at 100% and then drop to 0% will freeze the effect at that point.
The easiest way to achieve this would be to keyframe the "Time Scale" property under the "General" group of the particle simulator layer.