Physics group> Physics Time Factor

The Physics Time Factor sets the rate of the simulation or how quickly or slowly particles are generated by Particular. This control enables full control over physical time. The particles can be made to move faster, slow down, completely freeze or move backwards. This is useful for creating stop-time effects such as freezing an explosion or fireworks simulation in place and moving the camera to show a different view and then restarting the simulation.

The Physics Time Factor control in the Physics group.

How Time Factor works

To freeze time, the Physics Time Factor control has to be keyframed down to 0. If it is simply set to 0, no particles will ever be born. You can keyframe from 0 to another value to make the particles appear again. Higher values make the particle simulation rate go faster, while negative values will run the simulation in reverse.

Keyframe the Time Factor to 0 to stop particle movement.

Tutorial: Freezing Time with Physics Time Factor

Create a new comp (Ctrl/Cmd-N). Set the composition to be 640x480, with a frame rate of 30 and length of 5 seconds. Name it 'Main'. Create a new comp-sized Solid (Ctrl/Cmd-Y). Name it 'Particles', click the Make Comp Size button, and click OK. Apply Effect > Trapcode > Particular to the Solid.
To be able to navigate easily in the scene, let's create a composition camera. Select Layer > New > Camera and use the 28mm preset. Now we will keyframe the camera. Make sure that you have the Camera layer selected in the Timeline. Twirl down the Position layer property by hitting 'P'.

Go to Time 2:00 (2 seconds) and set a keyframe for Position (the default: 320,240,-499).
Go to Time 3:00 (3 seconds) and set a keyframe for Position: 0,240,-380.

Try a RAM preview. You will see the view of the camera shifts the perspective of the particles.

The camera rotates around the particles in 3D.

Now let's use the Physics Time Factor control to slow things down during the camera move. Select the 'Particles' layer in the Timeline. We are going to keyframe the Physics Time Factor control. Twirl down to the Physics > Physics Time Factor control. You can set your keyframes in the Effects palette or the Timeline.

Go to Time 1:00 (1 seconds) and set a keyframe with value 1.
Go to Time 1:20 (1 seconds and 20 frames) and set a keyframe value 0.
Go to Time 3:10 (3 seconds and 10 frames) and set a keyframe value 0.
Go to Time 4:00 (4 seconds) and set a keyframe with value 1.

TIP: To set the second 0 value, you can copy/paste the first 0 value's keyframe in the Timeline.

The timeline showing the keyframes for the Physics Time Factor control.
Try a RAM preview. You will see the particles stop moving halfway through the comp. You have successfully frozen time!

The final result shows the camera rotating around the particular effect and particles appear temporarily frozen in space.