xpConstraints are dynamic connections and forces affecting the way in which particles interact.

There are several constraint types, which can be layered for multiple effects, all within one xpConstraints object.

You can create stretchy springs between particles, add forces of attraction and repulsions and even simulate fluid properties like viscosity and surface tension.

Connection constraints can be seen in the viewport as yellow lines connecting the particles.

To see the constraints in the viewport you must enable Display Constraints in the Display tab of the xpEmitter.

For stiff constraints, more iterations and a higher number of subframe steps are required.

If you find that connections are not being made, or don't behave as expected, try adjusting these values in the Project Settings (CTRL+D), Simulation tab.

Connections Birth

Connections Distance

Connections Custom

Collisions

Forces