Use the Friction Force node to model collisions between particles and a background gas as a friction force that is proportional to the relative speed of the particles. Unlike the collision types that are available with the Collisions node, the Friction Force is deterministic and does not cause the particle velocity to change discontinuously over time.

Choose an option from the Specify list: Cross section and number density or Collision frequency.

If Cross section and number density is selected, enter the following:

If Collision frequency is selected, enter the Collision frequency ν (SI unit: 1/s). The default value is 2 × 106 Hz.

Enter coordinates for the Velocity field u (SI unit: m/s) based on space dimension.

Select an option from the Particles to affect list to apply the force to specific particles: All (the default), Single species, or Logical expression. The settings are described for the Force node.