PROCEDURE 

1. Define the particle properties: a downward acceleration is applied
2. Divide the surface UV to define particles starting point
3. Define a surface environment
4. Combine the particle settings and emitters to the system component; which requires a timer component to run
5. Use the engine component to run the system, which requires a timer component to run and a button to reset the simulation
6. Surfaceflow force component simulates liquid movement on surface
7. Use Set Velocity to keep the particles movement according to flow simulation
8. Use KillContain to kill the particles once it left the surface
9. Extract the route of particles with the GetParticlePositionHistory component
10. Select the particles which have already left the surface
11. Visualise the flow line density by recolouring the sub-surfaces