flotus1 
September 16, 2013 12:19 
Its the Brownian forces again...;)
I assume you have rather small particles.
The usual integration mechanisms when Brownian forces are involved only work (because of stability AND accuracy constraints) if the time step size is in the order of or smaller than the particle relaxation time.
For small particle Reynolds numbers ( ) the particle relaxation time is
.
Here is the particle diameter, is the particle density and is the dynamic viscosity of the fluid.
This relaxation time becomes small for small particles, leading to small time step sizes.
