two groups of particles with different sizes

Hi, everybody

I am trying to modify the tutorial to simulate the segregation of particles with different sizes. I tried a simple problem, a block with 90X90X1000, I poured a group particles with a diameter of 0.003 mm from z=0 to 350 mm, and then the other group with a diameter of 0.005 mm from z=350 to 700 mm. The result looks like that the larger particles can not fall down beneath z=350 to mix with the smaller particles, so do the small particles. Could somebody help me to solve this problem? What is the mistake i did in this case?

Thanks a lot!

Zhen Li

PS : In attachment is my in.liggghts data.

#particle insertion
group nve_group region reg
region bc block -0.045 0.045 -0.045 0.045 0 0.35 units box
fix ins nve_group pour 57296 1 1 vol 0.5 100 diam uniform 0.003 0.003 dens uniform 1000 1000 vel uniform 0. 0. 0. 0. -0.0 region bc

#particle insertion
group nve_group_2 region reg
region bc2 block -0.045 0.045 -0.045 0.045 0.35 0.7 units box
fix ins_2 nve_group_2 pour 12376 1 1 vol 0.5 100 diam uniform 0.005 0.005 dens uniform 1000 1000 vel uniform 0. 0. 0. 0. -0.0 region bc2