Thank you so much for your reply, Rob.

yes, now I remember that I read somewhere that fluent doesn't consider the particle diameter size when tracking them, and the particles are studied as point-particles which is actually the center of my particle (is that true? please correct me if I am wrong)

Based on what you said, I guess my first mesh layer height must be larger that my particle diameter(please correct me if I'm wrong). I am studying a range of particle sizes from 50nm to 10 microns. Do you think that it will be ok for tracking of those tiny particles if I generate a mesh with the first mesh layer height larger than 10 microns? (my hypothesis is that it should be fine and the trajectory of those small particles will be updated in more steps as they are passing through that mesh layer above the wall). Or do I have to generate different meshes for each of my particles? Of course, I should be mindful of the flow characterization limitations like y+ as well.

Now that we are talking about this issue, just to make sure, would you also please tell me, which one of these two is fluent's criterion for marking a particle as trapped on a wall:

1- the particle hitting the wall surface (this must be the correct one I think)

2- the particle entering the first mesh layer, right above the wall surface (the particle is marked as trapped as soon as it enters that mesh cell and no trajectory update happens in that cell)

Thank you so much for your time