Dependence of porosity on particle diameter

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Dependence of porosity on particle diameter
« on: September 08, 2022, 01:09:00 PM »
I am using GrainGeo to creat different homogeneous solid packings, each consisting of ideally spherical particles. One pack only consists of particle spheres of one diameter. I use the Pile-Mode and drop the particles one after the other until they have a stable position and the process ends when the pack is filled to the rim. This procedure works well in principle. However, it turns out that the porosity increases with the particle diameter. I am not yet able to understand this result. In theory, the porosity should be the same for ideal spheres regardless of the particle diameter (densest sphere packing). Otherwise, I could still understand that the porosity decreases with the particle diameter because with larger particles the weight force prevails over interparticle forces. I have so far explained it to myself as being due to the pack size, as wall effects play a greater role with larger particles. Accordingly, for larger packings, the porosity should converge to the densest packing regardless of the diameter.

Which effects are considered for the stable minimum? Are forces such as gravity and Van der Waals forces included? Or are only geometric factors taken into account?

Thanks in advance!
« Last Edit: September 08, 2022, 03:09:48 PM by Aaron Widera »

Re: Dependence of porosity on particle diameter
« Reply #1 on: October 25, 2022, 04:17:32 PM »
Dear Alex,
Thank you for your great question. In GrainGeo-Pile we use voxelized spheres for the piling process, which can lead to small resolution effects. More problems are introduced due to the boundary conditions. To achieve comparable results the computational domain needs to be rescaled e.g. larger spheres, domain larger. Otherwise, the global packing density decreases. To generate realistic packing densities without boundary effects the top and bottom area of the structure needs to be cropped. The images below show a set of spheres, which have been piled with GrainGeo and analyzed using MatDict. The plot of the solid volume fraction highlights the influence of the top and bottom boundary effect, which can be avoided if the structure is cropped at those locations.
Regarding the definition of the stable minimum in GrainGeo-Pile we only take geometric factors into account.

 We hope this helps you get the most out of GrainGeo simulations in your future work.