# Using pair colloid with pair granular together

Dear lammps users,

I want to simulate a system of colloidal particles. I wonder if I can use pair colloid with pair granular or not?
Pair granular is related to overlapping of particles, but the other is not valid when particles overlap.
I read one of the mail list posts (post with the title “effective mass and DLVO”) and they said it is possible to use DLVO together with granular Potential.
How lammps calculate them with hybrid/overlay?

Regards,

Setare

Dear Setare,

I haven’t read the list post “effective mass and DLVO”. I’ll have a look.
In any case, I am facing the same problem and what I am doing is to define another

pair_style that allows for particle overlap and get rid of the singularity that appears in the attractive part

of the Colloid potential when the particles come to contact.

I defined a pair-style of the type:

U_A= -A_CCa_1a_2/(a_1+a_2)/6/(h+z_0)

where h=r-a_1-a-2 is the surface-surface distance (r is the center-to-center distance)

and z_0 is the minimum approach distance that makes the potential finite for particles in contact.

(usually few angstroms…).

Then, this potential should combine fine with Granular.
Best,

Gianluca

Dear Gianluca,

Thanks for reply. I understand what you mean but How z-0 can be calculated for different systems with different sizes?

And how about overlapping two particles? We calculate contact forces (granular pairs) from the magnitude of overlapping of two particles.
And LAMMPS algorithm for calculating total potential is summation all potentials at each distance, right? Then how is it possible to calculate colloid potential when two particles are overlapping?

Setare

z_0 is the same as \sigma in the Lennard-Jones potentials - it differs between materials but I think we’d all have to argue about how well really know this number for different materials, typically you see something like 0.1-0.4 nm. The main point to your follow up question is that if you ignore the effect of surface deformation (Derjaguin Mueller Toporov model) than the attractive force truncates at contact (turns into a ramp for all overlaps with slope A_CCa_1a_2/(a_1+a_2)/6/(z_0)). The overlap doesn’t really matter in this whole thing. If surface deformation does matter one has to use the JKR (Johnson Kendall Roberts) model. You can differentiate the two by looking at the Tabor parameter.

You should also include the typical asperity size (replace the a_1*a_2/(a_1+a_2) with the typical radius of asperities rather than the approximated spherical shapes of the particles), since particles generally aren’t perfectly smooth. Indeed, that is the reason that the Tabor parameter is often less than 1 (DMT model). Note: the distance between particle should still be calculated using the approximated spherical shapes of the particles.

I encourage you guys to be reading Israelachvili’s book on surface science (or some other suitable reference) if you are not.

slight correction to aid readability: turns the attractive potential into a ramp for all overlaps with slope A_CCa_1a_2/(a_1+a_2)/6/(z_0)

Regards,