[lammps-users] Lubricated Friction problem

Hello members,

I am a physicist completely new to LAMMPS and would like to use it to simulate lubricated friction contacts. So far I’ve succeeded in 3d-ifying the very instructional 2d example included with the source code as well as using a more realistic potential. All this (including the sample geometry setup) can still be done via “atom_style atomic” and the “create_atoms”-command.

Next on the agenda would be to introduce a lubrication film between the friction partners. A moderately realistic lubrication film could consist of linear n-hexadecane molecules as described in a United Atom Model. These molecules, of course, will not like the constraints imposed on them by the “lattice” command used to set up the two metallic bodies.

Here come my questions:

  1. Is there any way to construct a reasonable sample consisting of two metal rubbing partners and an alkane lubricant within LAMMPS?

  2. If yes, does anyone have (and would be willing to share) some in.* files which could serve as an example?

  3. Does anyone have any sample in.* files (maybe including data.* files) to get me started on how to model a simple box of n-hexadecane molecules? So far, all I’ve come up with is a PDB-file of hexadecane for the basic geometry, but I haven’t actually figured out how to construct a proper sample consisting of a couple of tens (or hundreds) of molecules to play around with.

  4. Which would be a good tool to construct initial geometries which can then be fed into LAMMPS?

I have included the in.* file I’ve worked with so far for your reference.

I’d be glad for any suggestions you have!

Thanks a lot in advance,

Stefan

3d Iron single-asperity friction NVE

units metal
boundary p p s

atom_style atomic
lattice bcc 2.855324
region box block 0 30.0 0 16.0 0 20.0
create_box 4 box

pair_style eam/fs
pair_coeff * * Fe_mm.eam.fs Fe Fe Fe Fe

neighbor 0.3 bin
neigh_modify delay 5

region lo-fixed block INF INF INF INF INF 1.8
region lo-slab block INF INF INF INF INF 5.0
region above-lo block INF INF INF INF INF 5.0 side out
region hi-fixed block INF INF INF INF 18.2 INF
region hi-slab block INF INF INF INF 15.0 INF
region below-hi block INF INF INF INF 15.0 INF side out
region lo-asperity sphere 20 8 5 6
region hi-asperity sphere 10 8 15.0 6
region lo-half-sphere intersect 2 lo-asperity above-lo
region hi-half-sphere intersect 2 hi-asperity below-hi

create_atoms 1 region lo-slab
create_atoms 1 region hi-slab
create_atoms 2 region lo-half-sphere
create_atoms 3 region hi-half-sphere

group lo region lo-slab
group lo type 2
group hi region hi-slab
group hi type 3
group lo-fixed region lo-fixed
group hi-fixed region hi-fixed
group boundary union lo-fixed hi-fixed
group mobile subtract all boundary

set group lo-fixed type 4
set group hi-fixed type 4

temp controllers

compute new3d mobile temp
compute new2d mobile temp/partial 0 1 1

equilibrate

velocity mobile create 300.0 5812775 temp new3d
fix 1 all nve
fix 2 boundary setforce 0.0 0.0 0.0

fix 3 mobile temp/rescale 10 300.0 300.0 10.0 1.0
fix_modify 3 temp new3d

thermo 200
thermo_modify temp new3d

timestep 0.001
run 2000

friction

velocity hi-fixed set 0.05 0.0 0.0 sum yes

unfix 3
#unfix 1
#fix 1 all nvt 300.0 300.0 0.1 drag 0.3

fix 3 mobile temp/rescale 10 450.0 450.0 10.0 1.0
fix_modify 3 temp new2d

dump 1 all atom 2000 dump.fe_nve
thermo 2000
thermo_modify temp new2d

reset_timestep 0
run 5000000

Try sending an email to Gary Grest (gsgrest at sandia.gov). He has
several collaborators who build these kinds of systems routinely.
I think most people have their own codes that decorate a surface
with chains and output a LAMMPS input data file. I don't know of
tools that can do this automatically, unless you use a GUI builder
from some molecular design software package.

Steve