# Viscosity of spherical nanoparticle

Dear LAMMPS user,

I am modelling the metallic nanoparticle at liquid state (around melting temperature). I wonder if there is any method calculating the viscosity of a spherical nanoparticle?

Sincerely,
Truong

Dear LAMMPS user,

I am modelling the metallic nanoparticle at liquid state (around melting
temperature). I wonder if there is any method calculating the viscosity of
a spherical nanoparticle?

please explain, ​how can a single particle have a viscosity?​
you could compute the viscosity of the constituent material, but for that
you could just follow the provided howtos taking into account that
different materials require different system sizes and simulation times to
provide converged results.

http://lammps.sandia.gov/doc/Section_howto.html#howto-21

​axel.​

Dear Dr. Kohlmeyer,

In this case, we’re modelling metallic nanoparticles, i.e., cluster consisting thousands of atoms. We heat those to the melting temperature and wonder if there is any method calculating the viscosity of such spherical nanoparticles? In addition, non-periodic boundary conditions are applied in all directions.

Best Regards,
Truong

Dear Dr. Kohlmeyer,

In this case, we're modelling metallic nanoparticles, i.e., cluster
consisting thousands of atoms. We heat those to the melting temperature and
wonder if there is any method calculating the viscosity of such spherical
nanoparticles? In addition, non-periodic boundary conditions are applied in
all directions.

you are just repeating in more words what you already stated. that
doesn't help much. and you didn't answer my question. so let me
rephrase the problem:

the important question here is whether you are interested i the
viscosity of a liquid formed from many of such metallic nanoparticles,
or in the viscosity of the constituent material (i.e. the metal
itself).

i guessed from your previous e-mail, that you are most likely
interested in the second scenario, and in that case, you can just set
up a bulk system of the same material at the same conditions and
calculate the viscosity with the several methods that LAMMPS supports.
most likely such a study has already been done and you may be able to
look up the results from the corresponding publication.

axel.

I think you are interested in the first scenario, because you mention melting the nanoparticle, implying that you want to measure the internal viscosity of a nanodroplet. That is a very unusual question, but not completely crazy. I did some quick tests with examples/melt and observed that if you turn off PBC and melt the resultant cluster, the shear stress components fluctuate about zero. These time integral of the autocorrelation of these fluctuations is proportional to viscosity. See examples/VISCOSITY. I append my script below. I would expect the viscosity of the nanodroplet to be similar to that of the bulk melt, except when you get to very small droplets, and at that point you have two issues:

1. Statistical noise
2. Ill-defined volume

Aidan

# 3d Lennard-Jones melt

units lj

atom_style atomic

boundary f f f

lattice fcc 0.8442

region box block 0 100 0 100 0 100

region atoms block 45 55 45 55 45 55

create_box 1 box

create_atoms 1 region atoms

mass 1 1.0

velocity all create 2.0 87287

pair_style lj/cut 2.5

pair_coeff 1 1 1.0 1.0 2.5

neighbor 0.3 bin

neigh_modify every 20 delay 0 check no

fix 1 all nve

fix xwalls all wall/reflect xlo EDGE xhi EDGE

fix ywalls all wall/reflect ylo EDGE yhi EDGE

fix zwalls all wall/reflect zlo EDGE zhi EDGE

#dump id all atom 50 dump.melt

shell mkdir Jpgs

variable s equal logfreq(10,3,10)

dump 2 all image 1000 Jpgs/image.*.jpg type type &

axes yes 0.8 0.02 view 60 -30

dump_modify 2 pad 3 every v_s first yes

#dump 3 all movie 25 movie.mpg type type &