"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

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pair_style lubricate command :h3

[Syntax:]

pair_style lubricate mu squeeze shear pump twist cutinner cutoff T_target seed :pre

mu = dynamic viscosity (dynamic viscosity units)
squeeze = 0/1 for squeeze force off/on
shear = 0/1 for shear force off/on
pump = 0/1 for pump force off/on
twist = 0/1 for twist force off/on
cutinner = (distance units)
cutoff = outer cutoff for interactions (distance units)
T_target = desired temperature (temperature units)
seed = random number seed (positive integer) :ul

[Examples:]

pair_style lubricate 1.5 1 1 1 0 2.3 2.4 1.3 5878598
pair_coeff 1 1 1.8 2.0
pair_coeff * * :pre

pair_style lubricate 1.0 1 1 1 0 2.3 2.4 1.3 5878598
pair_coeff * *
variable mu equal ramp(1,2)
fix 1 all adapt 1 pair lubricate mu * * v_mu :pre

[Description:]

Style {lubricate} computes pairwise interactions between mono-disperse
spherical particles via this formula from "(Ball and Melrose)"_#Ball

:c,image(Eqs/pair_lubricate.jpg)

which represents the dissipation W between two nearby particles due to
their relative velocities in the presence of a background solvent with
viscosity mu.  Note that this is dynamic viscosity which has units of
mass/distance/time, not kinematic viscosity.

The viscosity mu can be varied in a time-dependent manner over the
course of a simluation, in which case in which case the pair_style
setting for mu will be overridden.  See the "fix adapt"_fix_adapt.html
command for details.

Rc is the outer cutoff specified in the pair_style command, the
translational velocities of the 2 particles are v1 and v2, the angular
velocities are w1 and w2, and n is the unit vector in the direction
from particle 1 to 2.  The 4 terms represent four modes of pairwise
interaction: squeezing, shearing, pumping, and twisting.  The 4 flags
in the pair_style command turn on or off each of these modes by
including or excluding each term.  The 4 coefficients on each term are
functions of the separation distance of the particles and the
viscosity.  Details are given in "(Ball and Melrose)"_#Ball, including
the forces and torques that result from taking derivatives of this
equation with respect to velocity (see Appendix A).

Unlike most pair potentials, the two specified cutoffs (cutinner and
cutoff) refer to the surface-to-surface separation between two
particles, not center-to-center distance.  Currently, this pair style
can only be used for mono-disperse extended spheres (same radii), so
that separation is r_ij - 2*radius, where r_ij is the center-to-center
distance between the particles.  Within the inner cutoff {cutinner},
the forces and torques are evaluated at a separation of cutinner.  The
outer {cutoff} is the separation distance beyond which the pair-wise
forces are zero.

A Langevin thermostatting term is also added to the pairwise force,
similar to that provided by the "fix langevin"_fix_langevin.html or
"pair_style dpd"_pair_dpd.html commands.  The target temperature for
the thermostat is the specified {T_target}.  The {seed} is used for
the random numbers generated for the thermostat.

The following coefficients must be defined for each pair of atoms
types via the "pair_coeff"_pair_coeff.html command as in the examples
above, or in the data file or restart files read by the
"read_data"_read_data.html or "read_restart"_read_restart.html
commands, or by mixing as described below:

cutinner (distance units)
cutoff (distance units) :ul

The two coefficients are optional.  If neither is specified, the two
cutoffs specified in the pair_style command are used.  Otherwise both
must be specified.

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[Mixing, shift, table, tail correction, restart, rRESPA info]:

For atom type pairs I,J and I != J, the two cutoff distances for this
pair style can be mixed.  The default mix value is {geometric}.  See
the "pair_modify" command for details.

This pair style does not support the "pair_modify"_pair_modify.html
shift option for the energy of the pair interaction.

The "pair_modify"_pair_modify.html table option is not relevant
for this pair style.

This pair style does not support the "pair_modify"_pair_modify.html
tail option for adding long-range tail corrections to energy and
pressure.

This pair style writes its information to "binary restart
files"_restart.html, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file.

This pair style can only be used via the {pair} keyword of the
"run_style respa"_run_style.html command.  It does not support the
{inner}, {middle}, {outer} keywords.

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[Restrictions:]

This style is part of the "colloid" package.  It is only enabled if
LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#2_3 section for more info.

This pair style requires that atoms be finite-size spheres with a
diameter, as defined by the "atom_style sphere"_atom_style.html
command.

Per-particle or per-type polydispersity is not yet supported by this
pair style; all particles must have the same diameter.

This pair style requires you to use the "communicate vel
yes"_communicate.html option so that velocites are stored by ghost
atoms.

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

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:link(Ball)
[(Ball)] Ball and Melrose, Physica A, 247, 444-472 (1997).
