"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 :pre

mu = viscosity (mass/distance/time 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) :ul

[Examples:]

pair_style lubricate 1.5 1 1 1 0 2.3 2.4
pair_coeff 1 1 1.8 2.0
pair_coeff * * :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
the background solvent.  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.  Details are given in "(Ball and Melrose)"_#Ball,
including the forces and torques that result from taking derivatives
of this equation (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 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.

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, per-atom energy/stress,
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 does not calculate per-atom energy and stress, as used
by the "compute epair/atom"_compute_epair_atom.html, "compute
stress/atom"_compute_stress_atom.html, and "dump custom"_dump.html
commands.

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.

Because this poential computes forces and torques on particles, the
atom style must support particles whose size is set via the
"shape"_shape.html command.  This is "atom_style"_atom_style.html
ellipsoid and dipole.  Since only spherical mono-disperse particles
are currently allowed for pair_style lubricate, this means the 3 shape
radii for all particle types must be the same.

[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).
