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pair_lubricate.html
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<HTML>
<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>pair_style lubricate command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style lubricate mu squeeze shear pump twist cutinner cutoff T_target seed
</PRE>
<UL><LI>mu = dynamic viscosity (dynamic viscosity units)
<LI>squeeze = 0/1 for squeeze force off/on
<LI>shear = 0/1 for shear force off/on
<LI>pump = 0/1 for pump force off/on
<LI>twist = 0/1 for twist force off/on
<LI>cutinner = (distance units)
<LI>cutoff = outer cutoff for interactions (distance units)
<LI>T_target = desired temperature (temperature units)
<LI>seed = random number seed (positive integer)
</UL>
<P><B>Examples:</B>
</P>
<PRE>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>
<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>
<P><B>Description:</B>
</P>
<P>Style <I>lubricate</I> computes pairwise interactions between mono-disperse
spherical particles via this formula from <A HREF = "#Ball">(Ball and Melrose)</A>
</P>
<CENTER><IMG SRC = "Eqs/pair_lubricate.jpg">
</CENTER>
<P>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.
</P>
<P>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 <A HREF = "fix_adapt.html">fix adapt</A>
command for details.
</P>
<P>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 <A HREF = "#Ball">(Ball and Melrose)</A>, including
the forces and torques that result from taking derivatives of this
equation with respect to velocity (see Appendix A).
</P>
<P>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 <I>cutinner</I>,
the forces and torques are evaluated at a separation of cutinner. The
outer <I>cutoff</I> is the separation distance beyond which the pair-wise
forces are zero.
</P>
<P>A Langevin thermostatting term is also added to the pairwise force,
similar to that provided by the <A HREF = "fix_langevin.html">fix langevin</A> or
<A HREF = "pair_dpd.html">pair_style dpd</A> commands. The target temperature for
the thermostat is the specified <I>T_target</I>. The <I>seed</I> is used for
the random numbers generated for the thermostat.
</P>
<P>The following coefficients must be defined for each pair of atoms
types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples
above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands, or by mixing as described below:
</P>
<UL><LI>cutinner (distance units)
<LI>cutoff (distance units)
</UL>
<P>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.
</P>
<HR>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
</P>
<P>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 <I>geometric</I>. See
the "pair_modify" command for details.
</P>
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
shift option for the energy of the pair interaction.
</P>
<P>The <A HREF = "pair_modify.html">pair_modify</A> table option is not relevant
for this pair style.
</P>
<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
tail option for adding long-range tail corrections to energy and
pressure.
</P>
<P>This pair style writes its information to <A HREF = "restart.html">binary restart
files</A>, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file.
</P>
<P>This pair style can only be used via the <I>pair</I> keyword of the
<A HREF = "run_style.html">run_style respa</A> command. It does not support the
<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
</P>
<HR>
<P><B>Restrictions:</B>
</P>
<P>This style is part of the "colloid" package. It is only enabled if
LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">Making
LAMMPS</A> section for more info.
</P>
<P>This pair style requires that atoms be finite-size spheres with a
diameter, as defined by the <A HREF = "atom_style.html">atom_style sphere</A>
command.
</P>
<P>Per-particle or per-type polydispersity is not yet supported by this
pair style; all particles must have the same diameter.
</P>
<P>This pair style requires you to use the <A HREF = "communicate.html">communicate vel
yes</A> option so that velocites are stored by ghost
atoms.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>
</P>
<P><B>Default:</B> none
</P>
<HR>
<A NAME = "Ball"></A>
<P><B>(Ball)</B> Ball and Melrose, Physica A, 247, 444-472 (1997).
</P>
</HTML>
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