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### pair_style lubricate command

Syntax:

    pair_style lubricate mu squeeze shear pump twist cutinner cutoff T_target seed

- 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)

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 * *

Description:

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

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.

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), 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 or pair_style dpd 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 command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands, or by mixing as described below:

- cutinner (distance units)
- cutoff (distance units)

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.

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 shift option for the energy of the pair interaction.

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

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

This pair style writes its information to binary restart files, 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 command. It does not support the inner, middle, outer keywords.

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 for more info.

This pair style requires that atoms store torque and a quaternion to represent their orientation, as defined by the atom_style. It also require they store a per-type shape. The particles cannot store a per-particle diameter or per-particle mass.

All the shape settings must be for finite-size spheres. They cannot be point particles, nor can they be aspherical. Additionally all the shape types must specify particles of the same size, i.e. a monodisperse system.

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

Related commands:

pair_coeff

Default: none

(Ball) Ball and Melrose, Physica A, 247, 444-472 (1997).