Dear LAMMPS community,

I had a look at the code of pair_lubricate.cpp / pair_lubricate_poly.cpp

and would like to raise some points with regard to the lubrication force

implementation.

Brief overview:

1) One additional "beta0" seems to be missing in 2nd squeeze term

2) Dimensionless gap distance definition different to literature

3) Inconsistency between "pair_lubricate_poly" and "pair_lubricate" for

"a_pu" term

In detail:

1) I think that there is a "beta" missing in the second term of the

squeeze term:

Current implementation (line 298 in pair_lubricate_poly.cpp):

"a_sq = ... +

(1.0+7.0*beta0+beta0*beta0)/5.0/pow(beta1,3.0)*log(1.0/h_sep);"

Comparing it to Kim & Karilla "Microhydrodynamics: Principles and

Selected Applications", it should be multiplied by an additional "beta0":

"a_sq = ... +

beta0*(1.0+7.0*beta0+beta0*beta0)/5.0/pow(beta1,3.0)*log(1.0/h_sep);"

2) Dimensionless gap distance "h_sep" in pair_lubricate_poly.cpp is:

"h_sep = (r - radi - radj)/radi"

In Kim & Karilla it is however:

"h_sep = 2*(r- radi - radj)/(radi+radj)"

This will affect the terms "log(1.0/h_sep)" and make them different to

the literature.

Was the former equation implemented deliberately like this and if so,

what is the reasoning behind it?

3) It seems that the leading order term for "a_pu" in pair_lubricate.cpp

is not consistent to "a_pu" in pair_lubricate_poly.cpp when monosized

spheres are chosen.

If we set "beta0 = 1" in pair_lubricate_poly, we obtain for the first

term in "a_pu = beta0*(4.0+beta0)/10.0/beta1/beta1*log(1.0/h_sep);"

(line 309):

"a_pu = 1/8*log(1.0/h_sep);"

Comparison to pair_lubricate.cpp (line 311):

"a_pu = ... *(3.0/160.0*log(1.0/h_sep));

I would have expected it to be the same.

Currently, I am having a look at all the resistance functions in Kim &

Karilla and it looks as if the implemented "a_pu" formula in

pair_lubricate_poly consists of the resistance functions "Y_11^B" and

"Y_12^B" instead of "Y_11^C" and "Y_12^C" (what I'd have expected).

I would be grateful if someone had any ideas to points 2) and 3).

Thank you in advance,

Tim