# Interfacial tension (IFT)

Dear users

Does anybody know how calculating Interfacial tension (IFT) between two immiscible liquids or liquid/gas in LAMMPS?

If yes, would you please let me know an illustrative work or paper?

Sincerely

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Shame not to catch the chance for self-promotion:
https://doi.org/10.1063/1.4937153
(look in “parameterization”)

The formula is well known, and dates back decades based on Kirkwood’s work (assumes integration an orthogonal unit cell going across the interface). The procedure for air/liquid or liquid/liquid is basically the same: use a unit cell with constant volume, and all liquids must be in equilibrium for it. Because fluctuations of the pressure tensor are big, expect slow converge and some sensitivity to cutoffs. Isotropic fluids (aside from the interface) will converge much better than oriented fluids like lipid membranes.

Specifically in LAMMPS, you can define the formula for gamma as an equal-style variable, and print it through a custom thermo output or fix props.

Giacomo

Sorry, I mistyped: it’s fix print, not fix props.

Liquid-vapor simulations of surface tension are easy to set up: https://doi.org/10.1063/1.2209240

The basic idea is that you set up a slab of the liquid in a simulation box with the direction normal to the surfaces elongated, from which you can compute the surface tension from the difference between the pressure tensor components normal and parallel to the interface. (The formulas are provided.) Note that if long-range dispersion (PPPM/disp) is used, then there is no need to include the long-range corrections that must otherwise be calculated from the density profiles (https://doi.org/10.1080/00268979500101371).

To do liquid-liquid surface tension calculations, you can measure each of the individual liquid-gas surface tensions, and subtract those from the value obtained for the combined liquid-liquid system. (The method is explained in more detail: https:doi.org/10.1021/ma802805y.

—AEI

My system is largely anisotropic, comprising many unsymmetric lipids in contact with the aqueous phase.

T
here are large fluctuations in pressure tensor when I try to simulate at NPT ensemble. So, I could not obtain reliable IFT values.

Is there any way to reduce those fluctuations?

S
increly

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Hi, Robert:

The best way to reduce fluctuations are the most straightforward: run larger systems and longer simulations.

Also, you should not be using NPT simulations to determine the surface tension. You can equilibrate using NPT, then deform the box to create the “head space” corresponding to the vapor phase, and then run in the NVT ensemble to get the surface tension.

—AEI