Surface tension: Difference between revisions

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The '''surface tension''',
The '''surface tension''',
<math> \gamma </math>, is a measure of the work required to create a surface.
<math> \gamma </math>, is a measure of the [[work]] required to create an [[interface]] between
two bulk phases.


== Thermodynamics ==  
== Thermodynamics ==  
Line 16: Line 17:


==Computer Simulation==
==Computer Simulation==
A review of the  different techniques that can be used to compute the surface (interface) tension can be found in the paper by Gloor ''et al.'' (Ref. 1).
Different techniques may be used to compute this quantity, such as the traditional [[stress | stress tensor]] route. More recently, several methods have been proposed which avoid the some times difficult calculation of the stress tensor, e.g., the [[test area method]] and [[wandering interface method]]. A review can be found in the paper by Gloor ''et al.'' <ref name="Gloor">[http://dx.doi.org/10.1063/1.2038827    Guy J. Gloor, George Jackson,    Felipe J. Blas and Enrique de Miguel    "Test-area simulation method for the direct determination of the interfacial tension of systems with continuous or discontinuous potentials", Journal of Chemical Physics '''123''' 134703 (2005)]</ref>.


==Liquid-Vapour Interfaces of one component systems ==
==Liquid-Vapour Interfaces of one component systems ==
=== Binder procedure===
=== Binder procedure===
Here, only an outline of the procedure is presented, more details can be found in Reference 2.
Here, only an outline of the procedure is presented, more details can be found in <ref>[http://dx.doi.org/10.1103/PhysRevA.25.1699 K. Binder "Monte Carlo calculation of the surface tension for two- and three-dimensional lattice-gas models", Physical Review A '''25''' pp. 1699 - 1709 (1982)]</ref>.
For given conditions of volume and temperature, the [[Helmholtz  energy function]] is computed as a function of the number of molecules, <math> A(N;V,T)</math>. The calculation is usually carried out using [[Monte Carlo]] simulation using [[periodic boundary conditions]]  
For given conditions of volume and temperature, the [[Helmholtz  energy function]] is computed as a function of the number of molecules, <math> A(N;V,T)</math>. The calculation is usually carried out using [[Monte Carlo]] simulation using [[periodic boundary conditions]]  
If liquid-vapour equilibrium occurs, a plot of the [[chemical potential]], <math> \mu \equiv (\partial A/\partial N)_{V,T} </math>,  
If liquid-vapour equilibrium occurs, a plot of the [[chemical potential]], <math> \mu \equiv (\partial A/\partial N)_{V,T} </math>,  
Line 36: Line 37:
:<math> \Omega (N) \equiv A(N)  - \mu_{eq} N = - p_{eq} V + \gamma {\mathcal A}(N)  </math>.
:<math> \Omega (N) \equiv A(N)  - \mu_{eq} N = - p_{eq} V + \gamma {\mathcal A}(N)  </math>.


For appropriate values of <math> N </math> one can estimate the value of the surface area, <math> {\mathcal A} </math> (See MacDowell ''et al.'', Ref. 3), and compute <math> \gamma </math> directly as:
For appropriate values of <math> N </math> one can estimate the value of the surface area, <math> {\mathcal A} </math> (See MacDowell ''et al.'' <ref>[http://dx.doi.org/10.1063/1.2218845  L. G. MacDowell, V. K. Shen, and J. R. Errington "Nucleation and cavitation of spherical, cylindrical, and slablike droplets and bubbles in small systems", Journal of Chemical Physics '''125'''  034705 (2006)]</ref>), and compute <math> \gamma </math> directly as:


:<math> \gamma = \frac{ \Omega(N) + p_{eq} V } { {\mathcal A}(N) } = \frac{ \Omega(N) - \frac{1}{2}(\Omega(N_l)+\Omega(N_v)) }{{\mathcal A}(N)} </math>
:<math> \gamma = \frac{ \Omega(N) + p_{eq} V } { {\mathcal A}(N) } = \frac{ \Omega(N) - \frac{1}{2}(\Omega(N_l)+\Omega(N_v)) }{{\mathcal A}(N)} </math>
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Taking into account the [[canonical ensemble]] definition (see above), one  computes the change in the [[Helmholtz energy function]] when a small (differential)
Taking into account the [[canonical ensemble]] definition (see above), one  computes the change in the [[Helmholtz energy function]] when a small (differential)
change of the surface area is performed at constant <math> V, T, </math> and <math> N </math>.
change of the surface area is performed at constant <math> V, T, </math> and <math> N </math>.
The explicit equations can be written in terms of the diagonal components of the [[pressure tensor]] of the system.
The explicit equations can be written in terms of the diagonal components of the [[pressure]] tensor of the system.
Mechanical arguments can also be invoked to arrive at equivalent conclusions (see Ref 1 for a detailed discussion of these issues).
Mechanical arguments can also be invoked to arrive at equivalent conclusions (see Ref <ref name="Gloor"></ref> for a detailed discussion of these issues).


=== System-size analysis ===
=== System-size analysis ===
The [[Finite size effects |system-size dependence]] of the results for <math> \gamma </math> have to be taken into account in order to  obtain  accurate results for [[Models |model systems]].
The [[Finite size effects |system-size dependence]] of the results for <math> \gamma </math> have to be taken into account in order to  obtain  accurate results for [[Models |model systems]].
Spurious effects that occur due to small system sizes can appear in the ''explicit interface'' methods. (See P.Orea ''et al.'' Ref. 4).
Spurious effects that occur due to small system sizes can appear in the ''explicit interface'' methods. (See P.Orea ''et al.'' <ref>[http://dx.doi.org/10.1063/1.2018640  Pedro Orea, Jorge López-Lemus, and  José Alejandre,  "Oscillatory surface tension due to finite-size effects", Journal of Chemical Physics '''123'''  114702 (6 pages) (2005)]</ref>).


== Mixtures ==
== Mixtures ==
 
Different ensembles can be used to compute the surface tension between two phases in the case of mixtures (See for example see Y. Zhang ''et al.'' <ref>[http://dx.doi.org/10.1063/1.469927  Yuhong Zhang, Scott E. Feller, Bernard R. Brooks, and Richard W. Pastor, "Computer simulation of liquid/liquid interfaces. I. Theory and application to octane/water", Journal of Chemical Physics, '''103''', pp. 10252-10266 (1995)]</ref>).
Different ensembles can be used to compute the surface tension between two phases in the case of mixtures (See for example Y. Zhang ''et al.'' Ref 5).
The simulation techniques are essentially the same as those for  one-component systems, but different ensembles can be more adequate.
 
For instance, for binary mixtures (with components 1 and 2), the [[isothermal-isobaric ensemble]], <math> N_1,N_2,p.T </math> is a ''more natural'' ensemble to compute
The simulation techniques are esentially the same as in one-component systems, but different ensembles can be more adequate.
<math> \gamma </math> using explicit interface techniques (See Ref. <ref>[http://dx.doi.org/10.1063/1.2751153  E. de Miguel, N. G. Almarza, and G. Jackson, "Surface tension of the Widom-Rowlinson model", Journal of Chemical Physics, '''127''', 034707  (10 pages)  (2007)]</ref>  as an example).  
For instance for binary mixtures, the [[isothermal-isobaric ensemble]], <math> N_1,N_2,P.T </math> ia a ''more natural'' ensemble to compute
In the case of the Binder technique
<math> \gamma </math> using explicit interface techniques (See Ref. 6 as an example).
the analysis can be carried out by fixing the total number of particles: <math> N \equiv N_1 + N_2 </math>, the [[pressure]], <math> p </math>, and the [[temperature]] <math> T </math>. Then
 
one will have to compute the variation of the adequate thermodynamic potential as a function of the composition, e.g. <math> x_1 = N_1/N </math>.


==See also==
==See also==
*[[Line tension]]
*[[Boundary tension]]
*[[Droplets]]
*[[Droplets]]
==References==
==References==
#[http://dx.doi.org/10.1063/1.2038827     Guy J. Gloor, George Jackson,    Felipe J. Blas and Enrique de Miguel    "Test-area simulation method for the direct determination of the interfacial tension of systems with continuous or discontinuous potentials", Journal of Chemical Physics '''123''' 134703 (2005)]
<references/>
#[http://dx.doi.org/10.1103/PhysRevA.25.1699 K. Binder "Monte Carlo calculation of the surface tension for two- and three-dimensional lattice-gas models", Physical Review A '''25''' pp. 1699 - 1709 (1982)]
;Related reading
#[http://dx.doi.org/10.1063/1.2218845 L. G. MacDowell, V. K. Shen, and J. R. Errington "Nucleation and cavitation of spherical, cylindrical, and slablike droplets and bubbles in small systems", Journal of Chemical Physics '''125''' 034705 (2006)]
*[http://dx.doi.org/10.1063/1.1747248     John G. Kirkwood and Frank P. Buff "The Statistical Mechanical Theory of Surface Tension", Journal of Chemical Physics '''17''' pp. 338-343 (1949)]
#[http://dx.doi.org/10.1063/1.2018640  Pedro Orea, Jorge López-Lemus, and José Alejandre,  "Oscillatory surface tension due to finite-size effects", Journal of Chemical Physics '''123''' 114702 (6 pages) (2005)]
*[http://dx.doi.org/10.1016/0021-9991(76)90078-4 Charles H. Bennett  "Efficient estimation of free energy differences from Monte Carlo data", Journal of Computational Physics  '''22''' pp. 245-268 (1976)]
#[http://dx.doi.org/10.1063/1.469927 Yuhong Zhang, Scott E. Feller, Bernard R. Brooks, and Richard W. Pastor, ''Computer simulation of liquid/liquid interfaces. I. Theory and application to octane/water'', Journal of Chemical Physics, 103, pp. 10252-10266 (1995) ]
*[http://dx.doi.org/10.1063/1.432627 J. Miyazaki, J. A. Barker and G. M. Pound "A new Monte Carlo method for calculating surface tension", Journal of Chemical Physics '''64'''  pp. 3364-3369 (1976)]
*[https://doi.org/10.1016/j.molliq.2016.11.103 A. Maslechko, K. Glavatskiy, V.L. Kulinskii "Surface tension of molecular liquids: Lattice gas approach", Journal of Molecular Liquids '''235''' pp. 119-125 (2017)]
*[https://doi.org/10.1016/j.molliq.2016.12.062 Stephan Werth, Martin Horsch and Hans Hasse "Molecular simulation of the surface tension of 33 multi-site models for real fluids", Journal of Molecular Liquids '''235''' pp. 126-134 (2017)]
*[https://doi.org/10.1063/1.5008473 T. Dreher, C. Lemarchand, L. Soulard, E. Bourasseau, P. Malfreyt, and N. Pineau "Calculation of a solid/liquid surface tension: A methodological study", Journal of Chemical Physics '''148''' 034702 (2018)]
 
==External links==
*[http://dx.doi.org/10.4249/scholarpedia.9218 Charles Pfister "Interface free energy", Scholarpedia, 5(2):9218 (2010)]
[[category: statistical mechanics]]
[[category: statistical mechanics]]

Latest revision as of 12:26, 30 January 2018

The surface tension, , is a measure of the work required to create an interface between two bulk phases.

Thermodynamics[edit]

In the Canonical ensemble the surface tension is formally given as:

;

where

  • is the Helmholtz energy function
  • is the number of particles
  • is the volume
  • is the temperature
  • is the surface area

Computer Simulation[edit]

Different techniques may be used to compute this quantity, such as the traditional stress tensor route. More recently, several methods have been proposed which avoid the some times difficult calculation of the stress tensor, e.g., the test area method and wandering interface method. A review can be found in the paper by Gloor et al. [1].

Liquid-Vapour Interfaces of one component systems[edit]

Binder procedure[edit]

Here, only an outline of the procedure is presented, more details can be found in [2]. For given conditions of volume and temperature, the Helmholtz energy function is computed as a function of the number of molecules, . The calculation is usually carried out using Monte Carlo simulation using periodic boundary conditions If liquid-vapour equilibrium occurs, a plot of the chemical potential, , as a function of shows a loop. Using basic thermodynamic procedures (Maxwell's equal area construction) it is possible to compute the densities of the two phases; at liquid-vapour equilibrium. Considering the thermodynamic limit for densities with the Helmholtz energy function will be:

where the quantities with the subindex "eq" are those corresponding to the fluid-phase equilibrium situation. From the previous equation one can write

.

For appropriate values of one can estimate the value of the surface area, (See MacDowell et al. [3]), and compute directly as:

where and are given by: and

Explicit interfaces[edit]

In these methods one performs a direct simulation of the two-phase system. Periodic boundary conditions are usually employed. Simulation boxes are elongated in one direction, and the interfaces are built (and expected to stay) perpendicular to such a direction. Taking into account the canonical ensemble definition (see above), one computes the change in the Helmholtz energy function when a small (differential) change of the surface area is performed at constant and . The explicit equations can be written in terms of the diagonal components of the pressure tensor of the system. Mechanical arguments can also be invoked to arrive at equivalent conclusions (see Ref [1] for a detailed discussion of these issues).

System-size analysis[edit]

The system-size dependence of the results for have to be taken into account in order to obtain accurate results for model systems. Spurious effects that occur due to small system sizes can appear in the explicit interface methods. (See P.Orea et al. [4]).

Mixtures[edit]

Different ensembles can be used to compute the surface tension between two phases in the case of mixtures (See for example see Y. Zhang et al. [5]). The simulation techniques are essentially the same as those for one-component systems, but different ensembles can be more adequate. For instance, for binary mixtures (with components 1 and 2), the isothermal-isobaric ensemble, is a more natural ensemble to compute using explicit interface techniques (See Ref. [6] as an example). In the case of the Binder technique the analysis can be carried out by fixing the total number of particles: , the pressure, , and the temperature . Then one will have to compute the variation of the adequate thermodynamic potential as a function of the composition, e.g. .

See also[edit]

References[edit]

Related reading

External links[edit]