code: def sigma(self):
          r'''Surface tension of the mixture at its current temperature and
          composition, in units of [N/m].
  
          For calculation of this property at other temperatures,
          or specifying manually the method used to calculate it, and more - see
          the object oriented interface :obj:`thermo.interface.SurfaceTensionMixture`;
          each Mixture instance creates one to actually perform the calculations.
  
          Examples
          --------
          >>> Mixture(['water'], ws=[1], T=300, P=1E5).sigma
          0.07176932405246211
          '''
          return self.SurfaceTensionMixture(self.T, self.P, self.zs, self.wsf sigma(self):
          r'''Surface tension of the mixture at its current temperature and
          composition, in units of [N/m].
  
          For calculation of this property at other temperatures,
          or specifying manually the method used to calculate it, and more - see
          the object oriented interface :obj:`thermo.interface.SurfaceTensionMixture`;
          each Mixture instance creates one to actually perform the calculations.
  
          Examples
          --------
          >>> Mixture(['water'], ws=[1], T=300, P=1E5).sigma
          0.07176932405246211
          '''
          return self.SurfaceTensionMixture(self.T, self.P, self.zs, self.ws)
docs: r'''Surface tension of the mixture at its current temperature and
          composition, in units of [N/m].
  
          For calculation of this property at other temperatures,
          or specifying manually the method used to calculate it, and more - see
          the object oriented interface :obj:`thermo.interface.SurfaceTensionMixture`;
          each Mixture instance creates one to actually perform the calculations.
  
          Examples
          --------
          >>> Mixture(['water'], ws=[1], T=300, P=1E5).sigma
          0.07176932405246211