code: def _surface(self, T):
          
          tau = 1-T/self.Tc
          sigma = 0
          for n, t in zip(self._surf["sigma"], self._surf["exp"]):
              sigma += n*tau**t
          return sigma
docs: Generic equation for the surface tension
  
          Parameters
          ----------
          T : float
              Temperature, [K]
  
          Returns
          -------
          σ : float
              Surface tension, [N/m]
  
          Notes
          -----
          Need a _surf dict in the derived class with the parameters keys:
              sigma: coefficient
              exp: exponent