def surface_tension(t_k=453.7):
    """Surface tension as a function of temperature

    Parameters
    ----------
    t_k : float, default=453.7
        K, temperature. Default is the melting point.

    Returns
    -------
    σ : float
        N m, surface tension

    Davison, H.W.
    "Complication of Thermophysical Properties of Liquid Lithium."
    NASA Technical Note TN D-4650.
    Cleveland, Ohio: Lewis Research Center, July 1968.

    "Using equation [below], the data are correlated with
     a standard deviation of +- 1.9 percent. The maximum difference
     between the data and the correlation is +5.2 percent."
    """
    σ = 0.447 - 1.07e-4 * t_k - 1.351e-8 * t_k**2
    return σ