def Morton(rhol, rhog, mul, sigma, g=g):
    """Calculates Morton number or `Mo` for a liquid and vapor with the
    specified properties, under the influence of gravitational force `g`.

    .. math::
        Mo = \\frac{g \\mu_l^4(\\rho_l - \\rho_g)}{\\rho_l^2 \\sigma^3}
        
    Parameters
    ----------
    rhol : float
        Density of liquid phase, [kg/m^3]
    rhog : float
        Density of gas phase, [kg/m^3]
    mul : float
        Viscosity of liquid phase, [Pa*s]
    sigma : float
        Surface tension between liquid-gas phase, [N/m]
    g : float, optional
        Acceleration due to gravity, [m/s^2]

    Returns
    -------
    Mo : float
        Morton number, [-]

    Notes
    -----
    Used in modeling bubbles in liquid.

    Examples
    --------
    >>> Morton(1077.0, 76.5, 4.27E-3, 0.023)
    2.311183104430743e-07

    References
    ----------
    .. [1] Kunes, Josef. Dimensionless Physical Quantities in Science and 
       Engineering. Elsevier, 2012.
    .. [2] Yan, Xiaokang, Kaixin Zheng, Yan Jia, Zhenyong Miao, Lijun Wang,
       Yijun Cao, and Jiongtian Liu. “Drag Coefficient Prediction of a Single 
       Bubble Rising in Liquids.” Industrial & Engineering Chemistry Research, 
       April 2, 2018. https://doi.org/10.1021/acs.iecr.7b04743.
    """
    mul2 = mul * mul
    return g * mul2 * mul2 * (rhol - rhog) / (rhol * rhol * sigma * sigma *
        sigma)
