def two_phase_dP_dz_gravitational(angle, alpha, rhol, rhog, g=g):
    r'''This function handles calculation of two-phase liquid-gas pressure drop
    due to gravitation for flow inside channels. This is a differential 
    calculation for a segment with an infinitesimal difference in elevation for
    use in performing integration over a pipe as shown in [1]_ and [2]_.
    
    .. math::
        -\left(\frac{dP}{dz} \right)_{grav} =  [\alpha\rho_g + (1-\alpha)
        \rho_l]g \sin \theta
        
    Parameters
    ----------
    angle : float
        The angle of the pipe with respect to the horizontal, [degrees]
    alpha : float
        Void fraction (area of gas / total area of channel), [-]
    rhol : float
        Liquid phase density, [kg/m^3]
    rhog : float
        Gas phase density, [kg/m^3]
    g : float, optional
        Acceleration due to gravity, [m/s^2]

    Returns
    -------
    dP_dz : float
        Gravitational component of pressure drop for two-phase flow, [Pa/m]
        
    Notes
    -----
        
    Examples
    --------    
    >>> two_phase_dP_dz_gravitational(angle=90, alpha=0.9685, rhol=1518, 
    ... rhog=2.6)
    493.6187084149995

    References
    ----------
    .. [1] Rohsenow, Warren and James Hartnett and Young Cho. Handbook of Heat
       Transfer, 3E. New York: McGraw-Hill, 1998.
    .. [2] Kim, Sung-Min, and Issam Mudawar. "Review of Databases and
       Predictive Methods for Pressure Drop in Adiabatic, Condensing and
       Boiling Mini/Micro-Channel Flows." International Journal of Heat and
       Mass Transfer 77 (October 2014): 74-97.
       doi:10.1016/j.ijheatmasstransfer.2014.04.035.
    '''
    angle = radians(angle)
    return g*sin(angle)*(alpha*rhog + (1. - alpha)*rhol)