def Confinement(D, rhol, rhog, sigma, g=g):
    """Calculates Confinement number or `Co` for a fluid in a channel of 
    diameter `D` with liquid and gas densities `rhol` and `rhog` and surface
    tension `sigma`, under the influence of gravitational force `g`.

    .. math::
        \\text{Co}=\\frac{\\left[\\frac{\\sigma}{g(\\rho_l-\\rho_g)}\\right]^{0.5}}{D}

    Parameters
    ----------
    D : float
        Diameter of channel, [m]
    rhol : float
        Density of liquid phase, [kg/m^3]
    rhog : float
        Density of gas phase, [kg/m^3]
    sigma : float
        Surface tension between liquid-gas phase, [N/m]
    g : float, optional
        Acceleration due to gravity, [m/s^2]

    Returns
    -------
    Co : float
        Confinement number [-]

    Notes
    -----
    Used in two-phase pressure drop and heat transfer correlations. First used
    in [1]_ according to [3]_.

    .. math::
        \\text{Co} = \\frac{\\frac{\\text{surface tension force}}
        {\\text{buoyancy force}}}{\\text{Channel area}}

    Examples
    --------
    >>> Confinement(0.001, 1077, 76.5, 4.27E-3)
    0.6596978265315191

    References
    ----------
    .. [1] Cornwell, Keith, and Peter A. Kew. "Boiling in Small Parallel 
       Channels." In Energy Efficiency in Process Technology, edited by Dr P. 
       A. Pilavachi, 624-638. Springer Netherlands, 1993. 
       doi:10.1007/978-94-011-1454-7_56.
    .. [2] Kandlikar, Satish G. Heat Transfer and Fluid Flow in Minichannels 
       and Microchannels. Elsevier, 2006.
    .. [3] Tran, T. N, M. -C Chyu, M. W Wambsganss, and D. M France. Two-Phase 
       Pressure Drop of Refrigerants during Flow Boiling in Small Channels: An 
       Experimental Investigation and Correlation Development." International 
       Journal of Multiphase Flow 26, no. 11 (November 1, 2000): 1739-54. 
       doi:10.1016/S0301-9322(99)00119-6.
    """
    return (sigma / (g * (rhol - rhog))) ** 0.5 / D
