def Capillary(V, mu, sigma):
    """Calculates Capillary number `Ca` for a characteristic velocity `V`,
    viscosity `mu`, and surface tension `sigma`.

    .. math::
        Ca = \\frac{V \\mu}{\\sigma}

    Parameters
    ----------
    V : float
        Characteristic velocity, [m/s]
    mu : float
        Dynamic viscosity, [Pa*s]
    sigma : float
        Surface tension, [N/m]

    Returns
    -------
    Ca : float
        Capillary number, [-]

    Notes
    -----
    Used in porous media calculations and film flow calculations.
    Surface tension may gas-liquid, or liquid-liquid.

    .. math::
        Ca = \\frac{\\text{Viscous forces}}
        {\\text{Surface forces}}

    Examples
    --------
    >>> Capillary(1.2, 0.01, .1)
    0.12

    References
    ----------
    .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
       Eighth Edition. McGraw-Hill Professional, 2007.
    .. [2] Kundu, Pijush K., Ira M. Cohen, and David R. Dowling. Fluid
       Mechanics. Academic Press, 2012.
    """
    return V * mu / sigma
