def Hicks(dp, voidage, vs, rho, mu, L=1):
    """Calculates pressure drop across a packed bed of spheres using a
    correlation developed in [1]_, as shown in [2]_. Twenty-third most accurate
    correlation overall in the review of [2]_. Also shown in [3]_.

    .. math::
        f_p = 6.8 \\frac{(1-\\epsilon)^{1.2}}{Re^{0.2}\\epsilon^3}

    .. math::
        f_p = \\frac{\\Delta P d_p}{\\rho v_s^2 L}

    .. math::
        Re = \\frac{\\rho v_s  d_p}{\\mu}

    Parameters
    ----------
    dp : float
        Particle diameter of spheres [m]
    voidage : float
        Void fraction of bed packing [-]
    vs : float
        Superficial velocity of the fluid (volumetric flow rate/cross-sectional 
        area)[m/s]
    rho : float
        Density of the fluid [kg/m^3]
    mu : float
        Viscosity of the fluid, [Pa*s]
    L : float, optional
        Length the fluid flows in the packed bed [m]

    Returns
    -------
    dP : float
        Pressure drop across the bed [Pa]

    Notes
    -----
    Valid in [1]_, [2]_, and [3]_ for a range of
    :math:`300 < Re_{Erg} < 60,000`.

    Examples
    --------
    >>> Hicks(dp=0.01, voidage=0.4, vs=1E-3, rho=1E3, mu=1E-3)
    3.631703956680737

    References
    ----------
    .. [1] Hicks, R. E. "Pressure Drop in Packed Beds of Spheres." Industrial
       Engineering Chemistry Fundamentals 9, no. 3 (August 1, 1970): 500-502.
       doi:10.1021/i160035a032.
    .. [2] Erdim, Esra, Ömer Akgiray, and İbrahim Demir. "A Revisit of Pressure
       Drop-Flow Rate Correlations for Packed Beds of Spheres." Powder
       Technology 283 (October 2015): 488-504. doi:10.1016/j.powtec.2015.06.017.
    .. [3] Allen, K. G., T. W. von Backstrom, and D. G. Kroger. "Packed Bed
       Pressure Drop Dependence on Particle Shape, Size Distribution, Packing
       Arrangement and Roughness." Powder Technology 246 (September 2013):
       590-600. doi:10.1016/j.powtec.2013.06.022.
    """
    Re = dp * rho * vs / mu
    fp = 6.8 * (1 - voidage) ** 1.2 / Re ** 0.2 / voidage ** 3
    return fp * rho * vs ** 2 * L / dp
