def drop_axis_ratio(D_eq):
    """
    Axis ratio of drops with respect to their diameter.

    Parameter:
    ==========
    D_eq: float
        Drop diameter.

    Return:
    =======
    axratio: float
        Axis ratio of drop.
    """
    if D_eq < 0.7:
        axratio = 1.0  # Spherical
    elif D_eq < 1.5:
        axratio = (
            1.173
            - 0.5165 * D_eq
            + 0.4698 * D_eq ** 2
            - 0.1317 * D_eq ** 3
            - 8.5e-3 * D_eq ** 4
        )
    else:
        axratio = (
            1.065
            - 6.25e-2 * D_eq
            - 3.99e-3 * D_eq ** 2
            + 7.66e-4 * D_eq ** 3
            - 4.095e-5 * D_eq ** 4
        )

    return 1.0 / axratio