def droplet_volume(length=100, diameter=250):
    """
    convert the length of a plug to volume in 250 um capillary.

    Parameters
    ----------
    l : float
        droplet or plug length
    d : float
        channel diameter

    Returns
    -------
    v : float
        volume of the object

    Examples
    --------
    the example of usage

    >>> droplet_volume(250,250)
    """
