def kn(dp, gas):
    """
    Calculate the Knudsen number of a particle.

    The Knudsen number determines the appropriateness of the continuum assumption.  If Kn >~1, then the continuum
    assumption is not appropriate for the problem solution.

    Parameters
    ----------
    dp:     float
            particle diameter in nm
    gas:    gas object
            Gas object used to determine the mean free path of the gas.

    Returns
    -------
    float
        Knudsen number

    """
    return 2*gas.l/dp