def calcKn(lm, Dp):
    """
    Returns the Knudsen Number (Kim et al., 2005)
    
    Parameters
    ----------
    lm : float
        gas mean free path (nm)
        
    Dp : paticle diameter (nm)
        
    Returns
    -------
    Kn : float
        Knudsen Number
    """
    #lm = lm *10**-8
    #Dp = Dp *10**-8
    
    Kn = (2*lm) / Dp
    
    return Kn