The primary function of density gradient centrifugation is to separate particles, either on the basis of their buoyancy density or their rate of sedimentation. For rate-zonal separations, the function of the gradient is to provide a gradient of viscosity which improves particle resolution while stabilizing the column from convection currents. For isopycnic separations, the important feature is that the maximum density of the gradient media is higher than that of the particles. An ideal density gradient media has the following properties:3 - See more at: http://www.sigmaaldrich.com/technical-documents/articles/biofiles/centrifugation-separations.html#sthash.qrhvnuJl.dpuf