Preparation of self-generating gradients

Iodixanol, like solutions of heavy metal salts (e.g. CsCl) can form a gradient from a solution of uniform density under the influence of the centrifugal field. Once the solute begins to sediment through the solvent a concentration gradient is formed which is opposed by back-diffusion of the solute. 

With a sufficiently high RCF, at equilibrium, the sedimentation of the solute is exactly balanced by the diffusion and the gradient is stable. It is possible to calculate the time for a selfgenerating gradient to reach equilibrium and it is described by the following equation:

where  r is the density at a point r cm from the axis of rotation,  i is the starting density of the homogeneous solution, rc is the distance in cm from the axis of rotation where the density of the gradient =  i , Q is the rotor speed in rpm and  o is a constant depending on the solute [1].