Occurs when two solvents with very different strengths — A is the weak solvent, and B is the strong solvent — are used with unmodified silica or alumina. The strong solvent (B) will be adsorbed preferentially by the active surface of the stationary phase until it is saturated; until this occurs, the weak solvent (A) will be enriched or demixed as it travels down the column. Eventually, when the entire column is saturated with solvent B, this solvent will be eluted, mixed with solvent A at the initial strength, and sample components will be eluted with the sudden change in solvent strength.