Lateral correlation of multivalent counterions is the universal mechanism of charge inversion

We review works on screening of a macroion, such as colloidal particle or double helix DNA, by multivalent counterions. Multivalent metal ions, charged micelles, short or long polyelectrolytes can play the role of multivalent counterions. Due to the strong Coulomb repulsion such multivalent counterions form a strongly correlated liquid resembling a Wigner crystal at the surface of the macroion. Even if the macroion is neutralized by this liquid, a newly arriving counterion creates in the liquid a correlation hole or image which attracts the ion to the surface. As a result, total charge of adsorbed counterions can substantially exceed the bare macroion charge, so that the macroion net charge inverts its sign. We discuss two previously suggested explanations of charge inversion and show that physics underlying both of them is intrinsically that of correlations, so that correlation is the universal force driving charge inversion.