The present article provides an overview of the recent progress in the direct
force measurements between individual pairs of colloidal particles in aqueous
salt solutions. Results obtained by two different techniques are being
highlighted, namely with the atomic force microscope (AFM) and optical
tweezers. One finds that the classical theory of Derjaguin, Landau, Verwey, and
Overbeek (DLVO) represents an accurate description of the force profiles even
in the presence of multivalent ions, typically down to distances of few
nanometers. However, the corresponding Hamaker constants and diffuse layer
potentials must be extracted from the force profiles. At low salt
concentrations, double layer forces remain repulsive and may become long
ranged. At short distances, additional short range non-DLVO interactions may
become important. Such an interaction is particularly relevant in the presence
of multivalent counterions.Comment: Submitted on 30th of May 2016 as invited article to Curr. Opinion
Colloid Interf. Sci. Edited by W. Ducker and P. Claesson. 15 Pages, 7 Figures
82 reference