Optical tweezers have been used to measure the potential, U(r), for strong interactions between particles at large separations, r. The electrostatic interactions between highly charged particles dispersed in electrolytes has been the subject of heated controversy in recent years . The well-established DLVO theory  predicts that spheres carrying the same sign charge should experience a screened-Coulomb repulsion when dispersed in a solvent such as water. Persistent discrepancies between predictions based on the DLVO theory and experimental observations have led some researchers to question even the qualitative form of the long-range electrostatic potential. In particular, results of some experiments would be most easily explained if the colloidal pair potential included a long-range attractive component not accounted for by the DLVO theory. Only very recently has the pair potential been measured with enough resolution to distinguish between competing theories.
Crocker and Grier [25, 26, 35] used optical tweezers to position and release otherwise isolated pairs of charge-stabilized colloidal spheres. Using roughly 50,000 trajectory steps to numerically solve the master equation for the spheres' Markovian dynamics , they were able to measure the pair potential with 50 nm spatial resolution and 2 meV energy sensitivity. A series of these measurements revealed purely repulsive interactions in quantitative agreement with predictions of the DLVO theory [26, 35]. Typical examples appear in Fig. 2. When confined to a thin layer between charged glass walls, the same spheres exhibit long ranged attractions inconsistent with the DLVO theory . Such attractions had been observed in imaging studies [37, 38] on confined colloidal monolayers, although their attribution to the confining walls was uncertain. Figure 3 shows that even a single nearby wall can induce an attractive interaction between a pair of spheres. Viewed in light of recent studies on metastable superheated colloidal crystals [39, 40] optical tweezer interaction measurements suggest that long-range attractions can arise in charge-stabilized suspensions as a many-body contribution to the effective pair potential. This view is supported by liquid structure calculations [41, 42] although not by recent simulation studies .