Their motions are captured in individual video fields at sec intervals and digitized. Each pair of consecutive images, such as the examples in Fig. Repeatedly trapping and releasing the spheres over a range of initial separations enables us to sample uniformly. The domain of integration is conveniently limited in practice by the core repulsion which suppresses at small , and by the finite range of the interaction which renders diagonal and independent of for large . Data from Ref.
Solid curves passing through the data points result from nonlinear least squares fits to Eq.
The screening length of is comparable to the spheres' diameters and corresponds to a total ionic strength around , a reasonable value for deionized water at . These numbers will be useful for comparison with results in Sec. 5 . Comparable results were obtained by Vondermassen et al. Sugimoto et al. In all cases, the measured pair potentials agree at least semi-quantitatively with predictions of the DLVO theory.