| Important material properties of colloidal suspensions such as their stability, rheology and microstructure are largely determined by the long-range forces acting between the suspension's microscopic constituent particles. The miniscule scale of these forces has long proved a barrier to a detailed understanding of colloidal suspensions. We surmount this barrier by combining recent advances in digital image processing and optical trapping to yield new techniques for measuring the forces acting on and between isolated colloidal spheres. We measure the electrostatic and hydrodynamic interactions between pairs of colloidal spheres using digital video microscopy and blinking optical tweezers. We also measure the interaction of a single colloidal sphere with a glass wall using total internal reflection microscopy. The interactions between isolated pairs of colloidal spheres agree quantitatively with the predictions of DLVO theory, charge renormalization theory and low-Reynolds number hydrodynamics. The interactions between pairs of spheres confined between glass walls, however, show an anomalous long-range attractive tail. We demonstrate that these attractions are due to the presence of the walls, and not intrinsic to the pair interaction of the spheres themselves, as suggested in the literature. Furthermore, we discuss the merits of possible mechanisms to account for these attractive interactions. |
