Imaging colloidal particles' motions offers valuable insights into processes such as structural phase transitions, aggregation and gelation, and complex fluids' response to external forces. Complementary information becomes available with the ability to manipulate individual colloidal particles. Optical tweezers introduced by Ashkin, Dziedzic and Chu  provide this capability with an unparalleled combination of precision, sensitivity and reproducibility.
An optical tweezer uses forces exerted by gradients in light intensity to drive small particles to the focal point of a tightly focused laser beam. A high degree of convergence is necessary to prevent the particles from being dispersed by radiation pressure. Consequently, most optical tweezers are built around optical microscopes, as shown in Fig. 1, taking advantage of the objective lens' high numerical aperture and optimized optics to achieve a diffraction-limited focus. A collimated laser beam entering the objective's back aperture comes to a focus and forms a trap in the microscope's focal plane. Typically, a few hundred microwatts of visible light suffices to localize a micron-scale dielectric particle. Multiple beams entering the objective's back aperture form multiple optical traps, with the desired configuration of beams being created with beam splitters , holograms [5,6], spatial light modulators  or by timesharing a single beam with high-speed deflectors [8,9].
The combination of optical tweezer manipulation and digital video microscopy makes possible precise measurements of colloidal interactions and dynamics [2,4]. In the following sections, we apply these methods to test long-accepted and still-evolving theories for colloidal interactions.