A colloid is a mesoscopic particle whose size typically ranges from about one nanometre to several micrometres. Often colloidal particles are dispersed in a host medium, typically a liquid or a gas. Dan Frenkel (2002) usefully defines a colloid by its behaviour, as the dynamics of colloids are significantly affected by their Brownian motion, which we have studied in Chapter 7. This definition encompasses nearly all of the objects to which the methods of optical tweezers have been applied, going from aerosols [Chapter 19] to nanoparticles [Chapter 23]. The interactions between particles in a colloidal suspension depend on the suspending solvent, as in the case of hydrodynamic interactions, electrostatic forces and depletion forces. These interactions can give rise to complex behaviours, as in the case of the hydrodynamic interactions between particles shown in Fig. 17.1. Optical tweezers provide an ideal tool for probing these colloidal interactions. In this Chapter, we review experiments that use optical traps to measure and exploit the coupling between colloidal particles in suspension, taking as examples experiments aimed at determining the hydrodynamic, electrostatic and depletion interactions.
17.1 Hydrodynamic interactions
17.2 Electrostatic interactions
17.3 Depletion interactions
17.4 Further reading