Phys. Rev. Lett. 108, 016101 (2012)

Charge-stabilized colloidal particles are in a delicate balance of attractive and repulsive interactions; variations in the particles' surface charge can lead to changes in interparticle distance and the destabilization of a colloidal suspension. Understanding charge transfer at the solid/liquid interface is not only of fundamental but also of practical importance, for example in applications such as electrophoretic inks. Filip Beunis and colleagues now reveal the charging dynamics of poly(methyl methacrylate) microspheres in dodecane using an improved optical tracking method. The researchers trap a microsphere with optical tweezers and record its oscillatory motion in a varying electric field. Sampling the oscillation amplitude at a high rate enables the researchers to detect charging in steps of the elementary charge and to study the dynamics of the process. They propose that charging is mediated by a few tens of surface sites on the microspheres. The researchers suggest that, with further improvements, their method could also be used to study charging in polar liquids.