Coupled electrorotation: two proximate microspheres spin in registry with an AC electric field.

We report a novel approach to micro- and nanoparticle rotation, uniting the fine translational control afforded by optical trapping with the flexibility and simplicity of dipole-field-induced coupled electrorotation (CER). Fluorescence imaging using a microparticle photopatterning technique was combined with optical trapping to quantify both the senses and speeds of rotation for individual pairs of particles. Laser tweezers allowed controlled positioning of a pair of particles within a dipole field while simultaneously providing an axis about which the particles rotated. The particle-particle interactions inherent in CER offer several distinct advantages compared with electrorotation in multipole fields. Results from several investigations highlight the utility of this approach, including quantification of rotation in spheres as small as 750 nm in diameter, observation of rotation rates as high as 1800 rpm, fabrication of coupled electrorotational "antigears", trapping and rotation of sphere dimers, and exploitation of the registry of sphere rotation to probe the dielectric properties of immobile objects.