416 ¹ WILEY-VCH-Verlag GmbH, 69451 Weinheim, Germany, 2002 1439-4235/02/03/05 $ 20.00+.50/0 CHEMPHYSCHEM 2002, 3, 416 ± 423 Coupled Electrorotation: Two Proximate Micro- spheres Spin in Registry with an AC Electric Field Garth J. Simpson,* [a] Clyde F. Wilson, [b] Karl-Heinz Gericke, [c] and Richard N. Zare* [b] 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. KEYWORDS: electrorotation ¥ fluorescence ¥ nanostructures ¥ photopatterning Introduction With the rapid expansion of studies utilizing microfluidic total analysis systems (∫labs on chips∫) and microelectromechanical systems (MEMS), interest is growing in the generation of noncontact methods to manipulate microscopic and nanoscopic particles. [1, 2] Several well established approaches are available for microparticle translational manipulation (for example optical trapping, [3±5] electrophoresis, [6, 7] and dielectrophoresis [2, 6, 8, 9] ). More recently, several ingenious approaches have been devel- oped to control rotational motion of microscale and nanoscale particles, including a number of purely optical methods. [10±18] Several groups have demonstrated that radiation pressure can induce rotation in anisotropic particles through an optical ∫windmill∫ effect. [10±13] Microfabrication of the particle structure has allowed for rational control over the direction of rotation in these devices. [11, 12] Radiation pressure-driven rotation rates of 420 rpm have been experimentally observed for microfabricated particles, [10] and rates of up to 2200 rpm have been reported for ground glass powder particles. [13] Optical trapping of birefrin- gent particles using circularly polarized light has also been demonstrated as a means to transfer angular momentum from the optical trap beam to an immobilized particle (in this case, photon spin angular momentum). [14, 15] Friese et al. [14] have demonstrated rotation rates for birefringent calcite particles in excess of 350 Hz (21000 rpm) and shown that these particles can be used to drive rotation in optically trapped microfabricated particles at rates of  0.2 Hz. [15] Utilizing Laguerre±Gaussian (LG) modes in optical trap beams, Simpson et al. [16, 17] have induced rotation in partially absorbing particles by transfer of a combination of spin and/or orbital photon angular momentum. Recently, Paterson et al. [18] have demonstrated optical trapping within the interference pattern of an LG beam and a plane- polarized beam as a means to either rotate rodlike particles or to revolve two or three particles about a central axis. Despite the advantages of purely optical, noncontact techni- ques in inducing rotational motion in particles, potential limitations of several of these techniques are related to the somewhat stringent requirements on the particle shape and/or optical properties. For example, the highest rotation rates have been achieved for particles prepared through physical grinding of bulk samples into powders. [13, 14] Although simple, this manufacturing approach does not provide much flexibility for rational design. Additionally, the most widely used microfabri- cation techniques are not amenable to the generation of birefringent particles, which limits the range of methods with this requirement. [14, 15] In the optical ∫windmill∫ methods, [10±13] the shape of the microparticle was prepared such that the particle could be both optically trapped and made to rotate by the same [a] Prof. G. J. Simpson Department of Chemistry Purdue University West Lafayette, IN 47907-1393 (USA) Fax: ( 1) 765-494-0239 E-mail: gsimpson@purdue.edu [b] Prof. R. N. Zare, C. F. Wilson Department of Chemistry Stanford University Stanford, CA 94305-5080 (USA) Fax: ( 1) 650-723-9262 E-mail: zare@stanford.edu [c] Prof. K.-H. Gericke Institut f¸r Physikalische und Theoretische Chemie der Technischen Universit‰t Braunschweig Hans-Sommer-Strasse 10 38106 Braunschweig (Germany) Supporting information for this article is available on the WWW under http://www.chemphyschem.com or from the author.