Hydrodynamic coupling of two rotating spheres trapped in harmonic potentials.

We theoretically study in detail the hydrodynamic coupling of two equal-sized colloidal spheres at low Reynolds numbers assuming the particles to be harmonically trapped with respect to both their positions and orientations. By taking into account the rotational motion, we obtain a rich spectrum of collective eigenmodes whose properties we determine on the basis of pure symmetry arguments. Extending recent investigations on translational correlations [J.-C. Meiners and S. R. Quake, Phys. Rev. Lett. 82, 2211 (1999)], we derive the complete set of autocorrelation and cross-correlation functions emphasizing the coupling of rotation to translation which we illustrate in a few examples. An important feature of our system is the self-coupling of translation and rotation of one particle mediated by the neighboring particle that is clearly visible in the appropriate autocorrelation function. This coupling is a higher-order effect and therefore not included in the widely used Rotne-Prager approximation for the hydrodynamic mobilities.