Tumbling of Quantum Dots: Rheo-Optics

Linear flow dichroism is shown to be a powerful tool to characterize the hydrodynamic dimensions of extremely small nonspherical colloids in solution. Dispersions of prolate and oblate quantum dots (QDs) are employed to investigate the validity of flow dichroism as a characterization tool. Shape-anisotropic QDs are important from an application perspective, where it is necessary to have a good knowledge of their hydrodynamic dimensions to predict and control their orientation during solution processing. Flow dichroism quantifies the tumbling motion of QDs in shear flow by optical means, which provides a characteristic signature of the particle shape, hydrodynamic friction, and size distribution. The effects of particle size and shape, size polydispersity, and shear rate on the temporal evolution of the flow-induced alignment are discussed in detail on the basis of numerical solutions of the Smoluchowski equation that describes the motion for the probability of the orientation of colloids in shear flow. It is shown that the combination of flow-dichroism experiments and the theoretical approach on the basis of the Smoluchowski equation provides a means to measure hydrodynamic aspect ratios and polydispersity, which for such small particles is not feasible with standard methods similar to light scattering. Flow dichroism will be useful not only for shape-anisotropic colloidal QDs, but also for other nanoscale systems.