(458d) Effective Density Sorting of Soft-Sphere Colloidal Particles

Colloidal suspensions often contain mixtures of particles that must be sorted by size or density. One route to sorting gram-size quantities of particles is sedimentation, which has been studied for decades from both dynamic and thermodynamic perspectives. Dynamic predictions for settling account for hydrodynamic, colloidal, and electrophoretic interactions at finite volume fractions, but the final sediments have been glassy. Thermodynamic studies have examined colloidal crystals, but primarily for the case of mono-disperse particles settling at low Peclet number (Pe < 0.1). An important question remains unanswered: What sediment structure results when bi-disperse or poly-disperse particles settle rapidly, but remain non-glassy in the final dense sediment? Here we show that bi-disperse colloids having soft-sphere interactions spontaneously arrange into two macroscopic layers after a sedimentation process at Peclet number greater than order unity. The layering can be explained without appealing to complex hydrodynamic or thermodynamic arguments; rather, the layering is readily explained simply by minimizing the free energy of the system, which is primarily the gravitational energy. Furthermore, minimizing the energy of the system leads one to expect an ?asederotrope?, an azeotrope-like mixture of particles in one of the layers that is seen in our experiments. Our results may lead to designs for preventing de-mixing of colloidal products during shipment, for improving sorting or size-refinement operations for colloidal particles of different size or density, for sorting mixtures of biological components, or for purifying mixtures of colloidal assemblies.