Sedimentation of colloidal plate-sphere mixtures and inference of particle characteristics from stacking sequences

Abstract

We investigate theoretically the effect of gravity on a plate-sphere colloidal mixture by means of an Onsager-like density functional to describe the bulk, and sedimentation path theory to incorporate gravity. We calculate the stacking diagram of the mixture for two sets of buoyant masses and different values of the sample height. Several stacking sequences appear due to the intricate interplay between gravity, the sample height, and bulk phase separation. These include the experimentally observed floating nematic sequence, which consists of a nematic layer sandwiched between two isotropic layers. The values of the thicknesses of the layers in a complex stacking sequence can be used to obtain microscopic information of the mixture. Using the thicknesses of the layers in the floating nematic sequence we are able to infer the values of the buoyant masses from the colloidal concentrations and vice versa. We also predict new phenomena that can be experimentally tested, such as a nontrivial evolution of the stacking sequence by increasing the sample height in which new layers appear either at the top or at the bottom of the sample.

Effect of sample height and particle elongation in the sedimentation of colloidal rods

“…The effect of varying the sample height (at fixed average colloidal concentration) in sedimentation of colloidal binary mixtures can be counterintuitive with new layers appearing at the top, at the bottom, or in the middle of the sample. 67,83,84 We study here the evolution of the stacking sequence by increasing the sample height in monodisperse colloidal systems. We select two illustrative average packing fractions Z = 0.452 and 0.50 in a suspension of elongated hard spherocylinders with L/D = 10.0, see vertical dotted lines in Fig.…”

“…The gravitational field can have a strong and far from trivial effect in sedimentation experiments, especially if the gravitational length is smaller or comparable to the height of the vessel, 67 which is often the case in colloidal science. To correctly extract information about bulk phenomena from sedimentation experiments it is essential to understand the effect of the gravitational field on the suspension.…”

Effect of sample height and particle elongation in the sedimentation of colloidal rods

“…The effect of varying the sample height (at fixed average colloidal concentration) in sedimentation of colloidal binary mixtures can be counterintuitive with new layers appearing at the top, at the bottom, or in the middle of the sample [67,83,84]. We study here the evolution of the stacking sequence by increasing the sample height in monodisperse colloidal systems.…”