Dynamics of charged gibbsite platelets in the isotropic phase

TL;DR

This study investigates the dynamic behavior of charged gibbsite platelets in DMSO, revealing significant decreases in diffusion coefficients and viscosity with increasing concentration, and models these effects using an effective charged sphere approach.

Contribution

It provides experimental measurements of diffusion and viscosity in charged platelet suspensions and compares them with a theoretical effective sphere model, highlighting the limitations of current modeling.

Findings

Diffusion coefficients decrease by over two orders of magnitude approaching the I/LC transition.

Static shear viscosity increases and exhibits strong shear-thinning with concentration.

Effective sphere model captures the enhancement of collective diffusion but underestimates self-diffusion declines.

Abstract

We report on depolarized and non-depolarized dynamic light scattering, static light scattering, and static viscosity measurements on interacting charged gibbsite platelets suspended in dimethyl sulfoxide (DMSO). The average collective and (long-time) translational self-diffusion coefficients, and the rotational diffusion coefficient, have been measured as functions of the platelet volume fraction \phi, up to the isotropic-liquid crystal (I/LC) transition. The non-depolarized intensity autocorrelation function, measured at low scattering wavenumbers, consists of a fast and a slowly decaying mode which we interpret as the orientationally averaged collective and translational self-diffusion coefficients, respectively. Both the rotational and the long-time self-diffusion coefficients decrease very strongly, by more than two orders of magnitude, in going from the very dilute limit to the I/LC transition concentration. A similarly strong decrease, with increasing \phi, is observed for the inverse zero-strain limiting static shear viscosity. With increasing \phi, increasingly strong shear-thinning is observed, accompanied by a shrinking of the low shear-rate Newtonian plateau. The measured diffusion coefficients are interpreted theoretically in terms of a simple model of effective charged spheres interacting by a screened Coulomb potential, with hydrodynamic interactions included. The disk-like particle shape, and the measured particle radius and thickness polydispersities, enter into the model calculations via the scattering amplitudes. The interaction-induced enhancement of the collective diffusion coefficient by more than a factor of 20 at larger volume fractions is well captured in the effective sphere model, whereas the strong declines both of the experimental translational and rotational self-diffusion coefficients are underestimated.