Floating nematic phase in colloidal platelet-sphere mixtures

Abstract

The phase behaviour of colloidal dispersions is interesting for fundamental reasons and for technological applications such as photonic crystals and electronic paper. Sedimentation, which in everyday life is relevant from blood analysis to the shelf life of paint, is a means to determine phase boundaries by observing distinct layers in samples that are in sedimentation-diffusion equilibrium. However, disentangling the effects due to interparticle interactions, which generate the bulk phase diagram, from those due to gravity is a complex task. Here we show that a line in the space of chemical potentials µi, where i labels the species, represents a sedimented sample and that each crossing of this sedimentation path with a binodal generates an interface under gravity. Complex phase stacks can result, such as the sandwich of a floating nematic layer between top and bottom isotropic phases that we observed in a mixture of silica spheres and gibbsite platelets.

Depletion-driven four-phase coexistences in discotic systems

“…It has been experimentally observed how addition of non-adsorbing polymers [39][40][41] or small spheres [42][43][44][45][46][47][48][49] to a colloidal platelet suspension enriches the phase behaviour. For a review, see [50].…”

The phase stacking diagram of colloidal mixtures under gravity

“…This maximal number of layers occurs if a sedimentation path crosses each binodal 2 + n b,i times, with n b,i the number of inflection points of each binodal. The occurrence of an inflection point in a binodal has been predicted in colloidal platelet-sphere mixtures [7]. It is also a natural consequence of a binodal that connects two phase transitions in the pure components of the mixture.…”

“…We choose ξ 1 = 0.805 mm and ξ 2 = 3.89 mm, which is compatible with, e.g., a colloidal mixture of gibbsite platelets (species 1: average diameter 214 nm and thickness 10 nm) and smectite (species 2: average diameter 300 nm and thickness 1 nm) dispersed in water at room temperature (see e.g. [7] and references therein). We obtain s = ξ 1 /ξ 2 = 0.207 or α = 192…”

“…Two sedimentation paths with the same slope but opposite directions correspond to stacking sequences with inverse order (e.g., AB and BA); in order to avoid this ambiguity we only consider the case m 1 > 0. [7] in mixtures of silica spheres and gibbsite platelets. In general, two consecutive phases in the stacking sequence coexist in bulk, but two non-consecutive phases may or may not coexist in bulk and, as the example demonstrates, the same phase (B) can reenter the sequence.…”

“…In the presence of gravity, one can define a height-and species-dependent local chemical potential [7,14,15]:…”