Phase behaviour and dynamics of suspensions of hard colloidal platelets

Abstract

In this thesis we aim to provide a many-sided answer to the question: what are the consequences of plate-like colloidal shape on a suspensions' physical properties? A central role in this investigation is played by the experimental model system of platelets which, building on the Van 't Hoff Laboratory's rich tradition with respect to synthesizing colloidal model systems, was developed for this purpose. Consisting of fairly monodisperse sterically stabilised gibbsite platelets, this system allows us to focus on the effect of plate-like particle shape while keeping the shape, size and interactions well-defined. In part I, we consider the dynamical properties of dilute suspensions. This yields the first results for the viscosity, sedimentation, and diffusion behaviour of such a platelet suspension as a function of the volume fraction. It will be interesting to see whether these experiments will inspire theoretical efforts towards calculating the f-coefficients in the viscosity, sedimentation, and diffusion, for which no predictions are yet available. In part II, we show that these suspensions of approximately hard platelets exhibit an isotropic to nematic phase transition. While being predicted by the Onsager theory and computer simulations, this observation clearly contrasts with the ubiquitous (but poorly understood) gel formation in suspensions of clay platelets. Furthermore, increasing the platelets' polydispersity leads to a broadening of the I-N coexistence region and the appearance of pronounced fractionation. We demonstrate that a remarkable feature of the observed I-N transition in these more polydisperse systems -where the nematic phase becomes the upper phase- can be explained on the basis of strong fractionation with respect to platelet thickness. Moreover, at high particle densities, the suspensions exhibit a nematic to columnar phase transition. Although predicted by simulations for monodisperse platelets, the stability of a columnar phase in suspensions with up to 25% polydispersity in diameter is remarkable in the light of the so-called terminal polydispersity for hard sphere crystallisation and hard rod smectisation. The stability of the columnar phase in these polydisperse systems is probably connected with the fact that the platelets are polydisperse in thickness too, which suppresses the relative stability of the competing liquid crystalline phase: the smectic. In the case of 25% polydispersity in diameter the columnar structure breaks down and a smectic-like structure appears upon increasing the density, whereas the columnar structure prevails for a lower value of the diameter polydispersity. In part III, we study the phase behaviour of mixtures of plate-like colloids and either non-adsorbing polymer or rod-like colloids. The phase behaviour of these mixtures is even richer than that of the pure platelet suspensions, as we find the appearance of an additional isotropic phase (in plate-polymer mixtures) and two additional (rod-rich) liquid crystalline phases (in rod-plate mixtures). The observed topology of these phase diagrams, which include the coexistence of up to five different phases, can be explained by the interplay between the employed types of species (plate, rod, or polymer) and their polydispersity. Looking back on the properties of the model system investigated in this thesis, it is clear that we have uncovered a number of features which are both fascinating and of fundamental interest. At the same time however, we must recognise that the role of the parameters varied in this study -such as polydispersity, attraction, and the presence of added colloidal species- does not yet allow us to understand the behaviour exhibited by the much more comple