Ageing in a system of polydisperse goethite boardlike particles showing rich phase behaviour.

Using microradian x-ray scattering and polarized light microscopy the rich liquid crystalline phase behaviour of a polydisperse system of chromium-modified goethite particles has been studied for five years. We observe that the particles stay highly mobile over years and the rich phase behaviour keeps developing in novel and even surprising ways. While in many other colloidal systems particle size polydispersity suppresses the formation of ordered phases, goethite particles form multiple coexisting ordered phases. The particle polydispersity problem is then solved by particle exchange between coexisting phases. One usually expects that a less ordered phase (e.g., nematic) is formed first while crystallization of the smectic and columnar crystals might take a longer time. For goethite particles we find the opposite, i.e. the nematic phase grows over years at the expense of a better ordered smectic phase. Moreover, SAXS patterns revealed peak splitting for both the smectic and the columnar phase, meaning that the system displays fractionated crystallization. We further discovered that the centred rectangular columnar phase spontaneously forms out of the simple rectangular columnar phase. The reverse transition is observed as well. We explain the ease of these martensitic transitions by showing how slight rotation and translation of the particles triggers the transition.

3D structure of nematic and columnar phases of hard colloidal platelets.

We present small angle x-ray scattering data of single-domain nematic and columnar liquid crystal phases in suspensions of sterically stabilized gibbsite platelets. The measurements are performed with different sample orientations to obtain information about the three-dimensional structure of the liquid crystalline phases. With the x-ray beam incident along the director of the nematic phase a strong correlation peak is observed corresponding to the side-to-side interparticle correlations, which suggests a columnar nematic structure. Upon sample rotation this side-to-side correlation peak of the nematic shifts to higher Q-values, suggesting the presence of strong fluctuations of small stacks of particles with different orientations, while the overall particle orientation is constant. In the hexagonal columnar phase, clear Bragg intercolumnar reflections are observed. Upon rotation, the Q-value of these reflections remains constant while their intensity monotonically decreases upon rotation. This indicates that the column orientation fluctuates together with the particle director in the columnar phase. This difference between the behaviour of the columnar and the nematic reflections upon sample rotation is used to assign the liquid crystal phase of a suspension consisting of larger platelets, where identification can be ambiguous due to resolution limitations.

Experimental observation of fractionated crystallization in polydisperse platelike colloids.

We have discovered that the long-term aging of the hexagonal columnar liquid-crystal phase of polydisperse gibbsite platelets leads to fractionated crystallization, that is, to the formation of coexisting columnar crystals with different periods. This process was revealed by microradian X-ray diffraction demonstrating the splitting of the Bragg intercolumnar reflections into sequences of sharper reflections. The fractionated crystallization was observed in a number of samples of sterically stabilized as well as charge-stabilized polydisperse gibbsite platelets.

Devitrification of the glassy state in suspensions of charged platelets.

Colloidal suspensions of charged gibbsite platelets at salt concentrations of 10(-2) M and below and with a sufficiently high particle concentration form a kinetically arrested, glassy state. We study the evolution of the glassy state in suspensions of three different gibbsite systems. Despite differences in size and polydispersity, we observe small, iridescent grains of the hexagonal columnar phase, for all these systems after periods of months to years. The connections between this devitrification phenomenon and the structure of the glassy state are discussed.

Experimental realization of biaxial liquid crystal phases in colloidal dispersions of boardlike particles.

Biaxial nematic and biaxial smectic phases were found in a colloidal model system of goethite (alpha-FeOOH) particles with a simple boardlike shape and short-range repulsive interaction. The macroscopic domains were oriented by a magnetic field and their structure was revealed by small angle x-ray scattering. In accordance with theoretical predictions, biaxiality appears in a system with particles that have a shape almost exactly in between rodlike and platelike. Our results suggest that biaxial phases can be readily obtained by a proper choice of the particle shape.

Microemulsion efficiency boosting and the complementary effect. 1. Structural properties.

Amphiphilic diblock copolymers added to microemulsions proved to enhance the efficiency of surfactants dramatically. The complementary effect of homopolymers is considered in the current work. A possible application of the homopolymer addition could be the viscosity tuning of the microemulsion without changing the considered bicontinuous phase. Furthermore, (homo)polymers are added for many other reasons in technical applications. A theory by Eisenriegler predicts a decreased efficiency when homopolymers are added. In further experiments, the simultaneous addition of homopolymers and diblock copolymers probes whether the two opposite effects superpose and allow for a compensation. Then, efficiency and viscosity are adjustable independently. Experimentally, phase diagrams are investigated and the microscopic structure is measured by small-angle neutron scattering. Within the presented models, both experimental methods are compared and discussed on the basis of the surfactant membrane bending moduli. The homopolymer effect is about 7 times larger than that theoretically predicted, and the superposition of the two polymer effects allows for a compensation with an optionally tunable viscosity.