Colloidal Mineral Liquid Crystals. Formation & Manipulation

The central topic of this thesis is the formation, manipulation and characterization of colloidal mineral liquid crystals. Liquid crystals are liquids containing ordered anisometric particles. A range of liquid crystalline phases exists, from solely orientationally ordered nematic phases to orientationally and 1D or 2D positionally ordered smectic and columnar phases. Liquid crystals have several applications but are mostly known for their application in LCD’s: Liquid Crystal Displays, which contain a nematic liquid crystal. Colloidal liquid crystals generally form spontaneously. For potential applications however, it is important to be able to controllably form and tune the desired liquid crystal phase. The key feature of the research described in this thesis is therefore how the formation of these liquid crystals can be manipulated. This thesis is split in two parts. Part I describes the mineral goethite, which consists of board-like colloids. Various methods to influence the liquid crystal phase behaviour are described. These include the earth gravitational field, a depletant and an external magnetic field. It is described how letting the colloids slowly sediment in the earth gravitational field solves the particle polydispersity problem. The nematic phase for example, was found to absorb particles that do not fit in the smectic phase. Additionally, observations of fractionated crystallization in both the smectic and the columnar phase are discussed. Another approach to influence the phase behaviour is by changing the inter-particle interactions by introducing a spherical depletant into the suspension. This thesis describes how the depletant-induced attraction between the particles promotes the formation of the rare biaxial (in which particles are orientationally ordered in three dimensions) smectic phase. The final method studied is an external magnetic field. We describe how the magnetic field can induce phase transitions between the three different nematic phases (biaxial, prolate and oblate uniaxial), depending on the particle aspect ratios. Additionally it is described how a magnetic field was used to reorient the goethite columnar phase and to study its reorientation pathway with synchrotron small angle X-ray scattering, revealing nanoshear between layers of particles. In Part II of this thesis the mineral gibbsite, consisting of plate-like colloids, is described. Although liquid crystals are generally studied with scattering techniques, in Part II we focus on direct space imaging of the liquid crystals with confocal laser scanning microscopy. This technique enables characterization of the liquid crystals in real-space and time at the particle level. This requires large and fluorescent particles. Different general synthesis methods to prepare such platelets are described. Additionally, results of a preliminary confocal study are discussed, which reveals the observation of single particles as well as their liquid crystalline phase. Since liquid crystals of larger particles are more challenging for X-ray scattering studies, a 3D X-ray scattering method is described which helps make unambiguous identification of the liquid crystal phase

Colloidal Mineral Liquid Crystals. Formation & Manipulation

The central topic of this thesis is the formation, manipulation and characterization of colloidal mineral liquid crystals. Liquid crystals are liquids containing ordered anisometric particles. A range of liquid crystalline phases exists, from solely orientationally ordered nematic phases to orientationally and 1D or 2D positionally ordered smectic and columnar phases. Liquid crystals have several applications but are mostly known for their application in LCD’s: Liquid Crystal Displays, which contain a nematic liquid crystal. Colloidal liquid crystals generally form spontaneously. For potential applications however, it is important to be able to controllably form and tune the desired liquid crystal phase. The key feature of the research described in this thesis is therefore how the formation of these liquid crystals can be manipulated. This thesis is split in two parts. Part I describes the mineral goethite, which consists of board-like colloids. Various methods to influence the liquid crystal phase behaviour are described. These include the earth gravitational field, a depletant and an external magnetic field. It is described how letting the colloids slowly sediment in the earth gravitational field solves the particle polydispersity problem. The nematic phase for example, was found to absorb particles that do not fit in the smectic phase. Additionally, observations of fractionated crystallization in both the smectic and the columnar phase are discussed. Another approach to influence the phase behaviour is by changing the inter-particle interactions by introducing a spherical depletant into the suspension. This thesis describes how the depletant-induced attraction between the particles promotes the formation of the rare biaxial (in which particles are orientationally ordered in three dimensions) smectic phase. The final method studied is an external magnetic field. We describe how the magnetic field can induce phase transitions between the three different nematic phases (biaxial, prolate and oblate uniaxial), depending on the particle aspect ratios. Additionally it is described how a magnetic field was used to reorient the goethite columnar phase and to study its reorientation pathway with synchrotron small angle X-ray scattering, revealing nanoshear between layers of particles. In Part II of this thesis the mineral gibbsite, consisting of plate-like colloids, is described. Although liquid crystals are generally studied with scattering techniques, in Part II we focus on direct space imaging of the liquid crystals with confocal laser scanning microscopy. This technique enables characterization of the liquid crystals in real-space and time at the particle level. This requires large and fluorescent particles. Different general synthesis methods to prepare such platelets are described. Additionally, results of a preliminary confocal study are discussed, which reveals the observation of single particles as well as their liquid crystalline phase. Since liquid crystals of larger particles are more challenging for X-ray scattering studies, a 3D X-ray scattering method is described which helps make unambiguous identification of the liquid crystal phase

Phase behaviour of lyotropic liquid crystals in external fields and confinement

This mini-review discusses the influence of external fields on the phase behaviour of lyotropic colloidal liquid crystals. The liquid crystal phases reviewed, formed in suspensions of highly anisotropic particles ranging from rod- to board- to plate-like particles, include nematic, smectic and columnar phases. The external fields considered are the earth gravitational field and electric and magnetic fields. For electric and magnetic fields single particle alignment, collective reorientation behaviour of ordered phases and field-induced liquid crystal phase transitions are discussed. Additionally, liquid crystal phase behaviour in various confining geometries, e.g. slit-pore, circular and spherical confinement will be reviewed

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

Diffuse scattering in random-stacking hexagonal close-packed crystals of colloidal hard spheres

Microradian X-ray diffraction from sedimentary colloidal crystals is studied using synchrotron radiation with photon energies of 12.4, 27, and 38 keV. Stacking disorder in these hard-sphere crystals leads to diffuse X-ray scattering along the Bragg scattering rods normal to the randomly stacked layers. We observed the appearance of diffuse scattering, shown to be induced by multiple scattering, along the secondary Bragg rods in between the stacking-independent true Bragg reflections. This effect can be reduced by measuring at higher X-ray energies

Diffuse scattering in random-stacking hexagonal close-packed crystals of colloidal hard spheres

Microradian X-ray diffraction from sedimentary colloidal crystals is studied using synchrotron radiation with photon energies of 12.4, 27, and 38 keV. Stacking disorder in these hard-sphere crystals leads to diffuse X-ray scattering along the Bragg scattering rods normal to the randomly stacked layers. We observed the appearance of diffuse scattering, shown to be induced by multiple scattering, along the secondary Bragg rods in between the stacking-independent true Bragg reflections. This effect can be reduced by measuring at higher X-ray energies

Dispersions and mixtures of particles with complex architectures in shear flow

We review the effect of shear flow on the phase behavior and structure of colloidal dispersions with increasing degree of complexity. We discuss dispersions of colloidal rods, stiff living polymers like wormlike micelles, and colloidal platelets. In addition, a review is presented on sheared binary dispersions. For all cases we discuss the interplay between thermodynamic instabilities and hydrodynamic instabilities