Search

We found, using polarization microscopy and small-angle X-ray scattering, that for goethite, a low polydispersity suffices to form two separate nematic phases, while previous theory showed that this is only possible for mixtures of particles with extremely different lengths or diameters. Applying a critical magnetic field, which induces some of the goethite nanorods to rotate, leads to sufficient excluded volume between the particles to cause macroscopic phase separation between two orthogonal nematic phases. The larger the polydispersity of the system, the broader the range of field strengths where nematic−nematic phase separation occurs. This is a new phase separation mechanism which is expected to lead to interesting interfacial phenomena.

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.

We demonstrate the suitability of polarization microscopy to study the recently discovered (parallel) nematic-(perpendicular) nematic phase separation. This novel type of phase transition is induced by applying an external magnetic field to a nematic liquid crystal of boardlike colloidal goethite and is due to an interplay between the intrinsic magnetic properties of goethite and the collective effect of liquid crystal formation. It is shown that the intense ochre colour of goethite does not preclude the use of polarization microscopy and interference colours, and that dichroism can give valuable qualitative information on the nature of the phases, their anchoring and their sedimentation and order parameter profiles. We also apply these techniques to study 'nematic-nematic tactoids': nematic droplets sedimenting within a nematic medium with mutually perpendicular orientations.

In this thesis the liquid crystal phase behavior of colloidal, boardlike, goethite (alpha-FeOOH) particles is described. Apart from the nematic phase, a smectic A phase is formed in systems with a low and high polydispersity. Strong fractionation occurs which is able to reduce the local length polydispersity by up to a factor two. The larger particles that do not fit into the smectic layers seem to be accommodated by the columnar phase, which is only observed in highly polydisperse systems. A special type of liquid crystals, the biaxial liquid crystals, is spontaneously formed if particles have specific dimensions, exactly in between rodlike and platelike, which confirms theoretical predictions. An unusual peak shape was observed in the smectic A phase. This is rationalized by the occurrence of sliding fluctuations, in which the layers undulate by sliding the particles along each other while keeping the director fixed. Goethite particles align along a small magnetic field, but in large fields the particles rotate to an orientation perpendicular to the field. This is observed in the isotropic and nematic phase. The columnar phase hardly reacts to a small field, but it aligns perpendicular to large fields. The smectic phase shows more complicated behavior. In systems with a low polydispersity it shows behavior similar to the isotropic and nematic phase. During reorientation the particle and layer orientation seem to be able to decouple. Systems with a high polydispersity align parallel to a small field as well, but in large fields the smectic phase transforms into an aligned columnar phase. Nematic-nematic phase separation can also occur in a magnetic field. When a field is applied around the critical magnetic field, where the particles rotate from parallel to perpendicular to the field, the nematic phase separates into two nematic phases with orthogonal orientations. Depending on the field strength the level of the isotropic-nematic interface rises or lowers, which seems to couple to the occurrence of nematic-nematic phase separation. Furthermore, from a certain field strength onwards nematic droplets are being formed in the aligned isotropic (paranematic) phase. In the paranematic phase also regions with different orientations coexist around the critical field strength. In the last part, modified goethite dispersions are discussed. The critical field strength of goethite particles modified with a few percent of the elements Cr, Al or Co replacing Fe is higher than that of the pure goethite particles. Apart from that, the phase behavior is similar to that of normal goethite. However, in a Cr-goethite dispersion a simple rectangular columnar phase is formed. In a large magnetic field it is transformed into the centered rectangular columnar phase, which is the structure that is usually found for goethite. This is rationalized in terms of entropic effects using a simple cell model. The last chapter shows the phase behavior of sterically stabilized goethite which is compared to that of charged goethite. The magnetic properties remain the same and the phase behavior is similar. However, in the sterically stabilized dispersions there is a stronger tendency to form gels.

Using high-resolution small-angle X-ray scattering, we observed a new type of the columnar phase with a simple rectangular (R(S)) structure in colloidal goethite dispersions. Furthermore, it displays a martensitic transition into the usual centered rectangular (R(C)) structure in an external magnetic field. The findings are rationalized in terms of entropic effects within a simple cell model. We interpret the results as an effect of the particle shape and the available degrees of freedom on the delicate balance between the space available for particle translations and rotations within the two structures.

The behavior of the smectic A phase of colloidal goethite in a magnetic field has been studied using small-angle X-ray scattering. It was found that in systems with a high polydispersity the smectic phase aligns parallel to a small field and it transforms into a columnar phase in high fields. In these systems a columnar phase is already present at higher concentrations without field. It apparently becomes more stable in a large magnetic field, when the particles orient perpendicular to the field. In systems with a low polydispersity the smectic phase aligns along a small magnetic field as well, but in high fields it reorients perpendicular to the field. The results suggest that the reorientation process follows a rather complex pathway. The particle orientation and the orientation of the layers seem to be able to decouple leading to smectic C-type deviations. No columnar phase is found in the low polydispersity systems without field nor does it form in a magnetic field.

We found, using polarization microscopy and small-angle X-ray scattering, that for goethite, a low polydispersity suffices to form two separate nematic phases, while previous theory showed that this is only possible for mixtures of particles with extremely different lengths or diameters. Applying a critical magnetic field, which induces some of the goethite nanorods to rotate, leads to sufficient excluded volume between the particles to cause macroscopic phase separation between two orthogonal nematic phases. The larger the polydispersity of the system, the broader the range of field strengths where nematic−nematic phase separation occurs. This is a new phase separation mechanism which is expected to lead to interesting interfacial phenomena.

Dispersions of board-like goethite (α-FeOOH) particles with short-range repulsive interaction form a versatile colloidal model system, showing a nematic, smectic A and columnar phase. In high magnetic fields a biaxial nematic phase is induced with the shortest dimension of the particles aligned along the field. Moreover, if particles have a shape almost exactly in between rod-like and plate-like they can spontaneously, without external magnetic field, form biaxial nematic and biaxial smectic A phases, which is in accordance with theoretical predictions. The macroscopic domains were oriented by a magnetic field and their structure was revealed by small-angle X-ray scattering. Our results suggest that biaxial phases can be readily obtained by a proper choice of the particle shape.

Biaxial nematic and biaxial smectic phases were found in a colloidal model system of goethite ( -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.

The behaviour of the different liquid crystalline phases observed in colloidal dispersions of goethite has been studied by small angle x-ray scattering. The particles align parallel to a small magnetic field and perpendicular to a large magnetic field, which was observed in the isotropic and nematic phase. The smectic phase also aligned parallel to a small magnetic field but it formed an aligned columnar phase with a distorted hexagonal structure in higher magnetic fields. The columnar phase mainly aligned in higher magnetic fields. Goethite particles were successfully modified with chromium. The Cr-modified goethite particles show similar phase behaviour as normal goethite particles but with different magnetic properties. The relative importance of the permanent and induced magnetic moment was found to have changed.

The effect of fractionation on the phase behavior of colloidal goethite dispersions with different polydispersities (17%, 35%, and 55% in length) has been studied by small angle x-ray scattering and transmission electron microscopy. All systems show at least nematic and smectic phases. The occurrence of the latter phase at such a high polydispersity is remarkable. It is shown that in the highly polydisperse systems strong fractionation occurs, which is able to reduce the local length polydispersity up to a factor of 2. A columnar phase was only found in the 35% and 55% polydisperse systems. It seems that the columnar phase accommodates the particles that do not fit into the smectic layers and, thus, reduces the length polydispersity within the smectic phase even further. The fact that a columnar phase was not found in the system of lowest polydispersity indicates that the smectic phase is the most stable phase at higher concentrations.

Summary in Dutch