Goethite liquid crystals and magnetic field effects

Abstract

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.