Kolloide in isotropen und anisotropen Fluiden: Wechselwirkungen und Mechanismen der Strukturbildung in Mischungen aus Flüssigkristallen und Kolloiden
Zusammenfassung der Projektergebnisse
The goal of our project was to investigate the transport mechanisms governing the behaviour of colloids dispersed in liquid crystals doped with isotropic solvents, with special regard to the mutual interdependence between phase transition dynamics and kinetics and colloid motion. Adding alkanes to the nematogen 5CB allowed us to simultaneously observe colloid motion in and advancing isotropic-nematic interfaces due to coexistence of isotropic and nematic domains during substantial temperature intervals. We were also able to estimate phase transition-induced intrinsic alkane transport and to relate it to colloid motion. PMMA as well as silica colloids were used to consider different surface anchoring and wetting properties. Optical tracking of single-particles and the corresponding interfaces in thin samples during slow cooling yielded different scenaria of colloid interaction: 1. Early-stage attraction, due to interaction with alkane released from growing droplets and asymmetric interfacial tension on the colloid surface. 2. Convective colloid motion patterns in the range of several micrometers from the isotropic-nematic interface can be attributed to the interplay between interfacial tension effects and advection due to alkane flow away from the droplets. 3. Advective motion due to large-scale hydrodynamic flow resulting from droplet coalescence and reduction of interfacial area. Surprisingly, all three patterns were observed simultaneously but could be distinguished during the most part of phase transition. That (1) and (2) were observed only in PMMA particles hints that modulation of interfacial tension by alkanes may stabilize colloidal networks exclusively formed in more concentrated PMMA suspensions. We also investigated such networks in 5CB-alkane media using a piezo-driven rheometer built for this purpose. With this device, even in very fragile networks shear can be applied without destruction. The observed f1/2 dependence of storage and loss moduli on frequency points to a large range of length scales suggestive of critical percolation. That time-temperature superposition was found to hold for frequency but not for the phase suggests that these networks undergo ageing close to phase transition. In order to investigate the influence of colloid-alkane on colloid-liquid crystal interaction during phase transition, we studied pre-transitional surface-induced ordering on modified glass and colloid surfaces by AFM. Whereas alkane decreases order in 5CB, we found that this effect depends on the length of alkane in 8CB. Moreover, it seems to be due to promotion of pre-smectic rather than pre-nematic order. When applying ac electric fields to separately analyze the dependence of colloid motion on interface movement and droplet growth, we discovered that droplet shape changes from spherical to oblate or prolate depending on field frequency and orientation. By measuring colloid velocities around the droplets, we characterized the electrohydrodynamic flow profile. This can be described by the "leaky dielectric" model; however, it remains to be elucidated how texture changes within droplets translate to flow patterns and how they couple to flow outside the droplet.
Projektbezogene Publikationen (Auswahl)
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Formation of S elf-Supporting Reversible Cellular Networks in Suspensions of
Colloids and Liquid Crystals; Langmuir 2005, 21, 4921-4930
D. Vollmer, G. Hinze, B. Ullrich, W.C. Poon, M.E. Gates, and A.B. Schofield
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Long Range Particle Transport in Liquid Crystal-Alkane Mixtures; Prog. Colloid Polym Sci. 2006, 133, 142-146
B. Ullrich, E. Ilska, N. Höhn, and D. Vollmer
