The grain-scale distribution and behaviour of melt and fluid in crystalline analogue systems

The production, segregation and migration of melt and aqueous fluids (henceforth called liquid) plays an important role for the transport of mass and energy within the mantle and the crust of the Earth. Many properties of large-scale liquid migration processes such as the permeability of a rock matrix or the initial segregation of newly formed liquid from the host-rock depends on the grain-scale distribution and behaviour of liquid. Although the general mechanisms of liquid distribution at the grain-scale are well understood, the influence of possibly important modifying processes such as static recrystallization, deformation, and chemical disequilibrium on the liquid distribution is not well constrained. For this thesis analogue experiments were used that allowed to investigate the interplay of these different mechanisms in-situ. In high-temperature environments where melts are produced, the grain-scale distribution in “equilibrium” is fully determined by the liquid fraction and the ratio between the solid-solid and the solid-liquid surface energy. The latter is commonly expressed as the dihedral or wetting angle between two grains and the liquid phase (Chapter 2). The interplay of this “equilibrium” liquid distribution with ongoing surface energy driven recrystallization is investigated in Chapter 4 and 5 with experiments using norcamphor plus ethanol liquid. Ethanol in contact with norcamphor forms a wetting angle of about 25°, which is similar to reported angles of rock-forming minerals in contact with silicate melt. The experiments in Chapter 4 show that previously reported disequilibrium features such as trapped liquid lenses, fully-wetted grain boundaries, and large liquid pockets can be explained by the interplay of the liquid with ongoing recrystallization. Closer inspection of dihedral angles in Chapter 5 reveals that the wetting angles are themselves modified by grain coarsening. Ongoing recrystallization constantly moves liquid-filled triple junctions, thereby altering the wetting angles dynamically as a function of the triple junction velocity. A polycrystalline aggregate will therefore always display a range of equilibrium and dynamic wetting angles at raised temperature, rather than a single wetting angle as previously thought. For the deformation experiments partially molten KNO3–LiNO3 experiments were used in addition to norcamphor–ethanol experiments (Chapter 6). Three deformation regimes were observed. At a high bulk liquid fraction >10 vol.% the aggregate deformed by compaction and granular flow. At a “moderate” liquid fraction, the aggregate deformed mainly by grain boundary sliding (GBS) that was localized into conjugate shear zones. At a low liquid fraction, the grains of the aggregate formed a supporting framework that deformed internally by crystal plastic deformation or diffusion creep. Liquid segregation was most efficient during framework deformation, while GBS lead to slow liquid segregation or even liquid dispersion in the deforming areas.

Liquid crystalline elastomers as stimuli-responsive microactuators

Liquid crystalline elastomers (LCEs) are known to perform a reversible change of shape upon the phase transition from the semi-ordered liquid crystalline state to the chaotic isotropic state. This unique behavior of these “artificial muscles” arises from the self-organizing properties of liquid crystals (mesogens) in combination with the entropy-elasticity of the slightly crosslinked elastomer network. In this work, micrometer-sized LCE actuators are fabricated in a microfluidic setup. The microtubular shear flow provides for a uniform orientation of the mesogens during the crosslinking, a perquisite for obtaining actuating LCE samples. The scope of this work was to design different actuator geometries and to broaden the applicability of the microfluidic device for different types of liquid crystalline mesogens, ranging from side-chain to main-chain systems, as well as monomer and polymer precursors. For example, the thiol-ene “click” mechanism was used for the polymerization and crosslinking of main-chain LCE actuators. The main focus was, however, placed on acrylate monomers and polymers with LC side chains. A LC polymer precursor, comprising mesogenic and crosslinkable side-chains was synthesized. Used in combination with an LC monomer, the polymeric crosslinker promoted a stable LC phase, which allowed the mixture to be isothermally handled in the microfluidic reactor. If processed without the additional LC components, the polymer precursor yielded actuating fibers. A suitable co-flowing continuous phase facilitates the formation of a liquid jet and lowers the tendency for drop formation. By modification of the microfluidic device, it was further possible to prepare core-shell particles, comprised of an LCE shell and filled with an isotropic liquid. In analogy to the heart, a hollow muscle, the elastomer shell expels the inner liquid core upon its contraction. The feasibility of the core-shell particles as micropumps was demonstrated. In general, the synthesized LCE microactuators may be utilized as active components in micromechanical and lab-on-chip systems.