Self-assembly of magnetic iron oxide nanoparticles into cuboidal superstructures

This chapter describes the synthesis and some characteristics of magnetic iron oxide nanoparticles, mainly nanocubes, and focus on their self-assembly into crystalline cuboids in dispersion. The influence of external magnetic fields, the concentration of particles, and the temperature on the assembly process is experimentally investigated

Effects of symmetry and novel geometries on observable properties of liquid crystal systems

From atoms to galaxies, symmetry plays a key role in providing structure and coherence to the laws of nature. The aim of this thesis is to investigate the effects of symmetry on a variety of liquid crystal systems. Liquid crystals are anisotropic fluids, in which the rigid and anisotropic constituent molecules have a strong tendency to form mesophases with long-range orientational order. Within this classification, there exists a rich variety of distinct mesophases with varying degrees of orientational and positional order. Tilted smectic liquid crystal phases, such as the smectic-C phase seen in calamitic liquid crystals, are usually treated using the assumption of biaxial orthorhombic symmetry. However, the smectic-C phase has monoclinic symmetry, thereby allowing a disassociation of the principal optic and dielectric axes based on symmetry and invariance principles. In this thesis, we demonstrate this by comparing optical and dielectric measurements for two materials with highly first order direct transitions from the nematic to the smectic-C phases. The results show a high difference between the orientations of the principal axes sets, which is interpreted as the existence of two distinct cone angles for optical and dielectric frequencies. Dispersion of microparticles in nematic liquid crystals offers novel means for controlling both their orientation and position through the combination of topology and external stimuli. In this thesis, we use double emulsions of water droplets inside radial nematic liquid crystal droplets to form various structures, ranging from linear chains to three-dimensional fractals. These systems are modelled as a formation of satellite droplets, distributed around a larger, central core droplet. Furthermore, we extend this reasoning to explain the formation of fractal structures. We show that a distribution of droplet sizes plays a key role in determining the symmetry properties of the resulting geometric structures. Finally, we disperse cuboid and triangular prism shaped particles in a nematic liquid crystal. Experimental observations are compared with numerical simulations to understand the influence of geometry and symmetry on the orientation and position of the particles, both with and without the application of electric fields. We find that a particle’s orientation depends on its aspect ratio and the applied voltage for both particle types. We show that geometric symmetry breaking plays a key role in the dynamics, which prompts the field induced rotation of the particles and allows the triangular prisms to travel perpendicular to the applied electric field

Chapter Potentials and Challenges of Additive Manufacturing Technologies for Heat Exchanger

The rapid development of additive manufacturing (AM) technologies enables a radical paradigm shift in the construction of heat exchangers. In place of a layout limited to the use of planar or tubular starting materials, heat exchangers can now be optimized, reflecting their function and application in a particular environment. The complexity of form is no longer a restriction but a quality. Instead of brazing elements, resulting in rather inflexible standard components prone to leakages, with AM, we finally can create seamless integrated and custom solutions from monolithic material. To address AM for heat exchangers we both focus on the processes, materials, and connections as well as on the construction abilities within certain modeling and simulation tools. AM is not the total loss of restrictions. Depending on the processes used, delicate constraints have to be considered. But on the other hand, we can access materials, which can operate in a much wider heat range. It is evident that conventional modeling techniques cannot match the requirements of a flexible and adaptive form finding. Instead, we exploit biomimetic and mathematical approaches with parametric modeling. This results in unseen configurations and pushes the limits of how we should think about heat exchangers today

Stochastic Surface Growth

Growth phenomena constitute an important field in nonequilibrium statistical mechanics. Kardar, Parisi, and Zhang (KPZ) in 1986 proposed a continuum theory for local stochastic growth predicting scale invariance with universal exponents and limiting distributions. For a special, exactly solvable growth model (polynuclear growth - PNG) on a one-dimensional substrate (1+1 dimensional) we confirm the known scaling exponents and identify for the first time the limiting distributions of height fluctuations for different initial conditions (droplet, flat, stationary). Surprisingly, these so-called Tracy-Widom distributions have been encountered earlier in random matrix theory. The full stationary two-point function of the PNG model is calculated. Its scaling limit is expressed in terms of the solution to a special Rieman-Hilbert problem and determined numerically. By universality this yields a prediction for the stationary two-point function of (1+1)-dimensional KPZ theory. For the PNG droplet we show that the surface fluctuations converge to the so-called Airy process in the sense of joint distributions. Finally we discuss the theory for higher substrate dimensions and provide some Monte-Carlo simulations.Wachstumsphänomene stellen ein wichtiges Teilgebiet der statistischen Mechanik des Nichtgleichgewichts dar. Die 1986 von Kardar, Parisi und Zhang (KPZ) vorgeschlagene Kontinuumstheorie sagt für lokales stochastisches Wachstum Skaleninvarianz mit universellen Exponenten und Grenzverteilungen vorher. Für ein spezielles, exakt lösbares, Wachstumsmodell (polynuclear growth - PNG) auf eindimensionalem Substrat (1+1 dimensional) werden die bekannten Skalenexponenten bestätigt und die Grenzverteilungen der Höhenfluktuationen bei verschiedenen Anfangsbedingungen (Tropfen, flach, stationär) erstmals identifiziert. Überraschenderweise sind diese sogenannten Tracy-Widom-Verteilungen aus der Theorie der Zufallsmatrizen bekannt. Die volle stationäre Zweipunkt-Funktion des PNG-Modells wird berechnet. Im Skalenlimes wird sie durch die Lösung eines speziellen Riemann-Hilbert-Problems ausgedrückt und numerisch bestimmt. Auf Grund der erwarteten Universalität erhält man somit eine Vorhersage für die stationäre Zweipunkt-Funktion der (1+1)-dimensionalen KPZ-Theorie. Für die Tropfengeometrie wird gezeigt, dass die Oberflächenfluktuationen im Sinne gemeinsamer Verteilungen gegen den sogenannten Airy-Prozess konvergieren. Schliesslich wird die Theorie für höhere Substratdimensionen diskutiert und durch Monte-Carlo-Simulationen ergänzt