Kaiserslautern - Fachbereich Physik
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Functional structures as well as materials provided by nature have always been a great source of inspiration for new technologies. Adapting and improving the discovered concepts, however, demands a detailed understanding of their working principles, while employing natural materials for fabrication tasks requires suitable functionalization and modification.
In this thesis, the white scales of the beetle Cyphochilus are examined in order to reveal unknown aspects of their light transport properties. In addition, the monomer of the material they are made of is utilized for 3D microfabrication.
White beetle scales have been fascinating scientists for more than a decade because they display brilliant whiteness despite their small thickness and the low refractive index contrast. Their optical properties arise from highly efficient light scattering within the disordered intra-scale network structure.
To gain a better understanding of the scattering properties, several previous studies have investigated the light transport and its connection to the structural anisotropy with the aid of diffusion theory. While this framework allows to relate the light scattering to macroscopic transport properties, an accurate determination of the effective refractive index of the structure is required. Due to its simplicity, the Maxwell-Garnett mixing rule is frequently used for this task, although its constraint to particle and feature sizes much smaller than the wavelength is clearly violated for the scales.
To provide a correct calculation of the effective refractive index, here, finite-difference time-domain simulations are used to systematically examine the impact of size effects on the effective refractive index. Deploying this simulation approach, the Maxwell-Garnett mixing rule is shown to break down for large particles. In contrast, it is found that a quadratic polynomial function describes the effective refractive index in close approximation, while its coefficients can be obtained from an empirical linear function. As a result, a simple mixing rule is reported that unambiguously surpasses classical mixing rules when composite media containing large feature sizes are considered. This is important not only for the accurate description of white beetle scales, but also for other turbid media, such as biological tissues in opto-biomedical diagnostics.
Describing light transport by means of diffusion theory moreover neglects any coherent effects, such as interference. Hence, their impact on the generation of brilliant whiteness is currently unknown. To shed a light on their role, spatial- and time-resolved light scattering spectromicroscopy is applied to investigate the scales and a model structure of them based on disordered Bragg stacks. For both structures the occurrence of weakly localized photonic modes, i.e., closed scattering loops, is observed, which is further verified in accompanying simulations. As shown in this thesis, leakage from these random photonic modes contributes at least 20% to the overall reflected light. This reveals the importance of coherent effects for a complete description of the underlying light transport properties; an aspect that is entirely missing in the purely diffusive transport presumed so far. Identifying the importance of weak localization for the generation of brilliant whiteness paves the way to further enhance the design of efficient optical scattering media, an issue that recently drawn great attention.
Unlike their plant-based counterparts, rigid carbohydrates, such as chitin, are currently unavailable for 3D microfabrication via direct laser writing, despite their great significance in the animal kingdom for the construction of functional microstructures. To overcome this gap, the monomeric unit of chitin, N-acetyl-D-glucosamine, is here functionalized to serve as a photo-crosslinkable monomer in a non-hydrogel photoresist. Since all previous photoresists based on animal carbohydrates are in the form of hydrogel formulations, a new group of photoresists is established for direct laser writing.
Moreover, it is exhibited that the sensitization effect, previously used only in the context of UV curing, can be successfully transferred to direct laser writing to increase the maximum writing speed. This effect is based on the beneficial combination of two photoinitiators.
In this, one photoinitiator is an efficient crosslinking agent for the monomer used, but a rather poor two-photon absorber. The other photoinitiator (called sensitizer) possesses, conversely, a much higher two-photon absorption coefficient at the applied wavelength but is not well suited as a crosslinking agent. In combination, the energy absorbed by the sensitizer is passed to the photoinitiator, resulting in the formation of radicals needed to start the polymerization. As this greatly increases the rate at which the photoinitiator is radicalized, resists containing a photoinitiator and a sensitizer are shown to outperform resists containing only one of the components. Deploying the sensitization effect in direct laser writing therefore offers a simple way to individually tune the crosslinking ability and the two-photon absorption properties by combining existing compounds, compared to the costly chemical synthesis of novel, customized photoinitiators.
Lubricated tribological contact processes are important in both nature and in many technical applications. Fluid lubricants play an important role in contact processes, e.g. they reduce friction and cool the contact zone. The fundamentals of lubricated contact processes on the atomistic scale are, however, today not fully understood. A lubricated contact process is defined here as a process, where two solid bodies that are in close proximity and eventually in parts in direct contact, carry out a relative motion, whereat the remaining volume is submersed by a fluid lubricant. Such lubricated contact processes are difficult to examine experimentally. Atomistic simulations are an attractive alternative for investigating the fundamentals of such processes. In this work, molecular dynamics simulations were used for studying different elementary processes of lubricated tribological contacts. A simplified, yet realistic simulation setup was developed in this work for that purpose using classical force fields. In particular, the two solid bodies were fully submersed in the fluid lubricant such that the squeeze-out was realistically modeled. The velocity of the relative motion of the two solid bodies was imposed as a boundary condition. Two types of cases were considered in this work: i) a model system based on synthetic model substances, which enables a direct, but generic, investigation of molecular interaction features on the contact process; and ii) real substance systems, where the force fields describe specific real substances. Using the model system i), also the reproducibility of the findings obtained from the computer experiments was critically assessed. In most cases, also the dry reference case was studied. Both mechanical and thermodynamic properties were studied -- focusing on the influence of lubrication. The following properties were studied: The contact forces, the coefficient of friction, the dislocation behavior in the solid, the chip formation and the formation of the groove, the squeeze-out behavior of the fluid in the contact zone, the local temperature and the energy balance of the system, the adsorption of fluid particles on the solid surfaces, as well as the formation of a tribofilm. Systematic studies were carried out for elucidating the influence of the wetting behavior, the influence of the molecular architecture of the lubricant, and the influence of the lubrication gap height on the contact process. As expected, the presence of a fluid lubricant reduces the temperature in the vicinity of the contact zone. The presence of the lubricant is, moreover, found to have a significant influence on the friction and on the energy balance of the process. The presence of a lubricant reduces the coefficient of friction compared to a dry case in the starting phase of a contact process, while lubricant molecules remain in the contact zone between the two solid bodies. This is a result of an increased normal and slightly decreased tangential force in the starting phase. When the fluid molecules are squeezed out with ongoing contact time and the contact zone is essentially dry, the coefficient of friction is increased by the presence of a fluid compared to a dry case. This is attributed to an imprinting of individual fluid particles into the solid surface, which is energetically unfavorable. By studying the contact process in a wide range of gap height, the entire range of the Stribeck curve is obtained from the molecular simulations. Thereby, the three main lubrication regimes of the Stribeck curve and their transition regions are covered, namely boundary lubrication (significant elastic and plastic deformation of the substrate), mixed lubrication (adsorbed fluid layers dominate the process), and hydrodynamic lubrication (shear flow is set up between the surface and the asperity). The atomistic effects in the different lubrication regimes are elucidated. Notably, the formation of a tribofilm is observed, in which lubricant molecules are immersed into the metal surface. The formation of a tribofilm is found to have important consequences for the contact process. The work done by the relative motion is found to mainly dissipate and thereby heat up the system. Only a minor part of the work causes plastic deformation. Finally, the assumptions, simplifications, and approximations applied in the simulations are critically discussed, which highlights possible future work.
With the transition of fluid-capillary-based “Lab on a chip 1.0″ concepts in analytical chemistry to “Lab on a chip
2.0″ approaches relying on distinct fluid droplets (“digital microfluidics”, DMF), the need for reliable methods for
droplet actuation has increasingly come into focus. One possible approach is based on “electrowetting on
dielectric” (EWOD). This technique has the disadvantage that any possible desired later positions of the droplets
on the chip have to be defined prior to chip realization because one of the EWOD electrode layers has to be
structured accordingly. “Optoelectrowetting” (OEW) goes a step further in the sense that the later droplet positions
do not have to be known before, and none of the electrode layers has to be structured. Instead, the
electrical parameters of the layer sequence can be altered locally by an impinging (and movable) light spot.
Although some research groups have succeeded in demonstrating OEW actuation of droplets, the optimization of
the relevant parameters of the layer sequence and the droplet – at least half a dozen parameters altogether – is
tedious and not straight-forward. In this contribution, for optimization purposes, the equations governing OEW
are revisited and altered again, e.g., by numerical implementation of the experimentally well-known saturation
of the contact angle change. Additionally, a Nelder-Mead algorithm is applied to find the parameters, on which
the optimization has to focus to maximize contact angle changes and, thus, mechanical forces on the droplets.
The numerical investigation yields diverse results, e.g., the finding that the droplet’s contact area on the
dielectric layer has a strong influence on the contact angle change and the question whether the droplet is pulled
or pushed. Moreover, the interplay between frequency and amplitude of the applied rectangular alternate voltage
is important for optimization.
In this work, we investigate and compare the condensation behavior of hydrophilic, hydrophobic, and biphilic microgrooved silicon samples etched by reactive ion etching. The microgrooves were 25 mm long and 17−19 μm deep with different
topologies depending on the etching process. Anisotropically etched samples had 30 μm wide rectangular microgrooves and silicon ridges between them. They were either left hydrophilic or covered with a hydrophobic fluorocarbon or photoresist layer.
Anisotropically etched samples consisted of 48 μm wide semicircular shaped microgrooves, 12 μm wide silicon ridges between them, and a 30 μm wide photoresist stripe centered on the ridges. The lateral dimensions were chosen to be much smaller than the capillary length of water to support drainage of droplets by coalescence rather than droplet sliding. Furthermore, to achieve a low thermal resistance of the periodic surface structure consisting of water-filled grooves and silicon ridges, the trench depth was also kept small. The dripped-off total amount of condensate (AoC) was measured for each sample for 12 h under the same boundary
conditions (chamber temperature 30 °C, cooling temperature 6 °C, and relative humidity 60%). The maximum increase in AoC of 15.9% (9.6%) against the hydrophilic (hydrophobic) reference sample was obtained for the biphilic samples. In order to elucidate their unique condensation behavior, in situ optical imaging was performed at normal incidence. It shows that the drainage of droplets from the stripe’s surface into the microgrooves as well as occasional droplet sliding events are the dominant processes to clear the surface. To rationalize this behavior, the Hough Circle Transform algorithm was implemented for image processing to receive
additional information about the transient droplet size and number distribution. Postprocessing of these data allows calculation
Understanding the mechanisms and controlling
the possibilities of surface nanostructuring is of crucial interest
for both fundamental science and application perspectives.
Here, we report a direct experimental observation
of laser-induced periodic surface structures (LIPSS) formed
near a predesigned gold step edge following single-pulse
femtosecond laser irradiation. Simulation results based on a
hybrid atomistic-continuum model fully support the experimental
observations. We experimentally detect nanosized
surface features with a periodicity of ∼300 nm and heights of
a few tens of nanometers.We identify two key components of
single-pulse LIPSS formation: excitation of surface plasmon
polaritons and material reorganization. Our results lay a
solid foundation toward simple and efficient usage of light
for innovative material processing technologies.
Visual–graphical representations are used to visualise information and are therefore key components of learning materials. An important type of convention-based representation in everyday contexts as well as in science, technology, engineering, and math (STEM) disciplines are vector field plots. Based on the cognitive theory of multimedia learning, we aim to optimize an instruction with symbolical-mathematical and visual-graphical representations in undergraduate physics education through spoken instruction combined with dynamic visual cues. For this purpose, we conduct a pre-post study with 38 natural science students who are divided into two groups and instructed via different modalities and with visual cues on the graphical interpretation of vector field plots. Afterward, the students rate their cognitive load. During the computer-based experiment, we record the participants’ eye movements. Our results indicate that students with spoken instruction perform better than students with written instruction. This suggests that the modality effect is also applicable to mathematical-symbolical and convention-based visual-graphical representations. The differences in visual strategies imply that spoken instruction might lead to increased effort in organising and integrating information. The finding of the modality effect with higher performance during spoken instruction could be explained by deeper cognitive processing of the material.
Indentation and Scratching with a Rotating Adhesive Tool: A Molecular Dynamics Simulation Study
(2022)
For the specific case of a spherical diamond nanoparticle with 10 nm radius rolling over a planar Fe surface, we employ molecular dynamics simulation to study the processes of indentation and scratching. The particle is rotating (rolling). We focus on the influence of the adhesion force between the nanoparticle and the surface on the damage mechanisms on the surface; the adhesion is modeled by a pair potential with arbitrarily prescribed value of the adhesion strength. With increasing adhesion, the following effects are observed. The load needed for indentation decreases and so does the effective material hardness; this effect is considerably more pronounced than for a non-rotating particle. During scratching, the tangential force, and hence the friction coefficient, increase. The torque needed to keep the particle rolling adds to the total work for scratching; however, for a particle rolling without slip on the surface the total work is minimum. In this sense, a rolling particle induces the most efficient scratching process. For both indentation and scratching, the length of the dislocation network generated in the substrate reduces. After leaving the surface, the particle is (partially) covered with substrate atoms and the scratch groove is roughened. We demonstrate that these effects are based on substrate atom transport under the rotating particle from the front towards the rear; this transport already occurs for a repulsive particle but is severely intensified by adhesion.
Nanostructured tantalum (Ta)-based dental implants have recently attracted significant attention thanks to their superior biocompatibility and bioactivity as compared to their titanium-based counterparts. While the biological and chemical aspects of Ta implants have been widely studied, their mechanical features have been investigated more rarely. Additionally, the mechanical behavior of these implants and, more importantly, their plastic deformation mechanisms are still not fully understood. Accordingly, in the current research, molecular dynamics simulation as a powerful tool for probing the atomic-scale phenomena is utilized to explore the microstructural evolution of pure polycrystalline Ta samples under tensile loading conditions. Various samples with an average grain size of 2–10 nm are systematically examined using various crystal structure analysis tools to determine the underlying deformation mechanisms. The results reveal that for the samples with an average grain size larger than 8 nm, twinning and dislocation slip are the main sources of any plasticity induced within the sample. For finer-grained samples, the activity of grain boundaries—including grain elongation, rotation, migration, and sliding—are the most important mechanisms governing the plastic deformation. Finally, the temperature-dependent Hall–Petch breakdown is thoroughly examined for the nanocrystalline samples via identification of the grain boundary dynamics.
Within this work, we report the results of nuclear inelastic scattering experiments of the low-spin phase of the iron(II) mononuclear SCO complex Fe[HBpz3]2 and density functional theory based calculations performed on a model molecule of the complex. We show that the calculated partial density of vibrational states based on the structure of a single iron(II) center which is linked by three pyrazole rings to borat is in good accordance with the experimentally obtained 57Fe-pDOS and assign the molecular vibrations to the prominent optical phonons.
Weyl points are point degeneracies that occur in momentum space of 3D periodic materials and are associated with a quantized topological charge. Here, the splitting of a quadratic (charge-2) Weyl point into two linear (charge-1) Weyl points in a 3D micro-printed photonic crystal is observed experimentally via Fourier-transform infrared spectroscopy. Using a theoretical analysis rooted in symmetry arguments, it is shown that this splitting occurs along high-symmetry directions in the Brillouin zone. This micro-scale observation and control of Weyl points is important for realizing robust topological devices in the near-infrared.