Complete list from the university bibliography

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Maity, Priyanka; Bittracher, Andreas; Koltai, Péter; Schumacher, Jörg
Collective variables between large-scale states in turbulent convection. - In: Physical review research, ISSN 2643-1564, Bd. 5 (2023), 3, S. 033061-1-033061-19

The dynamics in a confined turbulent convection flow is dominated by multiple long-lived macroscopic circulation states that are visited subsequently by the system in a Markov-type hopping process. In the present work, we analyze the short transition paths between these subsequent macroscopic system states by a data-driven learning algorithm that extracts the low-dimensional transition manifold and the related new coordinates, which we term collective variables, in the state space of the complex turbulent flow. We therefore transfer and extend concepts for conformation transitions in stochastic microscopic systems, such as in the dynamics of macromolecules, to a deterministic macroscopic flow. Our analysis is based on long-term direct numerical simulation trajectories of turbulent convection in a closed cubic cell at a Prandtl number Pr=0.7 and Rayleigh numbers Ra=10^6 and 10^7 for a time lag of 10^5 convective free-fall time units. The simulations resolve vortices and plumes of all physically relevant scales, resulting in a state space spanned by more than 3.5 million degrees of freedom. The transition dynamics between the large-scale circulation states can be captured by the transition manifold analysis with only two collective variables, which implies a reduction of the data dimension by a factor of more than a million. Our method demonstrates that cessations and subsequent reversals of the large-scale flow are unlikely in the present setup, and thus it paves the way for the development of efficient reduced-order models of the macroscopic complex nonlinear dynamical system.



https://doi.org/10.1103/PhysRevResearch.5.033061
Göring, Steve; Raake, Alexander
Image appeal revisited: analysis, new dataset, and prediction models. - In: IEEE access, ISSN 2169-3536, Bd. 11 (2023), S. 69563-69585

There are more and more photographic images uploaded to social media platforms such as Instagram, Flickr, or Facebook on a daily basis. At the same time, attention and consumption for such images is high, with image views and liking as one of the success factors for users and driving forces for social media algorithms. Here, “liking” can be assumed to be driven by image appeal and further factors such as who is posting the images and what they may show and reveal about the posting person. It is therefore of high research interest to evaluate the appeal of such images in the context of social media platforms. Such an appeal evaluation may help to improve image quality or could be used as an additional filter criterion to select good images. To analyze image appeal, various datasets have been established over the past years. However, not all datasets contain high-resolution images, are up to date, or include additional data, such as meta-data or social-media-type data such as likes and views. We created our own dataset “AVT-ImageAppeal-Dataset”, which includes images from different photo-sharing platforms. The dataset also includes a subset of other state-of-the-art datasets and is extended by social-media-type data, meta-data, and additional images. In this paper, we describe the dataset and a series of laboratory- and crowd-tests we conducted to evaluate image appeal. These tests indicate that there is only a small influence when likes and views are included in the presentation of the images in comparison to when these are not shown, and also the appeal ratings are only a little correlated to likes and views. Furthermore, it is shown that lab and crowd tests are highly similar considering the collected appeal ratings. In addition to the dataset, we also describe various machine learning models for the prediction of image appeal, using only the photo itself as input. The models have a similar or slightly better performance than state-of-the-art models. The evaluation indicates that there is still an improvement in image appeal prediction and furthermore, other aspects, such as the presentation context could be evaluated.



https://doi.org/10.1109/ACCESS.2023.3292588
Khamidullina, Liana; Seidl, Gabriela; Podkurkov, Ivan Alexeevich; Korobkov, Alexey Alexandrovich; Haardt, Martin
Enhanced solutions for the block-term decomposition in rank-(Lr, Lr, 1) terms. - In: IEEE transactions on signal processing, ISSN 1941-0476, Bd. 71 (2023), S. 2608-2621

The block-term decompositions (BTD) represent tensors as a linear combination of low multilinear rank terms and can be explicitly related to the Canonical Polyadic decomposition (CPD). In this paper, we introduce the SECSI-BTD framework, which exploits the connection between two decompositions to estimate the block-terms of the rank-(Lr, Lr, 1) BTD. The proposed SECSI-BTD algorithm includes the initial calculation of the factor estimates using the SEmi-algebraic framework for approximate Canonical polyadic decompositions via SImultaneous Matrix Diagonalizations (SECSI), followed by clustering and refinement procedures that return the appropriate rank-(Lr, Lr, 1) BTD terms. Moreover, we introduce a new approach to estimate the multilinear rank structure of the tensor based on the HOSVD and $k$-means clustering. Since the proposed SECSI-BTD algorithm does not require a known rank structure but can still take advantage of the known ranks when available, it is more flexible than the existing techniques in the literature. Additionally, our algorithm does not require multiple initializations, and the simulation results show that it provides more accurate results and a better convergence behavior for an extensive range of SNRs.



https://doi.org/10.1109/TSP.2023.3289730
Wegert, Laureen; Schramm, Stefan; Dietzel, Alexander; Link, Dietmar; Klee, Sascha
Three-dimensional light field fundus imaging: automatic determination of diagnostically relevant optic nerve head parameters. - In: Translational Vision Science & Technology, ISSN 2164-2591, Bd. 12 (2023), 7, 21, S. 1-16

Purpose: Morphological changes to the optic nerve head (ONH) can be detected at the early stages of glaucoma. Three-dimensional imaging and analysis may aid in the diagnosis. Light field (LF) fundus cameras can generate three-dimensional (3D) images of optic disc topography from a single shot and are less susceptible to motion artifacts. Here, we introduce a processing method to determine diagnostically relevant ONH parameters automatically and present the results of a subject study performed to validate this method. Methods: The ONHs of 17 healthy subjects were examined and images were acquired with both an LF fundus camera and by optical coherence tomography (OCT). The LF data were analyzed with a novel algorithm and compared with the results of the OCT study. Depth information was reconstructed, and a model with radial basis functions was used for processing of the 3D point cloud and to provide a finite surface. The peripapillary rising and falling edges were evaluated to determine optic disc and cup contours and finally calculate the parameters. Results: Nine of the 17 subjects exhibited prominent optic cups. The contours and ONH parameters determined by an analysis of LF 3D imaging largely agreed with the data obtained from OCT. The median disc areas, cup areas, and cup depths differed by 0.17 mm^2, -0.04 mm^2, and -0.07 mm, respectively. Conclusions: The findings presented here suggest the possibility of using LF data to evaluate the ONH. Translational Relevance: LF data can be used to determine geometric parameters of the ONH and thus may be suitable for future use in glaucoma diagnostics.



https://doi.org/10.1167/tvst.12.7.21
Labus Zlatanovic, Danka; Hildebrand, Jörg; Bergmann, Jean Pierre
The study of screw extrusion-based additive manufacturing of eco-friendly aliphatic polyketone. - In: Journal of materials research and technology, ISSN 2214-0697, Bd. 25 (2023), S. 4125-4138

Aliphatic polyketone is a new-age eco-friendly, high-performance engineering thermoplastic. However, its potential for replacing other polymers depends on its ability to be processed. Considering that the first aliphatic polyketone suitable for processing was developed relatively recently (2015), the material gained new research potential. In this paper screw extrusion-based process was developed for additive manufacturing of aliphatic polyketone. A detailed characterisation of the process and printed samples was done. It was shown that the extruder-base process can produce stable additive-manufactured parts depending on printing speed (process parameters). The interpass temperature has a significant influence on printing properties and it depends on printing speed (travel speed of building platform and extruder rotational speed). With the increase in the printing speed, the interpass temperature increases as well. If it is low causes insufficient heat for diffusion to occur causing delamination and if it is too high causes geometrical deviation of workpieces which leads to defects causing a reduction in inter-road strength. The tensile strength of specimens with raster angle 0˚ was 62.7 ± 1.4 MPa, which is slightly higher than the tensile strength of base material guaranteed by the supplier (60 MPa) while the elongation up to the first crack was 32.8 ± 4.6%. Iinter-road strength in specimens with a raster angle of 90˚ was 37.2 ± 0.8 MPa which is 62% of the base material while interpass temperature was 189 ± 3.3 ˚C.



https://doi.org/10.1016/j.jmrt.2023.06.223
Matthes, Sebastian; Glaser, Marcus; Vardo, Emina; Sauni Camposano, Yesenia Haydee; Jaekel, Konrad; Bergmann, Jean Pierre; Schaaf, Peter
Influence of extrinsic induced tensile stress on the self-propagating high-temperature synthesis of nanosized Al/Ni multilayers. - In: Journal of materials science, ISSN 1573-4803, Bd. 58 (2023), 24, S. 10085-10095

Reactive multilayer systems consisting of alternating nanoscale Al and Ni layers are applicable in joining, various pyrotechnic applications and thermal batteries. Since diffusion based high-temperature synthesis occurs without the presence of air, efforts have focused on investigating the understanding of the fundamental reaction processes and characteristics. The aim of this study is to expose the reactive multilayers to extrinsic induced tensile stress so that the self-propagating synthesis can proceed under these conditions. Further, the properties during and after the reaction will be investigated. Multilayers deposited by sputtering on Kapton® substrates with different bilayer- and total thicknesses as well as commercial Nanofoils® with thicknesses of 40 µm and 60 µm were used as samples. The investigations focused on the propagation velocity measured with a high-speed camera, the temperature regime determined with a high-speed pyrometer, and the formed phases after the synthesis examined via X-ray diffraction. The gained results of this study reveal important insights for the application of the reactive Al/Ni multilayer system in terms of stability or reliability related to propagation front velocity, maximum temperature and formed phases under induced external tensile stresses.



https://doi.org/10.1007/s10853-023-08618-w
Sharifi Ghazijahani, Mohammad; Cierpka, Christian
Flow structure and dynamics behind cylinder arrays at Reynolds number ∼100. - In: Physics of fluids, ISSN 1089-7666, Bd. 35 (2023), 6, 067125, S. 067125-1-067125-14

The flow behind nine different arrays of cylinders is experimentally investigated via Particle Image Velocimetry (PIV) at a Reynolds number of Re ∼ 100 based on the diameter of the cylinders. Each array consists of a column of four cylinders in front and three in the rear. The horizontal distance between the two columns and the vertical distance between the cylinders within each column are varied for H/D = [2, 4, 8] and V/D = [2, 4, 6], resulting in nine different arrays denoted as mVnH, where m corresponds to V/D and n stands for H/D. The PIV measurements are conducted for 15 s at 200 Hz frequency, corresponding to 39 to 360 vortex shedding events for the wakes in this study. Then, proper orthogonal decomposition is applied to the velocity fields to analyze the flow dynamics. All arrays show unsteady flow, and based on their flow structures, they are classified in to three main categories of single bluff body (SBB), transitional (TR), and co-shedding (CS) flow. SBB characteristics can be seen for 2V2H and 2V4H arrays, but the latter has more steady vortex shedding as the H/D increases from 2 to 4. Then, 2V8H and 4V2H have an asymmetric flow with several vortex streets and act as an intermediary stage in the shift from SBB to CS flow structure when the distances are increased. The highest total kinetic energy values and widest probability density functions of the velocity components are observed for this group. The five remaining arrays in the CS group have symmetric flow, with three or five vortex streets present behind. However, based on the distances, the frequency and phase synchronization of the vortex streets change considerably, which might have an important effect on, for example, the heat transfer or the structural load of the cylinders.



https://doi.org/10.1063/5.0155102
Mazétyté-Stasinskiené, Raminta; Kronfeld, Klaus-Peter; Köhler, Michael
Five-level structural hierarchy: microfluidically supported synthesis of core-shell microparticles containing nested set of dispersed metal and polymer micro and nanoparticles. - In: Particle & particle systems characterization, ISSN 1521-4117, Bd. 14 (2023), 10, 2300030, S. 1-13

This study presents the development of a hierarchical design concept for the synthesis of multi-scale polymer particles with up to five levels of organization. The synthesis of core-shell microparticles containing nested sets of dispersed metal and polymer micro- and nanoparticles is achieved through in situ photopolymerization using a double co-axial capillaries microfluidic device. The flow rates of the carrier, shell, and core phases are optimized to control particle size and result in stable core-shell particles with well-dispersed three-level composites in the shell matrix. The robustness and reversibility of these core-shell particles are demonstrated through five cycles of drying and re-swelling, showing that the size and structure of core-shell particles remain unchanged. Additionally, the permeability and mobility of dye molecules within the shell matrix are tested and showed that different molecular weight dyes have different penetration times. This study highlights the potential of microfluidics as a powerful tool for the controlled and precise synthesis of complex structured materials and demonstrates the versatility and potential of these core-shell particles for sensing applications as particle-based surface-enhanced Raman scattering (SERS).



https://doi.org/10.1002/ppsc.202300030
Isaac, Nishchay Angel; Schlag, Leslie; Ispas, Adriana; Reiprich, Johannes; Soydan, Alper K.; Moreira, Pedro H. O.; Thiele, Sebastian; Aliabadian, Bardia; Flock, Dominik; Knauer, Andrea; Jiménez, Juan J.; Bund, Andreas; Morales Sánchez, Francisco Miguel; Pezoldt, Jörg; Jacobs, Heiko O.
Novel gas phase route toward patterned deposition of sputter-free Pt/Al nanofoils. - In: Advanced Materials Technologies, ISSN 2365-709X, Bd. 8 (2023), 18, 2300448, S. 1-8

This article reports a new approach toward fabrication and directed assembly of nanoparticulate reactive system (Nanofoils) on patterned substrates. Different from current state-of-the-art, gas phase electrodeposition uses nanoparticles instead of atoms to form densely packed multilayered thin films at room temperature-pressure. On ignition, the multilayer system undergoes an exothermic self-propagating reaction. The numerous contact points between two metallic nanoparticulate layers aid in high heat release. Sub-10-nm Platinum (Pt) and Aluminum (Al) particles are synthesized through cathode erosion of metal electrodes in a flow of pure nitrogen gas (spark ablation). Pt/Al bilayer stacks with total thickness of 3–8 µm undergo self-propagating reaction with a 10.3 mm s−1 wavefront velocity on local ignition. The reaction wavefront is captured using high speed videography. Calorimetry studies reveal two exothermic peaks suggesting Pt/Al alloy formation. The peak at 135 ˚C has a higher calorific value of 150 mW g−1 while the peak at 400 ˚C has a 12 mW g−1 exothermic peak. X-ray diffraction study shows reaction-products are cubic Al2Pt with small quantities of orthorhombic Al6Pt and orthorhombic AlPt2. Electron microscopy studies help draw a correlation between film morphology, bimetallic interface, nanoparticle oxidation, and self-propagating reaction kinetics that is significant in broadening our understanding towards nanoparticulate reactive systems.



https://doi.org/10.1002/admt.202300448
Brokmann, Ulrike; Weigel, Christoph; Altendorf, Luisa-Marie; Strehle, Steffen; Rädlein, Edda
Wet chemical and plasma etching of photosensitive glass. - In: Solids, ISSN 2673-6497, Bd. 4 (2023), 3, S. 213-234

Photosensitive glasses for radiation-induced 3D microstructuring, due to their optical transparency and thermal, mechanical, and chemical resistance, enable the use of new strategies for numerous microscale applications, ranging from optics to biomedical systems. In this context, we investigated the plasma etching of photosensitive glasses after their exposure and compared it to the established wet chemical etching method, which offers new degrees of freedom in microstructuring control and microsystem fabrication. A CF4/H2 etching gas mixture with a constant volumetric flow of 30 sccm and a variable H2 concentration from 0% to 40% was utilized for plasma-based etching, while for wet chemical etching, diluted hydrofluoric acid (1% ≤ cHF ≤ 20%) was used. Therefore, both etching processes are based on a chemical etching attack involving fluorine ions. A key result is the observed reversion of the etch selectivity between the initial glassy and partially crystallized parts that evolve after UV exposure and thermal treatment. The crystallized parts were found to be 27 times more soluble than the unexposed glass parts during wet chemical etching. During the plasma etching process, the glassy components dissolve approximately 2.5 times faster than the partially crystalline components. Unlike wet chemical etching, the surfaces of plasma etched photostructured samples showed cone- and truncated-cone-shaped topographies, which supposedly resulted from self-masking effects during plasma etching, as well as a distinct physical contribution from the plasma etching process. The influences of various water species on the etching behaviors of the homogeneous glass and partially crystallized material are discussed based on FTIR-ATR and in relation to the respective etch rates and SNMS measurements.



https://doi.org/10.3390/solids4030014