Britta Nestler’s research while affiliated with Karlsruhe Institute of Technology and other places

Solid oxide fuel cells (SOFCs) are becoming increasingly important due to their high electrical efficiency, the flexible choice of fuels and relatively low emissions of pollutants. However, the increasingly growing demands for electrochemical devices require further performance improvements as for example by reducing degradation effects. Since it is well known that the 3D electrode morphology, which is significantly influenced by the underlying manufacturing process, has a profound impact on the resulting performance, a deeper understanding for the structural changes caused by modifications of the manufacturing process or degradation phenomena is desirable. In the present paper, we investigate the influence of the annealing time and the operating temperature on the 3D morphology of SOFC anodes using 3D image data obtained by focused-ion beam scanning electron microscopy, which is segmented into gadolinium-doped ceria, nickel and pore space. In addition, structural differences caused by manufacturing the anode via infiltration or powder technology, respectively, are analyzed quantitatively by means of various geometrical descriptors such as specific surface area, mean geodesic tortuosity, and constrictivity. The computation of these descriptors from 3D image data is carried out both globally as well as locally to quantify the heterogeneity of the anode structure.

The present work addresses the simulation of pore emptying during the drying of battery electrodes. For this purpose, a model based on the multiphase-field (MPF) method is used, since it is an established approach for modeling and simulating multiphysical problems. A model based on phase fields is introduced that takes into account fluid flow, capillary effects, and wetting behavior, all of which play an important role in drying. In addition, the MPF method makes it possible to track the movement of the liquid-air interface without computationally expensive adaptive mesh generation. The presented model is used to investigate pore emptying in real hard carbon microstructures. For this purpose, the microstructures of real dried electrodes are used as input for the simulations. The simulations performed here demonstrate the importance of considering the resolved microstructural information compared to models that rely only on statistical geometry parameters such as pore size distributions. The influence of various parameters such as different microstructures, fluid viscosity, and the contact angle on pore emptying are investigated. In addition, this work establishes a correlation between the capillary number and the breakthrough time of the solvent as well as the height difference of the solvent front at the time of breakthrough. The results indicate that the drying process can be optimized by doping the particle surface, which changes the contact angle between the fluids and the particles. Published by the American Physical Society 2025

Engineering materials are polycrystalline in nature, consisting of numerous single crystals interconnected through a three-dimensional interfacial network known as grain boundaries. Often essential in defining the performance and durability of materials, grain boundaries attract considerable attention during alloy development. Initially, we employ a multi-phase-field model and validate the phenomenon of grain-boundary grooving under isotropic energy conditions, with bulk diffusion as the dominant mass transport mechanism. Subsequently, we investigate the effects of interfacial surface anisotropy and crystal misorientation on groove formation. This present study focuses on the effects of interfacial surface anisotropy and crystal misorientation and, thus, allows us to draw comparisons between the effects of different physical phenomena on the grain-boundary behavior. It is observed that the groove kinetics accelerate as a result of fourfold anisotropy, with groove root deepening proportional to the imposed anisotropic strength. Furthermore, the phase-field results presented here align well with theoretical predictions. In addition, we briefly study on the effect of solid–solid anisotropy on the groove root position. We anticipate that the simulated liquid groove and its precise measurement will serve as important tools for studying the relative energies of grain boundaries.

Lateral flow assays (LFAs) have caught new attention in recent years due to extensive use in the containment of the COVID-19 pandemic. Especially the protein and fluid interactions with the nitrocellulose membrane structure are yet to be fully investigated, which affect the fluid and protein distribution of the test and control lines differently due to different adsorptive properties of fluids and proteins. Therefore, the relationship between fluid spread and protein distribution, respectively, and structure needs systematic evaluation. Two procedures were developed based on passive adsorption of complementary fluorescent dyes to investigate these phenomena. These procedures enabled three-dimensional visualization of the membrane structure, fluid as well as the protein spreading, respectively. Confocal laser scanning microscopy was applied after depositing picoliter and nanoliter volumes of the printing buffers containing fluorophore-labeled proteins (immunoglobulin G) and Oregon Green™ 488 onto the membrane using a high precision micro dispenser. The resulting data were correlated with the membrane's tortuosity and permeability. Inverse-proportional dependencies for the lateral spread of the fluid and protein adsorption with the structural parameters were observed. Additionally, surfactants [polysorbate 80 (PS80) and sodium dodecylbenzenesulfonate (SDBS), both at 0.1%] were added individually to the buffers, and the spread of the liquids was evaluated. Both surfactants increase the similarities between fluid and protein shape compared to the reference data. While SDBS increases the general lateral spread, PS80 does increase the penetration depth of the protein into the membrane, which could lead to reduced signal in LFAs.

When a fluid contacts solid surfaces, it can spread or slide, and the motion of the contact line involves a complex interplay between hydrodynamic and thermodynamic effects. Hydrodynamic theories, like the Huh & Scriven and Cox & Voinov models, assumeno-slip boundary condition, neglecting the macroscopic fluid slip at the fluid-solid interface. However, they cannot explain the contact line motion during droplet spreading whichthermodynamic theories is attribute to the microscopic surface diffusion of fluid molecules. To bridge these perspectives, we heed the physical origin of slip phenomenon by employing energy minimization principles to establish a force balance between solid-fluid friction, thermodynamic force, and viscous stress at the fluid-solid contact region. Our analysis reveals that slip is an intrinsic property, just like molecular surface diffusion, is also governedfluid-solid intermolecular interactions. Based on our model with slip, we extend the classical Huh & Scriven and Cox & Voinov theories by incorporating friction, enabling a more comprehensive understanding of droplet slide and kinetic induced contact angle hysteresis (CAH). Our results demonstrate that solid-fluid friction plays a vital role in momentum transfer between the substrate and the droplet, therefore, modifies the droplet internal fluid flow during wetting drastically. Under strong friction, classical wetting predictions with CAH are recovered, whereas weak friction suppresses internal fluid motion, leadingthe disappearance of CAH. These findings provide new insights into wetting dynamics, highlighting the importance of friction in slip behavior. This work has broad implications for droplet transport, surface engineering, and applications in microfluidics and coatings.

Optimal microstructure design of battery materials is critical to enhance the performance of batteries for tailored applications such as high power cells. Accurate simulation of the thermodynamics, transport, and electrochemical reaction kinetics in commonly used polycrystalline battery materials remains a challenge. Here, we combine state-of-the-art multiphase field modelling with the smoothed boundary method to accurately simulate complex battery microstructures and multi-phase physics. The phase-field method is employed to parameterize complex open pore cathode microstructures and we present a formulation to impose galvanostatic charging conditions on the diffuse boundary representation. By extending the smoothed boundary method to the multiphase-field method, we build a simulation framework which is capable of simulating the coupled effects of intercalation, anisotropic diffusion, and phase transitions in arbitrary complex polycrystalline agglomerates. This method is directly compatible with voxel-based data, e.g. from X-ray tomog-raphy. The simulation framework is used to study the reversible phase transitions in LiX NiO2 in dense and nanoporous agglomerates. Based on the thermodynamic consistency of phase-field approaches with ab-initio simulations and the open circuit potential, we reconstruct the Gibbs free energies of four individual phases (H1, M, H2 and H3) from experimental cycling data. The results show remarkable agreement with previously published DFT results. From charge simulations, we discover a strong influence of particle morphology on the phase transition behaviour, in particular a shrinking core-like behaviour in dense polycrystalline structures and a particle-by-particle mosaic behavior in nanoporous samples. Overall, the proposed simulation framework enables the detailed study of phase transitions in intercalation materials to enhance microstructure design and fast charging protocols.

Droplets exhibiting a myriad of shapes on surfaces are ubiquitous in both nature and industrial applications. In high-resolution manufacturing processes, e.g., semiconductor chips, precise control over wetting shapes is crucial for production accuracy. Despite the high demand for describing droplet wetting shapes and their transformations across a wide range of applications, a robust model for precisely depicting complex three-dimensional (3D) wetting droplet shapes on heterogeneous surfaces remains elusive. Herein, we fill this gap by developing a universal, high-precision model that accurately describes wetting shapes, including those with polygonal baselines and irregular footprints. Our model reveals the intricate wetting morphologies beyond the classic Young’s law and Cassie-Baxter-Wenzel models. Besides, it aligns quantitatively with physical simulations for various droplet volumes. This work provides a potential method to achieve highly complex morphologies of droplets via low-cost beforehand design of the surfaces, thereby opening up potential applications in 3D printing, printed electronics, and microfluidics.