arXiv subjects

Kerstin Eckert

Publications and source records attributed to Kerstin Eckert.

15 recordsLinked to original sources

arXiv2026

Bubble growth on arrays of micro-electrodes

Gas bubbles evolving on electrodes during water-electrolysis are blocking active reaction area, thus hindering mass transfer and raising Ohmic resistance. Unlike earlier models that prescribe a uniform current density on the wetted part of the electrode, we resolve the primary electric field, which allows the current density and the interfacial gas production to respond to the geometry of the electrode and the temporal evolution of the bubbles. Using three-dimensional geometrical volume-of-fluid (VOF) simulations with phase change in Basilisk, we examine the growth of single-bubbles on electrodes of different size and of multiple bubbles growing on arrays of catalytic electrode islands. The non-uniform current density and the associated Ohmic resistance significantly affect the growth dynamics. Unlike the case of a single bubble, the outer bubbles in case of electrode islands tend to drift outward during growth, thus delaying full electrode coverage and sustaining current. Footprint tracking and a theoretical analysis show that this drift is governed by the liquid advection driven by the growth of neighboring bubbles, scaling with their separation 1/d2, and modulated by the current-density asymmetry. These results show how electrode patterning and bubble spacing can be exploited to tailor the electric field distribution and reduce bubble-induced resistive losses during water electrolysis.

physics.flu-dyn↗

arXiv2026

Wetting effects on the dynamics of droplets and bubbles at surfaces

Dynamic wetting plays a fundamental role in the dynamics of droplets and bubbles at solid surfaces by influencing contact line motion and interfacial evolution. In this work, three representative wetting-controlled benchmarks, namely droplet splashing, bubble coalescence at solid surfaces, and bubble dynamics under shear flow, are investigated using a three-dimensional volume-of-fluid framework coupled with a recently developed dynamic wetting model based on contact line velocity reconstruction method [19]. The model is first validated against experimental observations and literature data for droplet splashing and bubble coalescence. It accurately reproduces the transient contact line evolution, splashing morphology, and coalescence dynamics. In particular, dynamic wetting suppresses the premature bubble detachment predicted by static wetting models and yields substantially improved agreement with experimental observations. In shear flow, contact angle hysteresis and contact line dissipation strongly influence bubble deformation, sliding, and detachment. These results demonstrate that accurate treatment of dynamic wetting is essential for predicting wetting-controlled droplets and bubbles involving rapid contact line motion, strong interfacial deformation, and topology changes.

physics.flu-dyn↗

arXiv2026

Bubble-induced versus thermodynamic voltage losses during pressurized alkaline water electrolysis

Understanding how bubbles influence the efficiency of water electrolysis is crucial to achieve economically competitive hydrogen, generated by renewable energy sources, such as wind and solar power. Water electrolysis is typically performed at high pressures to reduce the cost of energy-intensive mechanical compression of the produced H2. Thus, a better understanding of how the absolute pressure affects electrochemical performance and bubble size is necessary. In general, bubble sizes decrease as the pressure increases. Using different-sized pillar-patterned Ni electrodes generated by Direct Laser Writing, the detached bubble sizes can be modified even at elevated pressures. As the pillar size increases, the bubbles become larger at all pressures investigated from 1 to 6 bar. At a current density of -25 mA/cm2, the cathodic potential increases with pressure according to the thermodynamic voltage losses given by the Nernst equation (~ 23 mV at p = 6 bar). Surprisingly, increasing the current density to 100 mA/cm2 leads to a reduction of the overpotential by up to ~ 60 mV. Reduced bubble sizes at increased pressures minimize the losses caused by the bubbles, thereby compensating for the thermodynamic voltage penalty. Applying the Buckingham Π-theorem enables the derivation of dimensionless numbers to characterize the ratio of bubble-induced and thermodynamic voltage losses

cond-mat.mtrl-sci↗

arXiv2025

Monitoring of Fluid Transport in Low Temperature Water Electrolyzers and Fuel Cells: Emerging Technologies and Future Prospects

Low temperature water electrolyzers (LTWEs) and low temperature hydrogen fuel cells (LTFCs) present a promising technological strategy for the productions and usages of green hydrogen energy towards a net-zero world. However, the interactions of gas/liquid (fluid) transport and the intrinsic reaction kinetics in LTWEs/LTFCs present one of the key hurdles hindering high production rate and high energy conversion efficiency. Addressing these limitations requires analytical tools that are capable of resolving fluid transport across the heterogeneous, multiscale structures of operating LTWE and LTFC systems. This review provides a comprehensive overview of recent advancements in measurement technologies for investigating fluid transport. We first outline the technical requirements of such analytical systems, and assess the capabilities and limitations of established optical, X-rays and neutrons based imaging systems. We emphasis on emerging strategies that utilize integrated miniaturized sensors, ultrasound, and other alternative physical principles to achieve operando, high-resolution, and scalable measurements towards applications at device and system levels. Finally, we outline future directions in this highly interdisciplinary field, emphasizing the importance of next-generation sensing concepts to overcome the fluid transport hurdle, towards accelerating the deployment of green hydrogen technologies.

physics.app-ph↗

arXiv2025

A geometric interpolation scheme for applying dynamic wetting to three-dimensional volume of fluid simulations

This paper presents a three-dimensional framework for simulating dynamic wetting phenomena using the volume of fluid (VOF) method, implemented in Basilisk. A geometric interpolation scheme is developed to obtain an accurate and reliable value of the contact line velocity. To capture realistic wetting dynamics, a dynamic contact angle model is integrated that considers also contact angle hysteresis (CAH). The approach is validated against various experimental results, including droplet spreading, splashing, and sliding and demonstrates quantitative agreement with the three-dimensional wetting behavior observed. Additionally, a comparative analysis between dynamic and static contact angle models is performed.

physics.flu-dyn↗

arXiv2025

Scanning Acoustic Microscopy for Quantifying Bubble Evolution in Alkaline Water Electrolyzers

Improved understanding of gas/liquid transport in electrochemical gas-evolving systems is increasingly demanded for optimizing device performance. However, high-resolution measurement techniques for in-situ imaging remain limited. This work demonstrates the use of volumetric scanning acoustic microscopy (SAM) for quantifying hydrogen bubble evolution in porous nickel electrodes in a customized alkaline water electrolysis cell. By using high-frequency focused ultrasound, SAM enables volumetric imaging with high spatial resolution in the range of tens of micrometers. This allows the distribution of gas bubbles within the complex 3D architecture of porous electrodes to be resolved. Digital image processing methods are used to segment and quantify the gas content in the electrode. Thus, non-destructive SAM imaging is demonstrated to be an accessible and scalable analytical tool for the quantitative investigation of bubble evolution in operando electrochemical environments. Here, a solid foundation is established for future studies aimed at optimizing bubble dynamics and cell design under practically relevant operating conditions, ultimately contributing to higher electrolysis efficiencies.

physics.app-ph↗

arXiv2024

Dynamic equilibrium of electrochemical bubbles growing on micro-electrodes

In gas evolving electrolysis, bubbles grow at electrodes due to a diffusive influx from oversaturation generated locally in the electrolyte by the electrode reaction. When considering electrodes of micrometer-size resembling catalytic islands, bubbles are found to approach dynamic equilibrium states at which they neither grow nor shrink. Such equilibrium states are found at low oversaturation for both, pinning and expanding wetting regimes of the bubbles and are based on the balance of local influx near the bubble foot and global outflux. Unlike the stability of pinned nano-bubbles studied earlier, the Laplace pressure plays a minor role only. We extend the analytical solution of Zhang & Lohse (2023) by taking into account the non-uniform distribution of dissolved gas around the bubble obtained from direct numerical simulation. This allows us to identify the parameter regions of bubble growth, dissolution and dynamic equilibrium as well as to analyze the stability of the equilibrium states. Finally, we draw conclusions on how to possibly enhance the efficiency of electrolysis.

physics.flu-dyn↗

arXiv2024

Boosting electrode performance and bubble management via Direct Laser Interference Patterning

Laser-structuring techniques like Direct Laser Interference Patterning show great potential for optimizing electrodes for water electrolysis. Therefore, a systematic experimental study based on statistical design of experiments is performed to analyze the influence of the spatial period and the aspect ratio between spatial period and structure depth on the electrode performance for pure Ni electrodes. The electrochemically active surface area could be increased by a factor of 12 compared to a non-structured electrode. For oxygen evolution reaction, a significantly lower onset potential and overpotential ($\approx$-164 mV at 100 mA/cm$^2$) is found. This is explained by a lower number of active nucleation sites and, simultaneously, larger detached bubbles, resulting in reduced electrode blocking and thus, lower ohmic resistance. It is found that the spatial distance between the laser-structures is the decisive processing parameter for the improvement of the electrode performance.

cond-mat.mtrl-sci↗

arXiv2024

WhY shape matters: Hydrodynamics of a Y-shaped membraneless electrolyzer

A novel Y-shaped membraneless flow-through electrolyzer is introduced to achieve a homogeneous electrochemical reaction across the entire electrode in a cost-efficient cell design with effective product separation. Numerical simulations of the electrolyte flow and electrical current within the already known I- and T-shaped cells motivate the newly proposed Y-shape cell. Furthermore, a new design criterion is developed based on the balance between bubble removal and gas generation. As proof-of-concept experimental results using the Y-shaped electrolyzer are presented, showing homogeneous gas distributions across the electrode and efficient product separation by the electrolyte flow.

physics.flu-dyn↗

arXiv2024

Electrolyte spraying within H$_2$ bubbles during water electrolysis

Electrolytically generated gas bubbles can significantly hamper the overall electrolysis efficiency. Therefore it is crucial to understand their dynamics in order to optimise water electrolyzer systems. Here we demonstrate a distinct transport mechanism where coalescence with microbubbles drives electrolyte droplets, resulting from the fragmentation of the Worthington jet, into the gas phase during hydrogen evolution reaction, both in normal and microgravity environments. This indicates that the H$_2$ bubble is not only composed of hydrogen gas and vapor but also includes electrolyte fractions. Reminiscent of bursting bubbles on a liquid-gas interface, this behavior results in a flow inside the bubble, which is further affected by Marangoni convection at the gas-electrolyte interface, highlighting interface mobility. In the case of electrode-attached bubbles, the sprayed droplets form electrolyte puddles at the bubble-electrode contact area, affecting the dynamics near the three-phase contact line and favoring bubble detachment from the electrode. The results of this work unravel important insights into the physicochemical aspects of electrolytic gas bubbles, integral for optimizing gas-evolving electrochemical systems. Besides, our findings are essential for studying the limits of jet formation and rupture relevant to acid mist formation in electrowinning, generation of sea spray aerosols, impact of droplets on liquid surfaces, etc.

physics.flu-dyn↗

arXiv2024

Wettability-dependent dissolution dynamics of oxygen bubbles on Ti64 substrates

In this study, the dissolution of a single oxygen bubble on a solid surface, here Titianium alloy Ti64, in ultrapure water with different oxygen undersaturation levels is investigated. For that purpose, a combination of shadowgraph technique and planar laser-induced fluorescence is used to measure simultaneously the changes in bubble geometry and in the dissolved oxygen concentration around the bubble. Two different wettabilities of the Ti64 surface are adjusted by using plasma-enhanced chemical vapour deposition. The dissolution process on the solid surface involves two distinct phases, namely bouncing of the oxygen bubble at the Ti64 surface and the subsequent dissolution of the bubble, primarily by diffusion. By investigating the features of oxygen bubbles bouncing, it was found that the boundary layer of dissolved oxygen surrounding the bubble surface is redistributed by the vortices emerging during bouncing. This establishes the initial conditions for the subsequent second dissolution phase of the oxygen bubbles on the Ti64 surfaces. In this phase, the mass transfer of O2 proceeds non-homogenously across the bubble surface, leading to an oxygen accumulation close to the Ti64 surface. We further show that the main factor influencing the differences in the dynamics of O2 bubble dissolution is the variation in the surface area of the bubbles available for mass transfer, which is determined by the substrate wettability. As a result, dissolution proceeds faster at the hydrophilic Ti64 surface due to the smaller contact angle, which provokes a larger surface area.

physics.flu-dyn↗

arXiv2024

Learning to reconstruct the bubble distribution with conductivity maps using Invertible Neural Networks and Error Diffusion

Electrolysis is crucial for eco-friendly hydrogen production, but gas bubbles generated during the process hinder reactions, reduce cell efficiency, and increase energy consumption. Additionally, these gas bubbles cause changes in the conductivity inside the cell, resulting in corresponding variations in the induced magnetic field around the cell. Therefore, measuring these gas bubble-induced magnetic field fluctuations using external magnetic sensors and solving the inverse problem of Biot-Savart Law allows for estimating the conductivity in the cell and, thus, bubble size and location. However, determining high-resolution conductivity maps from only a few induced magnetic field measurements is an ill-posed inverse problem. To overcome this, we exploit Invertible Neural Networks (INNs) to reconstruct the conductivity field. Our qualitative results and quantitative evaluation using random error diffusion show that INN achieves far superior performance compared to Tikhonov regularization.

eess.IV↗

arXiv2022

Bubble size distribution and electrode coverage at porous nickel electrodes in a novel 3-electrode flow-through cell

A novel 3-electrode cell type is introduced to run parametrical studies of H$_2$ evolution in an alkaline electrolyte on porous electrodes. Electrochemical methods combined with a high-speed optical measurement system are applied simultaneously to characterize the electrodes and the bubble dynamics in terms of bubble size distribution and coverage of the working electrode. Three different cathodes made of expanded nickel are investigated at applied current densities of |j| = 10 to 200 mA cm$^{-2}$ without forced flow and at a flow rate of 5 ml min$^{-1}$. The applied current density is found to significantly influence both the size of detached bubbles and the surface coverage of the working electrode. The forced flow through the cathodes is found to strongly reduce the bubble size up to current densities of about 100 mA cm$^{-2}$, whereas the initial transient until the cathode surface is completely covered by bubbles is only marginally affected by the flow-through.

physics.flu-dyn↗

arXiv2022

H$_2$ bubble motion reversals during water electrolysis

The dynamics of hydrogen bubbles produced by water electrolysis in an acidic electrolyte are studied using electrochemical and optical methods. A defined cyclic modulation of the electric potential is applied at a microelectrode to produce pairs of interacting H$_2$ bubbles in a controlled manner. Three scenarios of interactions are identified and systematically studied. The most prominent one consists in a sudden reversal in the motion of the first detached bubble, its return to the electrode and finally its coalescence with the second bubble. Attested by Toepler's schlieren technique, an explanation of contactless motion reversal is provided by the competition between buoyancy and thermocapillary effects.

physics.flu-dyn↗

arXiv2021

Protein Enrichment by Flotation: Experiment and Modeling

Protein flotation is a process in which protein molecules are enriched by adsorption at rising bubbles. The bubbles then form a foam above the solution, where the liquid drains down and the dried foam, which is concentrated in protein, is extracted. Here, the recovery rate and purity of the extract depend on foam stability, surface coverage, bubble size, gas flow rate, etc. In this work, we performed flotation experiments using bovine serum albumin (BSA). In addition, an unsteady state simulation of the protein flotation process was carried out by numerically solving the liquid drainage equation in the foam. Thereby, the extracted liquid volume and protein concentration at the outlet were calculated with time. Required quantities such as foam stability, interface coverage or bubble size distribution were measured in corresponding experiments and were fed into the model. The experiments showed that the foam coalescence accelerates the liquid drainage leading to dryer extract and higher protein enrichment. The modeling also reproduced the liquid recovery and extract concentration of the flotation tests within a reasonable error range. The modeling solely relies on experimental inputs and does not require any tuning parameters. It can be further used for optimization or up-scaling of protein flotation.

cond-mat.soft↗