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.

This paper presents a three-dimensional framework for simulating dynamic wetting phenomena using the volume of fluid 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. The approach is validated against different numerical methods for computing the contact line velocity and various experimental results, including droplet spreading, splashing and sliding, where it demonstrates quantitative agreement with the three-dimensional wetting behavior observed. Additionally, a comparative analysis between dynamic and static contact angle models is performed.

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.

The evolution of gas bubbles on solid surfaces is known to affect the efficiency of various heat transfer and electrolysis processes. In this work, the over-saturation-driven growth of air bubbles nucleating at micro-cavities is investigated by direct numerical simulations, and the results are directly compared with experimental results obtained at silicon wafers of different wettability and size of the micro-cavities. A validated geometric volume-of-fluid method with adaptive mesh refinement is used to accurately capture the mass transfer at the interface. Applying proper boundary conditions then allows to accurately reproduce the two-stage wetting dynamics observed in experiments, where the contact line of the growing bubble initially remains pinned at the cavity edge before it later starts to move with constant contact angle. The bubble growth behavior obtained numerically is further compared and discussed with results from analytical approaches of an extended Scriven and the Popov solution, which give accurate results only at idealized initial conditions and low values of the oversaturation, where convective effects can be neglected. The simulation results obtained at surfaces of different wettability and cavity size allow to gain insight how the bubble behavior can be controlled by functionalization and structuring of the surface. The numerical approach presented in this work may easily be applied to more complex situations where theoretical solutions do not hold or experiments are not feasible.

Hypothesis The surface wettability influences the oversaturation-driven growth of gas bubbles on the surface via the contact angle. Larger contact angles on hydrophobic surfaces compared to hydrophilic ones lead to faster growth of bubbles nucleating at microcavities of identical size. Experiments Cylindric micro-cavities were etched in silicon wafers as nucleation sites. Afterward, the surfaces were functionalized to obtain different wettability characterized by contact angle. The growth of air bubbles was recorded using microscopic shadowgraphy. Findings The bubble growth at micro-cavities can be separated into a pinning stage and an expansion stage. On more hydrophobic surfaces, the duration of the pinning stage is shorter, and the bubble grows with a larger contact angle during the expansion stage, both contributing to faster bubble growth. A shape-dependent factor derived from simulations vividly describes the average mass flux into the bubble caps of given contact angles, providing a clear explanation of the impact of wettability on bubble growth.

Dynamic wetting is known to influence the dynamics of droplets and bubbles on surfaces. In this study, we present a numerical approach to simulate the oversaturation-driven growth of single bubbles on surfaces under the influence of dynamic wetting. An existing mass transfer model implemented in the Volume of Fluid (VOF) framework Basilisk is combined with a dynamic wetting model that includes contact angle hysteresis. Motivated by the initial dominance of surface tension during bubble growth, we explore the extent to which enlarged diffusion coefficients can be used to speed up the simulations. Hereby, for calculating the instantaneous contact angle, a properly rescaled value of the contact line velocity needs to be used. An extensive validation of the simulation method is conducted, including comparisons with experimental results. The impact of dynamic wetting on bubble growth is elucidated by comparisons with the bubble behavior at static wetting.

Nickel electrodes are widely used in alkaline water electrolysis, yet the remaining electrode overpotentials are leading to significant losses in electrochemical performance. These are partly due to the electrogenerated bubbles growing at the surface. Tuning the nickel surface for better bubble management is therefore of great relevance. Here, Direct Laser Writing is used to generate pillar-like surfaces with dual wetting behavior. This combines hydrophilic grooves with hydrophobic ripples on top of each pillar. Furthermore, the grooves show superspreading properties due to the capillary forces within them, which enables a fully wetted surface. The hydrophobic pillars serve as initial nucleation sites where the bubbles remain pinned during their growth. This results in a larger detachment size of the bubbles. In combination with an increase of the electrochemically active surface area by a factor of 9, a reduction of the overpotential for hydrogen evolution reaction of approximate to 24 % at-100 mAcm-2 is found.

Electrochemical water splitting leads to variations in the electrolyte concentration which might impact the surface tension at both hydrogen and oxygen bubbles. Although the potentially resulting solutal Marangoni convection has been predicted, it has not yet been observed in experiments. To discover solutal Marangoni convection, the potential applied during the oxygen evolution reaction at microelectrodes, as well as the electrolyte concentration, have been systematically varied. To capture the bubble dynamics and local variations of the refractive index, a combination of high-speed shadowgraphy, particle tracking velocimetry, and schlieren imaging was employed. The analysis has identified an exotic regime of oxygen bubbles at high potentials, characterized by slow growth that is interrupted by intermittent current spikes, causing fast growth. While the fast growth is expectedly dominated by thermocapillary convection, the slow growth displays a vortical flow that circulates in the reverse direction at a much slower velocity. This flow pattern is attributed to an increased sulfuric acid concentration at the bubble foot, induced by proton generation during oxygen evolution. This locally elevates the surface tension and causes the solutal Marangoni flow. These findings confirm the theoretical prediction of solutal effects in the bubble dynamics during water electrolysis and reveal the fascinating physics behind.

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 micrometre size resembling catalytic islands, direct numerical simulations show that bubbles may approach dynamic equilibrium states at which they neither grow nor shrink. These are found in under- and saturated bulk electrolytes during both pinning and expanding wetting regimes of the bubbles. The equilibrium is based on the balance of local influx near the bubble foot and global outflux. To identify the parameter regions of bubble growth, dissolution and dynamic equilibrium by analytical means, we extend the solution of Zhang & Lohse (2023) J. Fluid Mech.975, R3, by taking into account modified gas fluxes across the bubble interface, that result from a non-uniform distribution of dissolved gas. The Damköhler numbers at equilibrium are found to range from small to intermediate values. Unlike pinned nano-bubbles studied earlier, for micrometre-sized bubbles the Laplace pressure plays only a minor role. With respect to the stability of the dynamic equilibrium states, we extend the methodology of Lohse & Zhang (2015a) Phys. Rev. E91 (3), 031003(R), by additionally taking into account the electrode reaction. Under contact line pinning, the equilibrium states are found to be stable for flat nano-bubbles and for micro-bubbles in general. For unpinned bubbles, the equilibrium states are always stable. Finally, we draw conclusions on how to possibly enhance the efficiency of electrolysis.

This study presents both numerical and experimental analyses of enhanced mixing in a microflow system under the influence of a magnetic field. The research employed COMSOL Multiphysics for numerical simulations and Particle Image Velocimetry (PIV) for experimental validation. In the experimental microfluidic setup, permanent neodymium magnets were used to influence a laminar flow of water partially enriched with Ho(III) ions using the magnetic field. The findings confirmed that the strong interaction between Ho(III) ions and the magnetic field significantly affected the flow and may have resulted in vortex shedding downstream of the region with the highest magnetic field intensity. The numerical simulations demonstrated good agreement with the PIV experimental results. These findings suggest that it is possible to significantly enhance mixing in microflow systems without mechanical components, solely by exploiting the differences in the magnetic properties between the mixing substances. Traditionally, microreactors have been limited by mixing speeds governed by diffusion. These new results indicate the practical possibility of increasing mixing intensity in a cost-effective and safe manner.

While the dynamics of hydrogen bubbles during water electrolysis have been intensively studied in recent years, adequate insights into the dynamics of oxygen bubbles are still lacking. Therefore, this study presents a comparative analysis of hydrogen and oxygen bubble dynamics during potentiostatic water electrolysis in an acidic electrolyte. Complementary optical techniques, such as high-speed shadowgraphy, particle tracking velocimetry, and schlieren imaging are applied to measure geometric features of the evolving bubbles and the microscale Marangoni convection, as well as the refractive index field around the growing bubbles. Distinct differences between oxygen and hydrogen bubbles are found in the average current, in the Marangoni convection pattern, and in the degree of refractive index reduction at the bubble foot, suggesting a synergetic action of both thermal and solutal effects at oxygen bubbles.

Micro- and nano-sized conical structures hold great potential in tailoring the surface wettability and in enhancing electrocatalytic applications. However, their manufacturing process remains a formidable challenge. This work investigates whether and how magnetic fields could support the manufacture of such surfaces by electrodeposition. Through a combination of experiments and numerical simulations, we systematically investigate the magnetic field structuring effect at single dia- and ferromagnetic cones of mm size and at neighboring cones at varying distances. The results provide a first experimental proof of the magnetic-field-driven flow caused by the magnetization of the cones themselves and on the impact of neighboring cones, which helps to understand the resulting mass transfer at different scales. When a uniform external magnetic field is oriented perpendicular to the substrate, the magnetic gradient force enabled by the magnetization of ferromagnetic cones can generate a beneficial electrolyte flow and support the conical growth, thereby often dominating over the Lorentz and buoyancy forces arising from the electrode reactions. With a magnetic field of 200 mT and a current density of 16 mA/cm2, the deposit thickness at the cone tip can be increased by 73% for ferromagnetic and 9% for diamagnetic cones after 10 s of deposition. At smaller cone distances, the magnetic-field-driven flow becomes weaker. However, the magnetic gradient force could sustain a supportive flow to promote conical growth. Our study confirms that the magnetic gradient force is a promising means during electrodeposition for generating flow-enhanced mass transfer to support conical growth at arrays of small surface elevations.

Laser-structuring techniques like Direct Laser Interference Patterning show great potential for optimizing electrodes for water electrolysis. Therefore, a systematic experimental study 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. Using a statistical design of experiments approach, it is found that the spatial distance between the laser-structures is the decisive processing parameter for the improvement of the electrode performance. Thus, the electrochemically active surface area could be increased by a factor of 12 compared to a nonstructured electrode. For oxygen evolution reaction, a significantly lower onset potential and overpotential (≈ -164 mV at 100 mA cm-2) is found. This is explained by the superhydrophilic surface of the laser-structures and the influence of the structured surface on the bubble growth, which leads to a lower number of active nucleation sites and, simultaneously, larger detached bubbles. Combined with the fully wetted electrode surface, this results in reduced electrode blocking and thus, lower ohmic resistance.

Water electrolysis is a pivotal technology for enabling the green hydrogen economy, though its efficiency remains substantially limited by the dynamics of bubble evolution. This review comprehensively discusses recent advances and the critical role of computational simulations in elucidating the mechanisms of multiscale bubble evolutions during electrolysis. We begin by outlining the governing mechanisms and fundamental theories underlying the evolution of both individual and multiple bubbles. The core discussion is then divided into two domains: molecular-scale simulations, which reveal how nanoscale behaviors—such as nucleation, growth, stability, and detachment—are influenced by electrode properties and operational conditions; and continuum-scale simulations, which investigate bubble growth, stability, coalescence, and migration within porous transport layers and real electrolyzer configurations, highlighting key interactions between multiphase flow and mass transfer. Finally, we discuss persistent challenges in predictive multiscale modeling of bubble evolutions in water electrolysis under practical operating environments and propose promising directions for future research.

Electrochemical deposition in magnetic field gradients is a promising method to synthesize structured deposits by exploiting the magnetic field gradient force. Typically, magnetic field gradient templates with dimensions in the millimeter range are used, but a downscaling is desirable from fundamental and application points of view. In the present study, pulse reverse plating of copper is performed in combination with four successively downscaled magnetic field gradient templates, which consist of three iron wires with diameters of 1 mm, 500 mu m, 250 mu m, and 125 mu m. Structuring is demonstrated for all four templates, upon downscaling the Fe diameter within the templates. The deposited copper structures closely resemble the calculated profile of the magnetic field gradient term BOB. The application of more negative deposition potentials leads to improved structuring. Based on the analysis of the current transients, this effect is explained by the action of the curl of the magnetic field gradient force, which profits from the faster development of a steep Cu2+ concentration gradient due to the earlier transfer from a charge-transfer-controlled to a diffusion-limited deposition mode. Significant differences in morphology, ranging from needle-like growth to smooth deposits, are obtained, which vary with the magnetic field gradient profile and the deposition voltage. The results demonstrate that the combination of tailored magnetic field gradient templates and optimized electrochemical parameters offers an advanced route to control the shape and morphology of structured Cu deposits at the micrometer scale, with potential applications in microelectronics and catalysis.

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. Herein, we elucidate a distinct transport mechanism whereby electrolyte droplets are sprayed into H2 bubbles. These droplets arise from the fragmentation of the Worthington jet, which is engendered by the coalescence with microbubbles. The robustness of this phenomenon is corroborated under both normal and microgravity conditions. Reminiscent of bursting bubbles on a liquid-gas interface, electrolyte spraying results in a flow inside the bubble. This flow couples, in an intriguing way, with the thermocapillary convection at the bubble's surface, clearly underlining the high interfacial mobility. In the case of electrode-attached bubbles, the sprayed droplets form electrolyte puddles 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 physico-chemical aspects of electrolytic gas bubbles, integral for optimizing gas-evolving electrochemical systems.

The formation of single bubbles at nanoelectrodes during electrochemical reactions allows to accurately identify the critical nucleus for bubble formation. As demonstrated before, combining nanoelectrode experiments and an analysis approach based on classical nucleation theory (CNT) delivers useful insight into bubble nucleation. In this work we propose an alternative approach to analyze the critical nuclei by applying the nucleation theorem (NT), which is able to overcome the inherent shortcomings of CNT. The size of the critical nucleus can be calculated more accurately by fitting experimental data in a simple form of the NT. Simulating the local gas concentration using a finite element approach, and considering the effect of gas oversaturation on the interfacial tension and the real gas compressibility, we obtain a more realistic estimation of the critical nuclei morphology. With the NT-based analysis presented, we re-analyze the nucleation data reported before. The properties of the critical nuclei obtained here are roughly consistent with those obtained from the CNT-based approach. In addition, we confirm that the critical nucleus for bubble formation in high gas oversaturation is featured with a contact angle much larger than Young’s contact angle.

AbstractMagnetic fields may be utilized for controlling flow and mass transfer in fluids. This review focuses on lab-scale applications in weakly conducting aqueous liquids and summarizes work I was involved in over the past 20 years. Control can easily be achieved by applying a current and taking advantage of the Lorentz force. If magnetic ions or molecules are involved, also the Kelvin force can be utilized. The examples given range from flow control at hydrofoils in marine applications to electrochemical applications with gas evolution and of metal deposition.

Electric fields offer an easy means to manipulate liquid metal droplets. Now, directed droplet transfer between immersed electrodes is achieved in an alkaline electrolyte without electrical short circuit.