Unraveling dispersion and buoyancy dynamics around radial A + B → C reaction fronts: microgravity experiments and numerical simulations.

Radial Reaction-Diffusion-Advection (RDA) fronts for A + B → C reactions find wide applications in many natural and technological processes. In liquid solutions, their dynamics can be perturbed by buoyancy-driven convection due to concentration gradients across the front. In this context, we conducted microgravity experiments aboard a sounding rocket, in order to disentangle dispersion and buoyancy effects in such fronts. We studied experimentally the dynamics due to the radial injection of A in B at a constant flow rate, in absence of gravity. We compared the obtained results with numerical simulations using either radial one- (1D) or two-dimensional (2D) models. We showed that gravitational acceleration significantly distorts the RDA dynamics on ground, even if the vertical dimension of the reactor and density gradients are small. We further quantified the importance of such buoyant phenomena. Finally, we showed that 1D numerical models with radial symmetry fail to predict the dynamics of RDA fronts in thicker geometries, while 2D radial models are necessary to accurately describe RDA dynamics where Taylor-Aris dispersion is significant.

Pressure Changes Across a Membrane Formed by Coacervation of Oppositely Charged Polymer-Surfactant Systems.

We investigate the mass transfer and membrane growth processes during capsule formation by the interaction of the biopolymer xanthan gum with CnTAB surfactants. When a drop of xanthan gum polymer solution is added to the surfactant solution, a membrane is formed by coacervation. It encapsulates the polymer drop in the surfactant solution. The underlying mechanisms and dynamic processes during capsule formation are not yet understood in detail. Therefore, we characterized the polymer-surfactant complex formation during coacervation by measuring the surface tension and surface elasticity at the solution-air interface for different surfactant chain lengths and concentrations. The adsorption behavior of the mixed polymer-surfactant system at the solution-air interface supports the understanding of observed trends during the capsule formation. We further measured the change in capsule pressure over time and simultaneously imaged the membrane growth via confocal microscopy. The cross-linking and shrinkage during the membrane formation by coacervation leads to an increasing tensile stress in the elastic membrane, resulting in a rapid pressure rise. Afterward, the pressure gradually decreases and the capsule shrinks as water diffuses out. This is not only due to the initial capsule overpressure but also due to osmosis caused by the higher ionic strength of the surfactant solution outside the capsule compared to the polymer solution inside the capsule. The influence of polymer concentration and surfactant type and concentration on the pressure changes and the membrane structure are studied in this work, providing detailed insights into the dynamic membrane formation process by coacervation. This knowledge can be used to produce capsules with tailored membrane properties and to develop a suitable encapsulation protocol in technological applications. The obtained insights into the mass transfer of water across the capsule membrane are important for future usage in separation techniques and the food industry and allow us to better predict the capsule time stability.

Robust Reconstruction of the Void Fraction from Noisy Magnetic Flux Density Using Invertible Neural Networks.

Electrolysis stands as a pivotal method for environmentally sustainable hydrogen production. However, the formation of gas bubbles during the electrolysis process poses significant challenges by impeding the electrochemical reactions, diminishing cell efficiency, and dramatically increasing energy consumption. Furthermore, the inherent difficulty in detecting these bubbles arises from the non-transparency of the wall of electrolysis cells. Additionally, these gas bubbles induce alterations in the conductivity of the electrolyte, leading to corresponding fluctuations in the magnetic flux density outside of the electrolysis cell, which can be measured by externally placed magnetic sensors. By solving the inverse problem of the Biot-Savart Law, we can estimate the conductivity distribution as well as the void fraction within the cell. In this work, we study different approaches to solve the inverse problem including Invertible Neural Networks (INNs) and Tikhonov regularization. Our experiments demonstrate that INNs are much more robust to solving the inverse problem than Tikhonov regularization when the level of noise in the magnetic flux density measurements is not known or changes over space and time.

Neutron radiography of an anisotropic drainage flow.

Liquid drainage through foam is dominated by gravity, capillary, and viscous forces. The liquid is conducted by an isotropic network of Plateau borders; however, imposed stress changes the alignment of the foam's structural elements. Previous numerical simulations predicted that a vertical drainage flow will be deflected horizontally if the foam is sheared. We investigated this phenomenon by measuring the distribution of the liquid fraction within a foam formed in a flat rectangular cell. The foam was subjected to shear stress under a forced liquid supply from the top of the cell. Neutron radiographies of unchanged and sheared foam were analyzed to extract measurements of the liquid fraction. Deflections in the distribution of the drainage liquid were detected and found to be positively correlated with increasing foam shear.

The impact of an ultrasonic standing wave on the sorption behavior of proteins: Investigation of the role of acoustically induced non-spherical bubble oscillations.

HYPOTHESIS: Protein molecules adsorb on the air/liquid interface due to possessing a hydrophobic side. A full surface coverage is important in many processes such as in protein harvesting by foam fractionation. The adsorption of proteins in low concentration solutions is preceded by a relatively long time lag known as the induction period. This has been attributed to the formation of an adsorbed monolayer, which relies on the reorientation of the protein molecules. The reduction of the induction period can significantly facilitate the sorption process to reach full protein coverage. For this purpose acoustically induced non-spherical bubble oscillations can aid in the formation of the monolayer and enhance the sorption process. EXPERIMENT: In this study, low frequency ultrasound was used to induce non-spherical oscillations on an air bubble attached to a capillary. Profile analysis tensiometry was deployed to examine the effect of these non-spherical oscillations on the sorption dynamics of different proteins. FINDINGS: We observed that during the initial stages of adsorption, when the bubble surface is almost empty, non-spherical oscillations occur, which were found to significantly expedite the adsorption process. However, during later stages of the adsorption process, despite the continued presence of several sonication phenomena such as the primary radiation force and acoustic streaming, no change in adsorption behavior of the proteins could be noted. The occurrence, duration, and intensity of the non-spherical bubble oscillations appeared to be the sole contributing factors for the change of the sorption dynamics of proteins.

Functionalization of Ti64 via Direct Laser Interference Patterning and Its Influence on Wettability and Oxygen Bubble Nucleation.

The nucleation of bubbles on solid surfaces is an important phenomenon in nature and technological processes like electrolysis. During proton-exchange membrane electrolysis, the nucleation and separation of the electrically nonconductive oxygen in the anodic cycle plays a crucial role to minimize the overpotential it causes in the system. This increases the efficiency of the process, making renewable energy sources and the "power-to-gas" strategy more viable. A promising approach is to optimize gas separation by surface functionalization in order to apply a more advantageous interface to industrial materials. In this work, the connection between the wettability and bubble nucleation of oxygen is investigated. For tailoring the wettability of Ti64 substrates, the direct laser interference patterning method is applied. A laser source with a wavelength of 1064 nm and a pulse duration of 12 ps is used to generate periodic pillar-like structures with different depths up to ∼5 µm. The resulting surface properties are characterized by water contact angle measurement, scanning electron microscopy, confocal microscopy, and X-ray photon spectroscopy. It was possible to generate structures with a water contact angle ranging from 20° up to nearly superhydrophobic conditions. The different wettabilities are validated based on X-ray photon spectroscopy and the different elemental composition of the samples. The results indicate that the surface character of the substrate adapts depending on the surrounding media and needs more time to reach a steady state for deeper structures. A custom setup is used to expose the functionalized surfaces to oxygen-oversaturated solutions. It is shown that a higher hydrophobicity of the structured surface yields a stronger interaction with the dissolved gas. This significantly enhances the oxygen nucleation up to nearly 350% by generating approximately 20 times more nucleation spots, but also smaller bubble sizes and a reduced detachment rate.

Hydrogen Bubble Size Distribution on Nanostructured Ni Surfaces: Electrochemically Active Surface Area Versus Wettability.

Emerging manufacturing technologies make it possible to design the morphology of electrocatalysts on the nanoscale in order to improve their efficiency in electrolysis processes. The current work investigates the effects of electrode-attached hydrogen bubbles on the performance of electrodes depending on their surface morphology and wettability. Ni-based electrocatalysts with hydrophilic and hydrophobic nanostructures are manufactured by electrodeposition, and their surface properties are characterized. Despite a considerably larger electrochemically active surface area, electrochemical analysis reveals that the samples with more pronounced hydrophobic properties perform worse at industrially relevant current densities. High-speed imaging shows significantly larger bubble detachment radii with higher hydrophobicity, meaning that the electrode surface area that is blocked by gas is larger than the area gained by nanostructuring. Furthermore, a slight tendency toward bubble size reduction of 7.5% with an increase in the current density is observed in 1 M KOH.

Technological Pathways to Produce Compressed and Highly Pure Hydrogen from Solar Power.

Hydrogen (H2 ) produced from renewables will have a growing impact on the global energy dynamics towards sustainable and carbon-neutral standards. The share of green H2 is still too low to meet the net-zero target, while the demand for high-quality hydrogen continues to rise. These factors amplify the need for economically viable H2 generation technologies. The present article aims at evaluating the existing technologies for high-quality H2 production based on solar energy. Technologies such as water electrolysis, photoelectrochemical and solar thermochemical water splitting, liquid metal reactors and plasma conversion utilize solar power directly or indirectly (as carbon-neutral electrons) and are reviewed from the perspective of their current development level, technical limitations and future potential.

Electrolysis in reduced gravitational environments: current research perspectives and future applications.

Electrochemical energy conversion technologies play a crucial role in space missions, for example, in the Environmental Control and Life Support System (ECLSS) on the International Space Station (ISS). They are also vitally important for future long-term space travel for oxygen, fuel and chemical production, where a re-supply of resources from Earth is not possible. Here, we provide an overview of currently existing electrolytic energy conversion technologies for space applications such as proton exchange membrane (PEM) and alkaline electrolyzer systems. We discuss the governing interfacial processes in these devices influenced by reduced gravitation and provide an outlook on future applications of electrolysis systems in, e.g., in-situ resource utilization (ISRU) technologies. A perspective of computational modelling to predict the impact of the reduced gravitational environment on governing electrochemical processes is also discussed and experimental suggestions to better understand efficiency-impacting processes such as gas bubble formation and detachment in reduced gravitational environments are outlined.

Force balance of hydrogen bubbles growing and oscillating on a microelectrode.

Hydrogen evolution in acidic aqueous electrolytes was recently found to be characterized by a carpet of microbubbles covering the microelectrode and feeding the growth of the main bubbles by coalescence. Besides this, oscillatory behavior of the main bubbles was observed prior to departure. Extending earlier studies, this work delivers the forces acting on the main bubble more accurately by taking into account further geometric and electrochemical details measured during experiments. Combining simulation work and measurements makes it possible to confirm the role of an attractive electrical (Coulomb) force caused by the adsorption of hydrogen ions at the bubble interface and to obtain a better understanding of the bubble dynamics observed.

On the growth regimes of hydrogen bubbles at microelectrodes.

The growth of single hydrogen bubbles at micro-electrodes is studied in an acidic electrolyte over a wide range of concentrations and cathodic potentials. New bubble growth regimes have been identified which differ in terms of whether the bubble evolution proceeds in the presence of a monotonic or oscillatory variation in the electric current and a carpet of microbubbles underneath the bubble. Key features such as the growth law of the bubble radius, the dynamics of the microbubble carpet, the onset time of the oscillations and the oscillation frequencies have been characterized as a function of the concentration and electric potential. Furthermore, the system's response to jumps in the cathodic potential has been studied. Based on the analysis of the forces involved and their scaling with the concentration, potential and electric current, a sound hypothesis is formulated regarding the mechanisms underlying the micro-bubble carpet and oscillations.

Experimental techniques to study protein-surfactant interactions: New insights into competitive adsorptions via drop subphase and interface exchange.

Protein surfactant (PS) interactions is an essential topic for many fundamental and technological applications such as life science, nanobiotechnology processes, food industry, biodiesel production and drug delivery systems. Several experimental techniques and data analysis approaches have been developed to characterize PS interactions in bulk and at interfaces. However, to evaluate the mechanisms and the level of interactions quantitatively, e.g., PS ratio in complexes, their stability in bulk, and reversibility of their interfacial adsorption, new experimental techniques and protocols are still needed, especially with relevance for in-situ biological conditions. The available standard techniques can provide us with the basic understanding of interactions mainly under static conditions and far from physiological criteria. However, detailed measurements at complex interfaces can be formidable due to the sophisticated tools required to carefully probe nanometric phenomena at interfaces without disturbing the adsorbed layer. Tensiometry-based techniques such as drop profile analysis tensiometry (PAT) have been among the most powerful methods for characterizing protein's and surfactant's adsorption layers at interfaces via measuring equilibrium and dynamic interfacial tension and dilational rheology analysis. PAT provides us with insightful data such as kinetics and isotherms of adsorption and related surface activity parameters. However, the data analysis and interpretation can be challenging for mixed protein-surfactant solutions via standard PAT experimental protocols. The combination of a coaxial double capillary (micro flow exchange system) with drop profile analysis tensiometry (CDC-PAT) is a promising tool to provide valuable results under different competitive adsorption/desorption conditions via novel experimental protocols. CDC-PAT provides unique experimental protocols to exchange the droplet subphase in a continuous dynamic mode during the in-situ analysis of the corresponding interfacial adsorbed layer. The contribution of diffusion/convection mechanisms on the kinetics of the adsorption/desorption processes can also be investigated using CDC-PAT. Here, firstly, we review the commonly available techniques for characterizing protein-surfactant interactions in the bulk phase and at interfaces. Secondly, we give an overview for applications of the coaxial double capillary PAT setup for investigations of mixed protein-surfactant adsorbed layers and address recently developed protocols and analysis procedures. Exploring the competitive sequential adsorption of proteins and surfactants and the reversibility of pre-adsorbed layers via the subphase exchange are the particular experiments we can perform using CDC-PAT. Also the sequential and simultaneous competitive adsorption/desorption processes of some ionic and nonionic surfactants (SDS, CTAB, DTAB, and Triton) and proteins (bovine serum albumin (BSA), lysozyme, and lipase) using CDC-PAT are discussed. Last but not least, the fabrication of micro-nanocomposite layers and membranes are additional applications of CDC-PAT discussed in this work.

Interfacial Behavior of Particle-Laden Bubbles under Asymmetric Shear Flow.

The behavior of moving bubbles has mostly been studied in an axisymmetric flow field. To extend the knowledge to practical conditions, we investigate the interfacial and hydrodynamic properties of bubbles under asymmetric shear forces. Experiments are performed with a buoyant bubble at the tip of a capillary placed in a defined shear flow in the presence of surfactants, nanoparticles, and glass beads. The response of the interface to the surrounding asymmetric flow is measured under successive reduction of the surface area. Profile analysis tensiometry is utilized to investigate the dynamic surface tension and the surface rheology of the surfactant- and nanoparticle-laden interfaces. Microscopic particle image and tracking velocimetry are used to study the bulk flow and the interfacial mobility of the buoyant bubble. According to our results, the rotational component of the shear flow provokes an interfacial flow, which redistributes the adsorbed surfactants and particles at the interface. In the presence of NPSCs, a contiguous network of particles forms at the interface through densification of surface structures. We show that this interconnected nanoparticle network eventually stops the interfacial flow and decreases the mobility of the glass beads at the interface. The immobilization of the interface is characterized by a dimensionless number, defined as the ratio of the interfacial elasticity to bulk shear forces. This number provides an estimate of the interfacial forces required to impose interfacial immobility at a defined flow field. Our findings can serve as a basis to formulate boundary conditions for refined modeling and to predict the hydrodynamics of bubbles and droplets.

Enzymatic Hydrolysis of Triglycerides at the Water-Oil Interface Studied via Interfacial Rheology Analysis of Lipase Adsorption Layers.

The enzymatic hydrolysis of sunflower oil occurs at the water-oil interface. Therefore, the characterization of dynamic interfacial phenomena is essential for understanding the related mechanisms for process optimizations. Most of the available studies for this purpose deal with averaged interfacial properties determined via reaction kinetics and dynamic surface tension measurements. In addition to the classical approach for dynamic surface tension measurements, here, the evolution of the dilational viscoelasticity of the lipase adsorbed layer at the water-oil interface is characterized using profile analysis tensiometry. It is observed that lipase exhibits nonlinear dilational rheology depending on the concentration and age of the adsorbed layer. For reactive water-oil interfaces, the response of the interfacial tension to the sinusoidal area perturbations becomes more asymmetric with time. Surface-active products of the enzymatic hydrolysis of triglycerides render the interface less elastic during compression compared to the expansion path. The lipolysis products can facilitate desorption upon compression while inhibiting adsorption upon expansion of the interface. Lissajous plots provide an insight into how the hysteresis effect leads to different interfacial tensions along the expansion and compression routes. Also, the droplet shape increasingly deviates from a Laplacian shape, demonstrating an irreversible film formation during aging and ongoing hydrolysis reaction, which supports our findings via interfacial elasticity analysis.

Interfacial flow of a surfactant-laden interface under asymmetric shear flow.

HYPOTHESIS: The shear stress of the axisymmetric flow field triggers a nonuniform distribution of the surfactants at the surface of a rising bubble, known as stagnant cap. The formation of the stagnant cap gives rise to Marangoni stresses that reduce the mobility of the interface, which in return reduces the rising velocity. However, the conditions in technological processes usually deviate from the linear rise of a single bubble in a quiescent unbounded liquid. Asymmetric shear can act on the bubble surface e.g. due to the vorticity in the surrounding flow, bubble-bubble interactions, or influence of the reactor wall. A different surfactant distribution at the interface is expected under asymmetric shear, which can change the hydrodynamic behavior of the interface drastically. EXPERIMENTS: Here we conduct model experiments with a bubble or a drop at the tip of a capillary placed in a defined flow field. Thereby we investigate the influence of asymmetric shear forces on the interface in the presence of surfactants. Microscopic particle tracking velocimetry is employed to measure the velocity of the surfactant-laden interface for different degrees of asymmetry in the surrounding liquid flow. FINDINGS: We show a direct experimental observation of the circulating flow at the interface under asymmetric shear, which prevents the formation of the typical stagnant cap. Additionally, we reveal that the interface remains mobile regardless of the surfactant concentration. Our results confirm that increasing the degree of asymmetry increases the shear forces and thus the interfacial velocity.

Dynamics of single hydrogen bubbles at Pt microelectrodes in microgravity.

The dynamics of single hydrogen bubbles electrogenerated in acidic electrolytes at a Pt microelectrode under potentiostatic conditions is investigated in microgravity during parabolic flights. Three bubble evolution scenarios have been identified depending on the electric potential applied and the acid concentration. The dominant scenario, characterized by lateral detachment of the grown bubble, is studied in detail. For that purpose, the evolution of the bubble radius, electric current and bubble trajectories, as well as the bubble lifetime are comprehensively addressed for different potentials and electrolyte concentrations. We focus particularly on analyzing bubble-bubble coalescence events which are responsible for reversals of the direction of bubble motion. Finally, as parabolic flights also permit hypergravity conditions, a detailed comparison of the characteristic bubble phenomena at various levels of gravity is drawn.

Magnetically Induced Aggregation of Iron Oxide Nanoparticles for Carrier Flotation Strategies.

On the nanoscale, iron oxides can be used for multiple applications ranging from medical treatment to biotechnology. We aimed to utilize the specific properties of these nanoparticles for new process concepts in flotation. Magnetic nanoparticles were synthesized by alkaline coprecipitation, leading to a primary particle size of 9 nm, and coated with oleate. The nanomaterial was characterized for its superparamagnetism and its colloidal stability at different ionic strengths, with and without an external magnetic field. The nanomaterial was used for model experiments on magnetic carrier flotation of microplastic particles, based on magnetically induced heteroagglomeration. We were able to demonstrate the magnetically induced aggregation of the nanoparticles which allows for new flotation strategies. Since the nanomaterial has zero remanent magnetization, the agglomeration is reversible which facilitates the process control. Magnetic carrier flotation based on iron oxide nanoparticles can pave the way to promising new recycling processes for microplastic wastes.

Dynamics of Competitive Adsorption of Lipase and Ionic Surfactants at the Water-Air Interface.

Lipase is one of the most important enzymes playing a key role in many biological and chemical processes, in particular for fat hydrolysis in living systems and technological applications such as food production, medicine, and biodiesel production. As lipase is soluble in water, the major hydrolysis process occurs at the water-oil interface, where lipase can get in contact with the oil. To provide optimum conditions, the emulsification of the oil is essential to provide a large interfacial area which is generally done by adding surfactants. However, the presence of surfactants can influence the lipase activity and also cause competitive adsorption, resulting in a removal of lipase from the interface or its conformational changes in the solution bulk. Here we have studied the dynamics of competitive adsorption and interfacial elasticity of mixed solutions containing lipase and the anionic surfactant sodium dodecyl sulfate (SDS) or the cationic surfactant cetyltrimethylammonium bromide (CTAB), respectively, at the water-air interface. The experiments were performed with a special coaxial double capillary setup for drop bulk-interface exchange developed for the drop profile analysis tensiometer PAT with two protocols: sequential and simultaneous adsorption of single components and mixed systems. The results in terms of dynamic surface tension and dilational viscoelasticity illustrate fast and complete desorption of a preadsorbed CTAB and SDS layers via subphase exchange with a buffer solution. In contrast, the preadsorbed lipase layer cannot be removed either by SDS or CTAB from the interface during drop bulk exchange with a buffer solution due to the unfolding process and conformation evolution of the protein molecules at the interface. In the opposite case, lipase can remove preadsorbed SDS and CTAB. The dynamic surface tension and viscoelasticity data measured before and after subphase exchange show joint adsorption of lipase and CTAB in the form of complexes, while SDS is adsorbed in competition with lipase. The results are in good correlation with the determined surface charges of the lipase gained by computational simulations which show a dominant negatively charged surface for lipase that can interact with the cationic CTAB while partial positively charged regions are observed for the interaction with the anionic SDS.

Stability criterion for the magnetic separation of rare-earth ions.

The stability criterion for the magnetic separation of rare-earth ions is studied, taking dysprosium Dy(iii) ions as an example. Emphasis is placed on quantifying the factors that limit the desired high enrichment. During magnetic separation, a layer enriched in Dy(iii) ions is generated via the surface evaporation of an aqueous solution which is levitated by the Kelvin force. Later, mass transport triggers instability in the enriched layer. The onset time and position of the instability is studied using an interferometer. The onset time signals that an advective process which significantly accelerates the stratification of enrichment is taking place, although the initial phase is quasi-diffusion-like. The onset position of the flow agrees well with that predicted with a generalized Rayleigh number (Ra^{*}=0) criterion which includes the Kelvin force term acting antiparallel to gravity. Further three-dimensional analysis of the potential energy, combining magnetic and gravitational terms, shows an energy barrier that has to be overcome to initiate instability. The position of the energy barrier coincides well with the onset position of the instability.

X-ray particle tracking velocimetry in liquid foam flow.

In this work, we introduce a novel approach to measure the flow velocity of liquid foam by tracking custom-tailored 3D-printed tracers in X-ray radiography. In contrast to optical observations of foam flow in flat cells, the measurement depth equals 100 mm in the X-ray beam direction. Light-weight tracers of millimetric size and tetrapod-inspired shape are additively manufactured from stainless steel powder by selective laser melting. Matching with the foam structure and bubble size, these tracers follow the foam flow. An X-ray beam passes through the radiotransparent foam channel and is detected by an X-ray image intensifier. The X-ray transmission images show the two-dimensional projections of the radiopaque tracers. Utilizing particle tracking velocimetry algorithms, the tracer trajectories are measured with both high spatial (0.2 mm) and temporal (25 fps) resolution. Fine and coarse liquid foam flow of different velocities are studied in a partly curved channel with rectangular cross section. The simultaneous time-resolved measurements of the tracers' translational motion and their intrinsic rotation reveal both the local velocity and vorticity of the foam flow. In the semi-circular curved channel section, the rigid-body-like flow pattern is investigated. Moreover, a relaxation of the foam structure in the transition zone between straight and curved section is observed.