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.

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.

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.

Oscillating Hydrogen Bubbles at Pt Microelectrodes.

The dynamics of hydrogen bubbles produced via electrolysis in acidic electrolytes is studied in a combination of experiments and numerical simulations. A transition from monotonic to oscillatory bubble growth is observed after 2/3 of the bubble lifetime, if the electric potential exceeds -3 V. This work analyzes characteristic features of the oscillations in terms of bubble geometry, the thickness of the microbubble carpet, and the oscillation frequency. An explanation of the oscillation mechanisms is provided by the competition between buoyancy and electric force, the magnitude of which depends on the carpet thickness. Both the critical carpet thickness at detachment and the oscillation frequencies of the bubble as predicted by the model agree well with the experiment.

Mitigating Meniscus Instabilities in Solution-Sheared Polymer Films for Organic Field-Effect Transistors.

Semiconducting donor-acceptor copolymers are considered to be a promising material class for solution-coated, large-scale organic electronic applications. A large number of works have shown that the best-performing organic field-effect transistors (OFETs) are obtained on low-surface-energy substrates. The meniscus instabilities that occur when coating on such surfaces considerably limit the effective deposition speeds. This represents a limiting factor for the upscaling of device fabrication for mass production, an issue that needs to be addressed if organic electronic devices are ever to become commercially relevant. In this work, we present a method to increase the accessible window of coating speeds for the solution shearing of donor-acceptor semiconductor polymers for the fabrication of OFETs. By incorporating a piezo crystal that is capable of producing high-frequency vibrations into the coating head, we are able to mitigate contact line instabilities due to the depinning of the contact line, thereby suppressing the commonly encountered "stick-and-slip" phenomenon.

Cytocompatible, Injectable, and Electroconductive Soft Adhesives with Hybrid Covalent/Noncovalent Dynamic Network.

Synthetic conductive biopolymers have gained increasing interest in tissue engineering, as they can provide a chemically defined electroconductive and biomimetic microenvironment for cells. In addition to low cytotoxicity and high biocompatibility, injectability and adhesiveness are important for many biomedical applications but have proven to be very challenging. Recent results show that fascinating material properties can be realized with a bioinspired hybrid network, especially through the synergy between irreversible covalent crosslinking and reversible noncovalent self-assembly. Herein, a polysaccharide-based conductive hydrogel crosslinked through noncovalent and reversible covalent reactions is reported. The hybrid material exhibits rheological properties associated with dynamic networks such as self-healing and stress relaxation. Moreover, through fine-tuning the network dynamics by varying covalent/noncovalent crosslinking content and incorporating electroconductive polymers, the resulting materials exhibit electroconductivity and reliable adhesive strength, at a similar range to that of clinically used fibrin glue. The conductive soft adhesives exhibit high cytocompatibility in 2D/3D cell cultures and can promote myogenic differentiation of myoblast cells. The heparin-containing electroconductive adhesive shows high biocompatibility in immunocompetent mice, both for topical application and as injectable materials. The materials could have utilities in many biomedical applications, especially in the area of cardiovascular diseases and wound dressing.

Reversibly Assembled Electroconductive Hydrogel via a Host-Guest Interaction for 3D Cell Culture.

The study of cells responding to an electroconductive environment is impeded by the lack of a method, which would allow the encapsulation of cells in an extracellular matrix-like 3D electroactive matrix, and more challengingly, permit a simple mechanism to release cells for further characterization. Herein, we report a polysaccharide-based conductive hydrogel system formed via a ß-cyclodextrin-adamantane host-guest interaction. Oxidative polymerization of 3,4-ethylenedioxythiophene (EDOT) in the presence of adamantyl-modified sulfated alginate (S-Alg-Ad) results in bio-electroconductive polymer PEDOT:S-Alg-Ad, which can form hydrogel with poly-ß-cyclodextrin (Pß-CD). The PEDOT:S-Alg-Ad/Pß-CD hydrogels can be tuned on aspects of mechanical and electrical properties, exhibit self-healing feature, and are injectable. Electron microscopy suggested that the difference in stiffness and conductivity is associated with the nacre-like layered nanostructures when different sizes of PEDOT:S-Alg-Ad nanoparticles were used. Myoblast C2C12 cells were encapsulated in the conductive hydrogel and exhibited proliferation rate comparable to that in nonconductive S-Alg-Ad/Pß-CD hydrogel. The cells could be released from the hydrogels by adding the ß-CD monomer. Astonishingly, the conductive hydrogel can dramatically promote myotube-like structure formation, which is not in the non-electroconductive hydrogel. The ability to embed and release cells in an electroconductive environment will open new doors for cell culture and tissue engineering.

Noncovalently Assembled Electroconductive Hydrogel.

Cross-linking biomolecules with electroconductive nanostructures through noncovalent interactions can result in modular networks with defined biological functions and physical properties such as electric conductivity and viscoelasticity. Moreover, the resulting matrices can exhibit interesting features caused by the dynamic assembly process, such as self-healing and molecular ordering. In this paper, we present a physical hydrogel system formed by mixing peptide-polyethylene glycol and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate. This combinatorial approach, which uses different modular building blocks, could lead to high tunability on aspects of rheology and electrical impedance. The proposed physical hydrogel system is characterized by both a self-healing ability and injectability. Interestingly, the formation of hydrogels at relatively low concentrations led to a network of closer molecular packing of poly(3,4-ethylenedioxythiophene) nanoparticles, reflected by the enhanced conductivity. The biopolymer system can be used to develop three-dimensional cell cultures with incorporated electric stimuli, as evidenced by its contribution to the survival and proliferation of encapsulated mesenchymal stromal cells and their differentiation upon electrical stimulation.

Marangoni convection at electrogenerated hydrogen bubbles.

Electrolytic gas evolution is a fundamental phenomenon occurring in a large number of industrial applications. In these processes gas bubbles are formed at the electrode from a supersaturated solution. Since dissolved gases can change the surface tension, a gas concentration gradient may cause the surface tension to vary locally at the interface of the gas bubble. Surface tension gradients may also form due to temperature gradients generated by ohmic heating of the electrolyte. In both cases, the resulting shear stress imposes a convection in the electrolyte and the gas bubble (Marangoni effect). This phenomenon may influence the entire electrolytic gas evolution process, e.g., by an enhanced mass transfer. In this study, the first evidence of the Marangoni convection near growing hydrogen bubbles, generated by water electrolysis, is provided. Microscopic high speed imaging was applied to study the evolution of single hydrogen bubbles at a microelectrode. The convection near the interface of the growing bubble was measured by using a time-resolved Particle Tracking Velocimetry (PTV) technique. The results indicate a clear correlation between the magnitude of the Marangoni convection and the electric current.

Dynamics of Single Hydrogen Bubbles at a Platinum Microelectrode.

Bubble dynamics, including the formation, growth, and detachment, of single H2 bubbles was studied at a platinum microelectrode during the electrolysis of 1 M H2SO4 electrolyte. The bubbles were visualized through a microscope by a high-speed camera. Electrochemical measurements were conducted in parallel to measure the transient current. The periodic current oscillations, resulting from the periodic formation and detachment of single bubbles, allow the bubble lifetime and size to be predicted from the transient current. A comparison of the bubble volume calculated from the current and from the recorded bubble image shows a gas evolution efficiency increasing continuously with the growth of the bubble until it reaches 100%. Two different substrates, glass and epoxy, were used to embed the Pt wire. While nearly no difference was found with respect to the growth law for the bubble radius, the contact angle differs strongly for the two types of cell. Data provided for the contact point evolution further complete the image of single hydrogen bubble growth. Finally, the velocity field driven by the detached bubble was measured by means of PIV, and the effects of the convection on the subsequent bubble were evaluated.

Design and validation of a bioreactor for simulating the cardiac niche: a system incorporating cyclic stretch, electrical stimulation, and constant perfusion.

To simulate the cardiac niche, a bioreactor system was designed and constructed to incorporate cyclic stretch, rhythmic electrical stimulation, and constant perfusion. The homogeneity of surface strain distribution across the cell culture substrate was confirmed with ARAMIS deformation analysis. The proliferation marker, Ki-67, detected in human umbilical vein endothelial cells and 3-[4,5-dimethyl-thiazol-2-yl]-2,5-diphenyltetrazolium bromide cytotoxicity assay performed on human atrial fibroblasts confirmed biocompatibility of this novel device. Cyclic stretch treatment for 24 h resulted in the perpendicular alignment of human atrial fibroblasts. An electrical stimulation system containing carbon electrodes was characterized by electrochemical impedance spectroscopy and charge injection/recovery studies, which indicated that increased corrosive reactions were associated with a higher input voltage and prolonged pulse duration. Field stimulation delivered through this system could induce rhythmic contractions in adult rat ventricular myocytes, with contractile characteristics similar to those paced in a standard field stimulation chamber. In conclusion, this bioreactor provides a novel tool to study the interaction between physical stimulation and cardiac cell physiology.

Enrichment of Paramagnetic Ions from Homogeneous Solutions in Inhomogeneous Magnetic Fields.

Applying interferometry to an aqueous solution of paramagnetic manganese ions, subjected to an inhomogeneous magnetic field, we observe an unexpected but highly reproducible change in the refractive index. This change occurs in the top layer of the solution, closest to the magnet. The shape of the layer is in accord with the spatial distribution of the largest component of the magnetic field gradient force. It turns out that this layer is heavier than the underlying solution because it undergoes a Rayleigh-Taylor instability upon removal of the magnet. The very good agreement between the magnitudes of buoyancy, associated with this layer, and the field gradient force at steady state provides conclusive evidence that the layer formation results from an enrichment of paramagnetic manganese ions in regions of high magnetic field gradient.