Xuegeng Yang

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.

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.

Numerical simulation of the onset of mass transfer and convection in copper electrolysis subjected to a magnetic field

Numerical investigations have been performed in order to simulate the transient behaviour of copper electrolysis in a rectangular cell with vertical electrodes in a galvanostatic regime where the kinetics of the electrodes is controlled by charge-transfer. The transient behaviour observed for a binary electrolyte reproduces the temporal evolution of the concentration distribution measured in recent experimental work. Horizontal magnetic fields that vary linearly between the electrodes create Lorentz forces that either enhance or attenuate natural convection. The different time scales of natural convection and convection driven by the Lorentz force lead to interesting transient effects. The simulations performed for the case of an attenuating Lorentz force explain the dynamics of vertical inhomogeneities of the concentration boundary layer during the initial stages of electrolysis that were previously observed experimentally.

Magnetic Separation of Paramagnetic Ions From Initially Homogeneous Solutions

The concentration change in an initially homogeneous paramagnetic solution is studied interferometrically upon applying sufficiently strong inhomogeneous magnetic fields. For this purpose, five different magnetic field configurations are analyzed. Clear evidence is provided that an enrichment of paramagnetic ions occurs in the field gradient. In particular, we show that the shape of this enrichment layer maps the spatial distribution of the magnetic field gradient force.

Magnetic separation of Dy(III) ions from homogeneous aqueous solutions

The possibility to enrich paramagnetic dysprosium(III) ions in a magnetic field gradient is proved by means of interferometry, which may open the route for a magnetic separation of rare earth ions from aqueous solutions. The separation dynamics are studied for three different concentrations of DyCl3 and compared with those found recently in a sulphate solution of the 3d ion Mn(II). In view of the similar-sized hydration spheres for Dy(III) and Mn(II), the slower separation dynamics in DyCl3 is attributed to both a higher densification coefficient and the strong impact of Brownian motion due to the absence of ion-pair clusters.

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.