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.

Ionic size dependent electroviscous effects in ion-selective nanopores.

Pressure-driven flows of aqueous ionic liquids are characterized by electroviscosity-an increase in the effective (apparent) viscosity because of an induced back electric field termed streaming potential. In this work, we investigate the electrokinetic phenomenon of streaming potential mediated flows in ion-selective nanopores. We report a dramatic augmentation in the effective viscosity as attributable to the finite size effect of the ionic species in counterion-only systems. The underlying physics involves complex interaction between the concerned electrochemical phenomena and hydrodynamic transport in a confined fluidic environment, which we capture through a modified continuum based approach and validate using molecular dynamics simulations. We obtain an expression for the ionic-size dependent streaming potential pertinent to the physical situation being addressed. The corresponding estimations of effective viscosity implicate that the classical paradigm of point sized ions can give rise to gross underestimations of the flow resistance in counterion-only systems especially for negligible surface (Stern layer) conductivity and large fluidic slip at the surface.

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.