Transition from gravito- to electroconvective regimes in thin-layer electrodeposition

Coupling between Buoyancy Forces and Electroconvective Instability near Ion-Selective Surfaces

Recent investigations have revealed that ion transport from aqueous electrolytes to ion-selective surfaces is subject to electroconvective instability that stems from coupling of hydrodynamics with electrostatic forces. These systems inherently involve fluid density variation set by salinity gradients. However, the coupling between the buoyancy effects and electroconvective instability has not yet been investigated although a wide range of electrochemical systems are naturally prone to these interplaying effects. In this study we thoroughly examine the interplay of gravitational convection and chaotic electroconvection. Our results reveal that buoyant forces can significantly influence the transport rates, otherwise set by electroconvection, when the Rayleigh number Ra of the system exceeds a value Ra∼1000. We show that buoyancy forces can significantly alter the flow patterns in these systems. When the buoyancy acts in the stabilizing direction, it limits the extent of penetration of electroconvection, but without eliminating it. When the buoyancy destabilizes the flow, it alters the electroconvective patterns by introducing upward and downward fingers of respectively light and heavy fluids.

Nature of inclined growth in thin-layer electrodeposition under uniform magnetic fields

Electrochemical deposition (ECD) in thin cells in a vertical position relative to gravity, subject to an external uniform magnetic field, yields a growth pattern formation with dense branched morphology with branches tilted in the direction of the magnetic force. We study the nature of the inclined growth through experiments and theory. Experiments in ECD, in the absence of magnetic forces, reveal that a branch grows by allowing fluid to penetrate its tip and to be ejected from the sides through a pair of symmetric vortices attached to the tip. The upper vortices zone defines an arch separating an inner zone ion depleted and an outer zone in a funnel-like form with a concentrated solution through which metal ions are carried into the tip. When a magnetic field is turned on, vortex symmetry is broken, one vortex becoming weaker than the other, inducing an inclination of the funnel. Consequently, particles entering the funnel give rise to branch growth tilted in the same direction. Theory predicts, in the absence of a magnetic force, funnel symmetry induced through symmetric vortices driven by electric and gravitational forces; when the magnetic force is on, it is composed with the pair of clockwise and counterclockwise vortices, reducing or amplifying one or the other. In turn, funnel tilting modifies particle trajectories, thus, growth orientation.

Diffusion-limited and advection-driven electrodeposition in a microfluidic channel

Self-terminating electrochemical fabrication was used within a microfluidic channel to create a junction between two Au electrodes separated by a gap of 75 microm . During the electrochemical process of etching from the anode to deposition at the cathode, flow could be applied in the anode-to-cathode direction. Without applied flow, dendritic growth and dense branching morphologies were typically observed at the cathode. The addition of applied flow resulted in a densely packed gold structure that filled the channel. A computer simulation was developed to explore regimes where the diffusion, flow, and electric field between the electrodes individually dominated growth. The model provided good qualitative agreement relating flow to the experimental results. The model was also used to contrast the effects of open and closed boundaries and electric field strength, as factors related to tapering.

Simulations of transport regime in electrodeposition in different viscosity scenarios

In this work we study the effects of viscosity variations in thin-layer electrochemical deposition (ECD) under galvanostatic conditions through experimental measurements and theoretical modeling. The theoretical model, written in terms of dimensionless quantities, describes diffusive, migratory and convective ion transport in a fluid under galvanostatic conditions. Experiments reveal that as viscosity increases, convection decreases when the cell resistance remains constant. Our numerical model predicts that as viscosity increases, electroconvection becomes less relevant and concentration and convective fronts slow down. The time scaling of this phenomenon is studied and compared to previously reported low viscosity solution studies.

Ion transport in tumors under electrochemical treatment: in vivo, in vitro and in silico modeling

The electrochemical treatment of cancer (EChT) consists in the passage of a direct electric current through two or more electrodes inserted locally in the tumor tissue. The extreme pH changes induced have been proposed as the main tumor destruction mechanism. Here, we study ion transport during EChT through a combined modeling methodology: in vivo modeling with BALB/c mice bearing a subcutaneous tumor, in vitro modeling with agar and collagen gels, and in silico modeling using the one-dimensional Nernst-Planck and Poisson equations for ion transport in a four-ion electrolyte. This combined modeling approach reveals that, under EChT modeling, an initial condition with almost neutral pH evolves between electrodes into extreme cathodic alkaline and anodic acidic fronts moving towards each other, leaving the possible existence of a biological pH region between them; towards the periphery, the pH decays to its neutral values. pH front tracking unveils a time scaling close to t(1/2), signature of a diffusion-controlled process. These results could have significant implications in EChT optimal operative conditions and dose planning, in particular, in the way in which the evolving EChT pH region covers the active cancer cells spherical casket.

Noise-reduced electroless deposition of arrays of copper filaments

We report here a self-organized electroless deposition of copper in an ultrathin layer CuSO4 of electrolyte. Microscopically the branching rate of the copper deposits is significantly decreased, forming an array of smooth polycrystalline filaments. Compared with a conventional electrodeposition system, no macroscopic electric field is involved and the thickness of the electrolyte layer is greatly decreased. Therefore the electroless deposition takes place in a nearly ideal, two-dimensional diffusion-limited environment. We suggest that restriction of the thickness of the electrolyte film is responsible for the generation of smoother branches of the electrodeposits. Our data also show that even in a diffusion-limited scenario the aggregate morphology is not necessarily very ramified and fractal-like.

Morphological evolution in the electrodeposition of the Pb-Sn binary system

Morphological evolution in the electrodeposition of Pb-Sn binary system is studied. As the second component increases, the morphology of the codeposit changes from dendrite to ramification, to dense branch, and finally to fractal structure, respectively. The evolution arises from the influence of crystallographic texture, which leads to a splitting of dendritic tips and the formation of ramified morphology. This work provides direct evidence to explore the crystallographic influence on the morphological evolution in electrodeposition.

Three-dimensional nature of ion transport in thin-layer electrodeposition

A generalized three-dimensional model for ion transport in electrodeposition is introduced. Ion transport is mainly governed by diffusion, migration, and convection. When convection prevails, in particular, in the limiting case of gravity-driven convection, the model predicts concentration shells and convection rolls and their interaction mode with a deposit tip: shell and roll bend and surround the tip forming a three-dimensional envelope tube squeezed at the deposit tip. In the limiting case of electrically driven convection, a vortex ring and an electric spherical drop crowning the deposit tip are predicted. When gravity and electric convection are both relevant, the interaction of ramified deposits, vortex tubes and rings, and electric spherical drops, leading to complex helicoidal flow, is predicted. Many of these predictions are experimentally observed, suggesting that ion transport underlying dendrite growth is remarkably well captured by our model.