Nonclassical Crystallization Causes Dendritic and Band-Like Microscale Patterns in Inorganic Precipitates.

The self-organization of complex solids can create patterns extending hierarchically from the atomic to the macroscopic scale. A frequently studied model is the chemical garden system which consists of life-like precipitate shapes. In this study, we examine the thin walls of chemical gardens using microfluidic devices that yield linear Ni(OH)2 precipitate membranes. We observe distinct light-scattering patterns within the compositionally pure membranes, including disorganized spots, dendrites, and parallel bands. The bands are tilted with respect to the membrane axis and their spacing (20-100 µm) increases with increasing flow rates. Scanning electron microscopy reveals that the bands consist of submicron particles embedded in a denser material and these particles are also found in the reactant stream. We propose that dendrites and bands arise from the attachment of solution-borne nanoparticles. The bands are generated by particle-aggregation zones moving upstream along the slowly advancing membrane surface. The speed of the aggregation zones is proportional to the band distance and defines the system's dispersion relation. This speed-wavelength dependence and the flow-opposing motion of the aggregation zones are likely caused by low particle concentrations in the wake of the zones that only slowly recover due to Brownian motion and particle nucleation.

Shape Evolution of Precipitate Membranes in Flow Systems.

Chemical gardens are macroscopic structures that form when a salt seed is submerged in an alkaline solution. Their thin precipitate membranes separate the reactant partners and slow down the approach toward equilibrium. During this stage, a gradual thickening occurs, which is driven by steep cross-membrane gradients and governed by selective ion transport. We study these growth dynamics in microfluidic channels for the case of Ni(OH)2 membranes. Fast flowing reactant solutions create thickening membranes of a nearly constant width along the channel, whereas slow flows produce wedge-shaped structures that fail to grow along their downstream end. The overall dynamics and shapes are caused by the competition of reactant consumption and transport replenishment. They are reproduced quantitatively by a two-variable reaction-diffusion-advection model which provides kinetic insights into the growth of precipitate membranes.

Coupled Dynamics of Anode and Cathode in Proton-Exchange Membrane Fuel Cells.

An external reference electrode was used to monitor individually the evolution of the anodic and cathodic potentials during the stationary as well as oscillatory operation of a Direct Formic Acid Fuel Cell (DFAFC) and a Direct Methanol Fuel Cell (DMFC). Besides evidencing the large activation loss in both cells, we were able to observe how the oscillatory operation of such devices affects their cathodes. In fact, cathodic oscillations synchronized with the cell voltage dynamics were observed, hitherto never reported on fuel cells. We have addressed these phenomena taking into account two main coupling processes: through the proton concentration and a global coupling stemming from the control mode (potentiostatic or galvanostatic).