Shape Evolution of Precipitate Membranes in Flow Systems

Abstract

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.

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

“…As reported by earlier studies, the self-assembled barrier is permeable to hydroxide but nearly impermeable to nickel ions. [21][22][23] Accordingly, the membrane thickens strictly in the direction of the nickel solution while its hydroxide-facing surface remains stationary. The membrane width obeys w ¼ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi…”

“…, where D eff and t are an effective diffusion coefficient and time, respectively. [21][22][23] To ensure linear membranes for all flow rates and especially the problematic conditions of Q ! 1 mL/h, we first pre-grow a straight template membrane at Q = 1 mL/h for 1 h. [23] Then we switch the flow rate to the desired (lower or higher) value to produce the membrane portion of interest.…”

“…[17,18] Under the controlled conditions of laboratory experiments, similar shapes often arise from conditions that initially separate reactants and do not negate self-sustained gradients and diffusion fluxes. [19][20][21][22][23][24][25][26] Classic examples include Liesegang patterns that consist of periodic bands of crystallites, [27] related traveling precipitation-redissolution fronts, [28] and biomorphs [29][30][31] which include helical and funnel-like assemblies of metal carbonate nanorods.…”

“…A third, and arguably the most widely known, example is the chemical garden system (Figure S1). [10,[19][20][21][22][23]26,[32][33][34] These plant-like inorganic structures arise from self-organizing precipitation reactions between a metal salt and a solution containing silicates, hydroxides, carbonates, phosphates or similar anions. [10,26] During these reactions, the precipitates assemble thin membranes that compartmentalize the system and auto-regulate the exchange of reactants.…”

“…[19,32,33,35,36] In addition, pH gradients drive the transport of hydroxide ions across the membrane and steadily thicken the precipitate barrier. [21][22][23]36] Despite these qualitative insights, the mechanisms by which the chemical species combine to create chemical garden membranes have remained unknown. Understanding the nanoscopic processes of chemical garden growth could have far-reaching implications for fields such as biomineralization and materials science.…”