Growing Inorganic Membranes in Microfluidic Devices: Chemical Gardens Reduced to Linear Walls

Abstract

The hollow precipitate tubes in chemical gardens conserve the nonequilibrium conditions present during their formation and are an important example of molecular processes causing complex macroscopic self-organization. We report a greatly simplified experimental model of these structures that is based on the formation of an inorganic membrane in a microfluidic device. Within this device, we induce the precipitation of Mn­(OH)2 and other metal hydroxides at the reactive interface of steadily injected NaOH and MnCl2 solutions. The resulting precipitate wall extends along the entire length of the reactor channel and can be positioned at will, and its width increases strictly in the direction of the metal solution. These thickening dynamics obey a square root law. The corresponding effective diffusion coefficient is proportional to [OH–], shows a sigmoidal dependence on [Mn2+], and also depends on the precipitating metal ion. The precipitate wall is permeable to methylene blue and strongly adsorbs methyl orange. Electron and optical microscopy reveals decaying micrometer-sized perturbations and a 40 μm thick gel-like layer on the surface exposed to the Mn2+ solution. The wall growth is also followed by in situ Raman spectroscopy. Potential applications toward materials and origins-of-life research are discussed.

Wavy membranes and the growth rate of a planar chemical garden: Enhanced diffusion and bioenergetics

“…In addition, the wall thickening occurs strictly in the direction of slightly acidic, metal salt solution and not toward the alkaline silicate solution. Whereas this observation was originally made for silicate systems, the results from microfluidic growth experiments confirmed this finding for precipitation reactions between sodium hydroxide and various metal salt solutions including magnesium, manganese, iron, cobalt, and copper (28). Here we present experimental measurements on the growth of precipitate membranes in a microfluidic system together with an analytical model that captures key aspects of the growth dynamics.…”

“…Furthermore, one can reduce the dimensionality of the system by the injection of salt solutions from a point-like source into a thin, horizontal layer of silicate solution contained within a Hele-Shaw cell (25)(26)(27). This approach was taken further by studying the formation of precipitate membranes in a microfluidic device at the planar interface between two parallel, cocurrent reactive fluid streams (28). The…”

Microfluidic Production of Pyrophosphate Catalyzed by Mineral Membranes with Steep pH Gradients

“…The membrane initially formed at the reactive solution interface and thickened overtime in both directions while staying close to the middle of the serpentine‐shaped channel (see Movie S1 in the Supporting Information). As we can see from the micrograph in Figure b, the iron phosphate membrane was porous and heterogeneous and hence differed from the metal hydroxide membranes in our earlier studies . In previous studies the membranes showed no voids, acted as semi‐permeable membranes, and thickened only in the direction of the acidic metal salt solutions.…”

“…The polystyrene sheet was cut to the desired shape with an inexpensive electronic cutting tool (Silhouette Portrait) connected to a personal computer. The serpentine‐shaped channel started with a Y‐shaped pattern and had several semicircular‐ring segments connecting the other straight parts. The mixing part of the channel is 30 cm long, 3 mm wide, and approximately 200 μm high.…”

“…The thickness of the membrane is also significant in order to promote steep gradients across the wall. Recently, a methodology has been introduced by Batista et al . who demonstrated the formation of thin inorganic precipitate membranes in simple and inexpensive microfluidic devices.…”