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

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.