Tubular precipitation and redox gradients on a bubbling template

Tubular structures created by precipitation abound in nature, from chimneys at hydrothermal vents to soda straws in caves. Their formation is controlled by chemical gradients within which precipitation occurs, defining a surface that templates the growing structure. We report a self-organized periodic templating mechanism producing tubular structures electrochemically in iron-ammonium-sulfate solutions; iron oxides precipitate on the surface of bubbles that linger at the tube rim and then detach, leaving behind a ring of material. The acid-base and redox gradients spontaneously generated by diffusion of ammonia from the bubble into solution organize radial compositional layering within the tube wall, a mechanism studied on a larger scale by complex Liesegang patterns of iron oxides formed as ammonia diffuses through a gel containing FeSO4. When magnetite forms within the wall, a tube may grow curved in an external magnetic field. Connections with free-boundary problems in speleothem formation are emphasized.