Silica tubes in chemical gardens: Radius selection and its hydrodynamic origin

Chemical gardens consist of hollow silica fibers that form from silicate solution upon seeding with salt crystals or injection of salt solution.We investigate the outer radius of these tubes for steady and oscillatory growth dynamics.The radius increases with increasing injection rates and concentrations of cupric sulfate seed solution.For steady growth, we find that the tube radii are described quantitatively by the Poiseuille-flow characteristics of the buoyant jet of injected solution.The oscillatory regime gives rise to wider tubes and involves the cyclic expansion and detachment of a membrane-bound droplet at the growth point.The droplets' expansion rate equals the applied injection rate indicating that, in this growth regime, the fluid flow is constrained to the interior of the silica structures.