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. A variety of seemingly unrelated reaction-precipitation systems can give rise to the growth of mesoscopic tubular structures. Examples include hollow rust fibers [1,2] formed in corrosion processes and microscopic, needle-like tubes created during the setting of cement [3]. The latter example is closely related to the formation of silica tubes in chemical gardens which are a well-known demonstration experiment in physics and chemistry [4]. Chemical gardens involve the growth of colorful, plant-like fibers from aqueous solutions containing anions such as aluminate, borate, carbonate, or silicate [5–7]. These structures have diameters in the microand millimeter range and reach lengths of several decimeters. Tube growth can be induced by seeding the latter solutions with crystals of various soluble salts (e.g., CuSO4, MnCl2, FeCl3) excluding group (I) compounds [7]. Early references to chemical gardens can be traced back to the 17th century [8, 9]. In the 20th century, these structures attracted considerable interest because they were mistaken to be prototype models of simple life forms [10]. Recently, the study of chemical gardens has been experiencing a renaissance as tubular structures in the aluminosilicate system have been shown to be promising catalytic materials with an intriguing hierarchical nanostructure [11,12]. For (∗) E-mail: [email protected]