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

Abstract

Abstract. -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][6][7]. These structures have diameters in the micro-and 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., CuSO 4 , MnCl 2 , FeCl 3 ) 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

Genericity of confined chemical garden patterns with regard to changes in the reactants

“…Depending on the concentration of the injected solution, different growth regimes, such as popping, jetting, budding and ocean-ridge-like dynamics, have been observed. [25][26][27] Injection reduces the role played by osmosis during the growth and leads to the formation of regular tubes provided the concentration of metallic salt is not too large. Beyond some limit, wide bulging tubes are formed.…”

From Chemical Gardens to Chemobrionics

“…In an environment of finite vertical extent, the rising fluids spread out radially at the top surface, creating a recirculating toroidal cell around the jet. In laboratory experiments, the jet fluid may be removed at the top of the environment 41. This complex flow requires a two-dimensional solution of eq 1, with boundary conditions at the interface of the two fluids expressing continuity of stress, pressure and velocity; further conditions are needed at all the boundaries of the environment.Simple approximate solutions of eq 1 have been obtained by assuming a one-dimensional vertical flow, in both the jet and the environmental fluids and no net vertical transport of fluid in the environment; 41 these solutions focus on the flow in the jet and surroundings away from the bottom and top boundaries of the environment.…”

Tubular precipitation and redox gradients on a bubbling template

“…Precipitative film formation on bubbles has also been demonstrated in electrodeposition (9). Finally, tree-like ''silicate gardens'' (10-12) grow in sodium silicate solutions seeded with metal ion salts, possibly from osmotic stresses across a silicate gel membrane, and now can be studied in a very controlled manner (13).…”