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42798-AC10
Investigation of the Laws and Mechanisms Governing the Formation of Tubular Precipitation Structures
Reaction-precipitation processes are ubiquitous in nature and critical to many industrial processes. Some of the most striking examples involve the formation of permanent tubular structures such as the huge black “smokers” at hydrothermal vents on the ocean's floor. Examples at smaller lengthscales include tubular rust in corrosion systems, silica tubes in setting Portland cement and micro-tubes in the mineralized shells of marine algae.
These surprising tubular structures can only be understood by quantitative studies of specific experimental models. Such models ideally show pattern formation on convenient length and time scales such as millimeters and seconds, respectively. Our model of choice are the precipitation tubes found in “silica gardens” which form from readily available seeds, such copper salt crystals, and various solutions, including waterglass. Earlier, our laboratory developed a powerful technique that helped to overcome problems concerning the accurate control and reproducibility. This technique replaces the seed crystal by a “seed solution” which is injected at constant flow rates into a large volume of silicate so lution.
Using this methodology, we discovered three growth regimes that critically depend on the density difference between the waterglass in the reservoir and the injected cupric sulfate solution. For high density differences, hydrodynamics dominates the formation of the reaction-precipitation tubes that steadily grow around a stable jet of buoyant seed solution. For smaller density differences, the jetting regime switches to oscillatory, popping dynamics that involve the periodic formation and detachment of a solution filled, colloidal gel envelop at the growth point. For even smaller density differences, popping behavior changes to budding growth in which the droplet is not released but bursts to nucleate a new expanding bulge at the site of breach, namely, the budding regime.
Under the funding provided by our PRF grant, we have recently studied “reverse” conditions meaning that dense silicate solution is injected into lighter salt solutions containing metal ions such Cu(II). Under such conditions, four distinct growth regimes are observed and their stability in terms of flow rate and cupric sulfate concentration was investigated. Three of these growth regimes resemble behavior reported earlier for the injection of cupric sulfate into silicate solution. However “reverse” conditions reveal one distinctly different regime in which tube growth is limited by repetitive fracturing. The lengths of the broken-off tube segments and the times between subsequent break-off events were successfully described by log-normal distributions.
We also examined the composition of our reaction-precipitation tubes using various experimental methodologies including SEM/TEM, EDS, IR and Raman spectroscopy. The results provide information on the chemical composition and morphology of the tube walls. Specifically, we investigated the thickness of the wall of the resulting tubular structures. Typical values are around 10 micrometer. Moreover, we established that the wall is a gradient material with profound compositional changes between the outer and inner surface. Micro-Raman spectroscopy along with energy dispersive X-ray fluorescence data identify amorphous silica and copper (II) hydroxide as the main compounds within the inner and outer tube surfaces, respectively. Upon heating the blueish precipitates to approximately 150 degrees Celsius, the material turns black as copper (II) hydroxide reacts to copper (II) oxide. Moreover, we recorded high resolution transmission electron micrographs that reveal polycrystalline morphologies.
In a third study under this grant, we explored potential applications of our approach and specifically hierarchical structures in the tube material. For these investigations, we created precipitation tubes from the injection of zinc sulfate into silicate solution. The resulting structures were qualitatively similar to the ones formed by Cu(II) injection but contained zinc hydroxide on the inner tube wall. Mild to moderate heating of the harvested structures induced the formation of zinc oxide. This process was confirmed by x-ray diffraction measurements. SEM and HRTEM studies revealed further insights into the materials micro and nanostructure. Most importantly, we found that the resulting tubular material shows interesting photocatalytic and photoluminescence characteristics.
The latter studies employed tubes that were synthesized in the presence of buoyant gas bubbles that act as templates and directional guides during the synthesis process. These bubbles are injected into the early reaction interface where they get pinned. Remarkably, these bubbles stay attached to the interface between the reactant solutions during the injection-controlled tube growth. The resulting tubes are highly linear and their radius was found to be typically 80-90 percent of the radius of the employed gas bubble.
Additional studies are underway that aim to create micrometer-scale tubes in a controlled fashion. For these experiments, we create polymer beads using established emulsification techniques. Once prepared and washed, these beads are loaded with and submerged into the reactant solutions. Preliminary results indicate slow tube growth on a microscopic length scales with typically one to three tubes per bead.
