The study of matter far beyond equilibrium and its emergent complexity offers profound opportunities for the science of the 21st century. While our theoretical understanding of these phenomena has continued to increase during the past decades, there are only a few well established examples in materials science. These sparse examples include the frontal polymerization production of organic gels and resins as well as the self-propagating high-temperature synthesis of certain ceramics and superconductors.
There is no reason to believe that this lack of applications in materials science is intrinsic to the discipline. On the contrary, biology demonstrates unambiguously that the production and maintenance of complex materials benefit from nonequilibrium conditions and the controlled utilization of emergent structures and dynamics. Key examples are the hierarchical architectures of living systems, which span from molecular to macroscopic system length scales, and also the adaption to external constraints, specifically self-repair. However, can such features be reproduced with non-biological processes? The answer to this grand question clearly requires the careful development and study of appropriate experimental models. In this context, our workshop aims to discuss a particular class of chemical reactions that create permanent macroscopic patterns and structures from precipitation reactions. The two classic examples are Liesegang structures and “chemical gardens”. Our workshop aims to bring together scientists from different disciplines who share an interest in these as well as in closely related phenomena. The main focus will be the discussion of chemical gardens, their quantitative description, mathematical modeling, and potential use in materials science and engineering
The chemical-gardens system is as old as chemistry and has attracted the curiosity of scientists such as Isaac Newton. It involves the formation of hollow tubular solids in aqueous solution under ambient conditions at growth rates of mm/s to mm/day. The diameter of these hollow structures varies from micrometers to millimeters and seemingly involves no length limitation. Conventionally their self-organized synthesis is started from small salt crystals seeded into solutions containing sodium silicate (or other anions such as borates, phosphates, carbonates and sulfides). Tube growth can also be initiated by hydrodynamic injection or the use of pellets and microbeads. The reaction processes involve colloidal intermediates and are driven by the co-precipitation of amorphous silica and metal hydroxides (or oxides, sulfides, etc.) In the classic case of the dissolution of a seed crystal, the precipitates initially form a semipermeable, colloidal all-enveloping membrane which stretches and bursts due to osmotically controlled fluxes. The breach causes further precipitation but now around a buoyant jet of reactant solution, which explains qualitatively the formation of tubes at the system level. These macroscopic structures are self-healing as small damage sites seal themselves rapidly once the interior and exterior solutions begin to mix. Very similar materials form also in setting cement and certain corrosion systems. Furthermore some research groups have proposed precipitation tubes as the birth place of life. This “metabolism first” school further hypothesizes that steep pH gradients across the inorganic and often catalytic tube walls provided the chemical energy source for the necessary prebiotic processes.
We plan to encompass all of these aspects of chemical-gardens research in this workshop.