rXXXX American Chemical Society A dx.doi.org/10.1021/la104193q | Langmuir XXXX, XXX, 000–000 ARTICLE pubs.acs.org/Langmuir Chemical-Garden Formation, Morphology, and Composition. II. Chemical Gardens in Microgravity Julyan H. E. Cartwright,* ,† Bruno Escribano,* ,†,§ C. Ignacio Sainz-Díaz,* ,† and Louis S. Stodieck* ,‡ † Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Facultad de Ciencias, E-18071 Granada, Spain ‡ Department of Aerospace Engineering, BioServe Space Technologies, University of Colorado at Boulder, Boulder, Colorado 80309-0429, United States b S Supporting Information ABSTRACT: We studied the growth of metal-ion silicate chemical gardens under Earth gravity (1 g) and microgravity (μg) conditions. Identical sets of reaction chambers from an automated system (the Silicate Garden Habitat or SGHab) were used in both cases. The μg experiment was performed on board the International Space Station (ISS) within a temperature-controlled setup that provided still and video images of the experiment downlinked to the ground. Calcium chloride, manganese chloride, cobalt chloride, and nickel sulfate were used as seed salts in sodium silicate solutions of several concentrations. The formation and growth of osmotic envelopes and microtubes was much slower under μg conditions. In 1 g, buoyancy forces caused tubes to grow upward, whereas a random orientation for tube growth was found under μg conditions. ’ INTRODUCTION Chemical gardens, or silicate gardens, grow when a solid of a metal-ion salt is placed into a sodium silicate solution. 1 As the salt begins to dissolve in the silicate, it develops a colloidal semi- permeable membrane of metal silicate. Osmotic pressure pulls water from the silicate into the enveloping membrane, further dissolving the salt. The membrane is consequently inflated, forming what we term an osmotic envelope, until it ruptures and expels a jet of metal-ion solution. This solution has a different pH from that of the external solution and the silicate precipitates forming a hollow tube through which the metal-ion solution continues to flow, pumped by the semipermeable membrane. The tube continues growing at its tip by accretion of metal-ion precipitate until the initial salt is depleted. Hydroxide ions that enter through the osmotic membrane react with the metal ions and precipitate on the inner surface of the tubes as metal hydroxides. The morphology of these silicate gardens depends on the evolution of the combination of forced convection driven by osmotic pressure through the semipermeable membrane and free convection due to buoyancy forces. 1 Under normal gravity conditions, the tubes grow generally upward under a combina- tion of osmotic pressure and buoyancy forces, because the metal- ion salt solution is less dense than the surrounding silicate, so it is of interest to find out how the absence of buoyancy forces under conditions of microgravity affects chemical-garden growth. Buoyancy forces giving rise to convection are proportional to gravity. When chemical gardens are grown in microgravity, the buoyancy effect is diminished correspondingly and the tubes, driven by forced convection alone, would be expected to grow in arbitrary directions. Although chemical-garden growth is a long-studied phe- nomenon, 2-4 it is not yet well understood with regard to the physical and chemical variables that control the morphology of these microtubes. Gaining a greater insight into these pro- cesses would potentially be of value for materials sciences. Furthermore, the phenomenon is of interest for educational purposes, since in addition to having an attractive visual impact with students, chemical-garden growth is an excellent system in which to study the chemical processes of dissolution, precipita- tion, and crystallization interconnected with physical phenom- ena of fluid dynamics and osmosis. Previously, we explored salts with cations from the same group in the periodic table, that of group 2 of the alkaline-earth cations, and observed similar behavior but different growth rates. 5 We have also studied the behavior of salts within the same period, period 4, in the periodic table: 6 a study that we complete in this work with microgravity experiments. A sole experiment on chemical-garden growth in microgravity, which flew aboard the shuttle mission STS-55 in 1993, has been reported in the literature by Jones and Walter. 7 There was an earlier experiment than Jones and Walter’s flown on STS-47 in 1992, but inexplicably, the researchers reported only on the Received: October 18, 2010 Revised: January 9, 2011