The properties of two-dimensional arrays of micrometre-sized particles are of interest in relation to a wide range of phenomena, including self-organization and phase behaviour in colloid science and condensed-matter physics1,2,3, the behaviour of dusty plasmas4 and the templating of ordered structures for photonic applications5. Most studies have used pre-existing particles such as monodisperse latex spheres, which may be manipulated with electric or magnetic fields. In contrast, we report here an inorganic solution that spontaneously precipitates a self-organized two-dimensional colloid crystal at the air/water interface. A solution of calcium hydroxide exposed to air reacts with dissolved carbon dioxide to precipitate microcrystals of calcium carbonate in the form of calcite. These aggregate at the surface to form a disordered gel mat with fractal characteristics6. We find, however, that in aged solutions a second population of charged microcrystals with the ‘dogtooth spar’ morphology appears on the surface. These crystallites, which can be observed by optical microscopy, become organized into a regular triangular lattice. The competition between electrostatic and capillary forces between particles leads to lattice spacings of the order of 125 to 175 µm, 5 to 7 times the diameter of the particles. These structures are stable for around 24 h, but eventually aggregate with the fractal gel. The mechanism of their self-organization, as yet incompletely understood, might provide some insights into similar phenomena in colloid science2,3,7,8,9.
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The hospitality of the Naval Research Laboratory and partial support from the Office of Naval Research during a sabbatical visit of H.H.W. are gratefully acknowledged. We thank K.Hathaway, B. Gaber, P. Schoen, D. Turner and G. Lee for useful discussions and access to instruments used in the study. J.N.K. was a summer student whose visit to the Naval Research Laboratory was supported by an Undergraduate Minority Student Program Grant from the Chemical and Transport Systems Division, National Science Foundation.
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Wickman, H., Korley, J. Colloid crystal self-organization and dynamics at the air/water interface. Nature 393, 445–447 (1998). https://doi.org/10.1038/30930
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