8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.90Wed 711:10-14:00Unusual long-range repulsion between surfaces ofsilica-beads forming 2D hexagonal crystals insupercritical fluidsTakehito Koyama, 1 Shigeru Deguchi, 1 Sada-atsu Mukai, 2 Sayuki Ota, 1 and KaoruTsujii 31 JAMSTEC, 2-15 Natsushima-cho, 237-0061, Yokosuka, Japan2 Kyusyu University, Hukuoka, Japan3 Research Institute for Electronic Science, Hokkaido University, Sapporo, JapanIn supercritical fluids, intense density fluctuations appear around their gas/liquid critical points. Itis likely that the density fluctuations affect interactions between surfaces in the fluids, because fluidproperties such as dielectric constant and refractive index also fluctuate and modulate potentialsbetween the surfaces. The density fluctuations may also influence interactions between the fluidsand the surfaces, thereby alter properties of the surfaces such as the charge density. Interestingly,it should be possible to control the effects of the fluctuations to the interactions continuously bytemperature and pressure. However, nothing has been known as to how the density fluctuationsaffect surface forces. We studied structures of 2D hexagonal arrays of monodisperse silica beadsin supercritical ethanol (T c = 241 ◦ C, P c = 6.1 MPa), and found that the lattice constant of thearrays continuously changed with temperature and pressure near the ridge, leading to formation ofnon-close-packed hexagonal arrays. Very large interparticle gaps up to ∼ 10µm were observed inthe close vicinity of the ridge. The observations show that an anomalously long-range repulsionappeared between the silica surfaces in supercritical ethanol. The repulsion was suppressed whenan electrolyte (NaNO 3 ) was added to ethanol, indicating its electrostatic origin. This was alsoconfirmed by detailed analysis of the structures of the arrays. Similar observations were also madein supercritical acetone. Our results strongly suggest that the density fluctuation of supercriticalfluids plays a crucial role in the emergence of the anomalously long-range electrostatic repulsionbetween silica surfaces.90