Formation and Alignment of Elongated, Fractal-like Water-ice Grains in Extremely Cold, Weakly Ionized Plasma
Elongated, fractal-like water-ice grains are observed to form spontaneously when water vapor is injected into a weakly ionized laboratory plasma formed in a background gas cooled to an astrophysically relevant temperature. The water-ice grains form in 1–2 minutes, levitate with regular spacing, and are aligned parallel to the sheath electric field. Water-ice grains formed in plasma where the neutrals and ions have low mass, such as hydrogen and helium, are larger, more elongated, and more fractal-like than water-ice grains formed in plasmas where the neutrals and ions have high mass such as argon and krypton. Typical aspect ratios (length to width ratio) are as great as 5 while typical fractal dimensions are ~1.7. Water-ice grain lengths in plasmas with low neutral and ion masses can be several hundred microns long. Infrared absorption spectroscopy reveals that the water-ice grains are crystalline and so are similar in constitution to the water-ice grains in protoplanetary disks, Saturn's rings, and mesospheric clouds. The properties and behavior of these laboratory water-ice grains may provide insights into morphology and alignment behavior of water-ice grains in astrophysical dusty plasmas.
© 2015 American Astronomical Society. Received 2014 November 17; accepted 2015 February 6; published 2015 March 30. This material is based upon work supported by the US Department of Energy Office of Science under Award Number DE-SC0010471. We thank J. Goree for graciously loaning the long distance microscope lens used in this work. We also thank S. Shimizu for kindly providing information on the design of his experimental apparatus. The Eblana laser was purchased using the Caltech Engineering and Applied Science Discovery Fund.
Published - 0004-637X_802_2_112.pdf