How Do Gas Hydrates Spread on a Substrate?

Growth of gas hydrates as fast-growing polycrystalline crusts at interfaces between water and guest phases is well documented, but the mechanisms of hydrate growth on solid substrates are much less known. We report here on cyclopentane (CP) hydrate spreading on glass (fused silica) under CP. As seen for methane hydrate by Beltran and Servio (Cryst. Growth Des. 2010, 10, 4339-4347), CP hydrate grows on glass as a halo radiating from the contact line of a primary drop. Complementary optical microscopies at micron resolution here allow identification of the mechanisms of halo growth and melting. We conclude that forms of water on the substrate control halo spreading, namely, a precursor film near the contact line and a breath figure (dew) condensed from the CP (halo spreading at <= 2 mu m s(-1) at T approximate to 0 degrees C or subcooling similar to 7 degrees C), and leap-frogging (at similar to 10 mu m s(-1)) over secondary drops left behind by melting a previous halo. Halo thickening, about 5 nm s(-1), is attributed to water condensation, either incorporation of water dissolved in CP (like ablimation) or settling of water fog from the CP. Halos spread slower on untreated, compared to hydrophilic, glass, an effect attributed to the quantity of water present on the substrate; a similar trend is noted when the CP phase is not pre-equilibrated with water prior to the experiment. No hydrate halo was detected on hydrophobized (silane-treated) glass, where the breath figure is absent.