Roles of Wettability and Supercooling in the Spreading of Cyclopentane Hydrate over a Substrate.

Abstract

We use transmission optical microscopy to observe cyclopentane hydrate growth in sub-mm, open glass capillaries, mimicking cylindrical pores. The capillary is initially loaded with water and the guest fluid (cyclopentane) and thus possesses three menisci, that between water and cyclopentane (CP) in the middle and two menisci with the vapors at the ends. At temperatures T below the equilibrium temperature Teq ≈ 7 °C, the hydrate nucleates on the water-CP meniscus, rapidly coating it with an immobile, polycrystalline crust. Continued movement of the other two menisci provides insights into hydrate growth mechanisms, via the consumption and displacement of the fluids. On water-wet glass, the subsequent growth consists of a hydrate "halo" creeping with an underlying water layer on the glass on the CP side of the meniscus. Symmetrically, on CP-wet glass (silane-treated), a halo and a CP layer grow on the water side of the interface. No halo is observed on intermediate wet glass. The halo consists of an array of large monocrystals, over a thick water layer at low supercooling (ΔT = Teq - T below 5 K), and a finer, polycrystalline texture over a thinner water layer at higher ΔT. Furthermore, the velocity varies as ΔTα, with α ≈ 2.7, making the early stages of growth very similar to gas hydrate crusts growing over water-guest interfaces. Beyond a length in the millimeter range, the halo and its water layer abruptly decelerate and thin down to submicron thickness. The halo passes through the meniscus with the vapor without slowing down or change of texture. A model of the mass balance of the fluids helps rationalize all of these observations.