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 T ≈ 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 = T - T below 5 K), and a finer, polycrystalline texture over a thinner water layer at higher ΔT. Furthermore, the velocity varies as ΔT, 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.

Gas Hydrate Crystallization in Thin Glass Capillaries: Roles of Supercooling and Wettability

“…15−18 This growth is mediated by a thin wetting water layer between the substrate and the crust, as hypothesized 1−2 decades ago by Tabe and co-workers 15 and Beltran and Servio 16 and observed recently in one of our groups with cyclopentane as the guest phase. 19 In that study, the cyclopentane hydrate crust formed and grown initially on the water-cyclopentane meniscus in a thin glass capillary was observed to continue its growth beyond the contact line, advancing over the glass wall on the cyclopentane side of the meniscus, supported by a thin water sleeve between the crust and glass. The morphology and growth rate of the crust advancing on glass (the "halo") were observed to be similar to those of "free" hydrate crusts, i.e., growing at the water−guest (cyclopentane) interface, at least when the wetting water layer was not too thin.…”

Morphology Investigation on Cyclopentane Hydrate Formation/Dissociation in a Sub-Millimeter-Sized Capillary

“…The knowledge of small-scale features of hydrates in porous media is relatively limited. In fact, the microscopic observation and study of hydrates enables researchers to study and understand the behavior, characteristics, and rules of hydrate formation/dissociation [25][26][27][28]. Consequently, we conducted the formation and dissociation of CP hydrate in a special porous medium, i.e., a transparent sub-millimeter-sized capillary, in this work and investigated the features of CP hydrate in a capillary accordingly.…”

Small But Powerful Optically: Glass Microcapillaries for Studying Complex Fluids or Biological Systems with Submicrolitre Samples under Harsh Conditions

“…This last feature can be exploited to very sensitively detect thin layers with thicknesses commensurate with  creeping on the inner wall initially facing a liquid with an index close to that of the walli.e., the inner wall is initially invisible. An example is that of a few-µm -thick polycrystalline crust of cyclopentane hydrate (with refractive index of about of 1.35) creeping over the glass, where it displaces the cyclopentane initially present [12,13]. This crust nucleates and grows on the meniscus between water and a guest phase (here, cyclopentane) when the thermodynamic conditions are met (a temperature below 7°C), and then continues its growth beyond the contact line on glass, on the guest side of the meniscus, fed by an equally tenuous water film between the hydrate and the substrate [12,13].…”

“…An example is that of a few-µm -thick polycrystalline crust of cyclopentane hydrate (with refractive index of about of 1.35) creeping over the glass, where it displaces the cyclopentane initially present [12,13]. This crust nucleates and grows on the meniscus between water and a guest phase (here, cyclopentane) when the thermodynamic conditions are met (a temperature below 7°C), and then continues its growth beyond the contact line on glass, on the guest side of the meniscus, fed by an equally tenuous water film between the hydrate and the substrate [12,13]. In the example shown in Figure 4b and 8a, the hydrate crust becomes so thin and smooth after traveling a few hundreds of µm from the meniscus that it is no longer visible in the transmission image, but still apparent from the bright reflection on the inner wall.…”