Droplet-based millifluidics as a new tool to investigate hydrate crystallization: Insights into the memory effect

CO2 hydrate nucleation study: novel high-pressure microfluidic devices

This study presents the development and application of a novel high-pressure microfluidic system for investigating CO2 hydrate nucleation and growth, with applications for carbon capture and storage (CCS) technologies. Two distinct microchip geometries-a capillary channel chip (serpentine-shaped) and an advanced droplet trap chip- were respectively designed and evaluated. These microchips enable the generation, trapping, and observation of CO2 droplets or bubbles within aqueous systems under static and dynamic conditions. The capillary channel chip allows droplet storage in a single serpentine channel, whereas the droplet trap chip offers superior immobilization and control, preventing droplet/bubble displacement during CO2 hydrate formation. High-resolution optical imaging, coupled with precise pressure and temperature regulation and control, facilitated real-time visualization of CO2 hydrate crystallization at CO2-water interfaces under varying temperature and pressure conditions. Experimental results reveal the influence of geometry, flow dynamics, and hydrodynamics on hydrate morphology and growth. The high-pressure microfluidic setup provides an adaptable and scalable approach for studying hydrate behavior, offering valuable insights for investigating CO2 storage in geological formations.

Lab on a chip for a low-carbon future

Transitioning our society to a sustainable future, with low or net-zero carbon emissions to the atmosphere, will require a wide-spread transformation of energy and environmental technologies. In this perspective article, we describe how lab-on-a-chip (LoC) systems can help address this challenge by providing insight into the fundamental physical and geochemical processes underlying new technologies critical to this transition, and developing the new processes and materials required. We focus on six areas: (I) subsurface carbon sequestration, (II) subsurface hydrogen storage, (III) geothermal energy extraction, (IV) bioenergy, (V) recovering critical materials, and (VI) water filtration and remediation. We hope to engage the LoC community in the many opportunities within the transition ahead, and highlight the potential of LoC approaches to the broader community of researchers, industry experts, and policy makers working toward a low-carbon future.

Crystallization Behavior of the Hydrogen Sulfide Hydrate Formed in Microcapillaries

There are no reports on the hydrogen sulfide hydrate growth process and morphology in micropores due to the toxicity of hydrogen sulfide. In this study, the experimental measurements and dissociation enthalpies were provided to assess the effect of the microcapillary silica tube size on hydrogen sulfide hydrate dissociation conditions. To simulate micropore sediments, the H₂S hydrate growth processes and morphologies at different supercooling temperatures were observed in this study. The dissociation temperature depression of the hydrate crystal in the microcapillary was less than 0.001 °C, which shows that the stability of the hydrate is less affected by the microcapillary pore used in this study. The mass transfer from the gas phase to the liquid phase is easily blocked when the hydrogen sulfide hydrate shell covers the gas-water meniscus, causing the growth of the gas hydrate to be inhibited. The hydrate crystal morphology can be divided into fibrous, needle-like crystals and dendritic crystals when ΔT _sub > 12.7; the hydrate crystal morphology can be categorized as dendritic crystals and columnar crystals when ΔT _sub = 7.9-8.9, and the hydrate crystals can form polyhedral crystals when ΔT _sub = 7.9-8.9. Additionally, a new "bridging effect" that a hollow crystal which was filled with the gas phase can connect with two separated gas phases was found at low supercooling temperature.

Contactless probing of polycrystalline methane hydrate at pore scale suggests weaker tensile properties than thought

Methane hydrate is widely distributed in the pores of marine sediments or permafrost soils, contributing to their mechanical properties. Yet the tensile properties of the hydrate at pore scales remain almost completely unknown, notably the influence of grain size on its own cohesion. Here we grow thin films of the hydrate in glass capillaries. Using a novel, contactless thermal method to apply stress, and video microscopy to observe the strain, we estimate the tensile elastic modulus and strength. Ductile and brittle characteristics are both found, dependent on sample thickness and texture, which are controlled by supercooling with respect to the dissociation temperature and by ageing. Relating the data to the literature suggests the cohesive strength of methane hydrate was so far significantly overestimated.

The authors here report tensile properties of polycrystalline methane hydrate at the micron scale by applying a contactless, thermos-induced stress to a tenuous shell of hydrate grown in a thin glass capillary. The results suggest that the cohesive strength of methane hydrate in marine settings may be an order of magnitude less than currently thought.

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

We designed and implemented an experimental methodology to investigate gas hydrate formation and growth around a water-guest meniscus in a thin glass capillary, thus mimicking pore-scale processes in sediments. The glass capillary acts as a high-pressure optical cell in a range of supercooling conditions from 0.1 °C, i.e., very close to hydrate dissociation conditions, to ∼35 °C, very near the metastability limit. Liquid or gaseous CO₂ is the guest phase in most of the experiments reported in this paper, and N₂ in a few of them. The setup affords detailed microscopic observation of the roles of the key parameters on hydrate growth and interaction with the substrate: supercooling and substrate wettability. At low supercooling (less than 0.5 °C), a novel hydrate growth process is discovered, which consists of a hollow crystal originating from the meniscus and advancing on the guest side along the glass, fed by a thick water layer sandwiched between the glass and this crystal.

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

The formation, dissociation, and reformation of cyclopentane (CP) hydrate in a sub-millimeter-sized capillary were conducted in this work, and the morphology of CP hydrate was obtained during above processes, respectively. The influences of the supercooling degree, i.e., the hydrate formation driving force, on CP hydrate crystals’ aspect and growth rate were also investigated. The results demonstrate that CP forms hydrate with the water melting from ice at the interface between the CP and melting water at a temperature slightly above 273.15 K. With the action of hydrate memory effect, the CP hydrate in the capillary starts forming at the CP-water interface or CP–water–capillary three-phase junction and grows around the CP–water interface. The appearance and growth rate of CP hydrate are greatly influenced by the supercooling degree. It indicates that CP hydrate has a high aggregation degree and good regularity at a high supercooling degree (or a low formation temperature). The growth rate of CP hydrate crystals greatly increases with the supercooling degree. Consequently, the temperature has a significant influence on the formation of CP hydrate in the capillary. That means the features of CP hydrate crystals in a quiescent system could be determined and controlled by the temperature setting.

Microfluidic approaches for accessing thermophysical properties of fluid systems

Thermophysical properties of fluid systems under high pressure and high temperature conditions are highly desirable as they are used in many industrial processes both from a chemical engineering point of view and to push forward the development of modeling approaches.

Memory effect in tetra-n-butyl ammonium bromide semiclathrate hydrate reformation: the existence of solution structures after hydrate decomposition

The memory effect in TBAB semiclathrate hydrate reformation results from the residual solution structure composed of clusters and cluster aggregates.

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

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

Droplet coalescence at microchannel intersection chambers with different shapes

The influence of microchannel intersection chamber shape on the droplet coalescence process is investigated in this study. Three kinds of chamber shapes (half-round, triangle and camber) are designed to realize head-on droplet coalescence. The coalescence processes are visualized using a high-speed camera system and the internal flow patterns are resolved using a micro-PIV system. Experimental analyses on the coalescence position and coalescence time of droplets and the critical conditions are discussed. Both direct coalescence and late coalescence can be observed in the camber junction while only the late coalescence is present for the half-round and the triangle junction. The critical capillary number Ca* varies for different working systems or intersection shapes. Ca* in the camber junction is larger than that in the other two junctions for each working system and it decreases with the increase of the viscosity ratio for each intersection shape. Moreover, the characteristics of the velocity fields for different coalescence cases are analyzed for an in-depth understanding of the process.