Surface morphology effects on clathrate hydrate wettability.

HYPOTHESIS: Clathrate hydrates preferentially form at interfaces; hence, wetting properties play an important role in their formation, growth, and agglomeration. Experimental evidence suggests that the hydrate preparation process can strongly affect contact angle measurements, leading to the different results reported in the literature. These differences hamper technological progress. We hypothesize that changes in hydrate surface morphologies are responsible for the wide variation of contact angles reported in the literature. EXPERIMENTS: Experimental testing of our hypothesis is problematic due to the preparation history of hydrates on their surface properties, and the difficulties in advanced surface characterization. Thus, we employ molecular dynamics simulations, which allow us to systematically change the interfacial features and the system composition. Implementing advanced algorithms, we quantify fundamental thermodynamic properties to validate our observations. FINDINGS: We achieve excellent agreement with experimental observations for both atomically smooth and rough hydrate surfaces. Our results suggest that contact line pinning forces, enhanced by surface heterogeneity, are accountable for altering water contact angles, thus explaining the differences among reported experimental data. Our analysis and molecular level insights help interpret adhesion force measurements and yield a better understanding of the agglomeration between hydrate particles, providing a microscopic tool for advancing flow assurance applications.

Water Wettability Coupled with Film Growth on Realistic Cyclopentane Hydrate Surfaces.

Although the wettability of hydrate surfaces and hydrate film growth are key to understanding hydrate agglomeration and pipeline plugging, a quantitative understanding of the coupled behavior between both phenomena is lacking. In situ measurements of wettability coupled with film growth were performed for cyclopentane hydrate surfaces in cyclopentane at atmospheric pressure and temperatures between 1.5-6.8 °C. Results were obtained as a function of annealing (conversion) time and subcooling. Hydrate surface wettability decreased as annealing time increased, while hydrate film growth rate was unaffected by annealing time at any subcooling. The results are interpreted as a manifestation of the hydrate surface porosity, which depends on annealing time and controls water spreading on the hydrate surface. The wettability generally decreased as the subcooling increased because higher subcooling yields rougher hydrate surfaces, making it harder for water to spread. However, this effect is balanced by hydrate growth rates, which increase with subcooling. Also affecting the results, surface heating from heat release (from exothermic crystallization) allows excess surface water to promote spreading. The hydrate film growth rate on water droplets increased with subcooling, as expected from a higher driving force. At any subcooling, the instantaneous hydrate growth rate decreased over time, likely from heat transfer limitations. A new phenomenon was observed, where the angle at the three-phase point increases from the initial contact angle upon hydrate film growth, named the crystallization angle. This is attributed to the water droplet trying to spread while the thin film is weak enough to be redirected. Once the hydrate film grows and forms a "wall" around the droplet, it cannot be moved, and further growth yields a crater on the droplet surface, attributed to water penetrating the hydrate surface pore structures. This fundamental behavior has many flow assurance implications since it affects the interactions between the agglomerating hydrate particles and water droplets.