No abstract

AbstractMolecular dynamics (MD) analysis of methane hydrate is important for the application of methane hydrate technology. This study investigated the microstructure changes of sI methane hydrate and the laws of stress–strain evolution under the condition of compression and tension by using MD simulation. This study further explored the mechanical property and stability of sI methane hydrate under different stress states. Results showed that tensile and compressive failures produced an obvious size effect under a certain condition. At low temperature and high pressure, most of the clathrate hydrate maintained a stable structure in the tensile fracture process, during which only a small amount of unstable methane broke the structure, thereby, presenting a free-motion state. The methane hydrate cracked when the system reached the maximum stress in the loading process, in which the maximum compressive stress is larger than the tensile stress under the same experimental condition. This study provides a basis for understanding the microscopic stress characteristics of methane hydrate.

The study of changes in the related mechanical property and microscopic structure of methane hydrate during the decomposition process are of vital significance to its exploitation and comprehensive utilization. This paper had employed the molecular dynamics (MD) method to investigate the influence of defects on the microscopic structure and mechanical property of the sI methane hydrate system, and to discover the mechanical property for the defect-containing hydrate system to maintain its brittle materials. Moreover, the stress-strain curve of each system was analyzed, and it was discovered that the presence of certain defects in the methane hydrate could promote its mechanical property; however, the system mechanical property would be reduced when the defects had reached a certain degree (particle deletion rate of 9.02% in this study). Besides, the microscopic structures of the sI methane hydrate before and after failure were analyzed using the F3 order parameter value method, and it was found that the F3 order parameters near the crack would be subject to great fluctuations at the time of failure of the hydrate structure. The phenomenon and conclusions drawn in this study provide a basis for the study of the microscopic structure and mechanical characteristics of methane hydrate.

The interfacial properties of hydrate and its ambient play an important role in hydrate technique. In this paper, the molecular characteristics of H2O in hydrate/ice/liquid water mixture system are investigated based on molecular dynamics (MD) simulations. The structure I (sI) methane hydrate is partially heated to obtain the studied system. The properties including hydrogen bond, radial distribution function (RDF) and F3 order parameter (tetrahedral coordinated parameter of H2O) indicate that there is little difference of water structure in the hydrate region and ice/liquid water mixture region. The F4 order parameter (parameter based on H–O–O–H torsion angles of H2O) could be used to distinguish the different region. The value of F4 experiences the continuous change at interface between mixture region and hydrate region.

Development of disjoining pressure was performed to study the symmetric, Lennard-Jones (LJ) free thin films using molecular modelling. A methodology rooted from film thermodynamics was established to derive the disjoining pressure isotherm [Formula: see text], which is based on the surface tension at varied film thicknesses and can be viewed as a post-processing technique. The results showed that the disjoining pressure of LJ fluid is purely attractive. Compared with the complicated method reported previously, this methodology is demonstrated to be more convenient and readily applicable for other liquid films (e.g. water, aqueous thin films containing electrolyte or surfactants), meanwhile it can be applied at both low and high temperatures.