Yasuoka Laboratory



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講師Dr. Brian Barnes (Postdoctoral researcher, Center for Hydrate Research, Colorado School of Mines)
題目Simulation and Analysis of Clathrate Hydrate Nucleation
日時2013年2月28日(木) 11:00〜12:00
概要Despite their significance and ubiquity, the formation mechanism of hydrates remains poorly understood. The challenges associated with describing hydrate nucleation and growth mechanisms remain at the forefront of fields such as hydrogen storage, carbon dioxide sequestration, and hydrocarbon transportation and recovery. Molecular simulation of clathrate hydrate has provided significant advancements in our understanding of hydrate properties and formation. High-performance computing (HPC) advances have allowed the study via microseconds of atomic-scale molecular dynamics trajectories of clathrate hydrate nucleation. The simulations reveal the evolution of clathrate structures from initially phase-separated systems. Seemingly effortless and concerted ordering of the guest and water molecules marks the transition from disordering to ordering characteristic of clathrate hydrates. The topic of this talk is the quantification of the nucleation process with the goal of extracting mechanistic details to define the nucleation associated with specific molecular events that precede nucleation, reach a critical size, and result in growth. We will discuss what is known about clathrate hydrate nucleation and the tools we are using to quantify the nucleation process. Computational geometry concepts such as Voronoi tessellation and convex hulls are utilized to characterize the structuring of water and guest molecules forming hydrates, and gas-liquid interface structure. Applying tessellation to simulation trajectories of hydrate formation, and using a further selection criteria based on polyhedra volume and coordination number, we identify numbers and types of “cage-like” polyhedra. Voronoi tessellation is also shown to have utility in identifying an interface, and convex hull analysis is used to identify long-lived clusters contributing to nucleation. We will also present results using a "building block" order parameter analysis of hydrate trajectories. This order parameter is used to identify the onset of nucleation and estimate a critical nucleus size. Identification and use of these order parameters is key in the quantification of clathrate hydrate nucleation, providing the basic framework to further study heterogeneous nucleation, additional clathrate structures, and biased free energy calculations.