Abstract
Many properties of water, such as turbulent flow, are closely related to water clusters, whereas how water clusters form and transform in bulk water remains unclear. A hierarchical clustering method is introduced to search out water clusters in hydrogen bonded network based on modified Louvain algorithm of graph community. Hydrogen bonds, rings and fragments are considered as 1st-, 2nd-, and 3rd-level structures, respectively. The distribution, dynamics and structural characteristics of 4th- and 5th-level clusters undergoing non-shear- and shear-driven flow are also analyzed at various temperatures. At low temperatures, nearly 50% of water molecules are included in clusters. Over 60% of clusters remain unchanged between neighboring configurations. Obvious collective translational motion of clusters is observed. The topological difference for clusters is elucidated between the inner layer, which favors 6-membered rings, and the external surface layer, which contains more 5-membered rings. Temperature and shearing can not only accelerate the transformation or destruction of clusters at all levels but also change cluster structures. The assembly of large clusters can be used to discretize continuous liquid water to elucidate the properties of liquid water.
Highlights
Many properties of water, such as turbulent flow, are closely related to water clusters, whereas how water clusters form and transform in bulk water remains unclear
We aim to develop a graph-based hierarchical clustering method to provide a comprehensive overview of water clusters at all levels in liquid water undergoing both non-shear- and shear-driven flow
Molecular dynamics simulations are performed using periodic boundary conditions with 17,314 water molecules in a cubic box interacting through simple point charge (SPC/E) intermolecular potential[38]
Summary
Many properties of water, such as turbulent flow, are closely related to water clusters, whereas how water clusters form and transform in bulk water remains unclear. Water molecules are connected with each other by hydrogen bonds to form a three-dimensional network s tructure[4]. It is universally acknowledged that liquid water is considered as random three-dimensional hydrogen bond network continually undergoing topological reformation[12,13]. Water clusters are short-life and flickering which life spans are estimated from 10−10 to 10−11 s23,24 It remains a mystery how flickering water clusters form bulk water[25]. The characteristics of water clusters related to bulk motion can elucidate turbulent d iffusivity[26]. To form a bridge between macroscopic hydraulics and microscopic molecular dynamics, a bottom-up approach of searching out water clusters by molecular dynamics is necessary to explore detailed structure and dynamics of water clusters
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