12/7/2023 0 Comments Water viscosity versus temperature![]() The particle-particle particle-mesh algorithm is adopted to handle the long-range coulomb interactions. The cutoff distances for the LJ interactions and the electronic interactions are 10 and 12 Å, respectively. The periodic boundary condition is applied to all the three directions of the three-dimensional simulation system. The interactions between the carbon atoms and the oxygen atoms of the water molecules are calculated by the Lennard-Jones (LJ) potential with the main parameters σ CO = 3.28218 Å and ε CO = 0.11831 kcal mol -1. The water is simulated by the TIP4P-EW model, in which the SHAKE algorithm is used to constrain the bond length and angle of the water molecules. To save the computational cost, the carbon atoms of the SWCNTs and the graphite sheets are fixed. In this article, an open-source code Lammps is employed to conduct the MD simulations. The correlation coefficient between the viscosity calculated by the "Eyring-MD" method and that obtained from the numerical experiments (Stokes-Einstein relation) is about 0.99. Thus, by using Equations 1, 2, and 3, the viscosity of water can be predicted by the MD simulations. In which U coul and U van are the coulomb energy and the van der Waals energy extracted from the MD simulations. Here, the size effect on the viscosity of the confined water implies the influence of the diameter of SWCNTs. The objective of this study is to examine the size and the temperature effects on the water viscosity. In this article, we redetermine the coefficients in the "Eyring-MD" method through more numerical experiments and evaluate the viscosity of water inside SWCNTs at 298, 325, and 350 K. Recently, an "Eyring-MD" method was proposed to calculate the viscosity of water by using the MD simulations. This restricts the application of the classical continuum theory to the nanoflows. So far, the viscosity of fluids in nanoconfinement on a scale comparable to the molecular diameter is seldom explored owing to the extremely small scale on which the transport properties are difficult to be captured by experiments and the intrinsic limitations of the existing computational methods in the MD simulations. The previous results have identified that the water viscosity relies on the temperature and the characteristic length of the nanochannel. In classical continuum theory, the viscosity is an essential transport property and thereby has been extensively measured and computed. Hence, many researches focused on the unique feature of the confined fluid and its relationship with the continuum fluid. The previous studies have revealed that the flow behavior of water at the nanoscale strongly depends on the characteristic length of nanochannel, which implies that the classical continuum theory for the macroscopic fluid may be no longer applicable for the fluid confined in nanochannels. It is a significant topic for studying and designing the nanodevices such as the nanochannel for drug delivery and the membrane for water desalination. Water conduction through single-walled carbon nanotubes (SWCNTs) has been paid much attention in recent years. The present results should be instructive for understanding the coupling effect of the size and the temperature at the nanoscale. ![]() The results of the relative amount of the hydrogen bonds exhibit similar profiles with the curves of the relative viscosity. To demonstrate the rationality of the calculated relative viscosity, the relative amount of the hydrogen bonds of water confined in SWCNTs is also computed. Based on the computational results, a fitting formula is proposed to calculate the size- and temperature- dependent water viscosity, which is useful for the computation on the nanoflow. ![]() The results suggest that the relative viscosity of the confined water increases with increasing diameter and temperature, whereas the size-dependent trend of the relative viscosity is almost independent of the temperature. The influences of the diameter (size) of single-walled carbon nanotubes (SWCNTs) and the temperature on the viscosity of water confined in SWCNTs are investigated by an "Eyring-MD" (molecular dynamics) method.
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