Nanoconfined fluids exhibit specific characteristics different from bulk fluids that make them attractive for numerous applications in various fields. With the rapid development of advanced nanoscience and nanotechnology, significant improvement has been achieved in the study of nanoconfined fluids over recent decades. One key area of concern has been the transport properties of nanoconfined fluids, which has enabled in-depth scientific research on their flow dynamics and heat transfer. Nevertheless, despite extensive research, the underlying mechanism explaining the difference in the physical behavior between the bulk fluids and nanoconfined fluids has not been fully clarified. In recent research, nanoconfined water in graphene channels was reported to exhibit distinct characteristics desirable for water purification applications. This revelation opened doors for intensive studies on the transport properties of water confined in graphene nanochannels.
To this account, Professor Chengzhen Sun and Runfeng Zhou (PhD candidate) from Xi’an Jiaotong University together with Dr. Zhixiang Zhao from Xi’an Polytechnic University investigated the nanoscale effect of the thermal conductivity of water confined in graphene nanochannels based on equilibrium molecular dynamic (EMD) simulations with employing Greek-Kubo formula. Also, they studied and compared the thermal conductivities in the x-, y-, and z-directions. The main aim was to illustrate the anisotropic and size-dependent transport properties of nanoconfined water in graphene channels from the physical insights. Their research work is currently published in the research journal, International Journal of Heat and Mass Transfer.
According to the results, the thermal conductivity of nanoconfined water exhibited obvious anisotropy; that is, the perpendicular thermal conductivity (thermal conductivity in the z-direction) was lower than the longitudinal thermal conductivity (average thermal conductivity in the x- and y-directions). This was attributed to the partial trapping of the water molecules in the potential wells near the graphene walls, which inhibited the molecular collision in the z-direction while enhancing it in the x- and y-directions. Consequently, channel height exhibited remarkable effects on thermal conductivity. For instance, the perpendicular thermal conductivity decreased with an increase in the channel height while the perpendicular conductivity increased with an increase in the channel height. Furthermore, increasing the channel height resulted in an overall weakening of the contributions of the trapped water molecules in both the perpendicular and longitudinal directions, such that the thermal conductivity in the x-, y- and z-directions approach the bulk values.
In summary, the study investigated the thermal conductivity of confined-water in graphene nanochannels using EMD simulations with the Green-Kubo formula. Based on the analysis of the size-dependence of thermal conductivity, the authors reported that the thermal conductivity in the nanoconfined water is rather anisotropic. Parameters such as the channel height exhibited a remarkable influence on thermal conductivities in both x-, y- and z-directions. Overall, the study successfully identified the anisotropy and size dependence of thermal conductivity of confined water in graphene nanochannels and also clarified the underlying mechanism from the thermodynamics perspective. In a statement to Advances in Engineering, the authors said their findings would advance further research on the mass and energy transport of nanoconfined fluids that would benefit its various applications.
Zhao, Z., Sun, C., & Zhou, R. (2020). Thermal conductivity of confined-water in graphene nanochannels. International Journal of Heat and Mass Transfer, 152, 119502.