An empirical experimental observations and MD simulation data-based model for the material properties of confined fluids in nano/Angstrom size tubes

Abstract

Abstract The transport of fluids in nano/Angstrom-sized pores has gotten much attention because of its potential uses in nanotechnology, energy storage, and healthcare sectors. Understanding the distinct material properties of fluids in such close confinement is critical and dictate the fluid's behavior in determining flow dynamics, transport processes, and, ultimately, the performance of nanoscale devices. Remarkably, many researchers observed that the size of the geometry, such as confining nanotube diameter, exerts a profound and intriguing influence on the material properties, including on the critical parameters such as density, viscosity, and slip length. Many researchers tried to model viscosity $\eta$, density $\rho$, and slip $\lambda$ using various models with multiple dependencies on the tube-diameter. It is somewhat confusing and tough to decide which model is appropriate and can be incorporated in simulation. In this paper, we propose a simple single equation for each nanoconfined material property such as for density $\displaystyle \rho(D)/\rho_o = a+ b/(D-c)^n$, viscosity $\displaystyle \eta(D)/\eta_o = a+ b/(D-c)^n$, and the slip length $\lambda(D) = \lambda_1~D~e^{-n~D}+ \lambda_o$ (where $a,~b,~c,~n,~\lambda_1,~\lambda_o$ are the free fitting parameters). We model a wealth of previous experimental and MD simulation data from the literature using our proposed model for each material property of nanoconfined fluids. Our single proposed equation effectively captures and models all the data, even though many different models have been employed in literature to describe the same material property. Our proposed model exhibits exceptional agreement with multiple independent datasets from the experimental observations and MD simulations. Additionally, the model possesses continuity and continuous derivative, so that it is well-suited for simulations. The proposed models also obey the far boundary conditions, i.e., when tube-diameter $D \implies \infty$, the material properties approaches bulk properties of fluid. Because of models' simplicity, smooth, and generic nature, it holds promise to apply in simulations to design/optimize nanoscale-devices.