Relation Between Flow Enhancement Factor and Structure for Core-Softened Fluids Inside Nanotubes

Abstract

The relationship between enhancement flow and structure of core-softened fluids confined inside nanotubes has been studied using nonequilibrium molecular dynamics simulation. The fluid was modeled with different types of attractive and purely repulsive two length scale potentials. Such potentials reproduce in bulk the anomalous behavior observed for liquid water. The dual control volume grand canonical molecular dynamics method was employed to create a pressure gradient between two reservoirs connected by a nanotube. We show how the nanotube radius affects the flow enhancement factor for each one of the interaction potentials. The connection between structural and dynamical properties of the confined fluid is discussed, and we show how attractive and purely repulsive fluids exhibit distinct behaviors. A continuum to subcontinuum flow transition was found for small nanotube radius. The behavior obtained for the core-softened fluids is similar to what was recently observed in all-atom molecular dynamics simulations for classical models of water and also in experimental studies. Our results are explained in the framework of the two length scale potentials.

Effects of confinement on anomalies and phase transitions of core-softened fluids

“…[43][44][45][46] CS fluids confined in nanotubes also present interesting findings, similar to obtained in atomistic models for water, as the increase in diffusion coefficient and flux for narrow nanotubes associated to a layer to single-file transition and a discontinuity in the enhancement flow factor. [47][48][49] The drawback of these core-softened potentials is that due to the simplicity of the two length scales, they are not capable to reproduce the effects related to the third coordination shell of the anomalous fluid what might be relevant under confinement. 50 In addition to the relevance of the detail structure of the liquid, the structure of the confining system is also relevant since biological and physical materials do not exhibit the smoothness and regularity of the flat walls and tubes employed in the simulations.…”

Influence of disordered porous media on the anomalous properties of a simple water model

“…In order to answer these questions water-like atomistic or continuous effective potential models were explored. The confining geometries could be plates [27][28][29][30][31][32][33][34][35][36][37], one pore [38][39][40][41][42][43][44][45][46], and a disordered matrix [43,52,53,[55][56][57]. The results for the melting temperature obtained within these approaches are controversial, while results for SPC/E water show that the melting temperature for hydrophobic plates is lower than the melting for the unconfined system and higher than for the system confined by hydrophilic walls, for the mW model no difference between the melting temperatures due to the hydrophobicity [47] is found.…”

Enhanced flow of core-softened fluids through narrow nanotubes

“…41,42 Recently, it has been suggested that core-softened potential models are able to capture some of the anomalous behavior even under confinement. [43][44][45][46][47] In fact, the behavior of the diffusion constant for nanoconfined water, attributed to the hydrogen bonds, 48 was obtained by pure volumetric effects in these isotropic anomalous fluids. 46 Motivated by this proposition, in this paper we use NEMD simulation in order to explore the connection between the enhanced particle flow and the fluid-fluid interaction potential for fluids confined inside a nanopore.…”

“…46 Motivated by this proposition, in this paper we use NEMD simulation in order to explore the connection between the enhanced particle flow and the fluid-fluid interaction potential for fluids confined inside a nanopore. In order to avoid misleadings, we should address that in the literature, 25,27,47 it is usual to evaluate the flow enhancement factor, which can be defined as the ratio between the hydraulic conductivity obtained using molecular simulations and the hydraulic conductivity predicted by classical continuum models. In our paper, the enhanced flow is the anomalous increase of particle flow in narrow nanotubes, and will not be compared with the results of classical continuum theories.…”

“…The same NEMD model was used recently by us in the context of ionic channels 49 and core-softened fluids inside nanoporous. 47 We analyze only two length scales potential systems and therefore any comparison with the effects observed in alkanes where the fast flow is also observed should be taken with caution.…”