# Nanoscale Transport Involving Dense Fluids

**Introduction**

Due to the small spacing between atoms in a dense fluid, departures from Navier-Stokes behavior are observed at much smaller lengthscales in dense fluids compared to dilute systems of carriers (e.g. air at STP, phonon gases).

Our general objective is to provide fundamental understanding of transport at lengthscales that are sufficiently small for the Navier-Stokes description to be no longer valid. Of particular interest is transport in fluids under nanoconfinement as, for example, in small-scale lubrication flows and flow through nanoporous materials.

Using Molecular Dynamics Simulations and Molecular Mechanics arguments coupled to Mean Field theory, we have shown that standoff distances between a solid boundary and the first layer of fluid adjacent to the solid can be calculated in closed form for simple fluids [1]. The same work has shown that this standoff distance can be used to explain the lower density of fluids in nanotubes.

We are currently working on extending our understanding of the effect of fluid layering under nanoconfinement on transport (slip/ temperature jump at the fluid-solid interface). We have recently used MD simulations and Molecular Mechanics arguments, as well asymptotic analysts of the Nernst-Planck equation to study layering at the fluid-solid interface and in particular the density of the first fluid layer (closest to the solid) [2]. We have shown that the degree of layering is controlled by the Wall number which compares the interaction energy between the fluid and the wall with the fluid thermal energy. We also developed a scaling relation for predicting the density of the first fluid layer as a function of the system temperature and bulk density.

**Related Publications:**

1. Wang, G.J. and Hadjiconstantinou, N.G., Why are Fluid Densities so Low in Carbon Nanotubes?, *Physics of Fluids*, **27**, 052006, 2015. (
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2. Wang, G.J. and Hadjiconstantinou, N.G., Molecular Mechanics and Structure of the Fluid-Solid Interface in Simple Fluids, *Physical Review Fluids*, **2**, 094201, 2017.
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