Session J14: Nano Flows: Basic Physics (8:00am - 8:45am CST)

A Slippery Business: Explaining the Wide Range of Water Slip Lengths on Graphene using Molecular-Kinetic Theory

Since the early 2000s, the anomalous transport properties of water confined within carbon nanomaterial environments have attracted an immense amount of attention; the slip length of water has been one topic of particular interest. Accurate modeling of slip phenomena is critical for the design of a wide range of nanofluidic technologies. A scan through the molecular-dynamics (MD) literature reveals that reported slip lengths vary well over a factor of three. This raises the key question: Why do MD simulations yield such an enormous range of water slip lengths? In this work, we present a parsimonious answer to this question, based upon our recently developed molecular-kinetic theory (MKT) for slip. MKT provides an explicit expression that relates the magnitudes of slip phenomena to microscopic parameters describing the fluid-solid interface, along with fluid and solid material properties. We demonstrate that a significant amount of the variation in slip lengths reported in the literature is explainable within the framework of MKT, and can be directly attributed to different models used to simulate water. We briefly comment on a vision for canonical systems and protocol to measure hydrodynamic slip in MD simulations that would enable fair comparison of results.   

Substrates sort solute from solvent molecules

The dynamics of a monolayer of Lennard-Jones atoms driven by an external force over an atomically-spaced solid substrate is presented. As the magnitude of the external force is increased, two bifurcations are observed, from no-slip to defect slip and then to global slip. In the defect slip regime, while the majority of atoms remain stationary in their equilibrium positions, a small percentage of atoms propagate as nonlinear waves over the substrate. In defect slip regime, atoms do not follow the direction of the external driving force.In a solution, solvent and solute atoms follow different directions from one another, resulting in the solute atoms partitioning from the solvent. Using the monolayer dynamics, we aim to understand the physics of the liquid-solid interface and apply this knowledge for molecular-level separation mechanisms mediated by solid substrates.   

Counterintuitive flow from low-pressure bulk to high-pressure nanopore

In a typical pipe or tube, liquid always flows from high pressure to low pressure. Here, we present a counterintuitive phenomenon observed in molecular dynamics (MD) simulations: a passive evaporation-induced liquid flow from low-pressure bulk to high-pressure nanopore. In our simulations, the nanopore is formed between two parallel hydrophilic surfaces, and is connected to a conventional bulk liquid. At equilibrium (without any evaporation/flow), the pore pressure is an order magnitude high than that in bulk due to the strong solid-liquid interaction. Evaporation was achieved by removing atoms at the liquid-vapor interface, while the same amount of atoms were added in the bulk. A steady-evaporation state was reached associated with a constant liquid flow from bulk to nanopore. The bulk pressure maintained similar to that at equilibrium; while the pore pressure was reduced by around 25 atm, but still an order higher than that in bulk. This reduced pressure, from thermodynamically equilibrium state, drives liquid flow from low-pressure bulk to high-pressure nanopore. The work presented here has been published in J. of Phys. Chem. Lett., 11: 3637-3641 (2020).   

Viscous peeling of a graphene sheet

To get insights into the process of liquid-phase exfoliation of graphite into graphene, we study numerically the dynamics of propagation of a peeling front in a system composed of two adhered elastic sheets immersed in a liquid. The crack propagation is induced by lifting one of the edges with an assigned velocity $v$. A continuum model based on the lubrication theory is compared to non-equilibrium molecular dynamics (MD) simulations of a graphene-water system. We quantify the external peeling force by separating it into viscous and adhesive contributions. The continuum model predicts that for a sheet of length 100nm and a solvent of viscosity 10$^{\mathrm{-3}}$ Pa$.$s, the viscous contribution to the force is important after a threshold of pulling velocity $v $\textgreater 10m/s, a velocity relevant to MD simulations. We explore the effect of the fluid viscosity and slip length on this threshold and discuss several limitations of the continuum model. For example, while MD agrees with the continuum model at low peeling velocities, at higher velocities the viscous-dependent contribution to the pulling force predicted by MD is much higher than the one predicted by the continuum model. We will discuss possible causes for this discrepancy.