Session LG: Nano-Fluids I

Three-phase contact line at small scale

We investigate the area around an equilibrium three-phase contact line at a small scale by using a density functional approach. A typical system is made of a planar wall in contact with a Lennard-Jones gas below the critical temperature. The wall exerts an attractive force on the fluid molecules so that a thin film can usually form between the wall and the gas. We focus on two cases. When the chemical potential is smaller than its coexistence value and the system presents a phase transition with respect to the film thickness, we examine the area between the two equilibrium film thicknesses. It appears to be smooth and several molecular diameters long. When the chemical potential is at its coexistence value, computations of the equilibrium density profiles show a well formed contact angle whose value follows closely the Young equation.   

Mechanical role of phospolipid bilayers in synovial joint lubrication

Cartilaginous surfaces in synovial joints have been shown to be coated by multilamellar water and phospholipid bilayer arrangements. The interactions between the bilayers and synovial gel composed of hyaluronic acid (HA) and albumin forms the basis of the SAPL (surface active phospholipids) theory to explain joint lubrication. While considerable efforts have been devoted to the rheological properties of synovial gel, the ability of SAPL to sustain significant mechanical stress has received little attention. Using Gromacs molecular dynamics (MD) solver, we have characterized the response to compression and shear of multilamellar arrangements composed of water and dipalmitoylphosphatidylcholine (DPPC, a phospholipid in highest concentration in synovial fluid). MD simulations were performed with and without HA to isolate the dynamics that allow the multilamellar arrangement to retain its anisotropic structure beyond the normal gait pressure of 1.5 MPa. The critical normal compression that provokes the rupture of membranes and drainage of the system is a strong function of the thickness of the water layer and may exceed 15MPa due to complex underlying membrane dynamics.   

Understanding the structural properties of clusters in sheared aggregating systems using Brownian dynamics simulation

Nanoparticle synthesis in turbulent reactors subjects anoparticle aggregates to a homogeneous, time-varying shear flow. The shear flow results in anisotropic clusters and it is of interest to characterize the structural properties of these clusters and their effects on initiation and acceleration of aggregation, the restructuring of clusters, and their breakage. The anisotropic structure of a sheared cluster is characterized by the ratio of the major to minor axis length of the approximating ellipsoid oriented along the cluster moment of inertia tensor's principal axes. Brownian dynamics simulations show that shear flow dramatically changes the structure of aggregates by initiating the formation of more compact structures at smaller length scales perpendicular to the shear direction, and anisotropic, cigar--like structures along the shear direction. More compact clusters correspond to higher local volumetric potential energy density. Therefore, we classify the compactness and anisotropy of sheared clusters on a map of local volumetric potential energy density versus ratio of the principal values of the cluster's moment of inertia tensor. The effect of shear on breakage of clusters is characterized by the radius of gyration $R_g^{cr}$ of the largest stable aggregate for a given value of the imposed steady shear rate (P\'{e}clet number).   

Nanodroplet impact onto solid platinum surface: Spreading and bouncing

The impact of droplets onto solid surfaces is found in a huge variety of natural and technological applications, from rain drops splashing on the pavement, to material manufacturing by molten droplet deposition. Taking inspiration from existing microfluidic technologies (i.e. lab-on-chip), there is increasing interest in the use of nanodroplets (D $<$ 100 nm) for a number of applications such as drug delivery and semiconductor device manufacturing. However, as the size of the droplet is reduced into the nanoscale, the direct use of previously obtained macroscopic results is not guaranteed. At the nanoscale, important effects due to the molecular nature of the fluid, thermal fluctuations and reduced dimensionality can play a critical role in determining system dynamics. In this paper we present the results of large-scale, fully atomistic, three-dimensional molecular dynamics (MD) simulation of an argon nanodroplet (D = 18 nm, 54 000 atoms) impact onto a solid platinum surface, using the LAMMPS software package. The fluid argon is modeled using the well-known Lennard-Jones (LJ) potential, while the embedded-atom model (EAM) potential is used for the solid platinum. By varying both the impact velocities (10-1000 m/s) and the wettability of the solid surface a wide range of impact behaviors is observed, from smooth spreading, to bouncing recoil, pointing towards a wide array of potential applications.   

Properties of Nanofluids

Equilibrium molecular dynamic simulations are presented for different configurations of interactions between gold metal and liquid water. It makes use of state-of-the-art potentials to capture a broad spectrum of realistic physical phenomena at the interface. Thermodynamic properties, such as internal energy, heat capacities, isothermal compressibility, and coefficient of thermal expansion of the nanofluid are currently being analyzed. Transport properties, such as mass diffusion, viscosity and thermal conductivity, are also under investigation. Some of the results obtained thus far, seems to strongly diverge from the prediction of ideal mixture theories. The understanding of basic thermodynamic and transport effects in nanofluids is a stepping stone to further studies.   

Viscous Heating in Nanoscale Shear Driven Flows

Three-dimensional Molecular Dynamics (MD) simulations of heat and momentum transport in liquid Argon filled shear-driven nano-channels are performed using 6-12 Lennard-Jones potential interactions. Work done by the viscous stresses heats the fluid, which is dissipated through the channel walls, maintained at isothermal conditions via a recently developed interactive thermal wall model. Momentum transport in shear driven nano-flow is investigated as a function of the surface wettability ($\varepsilon _{wf} /\varepsilon )$, spatial variations in the fluid density, kinematic viscosity, shear- and energy dissipation rates are presented. Temperature profiles in the nano-channel are obtained as a function of the surface wettability, shear rate and the intermolecular stiffness of wall molecules. The energy dissipation rate is almost a constant for $\varepsilon _{wf} /\varepsilon <0.6$, which results in parabolic temperature profiles in the domain with temperature jumps due to the well known Kapitza resistance at the liquid/solid interfaces. Using the energy dissipation rates predicted by MD simulations and the continuum energy equation subjected to the temperature jump boundary conditions developed in [Kim et al., \textit{Journal of Chemical Physics}, 129, 174701, 2008], we obtain analytical solutions for the temperature profiles, which agree well with the MD results.   

Understanding the nanoscale liquid-solid interfacial phenomena in a Couette flow

Molecular dynamics simulations are used to study a nanoscale Couette flow and investigate the slip behavior at liquid-solid interfaces. We model liquid argon confined between two smooth rigid copper walls and the upper wall velocity is imposed to shear the fluid slab. The velocity fields, density distributions, flow boundary conditions and liquid structures/orderings are studied for walls ranging from hydrophobic to hydrophilic. We observe various flow boundary conditions ranging from pure slip to multi-layer locking in the simulations. The results show that, temperature, liquid-solid interaction parameter and shear rate are the major factor influencing the fluid structures at liquid-solid interfaces, thus determine the flow boundary condition. Different liquid states are named which are characterized by different liquid structures. Continuous transitions between states are also found. Furthermore, the positive correlation between shear rate and slip length is established and the temperature shift of the shear rate-slip length curve is observed. We confirm the unbounded slip length at high shear rates and correlates it to momentum transfer mechanism between liquid and solid atoms. In transient simulations, liquid may transit from several metastable states to a ground state under certain conditions.   

Electrokinetic transport in realistic nanochannels

When an electrolyte solution contacts with a solid surface, the surface will likely be charged through an electrochemical adsorption process. The surface charge in general varies with the local bulk ionic concentration, the pH value and the temperature of the solution, and even with the double layer interactions in the narrow channel. Most of the previous studies are based on a constant zeta potential or surface charge density assumption, which does not reflect the realistic charge status at interfaces and may lead to inaccurate predictions. In this work, we first develop a generalized model for electrochemical boundary conditions on solid-liquid interfaces, which can closely approximate the known experimental properties. We further present nonequilibrium molecular dynamic (NEMD) simulations of electrokinetic transport in nanochannels. We take silica and carbon as examples of channel materials. Both monovalent and multivalent ionic solutions are considered. The electrokinetic transport properties for realistic nanochannels are therefore studied and a multiscale analysis for a new energy conversion device is performed.   

Molecular Dynamics Simulation of Shock Waves Interacting with Nano-structures

Typical theoretical treatments of shock wave interactions are based on a continuum approach, which cannot resolve the spatial variations in solids with nano-scale porous structure.~ To investigate such interactions we have developed a molecular dynamics simulation model, based on~ Lennard-Jones interactions. A piston, modeled as a uni-directional repulsive force field translating at a prescribed velocity, impinges on a region of gas which is compressed to form a shock, which in turn is driven against an atomistic solid wall. Periodic boundary conditions are used in the directions orthogonal to the piston motion, and we have considered solids based on either atoms tethered to lattice sites by stiff springs, or on embedded atom potentials.~Velocity, temperature and stress fields are computed locally in both gas and solid regions, and displacements within the solid are interpreted in terms of its elastic constants.~ In this talk we present preliminary results, and the longer-term goal of this work is to understand energy transport and absorption in porous materials.