ORCID Profile
0000-0001-8454-3341
Current Organisation
RMIT University
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Theoretical and Computational Chemistry | Statistical Mechanics in Chemistry | Transport Properties and Non-Equilibrium Processes | Organic Chemistry | Theoretical And Computational Chemistry Not Elsewhere Classified | Theoretical and Computational Chemistry not elsewhere classified | Statistical Mechanics | Physical Organic Chemistry | Soft Condensed Matter | Thermodynamics and Statistical Physics | Biological Physics |
Expanding Knowledge in the Physical Sciences | Industrial Energy Conservation and Efficiency | Energy Conservation and Efficiency in Transport | Physical and Chemical Conditions of Water for Urban and Industrial Use | Expanding Knowledge in the Chemical Sciences | Medical Instruments | Lubricants | Chemical sciences | Physical sciences
Publisher: American Physical Society (APS)
Date: 14-09-2011
Publisher: AIP Publishing
Date: 10-06-2009
DOI: 10.1063/1.3149858
Abstract: A coarse-grained model for molecular dynamics simulations of polymer solutions with variable solvent quality is proposed. This model allows solvent quality to be varied over the whole range from very poor to very good solvent conditions by varying a single parameter. The model has several advantages. All interactions are short ranged and repulsive, making the model very computationally efficient compared with other explicit solvent models that include the long-ranged attractive part of the interactions the solvent is included explicitly, ensuring that the theta condition corresponds to a genuine cancellation of the solvent-mediated polymer-polymer interactions and hydrodynamic interactions and entanglement can occur for all solvent conditions. The theta point is determined and the conformational properties of a 50-bead chain system are investigated over the whole concentration range from the dilute limit to the melt as a function of solvent quality.
Publisher: American Physical Society (APS)
Date: 03-1993
Publisher: Elsevier BV
Date: 09-1995
Publisher: American Physical Society (APS)
Date: 25-03-2013
Publisher: American Physical Society (APS)
Date: 19-06-2008
Publisher: Elsevier BV
Date: 06-2008
Publisher: Society of Rheology
Date: 03-2014
DOI: 10.1122/1.4860355
Publisher: Elsevier BV
Date: 03-2004
Publisher: American Chemical Society (ACS)
Date: 03-05-2016
DOI: 10.1021/ACS.LANGMUIR.6B00791
Abstract: The extent of confinement effects on water is not clear in the literature. While some properties are affected only within a few nanometers from the wall surface, others are affected over long length scales, but the range is not clear. In this work, we have examined the dielectric response of confined water under the influence of external electric fields along with the dipolar fluctuations at equilibrium. The confinement induces a strong anisotropic effect which is evident up to 100 nm channel width, and may extend to macroscopic dimensions. The root-mean-square fluctuations of the total orientational dipole moment in the direction perpendicular to the surfaces is 1 order of magnitude smaller than the value attained in the parallel direction and is independent of the channel width. Consequently, the isotropic condition is unlikely to be recovered until the channel width reaches macroscopic dimensions. Consistent with dipole moment fluctuations, the effect of confinement on the dielectric response also persists up to channel widths considerably beyond 100 nm. When an electric field is applied in the perpendicular direction, the orientational relaxation is 3 orders of magnitude faster than the dipolar relaxation in the parallel direction and independent of temperature.
Publisher: IOP Publishing
Date: 16-04-2010
DOI: 10.1088/0953-8984/22/19/195105
Abstract: We present an extended analysis of the wavevector dependent shear viscosity of monatomic and diatomic (liquid chlorine) fluids over a wide range of wavevectors and for a variety of state points. The analysis is based on equilibrium molecular dynamics simulations, which involve the evaluation of transverse momentum density and shear stress autocorrelation functions. For liquid chlorine we present the results in both atomic and molecular formalisms. We find that the viscosity kernel of chlorine in the atomic representation is statistically indistinguishable from that in the molecular representation. The results further suggest that the real space viscosity kernels of monatomic and diatomic fluids depend sensitively on the density, the potential energy function and the choice of fitting function in reciprocal space. It is also shown that the reciprocal space shear viscosity data can be fitted to two different simple functional forms over the entire density, temperature and wavevector range: a function composed of n-Gaussian terms and a Lorentzian-type function. Overall, the real space viscosity kernel has a width of 3-6 atomic diameters, which means that the generalized hydrodynamic constitutive relation is required for fluids with strain rates that vary nonlinearly over distances of the order of atomic dimensions.
Publisher: Elsevier BV
Date: 07-2010
Publisher: American Physical Society (APS)
Date: 22-08-2017
Publisher: Royal Society of Chemistry (RSC)
Date: 1995
DOI: 10.1039/C39950001925
Publisher: AIP Publishing
Date: 11-04-2007
DOI: 10.1063/1.2715951
Abstract: In this paper the authors propose a novel method to study the local linear viscoelasticity of fluids confined between two walls. The method is based on the linear constitutive equation and provides details about the real and imaginary parts of the local complex viscosity. They apply the method to a simple atomic fluid undergoing zero mean oscillatory flow using nonequilibrium molecular dynamics simulations. The method shows that the viscoelastic properties of the fluid exhibit dramatic spatial changes near the wall-fluid boundary due to the high density in this region. It is also shown that the real part of the viscosity converges to the frequency dependent local shear viscosity sufficiently far away from the wall. This also provides valuable information about the transport properties in the fluid, in general. The viscosity is compared with predictions from the local average density model. The two methods disagree in that the local average density model predicts larger viscosity variations near the wall-fluid boundary than what is observed through the method presented here.
Publisher: American Physical Society (APS)
Date: 15-10-2007
Publisher: American Physical Society (APS)
Date: 12-1992
Publisher: Cambridge University Press
Date: 10-03-2017
Abstract: Written by two specialists with over twenty-five years of experience in the field, this valuable text presents a wide range of topics within the growing field of nonequilibrium molecular dynamics (NEMD). It introduces theories which are fundamental to the field - namely, nonequilibrium statistical mechanics and nonequilibrium thermodynamics - and provides state-of-the-art algorithms and advice for designing reliable NEMD code, as well as examining applications for both atomic and molecular fluids. It discusses homogenous and inhomogenous flows and pays considerable attention to highly confined fluids, such as nanofluidics. In addition to statistical mechanics and thermodynamics, the book covers the themes of temperature and thermodynamic fluxes and their computation, the theory and algorithms for homogenous shear and elongational flows, response theory and its applications, heat and mass transport algorithms, applications in molecular rheology, highly confined fluids (nanofluidics), the phenomenon of slip and how to compute it from basic microscopic principles, and generalized hydrodynamics.
Publisher: AIP Publishing
Date: 28-07-2013
DOI: 10.1063/1.4816514
Abstract: We use molecular dynamics simulations to investigate the linear and nonlinear density response functions for simple fluids under the influence of spatially periodic external fields. Using a direct Fourier space decomposition of the instantaneous microscopic density for the perturbed fluid we can clearly identify the distinct order of response. Using a single component sinusoidal longitudinal force for a set of wavelengths and litudes we show that in the linear response regime the proportionality between the external field litude and the density perturbation can be used to determine the linear density response function, and hence the pair correlation function, static liquid structure factor, and lowest order direct correlation function. We show also that for large external field litudes a single component external field can be used to determine the form for lowest order and second lowest order nonlinear response functions for restricted regions of the total response function spaces.
Publisher: AIP Publishing
Date: 18-02-2009
DOI: 10.1063/1.3077006
Abstract: Hyperbranched polymer melts have been simulated using a coarse-grained model and nonequilibrium molecular dynamics (NEMD) techniques. In order to determine the shear-induced changes in the structural properties of hyperbranched polymers, various parameters were calculated at different strain rates. The radii of gyration which characterize the size of the polymer were evaluated. The tensor of gyration was analyzed and results indicate that hyperbranched polymer molecules have a prolate ellipsoid shape under shear. As hyperbranched polymers have compact, highly branched architecture and layers of beads have increasing densities which might lead to an unusual distribution of mass, the distribution of beads was also studied. The distribution of terminal beads was investigated to understand the spatial arrangement of these groups which is very important for hyperbranched polymer applications, especially in drug delivery.
Publisher: American Physical Society (APS)
Date: 09-07-2010
Publisher: American Physical Society (APS)
Date: 06-09-2006
Publisher: Informa UK Limited
Date: 26-08-2016
Publisher: AIP Publishing
Date: 13-01-2012
DOI: 10.1063/1.3675904
Abstract: Data for the flow rate of water in carbon nanopores is widely scattered, both in experiments and simulations. In this work, we aim at precisely quantifying the characteristic large slip length and flow rate of water flowing in a planar graphene nanochannel. First, we quantify the slip length using the intrinsic interfacial friction coefficient between water and graphene, which is found from equilibrium molecular dynamics (EMD) simulations. We then calculate the flow rate and the slip length from the streaming velocity profiles obtained using non-equilibrium molecular dynamics (NEMD) simulations and compare with the predictions from the EMD simulations. The slip length calculated from NEMD simulations is found to be extremely sensitive to the curvature of the velocity profile and it possesses large statistical errors. We therefore pose the question: Can a micrometer range slip length be reliably determined using velocity profiles obtained from NEMD simulations? Our answer is “not practical, if not impossible” based on the analysis given as the results. In the case of high slip systems such as water in carbon nanochannels, the EMD method results are more reliable, accurate, and computationally more efficient compared to the direct NEMD method for predicting the nanofluidic flow rate and hydrodynamic boundary condition.
Publisher: American Physical Society (APS)
Date: 22-06-2015
Publisher: Elsevier BV
Date: 03-2003
Publisher: Springer Science and Business Media LLC
Date: 1998
Publisher: AIP Publishing
Date: 10-10-2013
DOI: 10.1063/1.4824140
Abstract: Thermophoresis is the movement of molecules caused by a temperature gradient. Here we report the results of a study of thermophoresis using non-equilibrium molecular dynamics simulations of a confined argon-krypton fluid subject to two different temperatures at thermostated walls. The resulting temperature profile between the walls is used along with the Soret coefficient to predict the concentration profile that develops across the channel. We obtain the Soret coefficient by calculating the mutual diffusion and thermal diffusion coefficients. We report an appropriate method for calculating the transport coefficients for binary systems, using the Green-Kubo integrals and radial distribution functions obtained from equilibrium molecular dynamics simulations of the bulk fluid. Our method has the unique advantage of separating the mutual diffusion and thermal diffusion coefficients, and calculating the sign and magnitude of their in idual contributions to thermophoresis in binary mixtures.
Publisher: Elsevier BV
Date: 02-1984
Publisher: American Physical Society (APS)
Date: 13-05-2008
Publisher: AIP Publishing
Date: 09-1996
DOI: 10.1063/1.473014
Publisher: Springer Science and Business Media LLC
Date: 04-2017
DOI: 10.1557/MRS.2017.61
Publisher: Springer Science and Business Media LLC
Date: 07-10-0001
Publisher: American Physical Society (APS)
Date: 25-07-2011
Publisher: American Chemical Society (ACS)
Date: 13-12-2017
Publisher: Informa UK Limited
Date: 23-09-2019
Publisher: Elsevier BV
Date: 03-1999
Publisher: AIP Publishing
Date: 03-02-2014
DOI: 10.1063/1.4862544
Abstract: We present a simple thermostatting method suitable for nanoconfined fluid systems. Two conventional strategies involve thermostatting the fluid directly or employing a thermal wall that couples only the wall atoms with the thermostat. When only a thermal wall is implemented, the temperature control of the fluid is true to the actual experiment and the heat is transferred from the fluid to the walls. However, for large or complex systems it can often be computationally prohibitive to employ thermal walls. To overcome this limitation many researchers choose to freeze wall atoms and instead apply a synthetic thermostat to the fluid directly through the equations of motion. This, however, can have serious consequences for the mechanical, thermodynamic, and dynamical properties of the fluid by introducing unphysical behaviour into the system [Bernardi et al., J. Chem. Phys. 132, 244706 (2010)]. In this paper, we propose a simple scheme which enables working with both frozen walls and naturally thermostatted liquids. This is done by superimposing the walls with oscillating particles, which vibrate on the edge of the fluid control volume. These particles exchange energy with the fluid molecules, but do not interact with wall atoms or each other, thus behaving as virtual particles. Their displacements violate the Lindemann criterion for melting, in such a way that the net effect would not amount to an additional confining surface. One advantage over standard techniques is the reduced computational cost, particularly for large walls, since they can be kept rigid. Another advantage over accepted strategies is the opportunity to freeze complex charged walls such as β-cristobalite. The method furthermore overcomes the problem with polar fluids such as water, as thermalized charged surfaces require higher spring constants to preserve structural stability, due to the effects of strong Coulomb interactions, thus inevitably degrading the thermostatting efficiency.
Publisher: AIP Publishing
Date: 15-12-1999
DOI: 10.1063/1.480428
Abstract: We comment on the paper by Khare et al. [J. Chem. Phys. 107, 2589 (1997)], which considers simulations of confined fluids under planar shear with and without the application of a thermostat. We suggest an alternative interpretation of some of their results, which leads to the same conclusion that applying a thermostat directly to a confined shearing fluid is not the preferred simulation procedure to follow.
Publisher: American Chemical Society (ACS)
Date: 12-03-2014
DOI: 10.1021/LA404805S
Abstract: In our recent work, J. Chem. Phys. 2013, 138, 154712, we demonstrated the feasibility of unidirectional pumping of water, exploiting translational-rotational momentum coupling using nonequilibrium molecular dynamics simulations. Flow can be sustained when the fluid is driven out of equilibrium by an external spatially uniform rotating electric field and confined between two planar surfaces exposing different degrees of hydrophobicity. The permanent dipole moment of water follows the rotating field, thus inducing the molecules to spin, and the torque exerted by the field is continuously injected into the fluid, enabling a steady conversion of spin angular momentum into linear momentum. The translational-rotational coupling is a sensitive function of the rotating electric field parameters. In this work, we have found that there exists a small energy dissipation region attainable when the frequency of the rotating electric field matches the inverse of the dielectric relaxation time of water and when its litude lies in a range just before dielectric saturation effects take place. In this region, that is, when the frequency lies in a small window of the microwave region around ∼20 GHz and litude ∼0.03 V Å(-1), the translational-rotational coupling is most effective, yielding fluid velocities of magnitudes of ∼2 ms(-1) with only moderate fluid heating. In this work, we also confine water to a realistic nanochannel made of graphene giving a hydrophobic surface on one side and β-cristobalite giving a hydrophilic surface on the other, reproducing slip-and-stick velocity boundary conditions, respectively. This enables us to demonstrate that in a realistic environment, the coupling can be effectively exploited to achieve noncontact pumping of water at the nanoscale. A quantitative comparison between nonequilibrium molecular dynamics and analytical solutions of the extended Navier-Stokes equations, including an external rotating electric field has been performed, showing excellent agreement when the electric field parameters match the aforementioned small energy dissipation region.
Publisher: AIP Publishing
Date: 23-11-2014
DOI: 10.1063/1.3184799
Abstract: The melt rheology of four hyperbranched polymer structures with different molecular weights has been studied using nonequilibrium molecular dynamics (NEMD). Systems were simulated over a wide range of strain rates to capture the crossover behavior from Newtonian to non-Newtonian regimes. Rheological properties including shear viscosity and first and second normal stress coefficients were computed and the transition to shear thinning was observed at different strain rates for hyperbranched polymers of different sizes. The results were consistent with previous findings from NEMD simulation of linear and dendritic polymers. Flow birefringence was characterized by taking into account both form and intrinsic birefringences, which result from molecular and bond alignment, respectively. The stress optical rule was tested and shown to be valid only in the Newtonian regime and violated in the strong flow regime where the rule does not take into account flow-induced changes of the microstructure.
Publisher: AIP Publishing
Date: 07-05-2015
DOI: 10.1063/1.4919654
Abstract: The structure and rheology of model polymer blends under planar elongational flow have been investigated through nonequilibrium molecular dynamics simulations. The polymeric blends consist of linear polymer chains (187 monomers per chain) and dendrimer polymers of generations g = 1 − 4. The number fraction, x, of the dendrimer species is varied (4%, 8%, and 12%) in the blend melt. We study the effect of extension rate, dendrimer generation, and dendrimer number fraction on pair distribution functions for different blend systems. We also calculate the extension-rate dependent radius of gyration and ratios of the eigenvalues of the gyration tensor to study the elongation-induced deformation of the molecules in the blend. Melt rheological properties including the first and second extensional viscosities are found to fall into the range between those of pure dendrimer and pure linear polymer melts, which are correlated with the mass fraction and generation of the dendrimers in the blend.
Publisher: AIP Publishing
Date: 22-11-2000
DOI: 10.1063/1.1319379
Abstract: We use nonequilibrium molecular dynamics to simulate steady state planar shear flow and planar elongational flow of fluids of small molecules at constant volume and temperature. The systems studied are Lennard–Jones diatomic molecules (chlorine), and a series of linear Lennard–Jones molecules with one, two, and four sites. In our simulations of planar elongational flow, we employ Kraynik–Reinelt periodic boundary conditions, which allow us to obtain precise values of the steady state planar elongational viscosity. We validate our application of Kraynik–Reinelt periodic boundary conditions by comparing the zero strain rate shear and elongational viscosities. The results show that the elongational viscosity is proportional to the shear viscosity in the zero strain rate limit, as expected. The viscosity, pressure, and internal energy of the atomic Lennard–Jones fluid show exactly the same behavior for the two types of flow when both sets of results are plotted against the second scalar invariant of the strain rate tensor. The results for the diatomic and four-site molecules show differences in the pressure, energy, and viscosity outside the Newtonian regime when plotted against the second scalar invariant of the strain rate tensor. The differences in the properties in the nonlinear regime increase with both strain rate and molecular length.
Publisher: Elsevier BV
Date: 10-2001
Publisher: Springer Science and Business Media LLC
Date: 03-1995
DOI: 10.1007/BF01441905
Publisher: AIP Publishing
Date: 15-09-2001
DOI: 10.1063/1.1396850
Publisher: American Physical Society (APS)
Date: 27-11-2013
Publisher: American Physical Society (APS)
Date: 08-1993
Publisher: Informa UK Limited
Date: 28-06-2020
Publisher: AIP Publishing
Date: 18-11-2014
DOI: 10.1063/1.4901721
Abstract: We present nonequilibrium molecular dynamics (NEMD) simulation results for the miscibility, structural properties, and melt rheological behavior of polymeric blends under shear flow. The polymeric blends consist of chemically identical linear polymer chains (187 monomers per chain) and dendrimer polymers of generations g = 1–4. The number fraction x of the dendrimer species is varied (4%, 8%, and 12%) in the blend melt. The miscibility of blend species is measured, using the pair distribution functions gDL, gLL, and gDD. All the studied systems form miscible blend melts under the conditions investigated. We also study the effect of shear rate \\documentclass[12pt]{minimal}\\begin{document}$\\dot{\\gamma }$\\end{document}γ̇ and dendrimer generation on inter-penetration between blend species for different blend systems. The results reveal that shear flow increases the interpenetration of linear chains toward the core of the dendrimers. We also calculate the shear-rate dependent radius of gyration and ratios of the eigenvalues of the gyration tensor to study the shear-induced deformation of the molecules in the blend. Melt rheological properties including the shear viscosity and first and second normal stress coefficients obtained from NEMD simulations at constant pressure are found to fall into the range between those of pure dendrimer and pure linear polymer melts.
Publisher: IOP Publishing
Date: 10-09-2008
Publisher: AIP Publishing
Date: 16-01-2013
DOI: 10.1063/1.4774095
Abstract: The extended Navier-Stokes theory accounts for the coupling between the translational and rotational molecular degrees of freedom. In this paper, we generalize this theory to non-zero frequencies and wavevectors, which enables a new study of spatio-temporal correlation phenomena present in molecular fluids. To discuss these phenomena in detail, molecular dynamics simulations of molecular chlorine are performed for three different state points. In general, the theory captures the behavior for small wavevector and frequencies as expected. For ex le, in the hydrodynamic regime and for molecular fluids with small moment of inertia like chlorine, the theory predicts that the longitudinal and transverse intrinsic angular velocity correlation functions are almost identical, which is also seen in the molecular dynamics simulations. However, the theory fails at large wavevector and frequencies. To account for the correlations at these scales, we derive a phenomenological expression for the frequency dependent rotational viscosity and wavevector and frequency dependent longitudinal spin viscosity. From this we observe a significant coupling enhancement between the molecular angular velocity and translational velocity for large frequencies in the gas phase this is not observed for the supercritical fluid and liquid state points.
Publisher: Informa UK Limited
Date: 20-04-1994
Publisher: American Physical Society (APS)
Date: 05-1995
Publisher: AIP Publishing
Date: 11-10-2010
DOI: 10.1063/1.3490664
Abstract: In this paper we evaluate the rotational viscosity and the two spin viscosities for liquid water using equilibrium molecular dynamics. Water is modeled via the flexible SPC/Fw model where the Coulomb interactions are calculated via the Wolf method which enables the long simulation times required. We find that the rotational viscosity is independent of the temperature in the range from 284 to 319 K. The two spin viscosities, on the other hand, decrease with increasing temperature and are found to be two orders of magnitude larger than that estimated by Bonthuis et al. [Phys. Rev. Lett. 103, 144503 (2009)] We apply the results from molecular dynamics simulations to the extended Navier–Stokes equations that include the coupling between intrinsic angular momentum and linear momentum. For a flow driven by an external field the coupling will reduce the flow rate significantly for nanoscale geometries. The coupling also enables conversion of rotational electrical energy into fluid linear momentum and we find that in order to obtain measurable flow rates the electrical field strength must be in the order of 0.1 MV m−1 and rotate with a frequency of more than 100 MHz.
Publisher: AIP Publishing
Date: 10-05-2001
DOI: 10.1063/5.0088127
Abstract: We derive the transient-time correlation function (TTCF) expression for the computation of phase variables of inhomogenous confined atomistic fluids undergoing boundary-driven planar shear (Couette) flow at constant pressure. Using nonequilibrium molecular dynamics simulations, we then apply the TTCF formalism to the computation of the shear stress and the slip velocity for atomistic fluids at realistic low shear rates, in systems under constant pressure and constant volume. We show that, compared to direct averaging of multiple trajectories, the TTCF method dramatically improves the accuracy of the results at low shear rates and that it is suitable to investigate the tribology and rheology of atomistically detailed confined fluids at realistic flow rates.
Publisher: AIP Publishing
Date: 16-05-2006
DOI: 10.1063/1.2192775
Abstract: We present a simple and direct derivation of the SLLOD equations of motion for molecular simulations of general homogeneous flows. We show that these equations of motion (1) generate the correct particle trajectories, (2) conserve the total thermal momentum without requiring the center of mass to be located at the origin, and (3) exactly generate the required energy dissipation. These equations of motion are compared with the g-SLLOD and p-SLLOD equations of motion, which are found to be deficient. Claims that the SLLOD equations of motion are incorrect for elongational flows are critically examined and found to be invalid. It is confirmed that the SLLOD equations are, in general, non-Hamiltonian. We derive a Hamiltonian from which they can be obtained in the special case of a symmetric velocity gradient tensor. In this case, it is possible to perform a canonical transformation that results in the well-known DOLLS tensor Hamiltonian.
Publisher: American Physical Society (APS)
Date: 08-1995
Publisher: AIP Publishing
Date: 11-04-2007
DOI: 10.1063/1.2714556
Abstract: The authors present the results from nonequilibrium molecular dynamics simulations for the structural and dynamical properties of highly confined linear polymer fluids undergoing planar Poiseuille flow. They study systems confined within pores of several atomic diameters in width and investigate the dependence of the density profiles, the mean squared radius of gyration, the mean squared end-to-end distance, streaming velocity, strain rate, shear stress, and streaming angular velocity as functions of average fluid density and chain length. Their simulation results show that, sufficiently far from the walls, the radius of gyration for molecules under shear in the middle of the pore follows the power law Rg=ANbν, where Nb is the number of bonds and the exponent has a value of 0.5 which resembles the value for a homogeneous equilibrium fluid. Under the conditions simulated, the authors find the onset of flat velocity profiles but with very little wall slippage. These flat profiles are most likely due to the restricted layering of the fluid into just one or two molecular layers for narrow pore widths compared to chain length, rather than typical plug-flow conditions. The angular velocity is shown to be proportional to half the strain rate in the pore interior when the chain length is sufficiently small compared to the pore width, consistent with the behavior for homogeneous fluids in the linear regime.
Publisher: AIP Publishing
Date: 14-11-2005
DOI: 10.1063/1.2110047
Abstract: The shear-rate dependence of viscosity is studied for model polymer melts containing various concentrations of spherical filler particles by molecular-dynamics simulations, and the results are compared with the experimental results for calcium-carbonate-filled polypropylene. Although there are some significant differences in scale between the simulated model polymer composite and the system used in the experiments, some important qualitative similarities in shear behavior are observed. The trends in the steady-state shear viscosities of the simulated polymer-filler system agree with those seen in the experimental results shear viscosities, zero-shear viscosities, and the rate of shear thinning are all seen to increase with filler content in both the experimental and simulated systems. We observe a significant difference between the filler volume fraction dependence of the zero-shear viscosity of the simulated system and that of the experimental system that can be attributed to a large difference in the ratio of the filler particle radius to the radius of gyration of the polymer molecules. In the simulated system, the filler particles are so small that they only have a weak effect on the viscosity of the composite at low filler volume fraction, but in the experimental system, the viscosity of the composite increases rapidly with increasing filler volume fraction. Our results indicate that there exists a value of the ratio of the filler particle radius to the polymer radius of gyration such that the zero-shear-rate viscosity of the composite becomes approximately independent of the filler particle volume fraction.
Publisher: Informa UK Limited
Date: 03-2007
Publisher: Elsevier BV
Date: 11-2007
Publisher: AIP
Date: 2008
DOI: 10.1063/1.2964719
Publisher: American Physical Society (APS)
Date: 26-07-2016
Publisher: AIP Publishing
Date: 19-04-2013
DOI: 10.1063/1.4801033
Abstract: Pumping of fluids confined to nanometer dimension spaces is a technically challenging yet vitally important technological application with far reaching consequences for lab-on-a-chip devices, biomimetic nanoscale reactors, nanoscale filtration devices and the like. All current pumping mechanisms require some sort of direct intrusion into the nanofluidic system, and involve mechanical or electronic components. In this paper, we present the first nonequilibrium molecular dynamics results to demonstrate that non-intrusive electropumping of liquid water on the nanoscale can be performed by subtly exploiting the coupling of spin angular momentum to linear streaming momentum. A spatially uniform rotating electric field is applied to water molecules, which couples to their permanent electric dipole moments. The resulting molecular rotational momentum is converted into linear streaming momentum of the fluid. By selectively tuning the degree of hydrophobicity of the solid walls one can generate a net unidirectional flow. Our results for the linear streaming and angular velocities of the confined water are in general agreement with the extended hydrodynamical theory for this process, though also suggest refinements to the theory are required. These numerical experiments confirm that this new concept for pumping of polar nanofluids can be employed under laboratory conditions, opening up significant new technological possibilities.
Publisher: MDPI AG
Date: 09-2018
DOI: 10.3390/PR6090144
Abstract: The fluid dynamics of macroscopic and microscopic systems is well developed and has been extensively validated. Its extraordinary success makes it tempting to apply Navier–Stokes fluid dynamics without modification to systems of ever decreasing dimensions as studies of nanofluidics become more prevalent. However, this can result in serious error. In this paper, we discuss several ways in which nanoconfined fluid flow differs from macroscopic flow. We give particular attention to several topics that have recently received attention in the literature: slip, spin angular momentum coupling, nonlocal stress response and density inhomogeneity. In principle, all of these effects can now be accurately modelled using validated theories. Although the basic principles are now fairly well understood, much work remains to be done in their application.
Publisher: AIP Publishing
Date: 11-02-2019
DOI: 10.1063/1.5079993
Abstract: We use nonequilibrium molecular dynamics to explore the effect of shear flow on heat flux. By simulating a simple fluid in a channel bounded by tethered atoms, the heat flux is computed for two systems: a temperature driven one with no flow and a wall driven, Couette flow system. The results for the temperature driven system give Fourier’s law thermal conductivity, which is shown to agree well with experiments. Through comparison of the two systems, we quantify the additional components of the heat flux parallel and normal to the walls due to shear flow. To compute the heat flux in the flow direction, the Irving-Kirkwood equations are integrated over a volume, giving the so-called volume average form, and they are also manipulated to get expressions for the surface averaged and method of planes forms. The method of planes and volume average forms are shown to give equivalent results for the heat flux when using small volumes. The heat flux in the flow direction is obtained consistently over a range of simulations, and it is shown to vary linearly with strain rate, as predicted by theory. The additional strain rate dependent component of the heat flux normal to the wall is obtained by fitting the strain rate dependence of the heat flux to the expected form. As a result, the additional terms in the thermal conductivity tensor quantified in this work should be experimentally testable.
Publisher: Springer Science and Business Media LLC
Date: 07-2004
Publisher: Elsevier BV
Date: 03-2003
Publisher: CSIRO Publishing
Date: 1983
DOI: 10.1071/CH9830609
Abstract: The adsorption of the photochemical sensitizer tris(2,2'-bipyridine)ruthenium(II) on dihexadecylphosphate vesicles has been determined as a function of the pH and ionic strength of the dispersing medium. The adsorption density is a maximum at pH 7.5 � 0.5 and with minimum ionic strength(pH adjusted by adding NaOH). The stability of the vesicles with respect to flocculation, in the presence of adsorbed sensitizer, is increased when the pH is raised from 3.3 to 7.5. The presence of an amine buffer at 5 x 10-2 at pH 7.8 reduces the adsorption density by more than 90%. The relevance of these results to the interpretation of photochemistry experiments (in conjunction with photochemical hydrogen production from water) is discussed.
Publisher: American Chemical Society (ACS)
Date: 12-10-2015
DOI: 10.1021/ACS.LANGMUIR.5B02237
Abstract: This paper introduces the fundamental continuum theory governing momentum transport in isotropic nanofluidic systems. The theory is an extension of the classical Navier-Stokes equation, and includes coupling between translational and rotational degrees of freedom as well as nonlocal response functions that incorporate spatial correlations. The continuum theory is compared with molecular dynamics simulation data for both relaxation processes and fluid flows, showing excellent agreement on the nanometer length scale. We also present practical tools to estimate when the extended theory should be used. It is shown that in the wall-fluid region the fluid molecules align with the wall, and in this region the isotropic model may fail and a full anisotropic description is necessary.
Publisher: American Physical Society (APS)
Date: 28-10-2009
Publisher: Elsevier BV
Date: 2023
Publisher: Springer Science and Business Media LLC
Date: 27-10-2011
Publisher: IOP Publishing
Date: 04-01-2008
Publisher: AIP Publishing
Date: 11-10-2010
DOI: 10.1063/1.3499745
Abstract: The nonlocal viscosity kernels of polymer melts have been determined by means of equilibrium molecular dynamics upon cooling toward the glass transition. Previous results for the temperature dependence of the self-diffusion coefficient and the value of the glass transition temperature are confirmed. We find that it is essential to include the attractive part of the interatomic potential in order to observe a strong glass transition. The width of the reciprocal space kernel decreases dramatically near the glass transition, being described by a deltalike function near and below the glass transition, leading to a very broad kernel in physical space. Thus, spatial nonlocality turns out to play an important role in polymeric fluids at temperatures near the glass transition temperature.
Publisher: Elsevier BV
Date: 11-2004
Publisher: AIP Publishing
Date: 22-10-2009
DOI: 10.1063/1.3247191
Abstract: Nonequilibrium molecular dynamics simulations were performed for a family of hyperbranched polymers of the same molecular weight but with different chain lengths between branches. Microscopic structural properties including mean squared radius of gyration, distribution of beads from the center of mass and from the core and the interpenetration function of these systems were characterized. A relationship between the zero shear rate mean squared radius of gyration and the Wiener index was established. The molecular and bond alignment tensors were analyzed to characterize the flow birefringence of these hyperbranched polymers. The melt rheology was also studied and the crossover from the Newtonian to non-Newtonian behavior was captured for all polymer fluids in the considered range of strain rates. Rheological properties including the shear viscosity and normal stress coefficients obtained from constant pressure simulations were found to be the same as those from constant volume simulations except at high strain rates due to shear dilatancy. A linear dependence of zero shear rate viscosities on the number of spacer units was found. The stress optical rule was shown to be valid at low strain rates with the stress optical coefficient of approximately 3.2 independent of the topologies of polymers.
Publisher: AIP Publishing
Date: 09-06-2003
DOI: 10.1063/1.1574776
Abstract: A detailed theory of the thermodynamics of linear viscoelastic fluids in steady shear is presented. This theory is exact and gives simple and easily computed expressions for the change in the internal energy and free energy due to the imposition of steady shear for a general linear viscoelastic fluid. It has strong similarities to the extended irreversible thermodynamics approach, but differs from it in a few crucial ways. An important element of our theory is the ision of the viscoelastic work into elastic and viscous parts, and the identification of these components as the reversible and irreversible parts of the work. This leads to a natural definition of reversible heat transfer that can be used in the definition of the steady-state entropy. We present the results of nonequilibrium molecular-dynamics computer simulations and evaluate the thermodynamic properties of a simple model fluid as a function of strain rate, showing how the entropy difference of the fluid between the equilibrium and shearing steady states can be computed. We find that, at low strain rates, the entropy change of a simple atomic fluid at constant temperature is positive and proportional to the square of the strain rate. A helpful geometrical interpretation of the decomposition of viscoelastic work into elastic and viscous components using a hysteresis loop construction is also discussed.
Publisher: American Physical Society (APS)
Date: 16-11-2012
Publisher: AIP Publishing
Date: 23-06-2015
DOI: 10.1063/1.4922831
Abstract: The rheology and molecular structure of a model bitumen (Cooee bitumen) under shear are investigated in the non-Newtonian regime using non-equilibrium molecular dynamics simulations. The shear viscosity, normal stress differences, and pressure of the bitumen mixture are computed at different shear rates and different temperatures. The model bitumen is shown to be a shear-thinning fluid at all temperatures. In addition, the Cooee model is able to reproduce experimental results showing the formation of nanoaggregates composed of stacks of flat aromatic molecules in bitumen. These nanoaggregates are immersed in a solvent of saturated hydrocarbon molecules. At a fixed temperature, the shear-shinning behavior is related not only to the inter- and intramolecular alignments of the solvent molecules but also to the decrease of the average size of the nanoaggregates at high shear rates. The variation of the viscosity with temperature at different shear rates is also related to the size and relative composition of the nanoaggregates. The slight anisotropy of the whole s le due to the nanoaggregates is considered and quantified. Finally, the position of bitumen mixtures in the broad literature of complex systems such as colloidal suspensions, polymer solutions, and associating polymer networks is discussed.
Publisher: AIP Publishing
Date: 04-01-2022
DOI: 10.1063/5.0054681
Abstract: Using non-equilibrium molecular dynamics simulations, we demonstrate the effect of concentration and alkali cation types (K+, Na+, and Li+) on the hydrodynamic slip of aqueous alkali chloride solutions in an uncharged graphene nanochannel. We modeled the graphene–electrolyte interactions using the potential of Williams et al. [J. Phys. Chem. Lett. 8, 703 (2017)], which uses optimized graphene–ion Lennard-Jones interaction parameters to effectively account for surface and solvent polarizability effects on the adsorption of ions in an aqueous solution to a graphene surface. In our study, the hydrodynamic slip exhibits a decreasing trend for alkali chloride solutions with increasing salt concentration. The NaCl solution shows the highest reduction in the slip length followed by KCl and LiCl solutions, and the reduction in the slip length is very much dependent on the salt type. We also compared the slip length with that calculated using a standard unoptimized interatomic potential obtained from the Lorentz–Berthelot mixing rule for the ion–carbon interactions, which is not adjusted to account for the surface and solvent polarizability at the graphene surface. In contrast to the optimized model, the slip length of alkali chloride solutions in the unoptimized model shows only a nominal change with salt concentration and is also independent of the nature of salts. Our study shows that adoption of the computationally inexpensive optimized potential of Williams et al. for the graphene–ion interactions has a significant influence on the calculation of slip lengths for electrolyte solutions in graphene-based nanofluidic devices.
Publisher: American Physical Society (APS)
Date: 06-07-2015
Publisher: American Physical Society (APS)
Date: 14-08-2020
Start Date: 06-2019
End Date: 06-2023
Amount: $380,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2020
End Date: 12-2025
Amount: $410,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2006
End Date: 12-2009
Amount: $259,570.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2012
End Date: 12-2016
Amount: $315,000.00
Funder: Australian Research Council
View Funded Activity