360 research outputs found
Deconstructing temperature gradients across fluid interfaces: the structural origin of the thermal resistance of liquid-vapor interfaces
The interfacial thermal resistance determines condensation-evaporation
processes and thermal transport across material-fluid interfaces. Despite its
importance in transport processes, the interfacial structure responsible for
the thermal resistance is still unknown. By combining non-equilibrium molecular
dynamics simulations and interfacial analyses that remove the interfacial
thermal fluctuations we show that the thermal resistance of liquid-vapor
interfaces is connected to a low density fluid layer that is adsorbed at the
liquid surface. This thermal resistance layer (TRL) defines the boundary where
the thermal transport mechanism changes from that of gases (ballistic) to that
characteristic of dense liquids, dominated by frequent particle collisions
involving very short mean free paths. We show that the thermal conductance is
proportional to the number of atoms adsorbed in the TRL, and hence we explain
the structural origin of the thermal resistance in liquid-vapor interfaces.Comment: 4 pages, 4 figures, and supplementary informatio
The influence of surface roughness on the adhesive interactions and phase behavior of suspensions of calcite nanoparticles
We investigate the impact of nanoparticle roughness on the phase behaviour of
suspensions in models of calcium carbonate nanoparticles. We use a Derjaguin
approach that incorporates roughness effects and interactions between the
nanoparticles modelled with a combination of DLVO forces and hydration forces,
derived using experimental data and atomistic molecular dynamics simulations,
respectively. Roughness effects, such as atomic steps or terraces appearing in
mineral surfaces result in very different effective inter-nanoparticle
potentials. Using stochastic Langevin Dynamics computer simulations and the
effective interparticle interactions we demonstrate that relatively small
changes in the roughness of the particles modify significantly the stability of
the suspensions. We propose that the sensitivity of the phase behavior to the
roughness is connected to the short length scale of the adhesive attraction
arising from the ordering of water layers confined between calcite surfaces.
Particles with smooth surfaces feature strong adhesive forces, and form gel
fractal structures, while small surface roughness, of the order of atomic steps
in mineral faces, stabilize the suspension. We believe that our work helps to
rationalize the contrasting experimental results that have been obtained
recently using nanoparticles or extended surfaces, which provide support for
the existence of adhesive or repulsive interactions, respectively. We further
use our model to analyze the synergistic effects of roughness, pH and ion
concentration on the phase behavior of suspensions, connecting with recent
experiments using calcium carbonate nanoparticles
Solvent-mediated interactions between nanoparticles at fluid interfaces
We investigate the solvent mediated interactions between nanoparticles
adsorbed at a liquid-vapor interface in comparison to the solvent mediated
interactions in the bulk liquid and vapor phases of a Lennard-Jones solvent.
Molecular dynamics simulation data for the latter are in good agreement with
results from integral equations in the reference functional approximation and a
simple geometric approximation. Simulation results for the solvent mediated
interactions at the interface differ markedly from the interactions of the
particles in the corresponding bulk phases. We find that at short interparticle
distances the interactions are considerably more repulsive than those in either
bulk phase. At long interparticle distances we find evidence for a long-ranged
attraction. We discuss these observations in terms of interfacial interactions,
namely, the three-phase line tension that would operate at short distances, and
capillary wave interactions for longer interparticle distances.Comment: 22 pages, 6 figure
Polarisation of water under thermal fields: the effect of the molecular dipole and quadrupole moments.
The investigation of the behaviour of water under thermal fields is important to understand thermoelectricity of solutions, aqueous suspensions, bioelectric effects or the properties of wet materials under spatially inhomogeneous temperature conditions. Here we discuss the response of bulk water to external thermal fields using non-equilibrium molecular dynamics simulations, and five widely used forcefields: TIP4P/2005, TIP4P/2005f, OPC, SPC/E and TIP3P. These models all show the thermal polarisation (TP) effect in bulk water, namely the build-up of an electrostatic field induced by the temperature gradient. The strength of this effect is ∼0.1-1 mV K-1 at near-standard conditions for all forcefields, supporting the generality of TP. Moreover, all the models predict a temperature inversion of the polarisation field, although the inversion temperatures vary significantly across different models. We rationalise this result by deriving theoretical equations that describe the temperature inversion as a balance of the isobaric thermal expansion, dipole orientation in the thermal field and the ratio of the molecular dipole/quadrupole moments. Lower ratios lead to higher inversion temperatures. Based on our results, we conclude that the accuracy of the forcefields describing the TP effect decreases as, TIP4P/2005 ∼ TIP4P/2005f ∼ OPC > SPC/E > TIP3P. At coexistence conditions, the inversion temperature is expected to be around 400 K. Furthermore, we establish a correlation between the TP inversion temperature and the temperature corresponding to the minimum of the liquid-vapour surface potential of water
Thermophysical properties of water using reactive force fields.
The widescale importance and rich phenomenology of water continue to motivate the development of computational models. ReaxFF force fields incorporate many characteristics desirable for modeling aqueous systems: molecular flexibility, polarization, and chemical reactivity (bond formation and breaking). However, their ability to model the general properties of water has not been evaluated in detail. We present comprehensive benchmarks of the thermophysical properties of water for two ReaxFF models, the water-2017 and CHON-2017_weak force fields. These include structural, electrostatic, vibrational, thermodynamic, coexistence, and transport properties at ambient conditions (300Â K and 0.997Â g cm-3) and along the standard pressure (1Â bar) isobar. Overall, CHON-2017_weak predicts more accurate thermophysical properties than the water-2017 force field. Based on our results, we recommend potential avenues for improvement: the dipole moment to quadrupole moment ratio, the self-diffusion coefficient, especially for water-2017, and the gas phase vibrational frequencies with the aim to improve the vibrational properties of liquid water
Mass dipole contribution to the isotopic Soret effect in molecular mixtures
Temperature gradients induce mass separation in mixtures in a process called thermal diffusion and are quantified by the Soret coefficient ST. Thermal diffusion in fluid mixtures has been interpreted recently in terms of the so-called (pseudo-)isotopic Soret effect but only considering the mass and moment of inertia differences of the molecules. We demonstrate that the first moment of the molecular mass distribution, the mass dipole, contributes significantly to the isotopic Soret effect. To probe this physical effect, we investigate fluid mixtures consisting of rigid linear molecules that differ only by the first moment of their mass distributions. We demonstrate that such mixtures have non-zero Soret coefficients in contrast with ST = 0 predicted by current formulations. For the isotopic mixtures investigated in this work, the dependence of ST on the mass dipole arises mainly through the thermal diffusion coefficient DT. In turn, DT is correlated with the dependence of the molecular librational modes on the mass dipole. We examine the interplay of the mass dipole and the moment of inertia in defining the isotopic Soret effect and propose empirical equations that include the mass dipole contribution
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