71 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
The impact of the interfacial Kapitza resistance on colloidal thermophoresis
Thermal gradients impart a force on colloidal particles pushing the colloids
towards cold or hot regions, a phenomenon called thermophoresis. Existing
theories describe thermophoresis by considering the local perturbation of the
thermal field around the colloid. While these approaches incorporate
interfacial surface free energies, they have consistently ignored the impact of
the Kapitza resistance associated with the colloid-solvent interface. We
propose a theoretical approach to include interfacial Kapitza resistance
effects, and we test the new equations using non-equilibrium molecular dynamics
simulations. We demonstrate that the Kapitza resistance influences the local
thermal field around a colloid, modifying the Soret coefficient. We conclude
that interfacial thermal conductance effects must be included to describe
thermophoresis.Comment: Main paper/: 6 pages, 4 figures; Supplementary: 6 pages, 6 figure
Theory and simulation of central force model potentials: Application to homonuclear diatomic molecules
14 pags., 14 figs., 6 tabs.Structure and thermodynamics of fluids made of particles that interact via a central force model potential are studied by means of Monte Carlo simulations and integral equation theories. The Hamiltonian has two terms, an intramolecular component represented by a harmonic oscillatorlike potential and an intermolecular interaction of the Lennard-Jones type. The potential does not fulfill the steric saturation condition so it leads to a polydisperse system. First, we investigate the association (clustering) and thermodynamic properties as a function of the potential parameters, such as the intramolecular potential depth, force constant, and bond length. It is shown that the atomic hypernetted chain (HNC) integral equation provides a correct description of the model as compared with simulation results. The calculation of the HNC pseudospinodal curve indicates that the stability boundaries between the vapor and liquid phases are strongly dependent on the bond length and suggests that there might be a direct gas-solid transition for certain elongations. On the other hand, we have assessed the ability of the model to describe the thermodynamics and structure of diatomic liquids such as N2 and halogens. To this end we have devised a procedure to model the intramolecular potential depth to reproduce the complete association limit (i.e., an average number of bonds per particle equal to one). This constraint is imposed on the Ornstein-Zernike integral equation in a straightforward numerical way. The structure of the resulting fluid is compared with results from molecular theories. An excellent agreement between the HNC results for the associating fluid and the reference interaction site model (RISM)-HNC computations for the atom-atom model of the same fluid is obtained. There is also a remarkable coincidence between the simulation results for the molecular and the associating liquids, despite the polydisperse character of the latter. The stability boundaries in the complete association limit as predicted by the HNC integral equation have been computed for different bond lengths corresponding to real molecular liquids. These boundaries appear close to the experimental liquid branch of the vapor-liquid coexistence line of the molecular systems under consideration. © 1996 American Institute of Physics.This work was partially supported by Grants No. PB93-
0085 and PB94-0112 furnished by the Direccion General de
Investigacion Cientıfica y Tecnologica ~DGICYT! of Spain.
FB acknowledges a predoctoral fellowship awarded by the
Universidad Complutense de Madrid
On the Thermodynamic Efficiency of Ca2+-ATPase Molecular Machines
AbstractExperimental studies have shown that the activity of the reconstituted molecular pump Ca2+-ATPase strongly depends on the thickness of the supporting bilayer. It is thus expected that the bilayer structure will have an impact on the thermodynamic efficiency of this nanomachine. Here, we introduce a nonequilibrium-thermodynamics theoretical approach to estimate the thermodynamic efficiency of the Ca2+-ATPase from analysis of available experimental data about ATP hydrolysis and Ca2+ transport. We find that the entropy production, i.e., the heat released to the surroundings under working conditions, is approximately constant for bilayers containing phospholipids with hydrocarbon chains of 18–22 carbon atoms. Our estimates for the heat released during the pump operation agree with results obtained from separate calorimetric experiments on the Ca2+-ATPase derived from sarcoplasmic reticulum. We show that the thermodynamic efficiency of the reconstituted Ca2+-ATPase reaches a maximum for bilayer thicknesses corresponding to maximum activity. Surprisingly, the estimated thermodynamic efficiency is very low, ∼12%. We discuss the significance of this result as representative of the efficiency of other nanomachines, and we address the influence of the experimental set-up on such a low efficiency. Overall, our approach provides a general route to estimate thermodynamic efficiencies and heat dissipation in experimental studies of nanomachines
Thermophoretic torque in colloidal particles with mass asymmetry
We investigate the response of anisotropic colloids suspended in a fluid under a thermal field. Using nonequilibrium molecular dynamics computer simulations and nonequilibrium thermodynamics theory, we show that an anisotropic mass distribution inside the colloid rectifies the rotational Brownian motion and the colloids experience transient torques that orient the colloid along the direction of the thermal field. This physical effect gives rise to distinctive changes in the dependence of the Soret coefficient with colloid mass, which features a maximum, unlike the monotonic increase of the thermophoretic force with mass observed in homogeneous colloids
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