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 (300 K and 0.997 g cm^-3) and along the standard pressure (1 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.

Optical Tweezers: Phototoxicity and Thermal Stress in Cells and Biomolecules

For several decades optical tweezers have proven to be an invaluable tool in the study and analysis of myriad biological responses and applications. However, as with every tool, they can have undesirable or damaging effects upon the very sample they are helping to study. In this review the main negative effects of optical tweezers upon biostructures and living systems will be presented. There are three main areas on which the review will focus: linear optical excitation within the tweezers, non-linear photonic effects, and thermal load upon the sampled volume. Additional information is provided on negative mechanical effects of optical traps on biological structures. Strategies to avoid or, at least, minimize these negative effects will be introduced. Finally, all these effects, undesirable for the most, can have positive applications under the right conditions. Some hints in this direction will also be discussed.

Microscopic analysis of thermo-orientation in systems of off-centre Lennard-Jones particles

When fluids of anisotropic molecules are placed in temperature gradients, the molecules may align themselves along the gradient: this is called thermo-orientation. We discuss the theory of this effect in a fluid of particles that interact by a spherically symmetric potential, where the particles' centres of mass do not coincide with their interaction centres. Starting from the equations of motion of the molecules, we show how a simple assumption of local equipartition of energy can be used to predict the thermo-orientation effect, recovering the result of Wirnsberger et al. [Phys. Rev. Lett. 120, 226001 (2018)]. Within this approach, we show that for particles with a single interaction centre, the thermal centre of the molecule must coincide with the interaction centre. The theory also explains the coupling between orientation and kinetic energy that is associated with this non-Boltzmann distribution. We discuss deviations from this local equipartition assumption, showing that these can occur in linear response to a temperature gradient. We also present numerical simulations showing significant deviations from the local equipartition predictions, which increase as the centre of mass of the molecule is displaced further from its interaction centre.

Effects of nanoparticle heating on the structure of a concentrated aqueous salt solution

The effects of a rapidly heated nanoparticle on the structure of a concentrated aqueous salt solution are studied using molecular dynamics simulations. A diamond-like nanoparticle of radius 20 Å is immersed in a sodium-chloride solution at 20% above the experimental saturation concentration and equilibrated at T = 293 K and P = 1 atm. The nanoparticle is then rapidly heated to several thousand degrees Kelvin, and the system is held under isobaric-isoenthalpic conditions. It is observed that after 2-3 ns, the salt ions are depleted far more than water molecules from a proximal zone 15-25 Å from the nanoparticle surface. This leads to a transient reduction in molality in the proximal zone and an increase in ion clustering in the distal zone. At longer times, ions begin to diffuse back into the proximal zone. It is speculated that the formation of proximal and distal zones, and the increase in ion clustering, plays a role in the mechanism of nonphotochemical laser-induced nucleation.

Theoretical Prediction of Thermal Polarization

We present a mean-field theory to explain the thermo-orientation effect in an off-center Stockmayer fluid. This effect is the underlying cause of thermally induced polarization and thermally induced monopoles, which have recently been predicted theoretically. Unlike previous theories that are based either on phenomenological equations or on scaling arguments, our approach does not require any fitting parameters. Given an equation of state and assuming local equilibrium, we construct an effective Hamiltonian for the computation of local Boltzmann averages. This simple theoretical treatment predicts molecular orientations that are in very good agreement with simulation results for the range of dipole strengths investigated. By decomposing the overall alignment into contributions from the temperature and density gradients, we shed further light on how the nonequilibrium result arises from the competition between the two gradients.

Numerical evidence for thermally induced monopoles

Electric charges are conserved. The same would be expected to hold for magnetic charges, yet magnetic monopoles have never been observed. It is therefore surprising that the laws of nonequilibrium thermodynamics, combined with Maxwell's equations, suggest that colloidal particles heated or cooled in certain polar or paramagnetic solvents may behave as if they carry an electric/magnetic charge. Here, we present numerical simulations that show that the field distribution around a pair of such heated/cooled colloidal particles agrees quantitatively with the theoretical predictions for a pair of oppositely charged electric or magnetic monopoles. However, in other respects, the nonequilibrium colloidal particles do not behave as monopoles: They cannot be moved by a homogeneous applied field. The numerical evidence for the monopole-like fields around heated/cooled colloidal particles is crucial because the experimental and numerical determination of forces between such colloidal particles would be complicated by the presence of other effects, such as thermophoresis.

Molecular alignment in molecular fluids induced by coupling between density and thermal gradients

A confined apolar dumbbell fluid subject to both thermal gradients and an external force shows orientational preferences that depend on both contributions.

Heat conduction in double-walled carbon nanotubes with intertube additional carbon atoms

Theoretical insights into the heat transfer performance and its reduction mechanisms in double-walled carbon nanotubes with intertube additional carbon atoms.

Water polarization induced by thermal gradients: the extended simple point charge model (SPC/E)

We investigate the non-equilibrium response of extended simple point charge (SPC/E) water to thermal gradients. Using non-equilibrium molecular dynamics simulations, we show that SPC/E water features the thermo-polarization orientation effect, namely, water becomes polarized as a response to a thermal gradient. The polarization field increases linearly with the thermal gradient, in agreement with predictions of non-equilibrium thermodynamics theory. This observation confirms the generality of the thermo-polarization effect, first reported using the Modified Central Force Model (MCFM), and shows this physical effect is present irrespective of the water model details, in particular, dipole moment magnitude and model flexibility. The magnitude of the effect is the same for both models, although the sign of the electrostatic field is reversed in going from the MCFM to the SPC/E model. We further analyze the impact that the molecular geometry and mass distribution has on the magnitude of the polarization. Our results indicate that the thermo-polarization effect should be observed in a wide range of polar fluids, including fluids where hydrogen bonding is not present. Using various molecular models, we show that the polarization of these fluids under appropriate thermodynamic conditions can be of the same order or stronger than in water.

Thermo-molecular orientation effects in fluids of dipolar dumbbells

Plots of first-order (left) and novel second-order (right) thermomolecular orientation effects in fluids of dipolar dumbbells.

Enhancement of the thermal polarization of water via heat flux and dipole moment dynamic correlations

It has been recently shown that liquid water polarizes as a response to a temperature gradient. This polarization effect can be significant for temperature gradients that can be achieved at micro and nanoscales. In this paper we investigate the dependence of the polarization response of liquid and supercritical water at different thermodynamic conditions using both equilibrium and nonequilibrium molecular dynamics simulations for the extended point charge water model. We find that the thermal polarization features a nonmonotonic behavior with temperature, reaching a maximum response at specific thermodynamic states. We show that the thermal polarization is maximized when the density of states of the heat flux and dipole moment correlation functions feature the strongest overlap. The librational modes of water are shown to play an important role in determining this behavior as well as the heat transport mechanism in water. The librational frequencies show a significant dependence with temperature and pressure. This dependence provides a microscopic mechanism to explain the observed maximization of the thermal-polarization effect. Our work provides new microscopic insights on the mechanism determining the orientation of polar fluids under thermal gradients, as well as new strategies to maximize their orientation by manipulating the dynamic correlations between the heat flux and the sample dipole moment.

Out-of-equilibrium one-dimensional disordered dipole chain

We consider a chain of one-dimensional dipole moments connected to two thermal baths with different temperatures. The system is in nonequilibrium steady state and heat flows through it. Assuming that fluctuation of the dipole moment is a small parameter, we develop an analytically solvable model for the problem. The effect of disorder is introduced by randomizing the positions of the dipole moments. We show that the disorder leads to Anderson-like transition from conducting to a thermal insulating state of the chain. It is shown that considered chain supports both ballistic and diffusive heat transports depending on the strength of the disorder. We demonstrate that nonequilibrium leads to the emergence of the long-range order between dipoles along the chain and make the conjecture that the interplay between nonequilibrium and next-to-nearest-neighbor interactions results in the emergence of long-range correlations in low-dimensional classical systems.

Alkali halide solutions under thermal gradients: soret coefficients and heat transfer mechanisms

We report an extensive analysis of the non-equilibrium response of alkali halide aqueous solutions (Na(+)/K(+)-Cl(-)) to thermal gradients using state of the art non-equilibrium molecular dynamics simulations and thermal diffusion forced Rayleigh scattering experiments. The coupling between the thermal gradient and the resulting ionic salt mass flux is quantified through the Soret coefficient. We find the Soret coefficient is of the order of 10(-3) K(-1) for a wide range of concentrations. These relatively simple solutions feature a very rich behavior. The Soret coefficient decreases with concentration at high temperatures (higher than T ∼ 315 K), whereas it increases at lower temperatures. In agreement with previous experiments, we find evidence for sign inversion in the Soret coefficient of NaCl and KCl solutions. We use an atomistic non-equilibrium molecular dynamics approach to compute the Soret coefficients in a wide range of conditions and to attain further microscopic insight on the heat transport mechanism and the behavior of the Soret coefficient in aqueous solutions. The models employed in this work reproduce the magnitude of the Soret coefficient, and the general dependence of this coefficient with temperature and salt concentration. We use the computer simulations as a microscopic approach to establish a correlation between the sign and magnitude of the Soret coefficients and ionic solvation and hydrogen bond structure of the solutions. Finally, we report an analysis of heat transport in ionic solution by quantifying the solution thermal conductivity as a function of concentration. The simulations accurately reproduce the decrease of the thermal conductivity with increasing salt concentration that is observed in experiments. An explanation of this behavior is provided.

Thermomolecular orientation of nonpolar fluids

We investigate the response of molecular fluids to temperature gradients. Using nonequilibrium molecular dynamics computer simulations we show that nonpolar diatomic fluids adopt a preferred orientation as a response to a temperature gradient. We find that the magnitude of this thermomolecular orientation effect is proportional to the strength of the temperature gradient and the degree of molecular anisotropy, as defined by the different size or mass of the molecular atomic sites. We show that the preferred orientation of the molecules follows the same trends observed in the Soret effect of binary mixtures. We argue this is a general effect that should be observed in a wide range of length scales.