Water under temperature gradients: polarization effects and microscopic mechanisms of heat transfer

We investigate, using non-equilibrium molecular dynamics simulations and theory, the response of molecular fluids confined in slit pores under the influence of a thermal gradient and/or an applied force. The applied force which has the same functional form as a gravitational force induces an inhomogeneous density in the confined fluid, which results in a net orientation of the molecules with respect to the direction of the force. The orientation is qualitatively similar to that induced by a thermal gradient. We find that the average degree of orientation is proportional to the density gradient of the fluid in the confined region and that the orientation increases with the magnitude of the force. The concurrent application of the external force and the thermal gradient allows us to disentangle the different mechanisms leading to the thermal orientation of molecular fluids. One mechanism is connected to the density variation of the fluid, while the second mechanism can be readily observed in molecular fluids consisting of molecules with mass or size asymmetry, even in the absence of a density gradient, hence it is connected to the application of the thermal gradient only.

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“…This polarization is similar to that found in water, as first reported in Ref. 9 and studied further in subsequent works [10][11][12][13][14].…”

Section: Introductionsupporting

confidence: 71%

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

Daub

Tafjord

Kjelstrup

et al. 2016

Phys. Chem. Chem. Phys.

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“…We can compare, for example, the largest system with q/e = ±1 in Figure 5, in a thermal gradient very close to that of system 4a we obtain E max = 2.9 × 10 7 V/m. In summary, regardless of the molecular dipole strength, in a temperature gradient ∇T = 1 K/Å we can expect an induced field strength |E| ∼ 2.5×10 7 V/m, perhaps an order of magnitude less than what has been computed for water [5][6][7].…”

Section: Resultsmentioning

confidence: 62%

Thermo-molecular orientation effects in fluids of dipolar dumbbells

Daub

Åstrand

Bresme

2014

Phys. Chem. Chem. Phys.

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We use molecular dynamics simulations in applied thermal gradients to study thermomolecular orientation (TMO) of size-asymmetric dipolar dumbbells with different molecular dipole moments.We find that the direction of the TMO is the same as in apolar dumbbells of the same size, i.e. the smaller atom in the dumbbell tends to orient towards the colder temperature. The ratio of the electrical polarization to the magnitude of the thermal gradient does not vary much with the magnitude of the molecular dipole moment. We also investigate a novel second order TMO that persists even in size-symmetric dipolar dumbbells where molecules have a slight tendency to orient perpendicular to the gradient except very close to the hot region, where (anti-)parallel orientations are preferred. Finally, we investigate rotational correlation functions and characteristic rotational times in these systems in an attempt to model possible spectroscopic signatures of TMO in experiments. Although we cannot detect any difference in integrated rotational times between equilibrium simulations and simulations in a thermal gradient, more careful modelling of the anisotropic rotational dynamics in the thermal gradient may be more successful.

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“…The general conclusion from these studies is that the thermal conductivity is overestimated, typically by about 20%, at temperatures close to 300 K and 1 bar pressure. Some of these studies have given an indication that the models can reproduce the anomalous increase of the thermal conductivity with temperature 18,21,22 .…”

Section: Introductionmentioning

confidence: 99%

“…The thermal conductivity of water has been computed before using a variety of techniques and models [17][18][19][20][21][22] . Most of these studies have focused on a specific temperature and density or a narrow range of thermodynamic states.…”

Section: Introductionmentioning

confidence: 99%

Nonequilibrium molecular dynamics simulations of the thermal conductivity of water: A systematic investigation of the SPC/E and TIP4P/2005 models

Römer

Lervik²,

Bresme

2012

The Journal of Chemical Physics

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We report an extensive non equilibrium molecular dynamics investigation of the thermal conductivity of water using two of the most accurate rigid non polarizable empirical models available, SPC/E and TIP4P/2005. Our study covers liquid and supercritical states. Both models predict the anomalous increase of the thermal conductivity with temperature and the thermal conductivity maximum, hence confirming their ability to reproduce the complex anomalous behaviour of water. The performance of the models strongly depends on the thermodynamic state investigated, and best agreement with experiment is obtained for states close to the liquid coexistence line and at high densities and temperatures. Considering the simplicity of these two models the overall agreement with experiments is remarkable. Our results show that explicit polarizability and molecular flexibility are not needed to reproduce the anomalous heat conduction of water.

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Water under temperature gradients: polarization effects and microscopic mechanisms of heat transfer

Cited by 30 publications

References 42 publications

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

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

Thermo-molecular orientation effects in fluids of dipolar dumbbells

Nonequilibrium molecular dynamics simulations of the thermal conductivity of water: A systematic investigation of the SPC/E and TIP4P/2005 models

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