The heat conductivity of liquid crystal phases of a soft ellipsoid string-fluid evaluated by molecular dynamics simulation

Abstract

We have applied a nonequilibrium molecular dynamics heat flow algorithm to calculate the heat conductivity of a molecular model system, which forms uniaxial and biaxial nematic liquid crystals. The model system consists of a soft ellipsoid string-fluid where the ellipsoids interact according to a repulsive version of the Gay-Berne potential. On compression, this system forms discotic or calamitic uniaxial nematic phases depending on the dimensions of the molecules, and on further compression a biaxial nematic phase is formed. In the discotic nematic phase, the heat conductivity has two components, one parallel and one perpendicular to the director, where the last mentioned component is the largest one. This order of magnitudes is reversed in the calamitic nematic phase. In the biaxial nematic phase there are three components of the heat conductivity, one in the direction around which the long axes of the molecules are oriented, this is the largest component, another one in the direction around which the normals of the broadsides of the molecules are oriented, this is the smallest component, and one in the direction perpendicular to these two directions with a magnitude in between those of the first mentioned components. The relative magnitudes of the components of the heat conductivity span a fairly wide interval so it should be possible to use the model to parameterise experimental data.

Large-scale molecular dynamics investigation of geometrical features in nanoporous Si

“…6b, respectively), the HCACF behaviour is initially similar to the equivalent clustered systems (in brown for voids, and orange or cyan for pores), with the exception that there is a marked dip in the autocorrelation function after ~10 ps, corresponding to slightly anti-correlated behaviour in the heat-flux, and which reduces the thermal conductivity. This interesting effect is a characteristic of some liquid and amorphous materials [50][51][52], and could possibly be due to oscillatory behaviour in the HCACF resulting from ballistic phonons moving back and forth [53]. It is interesting that our results suggest that voids/pores materials exhibit anticorrelated heat-flux behaviour, suggesting liquid, amorphous, or oscillatory behaviour, which we will be investigating in future work.…”

Heat current anticorrelation effects leading to thermal conductivity reduction in nanoporous Si

“…Herein, we report on the emergence of anticorrelated (AC) specular phonon scattering (and thus heat flux) as a result of heat trapping between the pores, which can provide up to ~ 80% additional reduction in thermal conductivity for specific nanoporous geometries. Anticorrelated heat flux has been observed in amorphous and fluid materials, but not in crystalline materials [20][21][22][23]. The AC effect can benefit thermoelectric applications, but also find wider application for the control and manipulation of heat carrying phonons in nanophononic metamaterials in general.…”

Twist viscosities and flow alignment of biaxial nematic liquid crystal phases of a soft ellipsoid-string fluid studied by molecular dynamics simulation

“…13 We have chosen to study the last mentioned system because part of the phase diagram is available for it and its heat conductivity has been determined in a previous study. 15 Even though this system cannot be expected to yield quantitative results it should be qualitatively correct. Once experimental results for the twist viscosity of a biaxial nematic liquid crystal become available it will be possible to parameterise them by using this model.…”

“…The shear viscosity, twist viscosity and director alignment angle have been evaluated for this system but only at one state point 14 and a thorough study of its heat conductivity has been carried out. 15 There are basically two ways of evaluating the viscosities and other transport coefficients of a molecular model system by computer simulation methods: equilibrium Green-Kubo methods on one hand and nonequilibrium molecular dynamics (NEMD) methods on the other hand. 16,17 In the former case the integrals of the time correlation function for the transport coefficients are evaluated by an equilibrium molecular dynamics simulation.…”