Local thermal transport of liquid alkanes in the vicinity of α-quartz solid surfaces and thermal resistance over the interfaces: A molecular dynamics study

Thermal transport in liquid $n$-alkanes in the vicinity of α-quartz substrates and thermal boundary resistance between the liquid $n$-alkanes and the α-quartz substrates have been investigated using nonequilibrium molecular dynamics simulations. The study considers two liquid alkanes, methane and decane, and three crystal orientations of α-quartz substrate terminated with -H or -OH groups. The local thermal conductivity (LTC), defined in the same manner as with macroscopic thermal conductivity, is used to measure the efficiency of thermal energy transport of the liquids in the vicinity of the solid surface. The variations in the LTC of the liquid alkanes in the layered region next to the surface of the substrate were examined. The modeled LTC values of the alkanes were found to oscillate in the solid-liquid interface region. These fluctuations were typically proportional to the oscillations in the local density profile. The correlation between the thermal conductivity and density was linear in the bulk liquid region. The correlation between LTC and local density in the first adsorption layer is not a straightforward extension of that of the bulk liquid, which is mostly due to the specific molecular-scale ordering structure that occurs in the liquids formed by the proximity of the solid substrate. Thermal boundary resistance between the liquid alkanes and the quartz substrate was also evaluated. It was observed that thermal boundary resistance is relatively large when the in-plane molecular-scale structure in the first adsorption layer is sparse, and is lower when the liquid structure is dense in the adsorption layer.

Figure 1

Schematic of the α-quartz (001)–methane NEMD system. Heat fluxes are generated from the heat source to the heat sink direction. $\mathrm{\Delta }L$ is the thickness of heat source and $2\mathrm{\Delta }L$ is the thickness of the heat sink.

Figure 2

Temperature distributions in liquid methane along the $z$ axis when in contact with (001), (011), and (100) crystal planes terminated with (a) -H groups and (b) -OH groups.

Figure 3

Variation of thermal conductivity in liquid methane in contact with an OH-terminated (011) crystal plane; error bars were calculated as described in the text. Density profile is shown with a dashed line.

Figure 4

Thermal conductivity distributions in liquid methane in contact with (001), (011), and (100) crystal planes terminated with (a) -H groups and (b) -OH groups. Density profiles are shown with dashed lines.

Figure 5

Thermal conductivity distributions in liquid decane in contact with (001), (011), and (100) crystal planes terminated with (a) -H groups and (b) -OH groups. Density profiles are shown with dashed lines.

Figure 6

Correlation between thermal conductivity and density in the bulk liquid region and in the solid-liquid interface region for (a) methane and (b) decane. Experimental values [45] of thermal conductivity versus density for both alkanes are also shown. Linear fits are included for the bulk data sets; the correlation coefficients $R^2$ for methane and decane are given in the figure.

Figure 7

Thermal boundary resistance at the solid-liquid interface between methane and decane and α-quartz substrates with (001), (100), and (011) crystal surfaces.

Figure 8

In-plane density profiles of methane in first adsorption layer in the vicinity of OH-terminated and H-terminated surfaces for (a) (001), (b) (011), and (c) (100) crystal planes.