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.

Figure 1

Tangent sphere models used in this work, with size ratios ${\sigma }_2/{\sigma }_1=1$ (a), $3/4$ (b), $2/3$ (c), $1/2$ (d), $1/3$ (e). The arrow defines the molecule orientation. The vector points from the bigger to the smaller site, or from the heavier to the lighter site in the case of asymmetry in mass.

Figure 2

(a) Orientation of the diatomic molecule versus the reduced temperature for different size ratios. From bottom to top ${\sigma }_2/{\sigma }_1=1$, $3/4$, and $1/3$. The mass in all cases is $m_2/m_1=1$ and the temperature gradient $\nabla T^{*}=0.08$. The inset shows the same plot but for the product $⟨\cos ({\theta }_x)⟩T^{*}$ in the $y$ axis. This plot is used to test the NET equations derived in the Letter. See main text for details and discussion. (b) Dependence of the orientation with particle packing fraction for ${\sigma }_2/{\sigma }_1=1/2$ as a function of the temperature gradient, $\nabla T^{*}=0.08$, 0.12 and 0.14. The average temperature for all the systems is $⟨T^{*}⟩=1.3$. The inset shows the rescaled plot for the orientation, i.e., $⟨\cos ({\theta }_x)⟩/(\nabla T^{*}/0.08)$, to test the NET equations derived in the Letter. See discussion in the main text for details.

Figure 3

Correlation between the orientation of the diatomic molecules and the Soret coefficient, $s_T^{*}=s_T\epsilon /k_B$, of the corresponding binary mixture. $s_T^{*}<0$ corresponds to ${\sigma }_1\ne {\sigma }_2$ (see labels in the figure for size ratios), and $m_1=m_2$. $s_T^{*}>0$ corresponds to ${\sigma }_1={\sigma }_2$ and $m_1\ne m_2$ (see labels in the figure for mass ratios). All the simulations were performed at the average packing fraction $\eta =0.37$, temperature gradients, $\nabla T^{*}=0.08$, and average temperatures $⟨T^{*}⟩=1.3$ (filled symbols) and $⟨T^{*}⟩=1.0$ (open symbols).