Combined effects of molecular geometry and nanoconfinement on liquid flows through carbon nanotubes

Molecular dynamics simulations are carried out to investigate the geometry effects of diatomic molecules on liquid flows in carbon nanotubes (CNTs). Oxygen molecules are considered as the fluid inside armchair ($n,n$) ($n=6–20$) CNTs. The simulated fluid temperature and bulk pressure for the liquid state are $T=133$ K and ${\rho }_b=1346{\mathrm{kg}/\mathrm{m}}^3$, respectively. In the agglomerated molecular cluster, nanoconfinement-induced structural changes are observed. As the CNT diameter decreases, it is confirmed that the flow rate significantly increases with irregular trends (discontinuity points in the profiles). From the discussion of the structure of the agglomerated fluid molecules, it is found that those trends are not simply caused by the structural changes. The main factor to induce the irregularity is confirmed to be the interlayer molecular movement affected by the combination of the molecular geometry and the arrangement of the multilayered structure.

Figure 1

Geometry of oxygen molecule; open and filled circles denote centres of mass of oxygen atoms and molecule, respectively.

Figure 2

Geometry of single-walled carbon nanotube.

Figure 3

Phase diagram of oxygen: filed circle denotes the simulated condition for the liquid state; $T^{*}=\left(k_{B}T\right)/\varepsilon$ and ${\rho }^{*}={\sigma }^3{\rho }_b/m, k_B$ is the Boltzmann constant.

Figure 4

Snapshots of structures of liquid oxygen flows inside CNTs at $T=133$ K and ${\rho }_b=1346{\mathrm{kg}/\mathrm{m}}^3$; axial cross-sectional and side views. In the side views, carbon atoms are removed for clarity and the flow direction is from left to right.

Figure 5

Axial velocity $u$ and number density $N$ profiles of liquid oxygen flows inside CNTs at $T=133$ K and ${\rho }_b=1346{\mathrm{kg}/\mathrm{m}}^3$; (a)–(h) correspond to the (7,7), (8,8), (9,9), (10,10), (13,13), (14,14), (15,15), and (20,20) CNTs; $u_f=R^2{N}^{*}f/{\mu }_{\infty }, {\mu }_{\infty }$ and $N^{*}$ are the bulk shear viscosity and the averaged number density, respectively; solid black lines and solid black circles denote number density and velocity distributions by the MD simulations, respectively; red broken lines are the velocity fitting lines by the method of Yasuoka et al. [32].

Figure 6

Flow rate enhancement of liquid oxygen flows inside CNTs at $T=133$ K and ${\rho }_b=1346{\mathrm{kg}/\mathrm{m}}^3$. Argon flow results of Yasuoka et al. [32] at $T=121$ K and ${\rho }_b=1344.3{\mathrm{kg}/\mathrm{m}}^3$ are also plotted.

Figure 7

Cross-sectional views of trajectories of liquid oxygen molecules.

Figure 8

Interlayer momentum of liquid oxygen flows and index of the layered structure; layer 0.5, 1, 1.5, 2, and 2.5 correspond to the single core, single layer (ring), single layer with a center core, double layer, and double layer with a center core structures, respectively.

Figure 9

Comparison of number density profiles of liquid oxygen flows inside CNTs at $T=133$ K and ${\rho }_b=1346{\mathrm{kg}/\mathrm{m}}^3$; (a)–(c) correspond to the (6,6), (7,7), and (8,8) CNTs; solid black lines and red broken lines indicate the molecular mass-center number density and atomic mass-center-based number density profiles, respectively.

Figure 10

Definition of orientational angles: ${\theta }_r,{\theta }_{\phi },{\theta }_z$; $r$ and $z$ axes are the cylindrical coordinate axes of a CNT, sites $p_1,p_2$ are the atomic mass centers of an oxygen molecule.

Figure 11

Orientational angles of oxygen molecules inside CNTs at $T=133$ K and ${\rho }_b=1346{\mathrm{kg}/\mathrm{m}}^3$; (a) mean orientational angles, (b) standard deviation of the orientational angles.