Contact angles, ordering, and solidification of liquid mercury in carbon nanotube cavities

Optimized model potentials for mercury-mercury and mercury-carbon interactions are used in molecular dynamics simulations to study wetting and solidification of liquid mercury encapsulated in single-walled carbon nanotubes. The contact angle of mercury in the nanotube cavity increases linearly with wall curvature. The solid-liquid transition becomes less well defined as nanotube diameter decreases, while the melting temperature drops exponentially. A concentric cylindrical-shell structure is predicted for solidified mercury in small (20,20) nanotubes, while a polycrystalline structure appears in larger (40,40) nanotubes.

Figure 1

(a) Snapshot of an equilibrated mercury drop (4000 atoms) on a graphene sheet (left) with averaged mercury density contour (right) with contact angle determination. (b) Snapshot of an equilibrated mercury slug (12 151 atoms) in a (40,40) single-walled nanotube (top) with corresponding density contour (bottom). Note similarities in ordering of the liquid near the graphene sheet and nanotube walls.

Figure 2

Radial density profile of mercury as a function of the distance from a solid surface. Profiles are shown for a drop resting on a graphene sheet (dotted line), a drop confined between two graphene sheets separated by $54.24\mathrm{\AA }$ (dashed line), and a drop in a (40,40) single-walled nanotube of $\text{diameter}=54.24\mathrm{\AA }$ (solid line).

Figure 3

(a) Outline of menisci for mercury slugs inside the indicated single-walled carbon nanotubes as a function of the distance from the tube centerline. Each shape is fitted with an arc (dotted line). (b) Mercury contact angles obtained from (a) as a function of nanotube curvature.

Figure 4

Total energy of a mercury slug ($7.9\mathrm{nm}$ long) encapsulated in the cavity of (20,20) and (40,40) single-walled nanotubes, calculated as a function of temperature after equilibration.

Figure 5

Solidification temperature as a function of single-walled nanotube radius for a fixed-length mercury slug encapsulated in the nanotube.

Figure 6

(a) Front view of typical equilibrium configurations of encapsulated mercury in a (20,20) single-walled nanotube at various temperatures above and below the solidification point $(\sim 173\mathrm{K})$. Only the centers of mass of the mercury atoms are depicted in the snapshots to facilitate observation. (b) Radial density profiles for the indicated temperatures, averaged over many snapshots.

Figure 7

Radial snapshots of solidified mercury, encapsulated in (20,20), (30,30), and (40,40) single-walled nanotubes at a representative temperature below the solidification point for each nanotube.