Current inversions induced by resonant coupling to surface waves in a nanosized water pump

We conducted a molecular dynamics simulation to investigate current inversions in a nanosized water pump based on a single-walled carbon nanotube powered by mechanical vibration. It was found that the water current depended sensitively on the frequency of mechanical vibration. Especially in the resonance region, the nanoscale pump underwent reversals of the water current. This phenomenon was attributed to the dynamics competition of the water molecules in the two sections (the left and right parts) divided by the vibrating atom and the differences in phase and decay between the two mechanical waves generated by mechanical vibration and propagating in opposite directions toward the two ends of the carbon nanotube. Our findings provide an insight into water transportation through nanosized pumps and have potential in the design of high-flux nanofluidic systems and nanoscale energy converters.

Figure 1

Upper panel: snapshot of the simulation system. Two reservoirs are connected by an uncapped (10,10) SWNT combined with two carbon planes. The red and white balls represent the oxygen and hydrogen atoms in water molecules, respectively. A vibrating atom of the SWNT (10,10) vibrates along the $x$ axis periodically. Lower panel: zoomed side view of the SWNT. Carbon atoms in the same row as the vibrating atom are labeled with numbers. All molecular visualizations were generated using the VMD software package [43].

Figure 2

Average water flux as a function of the vibration frequency.

Figure 3

Oscillatory motion and wave propagation in the (10,10) SWNT for sys1. (a), (c), (e) Displacement $x-x_0$ versus time for the 7th, 10th, 11th, 12th, 13th, 14th, and 17th carbon atoms in one period under $f=500\mathrm{GHz}$ (a), $f=1666.7\mathrm{GHz}$ (c), $f=2000\mathrm{GHz}$ (e). $x_0$ is the initial $x$ coordinate of the 12th carbon atom, which is the forced-vibration atom. (b), (d), (f): Wave propagation in the SWNT generated by mechanical vibration of the 12th carbon atom under $f=500\mathrm{GHz}$ (b), $f=1666.7\mathrm{GHz}$ (d), $f=2000\mathrm{GHz}$ (f). T is the period. (g): Black bars denote the vibration regions of the carbon atoms in the same top row as the vibrating atom. The height of the bar is twice the vibration amplitude of the carbon atom.

Figure 4

Oscillatory motion and wave propagation for sys2. (a), (c), (e), (g) Displacement $x-x_0$ versus time for the 7th, 10th, 11th, 12th, 13th, 14th, and 17th carbon atoms in one period under $f=500\mathrm{GHz}$ (a), $f=1851.9\mathrm{GHz}$ (c), $f=2777.8\mathrm{GHz}$ (e), $f=3333.3\mathrm{GHz}$ (g). $x_0$ is the initial $x$ coordinate of the 12th carbon atom, which is the vibrating atom. (b), (d), (f), (h) Wave propagation in the SWNT generated by mechanical vibration of the 12th carbon atom under $f=500\mathrm{GHz}$ (b), $f=1851.9\mathrm{GHz}$ (d), $f=2777.8\mathrm{GHz}$ (f), $f=3333.3\mathrm{GHz}$ (h). T is the period. (i): Black bars denote the vibration regions of the carbon atoms in the same top row as the vibrating atom. The height of the bar is twice the vibration amplitude of the carbon atom.

Figure 5

Oscillatory motion and wave propagation for sys3. (a),(c) Displacement $x-x_0$ versus time for the 15th, 21st, 22nd, 23rd, 24th, 25th, and 31st carbon atoms in one period under $f=500\mathrm{GHz}$ (a), $f=1785.7\mathrm{GHz}$ (c). $x_0$ is the initial $x$ coordinate of the 23rd carbon atom, which is the vibrating atom. (b), (d) Wave generated by mechanical vibration of the 23rd carbon atom under $f=500\mathrm{GHz}$ (b), $f=1785.7\mathrm{GHz}$ (d). T is the period. (e): Black bars denote the vibration regions of the carbon atoms in the same top row as the vibrating atom. The height of the bar is twice the vibration amplitude of the carbon atom.

Figure 6

Water distribution along the nanotube axis for sys1 (a), sys2 (b), and sys3 (c).

Figure 7

Hydrogen bonds between water molecules inside the CNT for different frequencies. ξ is the average number of hydrogen bonds per water molecule for the left part or right part of the SWNT. (a), (b), and (c) are results for sys1, sys2, and sys3, respectively.