Carbon Nanotubes as Thermally Induced Water Pumps

Abstract

Thermal Brownian motors (TBMs) are nanoscale machines that exploit thermal fluctuations to provide useful work. We introduce a TBM-based nanopump which enables continuous water flow through a carbon nanotube (CNT) by imposing an axial thermal gradient along its surface. We impose spatial asymmetry along the CNT by immobilizing certain points on its surface. We study the performance of this molecular motor using molecular dynamics (MD) simulations. From the MD trajectories, we compute the net water flow and the induced velocity profiles for various imposed thermal gradients. We find that spatial asymmetry modifies the vibrational modes of the CNT induced by the thermal gradient, resulting in a net water flow against the thermal gradient. Moreover, the kinetic energy associated with the thermal oscillations rectifies the Brownian motion of the water molecules, driving the flow in a preferred direction. For imposed thermal gradients of 0.5-3.3 K/nm, we observe continuous net flow with average velocities up to 5 m/s inside CNTs with diameters of 0.94, 1.4, and 2.0 nm. The results indicate that the CNT-based asymmetric thermal motor can provide a controllable and robust system for delivery of continuous water flow with potential applications in integrated nanofluidic devices.

Spontaneous directional motion of water molecules in single-walled carbon nanotubes with a stiffness gradient

“…In the NVE ensemble, no external thermostat was applied. Therefore, our directional motion is spontaneously achieved by the stiffness gradient without applying external driving forces, which is different from the motion realized by an external thermal gradient 13,15 and stiffness gradient induced by a thermal gradient. 42 It is worth noting that although the motion of water molecules inside the CNT is a spontaneous process without resorting to any active driving source, a certain amount of energy is still required to push water molecules entering the CNT, which is the general behaviour for any type of water channel.…”

“…Since the demonstration of the excellent behavior of carbon nanotubes (CNTs) as a water transportation channel by Hummer et al 2 in 2001, many nanoscale water channels based on CNTs have been reported, and important progress has been made in understanding their transport behaviors. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] For example, for nanoscale channels based on CNTs, the structure of nanotubes 16 and dopant atoms 17 may greatly affect the water permeation rate. In multi-walled carbon nanotubes (MWCNTs), the nanotube type and layer number may also exert strong inuence on the water transport behaviors.…”

“…The directional motion of water molecules in CNTs has been typically realized by active means, including applications of pressure, 16,25 electrical currents 30 (or charge 12,31 ), selective heating 13,15 or complicated chemical reactions. 32 For instance, Ma et al 25 found that the water ow velocity increases linearly with pressure between the two ends of a CNT.…”

Effect of an external electric field on capillary filling of water in hydrophilic silica nanochannels

“…To elucidate the effect of externally applied electric fields in capillary filling of water in hydrophilic silica nanopores, we conduct a series of all-atom MD simulations using FAST-TUBE, a molecular dynamics simulation package which has widely been used to investigate fundamental fluid dynamics under nanoconfinement. 29,42,45,[63][64][65] Water molecules are described using the simple point charge SPC/E model 66 and the silica atoms by the TTAMm model developed by Guissani and Guillot 67 which is a modification of the original TTAM model. 68 Water-silica interactions are described using Coulomb and Buckingham potentials calibrated in our previous study 69 wherein the experimental value of the water contact angle reported by Thamdrup et al 70 in studies of capillary filling is used as criterion to calibrate the potential parameters.…”

The speed-locking effect of particles on a graphene layer with travelling surface wave

“…At first, the thermophoric phenomena along a carbon nanotube [ 1 – 5 ] or a graphene ribbon [ 6 – 10 ] have been extensively investigated. Thermal fluctuations are confirmed to enable continuous water flow through a carbon nanotube (CNT) by imposing an axial thermal gradient along its surface [ 11 – 13 ]. Nonequilibrium molecular dynamics simulations are done to explore the feasibility of utilizing a thermal gradient on a large graphene substrate to control the motion of a small graphene nanoflake [ 6 ].…”