Simulation of water impregnation through vertically aligned CNT forests using a molecular dynamics method

Abstract

The flow rate of water through carbon nanotube (CNT) membranes is considerably large. Hence, CNT membranes can be used in nanofluidic applications. In this work, we performed a molecular dynamics (MD) simulation of the introduction of water into CNTs in the CNT membranes, especially in vertically aligned CNT forests. The results showed that the Knudsen number (Kn) increased with an increasing volume fraction of CNT (VC) and was greater than 10−3 for each VC. Beyond this value, the flow became a slip flow. Further, the permeability increased as VC increased in the actual state calculated by the MD simulation, whereas the permeability in the no-slip state predicted by the Hagen–Poiseuille relationship decreased. Thus, a clear divergence in the permeability trend existed between the states. Finally, the flow enhancement ranged from 0.1 to 23,800, and the results show that water easily permeates as VC increases.

Nanojunction Effects on Water Flow in Carbon Nanotubes

“…Since we study water imbibition in small CNTs, the contact angle (CA) of water with the internal surface of the nanotubes is important. The CA of water on the surface of CNTs has been reported to be 26 , 50 , 51 , θ ≈ 57°, which is in the middle of the range of the experimental values of the CA on graphite 52 . Using MD simulation, we also computed the CA for water droplets in (20, 20) and (30, 30) CNTs, using the method proposed by Werder et al .…”

“…The timestep was 2 fs, and all the simulations were at 300 K. We used the TIP3P model of water 49 . We used the Amber-96 force field, which has been used extensively in the past to represent water-carbon interaction in CNTs 26 , 50 , 51 .…”

Elucidation of high permeability water among VACNFs using molecular dynamics

“…In addition, although flow investigations in CNTs have been conducted analytically and experimentally, flow investigations outside CNTs have rarely been conducted. In the analytical investigation of the impregnation of external CNTs, water impregnation simulations in VACNFs were conducted to investigate the flow between CNTs 15 . Moreover, the applicability of a continuous mechanics model based on the Gebart equation 16 was derived using a assumed Hagen–Poiseuille flow with no-slip condition with respect to the permeability coefficient and evaluation of the flow trend in the nanosized pores have been investigated 15 .…”

Evaluation of permeability applicability based on continuum mechanics law in fluid flow through graphene membrane

“…Although it does not permeate a fluid such as He, a permeability of 10 10 times larger than in the case of water has been reported. However, because a flow at the nanoscale has a relatively large influence on the wall surface, it is empirically known that such a flow deviates from the basic continuum law, including a continuous equation or non-slip condition 7 . Therefore, even in the flow in a graphene filtration membrane, these basic laws cannot be applied, and there is a possibility that the desired permeation level cannot be obtained.…”

“…It is known that such deviations begin to occur when the Knudsen number (dimensionless number divided by the liquid’s molecular diameter based on the characteristic length of the flow path) exceeds 10 −3 8,9 . Particularly within the range of 10 −3 –10 −1 , it is empirically known that the continuum model can be applied by providing the wall slip boundary conditions 7,10–13 . For example, Bhatia et al .…”