Nanoscale fluid flow at solid surfaces
The development of nanofluidic devices has recently revived the interest in problems of flow slip at solid interfaces. Although no-slip remains a trustworthy boundary condition for large-scale flows, significant liquid slip at solid surfaces is observed in micro/nanoscale flows. However, how this slip occurs is unclear. We have made an integrated theoretical and computational effort to better understand the molecular mechanisms of liquid slip at smooth surfaces and to identify the key parameters that control the slip of liquid at rough surfaces.
Possible distance each water molecule within the first layer next to the carbon nanotube wall can travel during a time period of 20 ps (time interval = 5 ps), reproduced from Physical Rev E 83, 036316 (2011)
By simulating the short-term one-dimensional Brownian motions of water columns inside carbon nanotubes (CNT) using molecular dynamics, we obtained the thorough characteristics of the water column vibration. It was confirmed that although water molecules can form layered structures near the CNT walls, nanoscale water columns do have the ability to slip at atomically smooth surface spontaneously caused by the thermal activation at room temperature. By comparing the thermal-induced random driving force and potential energy-induced resistance, we predicted the lower bound of the critical water column length conservatively, under which the slip of water column inside carbon nanotube can spontaneously occur under any given pressure drop at room temperature. The results have been reported in Microfluidics and Nanofluidics.
With the computing power of millions of volunteer computers globally, we carried out extensive MD simulations of water flowing through CNTs at velocities which were directly comparable to experiments. As reported in Nature Nanotechnology, we observed a novel phenomenon, temporal oscillations of shear stress at the interface. We found that the lowest odd-index longitudinal modes of the innertube produce the oscillating behaviour. We also demonstrated an enhancement in the diffusion coefficient of more than 300% due to the coupling between CNT phonon modes and confined water.
These results could have profound implications for phenomena such as transport of ions through CNTs and boron nitride nanotubes, and the functioning of biological channels like aquaporin.
Luming Shen
