Water flow enhancement in hydrophilic nanochannels.

All published reports on fluid flow enhancement and water slippage are associated with hydrophobic surfaces, such as carbon nanotubes. Here, we investigate water flow in hydrophilic alumina nanochannels with diameters ranging from 20 nm to 100 nm. For the smallest channels tested, the water permeability is more than double than the theoretical prediction using the Hagen-Poiseuille equation. Though such an enhancement is significantly smaller than what has been measured in carbon nanotubes, it clearly shows that flow enhancement and water slippage occurs on hydrophilic surfaces as well, contrary to existing theoretical models. To the authors' knowledge, it is the first experimental demonstration of water slippage on hydrophilic surfaces. The results show the dependence of the flow enhancement on the surface chemistry, diameter and length of the nanochannel.