Nanofluidics is a fascinating field with great potentials for applications in science and engineering because of rich unveiled striking phenomena such as superfast flows in carbon nanotubes, fluid slip, and the Debye layer overlap. Those new discoveries benefit from the small size of nanoconfinements due to the high surface-area-to-volume ratio. However, the nanoscale confinement in turn raises considerable challenges. One is that a high infiltration pressure is usually needed to drive fluids into hydrophobic nanochannels. For carbon nanotubes, it has been shown that the infiltration pressure can be as high as 300 MPa. Such a high pressure poses challenges for the applications of nanofluidic systems. To find solutions to reduce the infiltration pressure, systematic investigatio...[ Read more ]

Nanofluidics is a fascinating field with great potentials for applications in science and engineering because of rich unveiled striking phenomena such as superfast flows in carbon nanotubes, fluid slip, and the Debye layer overlap. Those new discoveries benefit from the small size of nanoconfinements due to the high surface-area-to-volume ratio. However, the nanoscale confinement in turn raises considerable challenges. One is that a high infiltration pressure is usually needed to drive fluids into hydrophobic nanochannels. For carbon nanotubes, it has been shown that the infiltration pressure can be as high as 300 MPa. Such a high pressure poses challenges for the applications of nanofluidic systems. To find solutions to reduce the infiltration pressure, systematic investigations of water infiltration into hydrophobic nanochannels through molecular dynamics (MD) simulations are conducted. It is found that the classic Young-Laplace equation is invalid for nanochannels due to the entrance energy barrier. As the channel surface is tuned from superhydrophobic to hydrophobic, the infiltration pressure is greatly reduced by a factor of 7.

Another challenge is to effectively regulate fluid flow in a specific direction with fixed structures at the nanoscale. Conventional microvalves can be used for flow control. However, they usually contain moving parts, which require external actuations to provide driving forces, cause reliability issues at the nanoscale, and damage other delicate molecules. Microscopic fluidic diodes without moving parts, analogous to that of electronic diodes, if they can be developed, would significantly advance micro/nanofluidic technologies. Inspired by the work on the infiltration pressure, three novel, passive nanofluidic diodes (no moving parts) for simple fluids using heterogeneous, nested, and non-uniform nanochannels are developed. It is shown that the fluidic diodes can achieve unidirectional water flows in a wide range of pressure drops. The pressure drop range for the fluidic diodes can be varied by modifying the surface wettability and channel structure. The fluidic diodes can be used for flow control in integrated micro- and nanofluidic devices and fluidic logic operations.