Spontaneous liquid outflow from hydrophobic nanopores: Competing liquid–solid and liquid–gas interactions

Abstract

Understanding liquid flow behavior in a nanoenvironment is of utmost importance for a vast array of biological and engineering applications. Both the liquid–solid interaction (LSI) and liquid–gas interaction (LGI) significantly affect the liquid outflow from hydrophobic nanopores in a liquid–gas–nanopore system. However, the individual contribution of these interactions on the spontaneous liquid outflow from the hydrophobic nanopores is not clear. To fill this scientific gap, we present an experimental study of the competition between the LSI and LGI on the nanoscale liquid outflow behavior in a liquid nanofoam (LN) system. LN samples, consisting of water and nanoporous silicas with similar pore size distribution but different average pore sizes, are subjected to cyclic pressure-induced liquid infiltration-outflow tests with or without the presence of a gas phase. Considerable difference in the liquid outflow behavior has been observed as pore size varies, indicating the size effect on the competition between the LSI and LGI. In smaller nanopores, the stronger LSI dominates the liquid outflow behavior as the LGI is diminished due to the nanoconfinement enhanced gas oversolubility. In contrast, in larger nanopores, LGI becomes a stronger driving force for the liquid outflow. These fundamental findings provide important insights on liquid flow behavior in a nanoenvironment and can facilitate a rational design of high performance nanofluidics-based systems.