ABSTRACT
We have fabricated and characterized a novel superhydrophobic system, a meshlike porous superhydrophobic membrane with solid area fraction Φ(s), which can maintain intimate contact with outside air and water reservoirs simultaneously. Oscillatory hydrodynamic measurements on porous superhydrophobic membranes as a function of Φ(s) reveal surprising effects. The hydrodynamic mass oscillating in phase with the membranes stays constant for 0.9â²Φ(s)≤1, but drops precipitously for Φ(s)<0.9. The viscous friction shows a similar drop after a slow initial decrease proportional to Φ(s). We attribute these effects to the percolation of a stable Knudsen layer of air at the interface.
ABSTRACT
A solid body undergoing oscillatory motion in a fluid generates an oscillating flow. Oscillating flows in Newtonian fluids were first treated by G.G. Stokes in 1851. Since then, this problem has attracted much attention, mostly due to its technological significance. Recent advances in micro- and nanotechnology require that this problem be revisited: miniaturized mechanical resonators with linear dimensions in microns and sub-microns-microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), respectively-give rise to oscillating flows when operated in fluids. Yet flow parameters for these devices, such as the characteristic flow time and length scales, may deviate greatly from those in Stokes' solution. As a result, new and interesting physics emerges with important consequences to device applications. In this review, we shall provide an introduction to this area of fluid dynamics, called high-frequency nanofluidics, with emphasis on both theory and experiments.
