Nanobubbles and gas dynamics during capillary filling of nanochannels

Abstract

This paper focuses on capillary filling at the nanoscale where deviations to the Washburn’s classical theory are observed. Imbibition experiments in microfabricated silicon-glass nanochannels with low aspect ratio (width >> depth and depths going from 400 nm down to 20 nm) are performed for several liquids. In all cases, as predicted by the Washburn’s law, liquid invasion front location evolves as the square root of time. However, filling kinetics slowdown compared to the Washburn’s law is measured in nanochannels for depths below ~ 100 nm. Furthermore, below a liquid-dependent depth threshold, we observe spontaneous bubbles formation behind the advancing meniscus. Bubbles dynamics (formation conditions and lifetime) are analyzed thanks to our experimental data involving several liquids and nanochannels depths. Viscous resistance induced by the bubbles presence is estimated using an effective medium approach. Conjointly, gas flow ahead of the advancing meniscus is modeled considering the gas as viscous and compressible. Influence of these effects on the filling kinetics is discussed.