Quantitative characterization of liquids flowing in geometrically controlled sub-100 nm nanofluidic channels

With development of nanotechnologies, applications exploiting nanospaces such as single-molecule analysis and high-efficiency separation have been reported, and understanding properties of fluid flows in 10¹ nm to 10² nm scale spaces becomes important. Nanofluidics has provided a platform of nanochannels with defined size and geometry, and revealed various unique liquid properties including higher water viscosity with dominant surface effects in 10² nm spaces. However, experimental investigation of fluid flows in 10¹ nm spaces is still difficult owing to lack of fabrication procedure for 10¹ nm nanochannels with smooth walls and precisely controlled geometry. In the present study, we established a top-down fabrication process to realize fused-silica nanochannels with 10¹ nm scale size, 10⁰ nm roughness and rectangular cross-sectional shape with an aspect ratio of 1. Utilizing a method of mass flowmetry developed by our group, accurate measurements of ultra-low flow rates in sub-100 nm nanochannels with sizes of 70 nm and 100 nm were demonstrated. The results suggested that the viscosity of water in these sub-100 nm nanochannels was approximately 5 times higher than that in the bulk, while that of dimethyl sulfoxide was similar to the bulk value. The obtained liquid permeability in the nanochannels can be explained by a hypothesis of loosely structured liquid phase near the wall generated by interactions between the surface silanol groups and protic solvent molecules. The present results suggest the importance of considering the species of solvent, the surface chemical groups, and the size and geometry of nanospaces when designing nanofluidic devices and membranes.