Direct visualization of fluid dynamics in sub-10 nm nanochannels.

Optical microscopy is the most direct method to probe fluid dynamics at small scales. However, contrast between fluid phases vanishes at ∼10 nm lengthscales, limiting direct optical interrogation to larger systems. Here, we present a method for direct, high-contrast and label-free visualization of fluid dynamics in sub-10 nm channels, and apply this method to study capillary filling dynamics at this scale. The direct visualization of confined fluid dynamics in 8-nm high channels is achieved with a conventional bright-field optical microscope by inserting a layer of a high-refractive-index material, silicon nitride (SiN), between the substrate and the nanochannel, and the height of which is accurately controlled down to a few nanometers by a SiO spacer layer. The SiN layer exhibits a strong Fabry-Perot resonance in reflection, providing a sharp contrast between ultrathin liquid and gas phases. In addition, the SiN layer enables robust anodic bonding without nanochannel collapse. With this method, we demonstrate the validity of the classical Lucas-Washburn equation for capillary filling in the sub-10 nm regime, in contrast to the previous studies, for both polar and nonpolar liquids, and for aqueous salt solutions.