1. Motivation / Overview
Nanofluidics is a rapidly emerging and important research field in nano-bio science and technology1. Despite its importance, liquid properties at the nano-scale are not fully understood. There have been several efforts to define and control nano-scale fluidic behavior by making nanochannels2. However, they suffer from difficulties in controlling the extrusion fluid of the liquid and defining nanofluidics through nano-aperture in ambient conditions due to external high atmospheric pressure coming from the geometry of nano-constructions. Here, we first demonstrate nano-scaled liquid extrusion through a nano-aperture pipette (30 nm) based on the optical microscope combined quartz tuning fork – atomic force microscopy (OM QTF-AFM) system in ambient conditions3.
1. W. Sparreboom, A. van den Berg and J. C. T. Eijkel, “Principles and applications of nanofluidic transport”, Nature Nanotech. 332, 713-720 (2009) [Link]
2. Nevin Naguib, Haihui Ye, Yury Gogotsi, Almila G. Yazicioglu, Constantine M. Megaridis, and Masahiro Yoshimura,”Observation of water confined in nanometer channels of closed carbon nanotubes”, nano Lett. 4, 2237-2243 (2004) [Link]
3. F. J. Giessibl, “Advances in atomic force microscopy”, Rev. Mod. Phys. 75, 949-983 (2003) [Link]
2. Nanopipette – Pulled Nanopipette
A pencil-shaped nano-aperture glass pipette which was fabricated using thermal heating followed by mechanical pulling (P-2000, Sutter Instrument Co.)
When dye molecules pass through the nano-aperture, the dye molecules are aggregated by the interplay of hydrophobic and electrostatic interactions. The aggregates were small enough to pass through the 30 nm aperture, but it was difficult to define their sizes using a fluorescence microscope because of the blowing effect.
3. Experimental Setup
Using the naturally formed capillary condensation on the apex of the nanopipette, the filled liquid inside the nanopipette can be connected and ejected onto the outside surface by an applied electric field between the inserted electrode inside the nanopipette and the substrate.
(ⅰ) forming confined nano-water (ⅱ) applying electric field (ⅲ) capturing the extrusion images.
4. Results and Discussion
The rupture distance can be interpreted as the amount and size of the water nano-meniscus. After retraction, the elongated lengths of each meniscus before rupture were approximately 1 nm, 63 nm, and 171 nm, respectively.
At 12 V, the output QTF signal was suddenly decreased and the rupture occurred at a significantly elongated separation of 161 nm, resulting from extraction of the filled liquid solution by the electrostatic force.
After 50 s, the each spread areas (the spread speeds) of the liquids were 243.3 μm2 (176 nm/s), 167.4 μm2 (146 nm/s), and 254.5 μm2 (180 nm/s), and each liquid’s flow rates were 12.4 pL/s, 4.5 pL/s, and 12.2 pL/s, respectively.
One may also combine this nanopipette-based nanofluidics system with fluorescence microscope for investigating the organic molecule’s behavior through the nano-scaled liquid channel.