Micro-Nano-Meso Scale Mechanics

Nanofluidics

The nanofluidics portion of the nano-micro-meso scale mechanics group is concerned primarily with fluid flow interactions with carbon nanotubes. By utilizing carbon nanotubes (our's are of the order of 20 nanometers in diameter), we aim to study the effects on the carbon nanotubes of nanoscale fluid flow, and also the flow induced by the presence of these structures. Wherever possible, we direct this work toward useful device applications in addition to elucidation of basic mechanics.

In our work, the term nanocarpet is used to describe a nanostructured surface comprised of densely packed, well-aligned, and vertically oriented carbon nanotubes supported on a substrate. A nanocarpet is quite different from other nanostructured (significant features of submicron or smaller size) surfaces currently being studied elsewhere, such as submicron-pitch silicon post surfaces (nanoturf), aligned silicon nanorod arrays, nanosphere lithography created nanobowl arrays, superhydrophobic block-copolymer surfaces, or hydrophobized silicon oxide micro-post arrays. (See images above.)

We form our nanocarpets by self-assembly using thermal chemical vapor deposition (CVD) carried out in a quartz tube furnace. Reaction gases (typically ethylene and hydrogen) flow over the catalyst (usually iron) coated substrates (usually Si or quartz) at reaction temperature (typically 750 C) for a specified time, which controls nanocarpet height.  Scanning electron microscope (SEM) characterization is carried out to determine height, packing, orientation, and growth uniformity of nanocarpet samples. (See images above.)

Properties of nanocarpets grown in this way include super-hydrophobicity (highly non-wettable by pure water), high opacity to visible light (blocks laser light), and good electrical and thermal conductivity.

Objective
We have discovered that nanocarpets can be patterned by fluid forces, especially surface tension, and this presents a method for pattern formation of nanoscale fibers on a surface, which we have termed capillography. First, we wet the nanocarpet in a prescribed manner (drops, dipping, immersion, etc) with a liquid (aqueous or non-), and then remove the fluid by evaporation or withdrawal. This leaves permanent patterns in the dried nanocarpet of rearranged carbon nanotubes, in bowl-like shapes (nests), long trenches, and polygonal bowls. The goal of this project is to elucidate the mechanism of this pattern formation process and develop it as a general self-assembly patterning method. (See images above.)

Key Features
A very important characteristic of this method is that it is fully scalable to very large areas because it is based on self-assembly and does not require any top-down fabrication such as photolithography. It also has the capability of dual functional patterning by deposition of some liquid-borne materials (particles) simultaneously with patterning of the nanoscale structures (in this case the carbon nanotubes). (See image above.)

Potential Applications
This work has many potential applications including memory and data storage, tissue scaffolding, heat transfer, field emission and high density displays in addition to giving rise to a vast array of never observed nanofluidic phenomena.