'Nailing' superlyophobic surfaces with nanotechnology
(Nanowerk Spotlight) When raindrops splash against your window you probably get frustrated because the weather has turned bad again. Physicists and material engineers, on the other hand, are quite fascinated by the process of 'wetting.'
What happens when a fluid is brought in contact with a solid surface is much more complex than you might guess from just looking at your wet window. In physical terms, the process of wetting is driven by the minimum free energy principle - the liquid tends to wet the solid because this decreases the free energy of the system (in this case the system consists of a liquid plus solid).
For low-surface-tension liquids the minimum free energy is achieved only when the liquid completely wets the solid. Understanding these mechanics, and using nanotechnology to structure surfaces to control wetting, has a far-reaching impact for many objects and products in our daily lives - by preventing wear on engine parts or fabricating more comfortable contact lenses, better prosthetics, and self-cleaning materials. The primary measurement to determine wettability is the angle between the solid surface and the surface of a liquid droplet on the solid's surface.
For example, a droplet of water on a hydrophobic surface would have a high contact angle, but a liquid spread out on a hydrophilic surface would have a small one. Surfaces where the contact angle is approaching 180° are called superhydrophobic and surfaces where the contact angle is approaching 0° are called superhydrophilic.
Advanced material engineering techniques can structure surfaces that allow dynamic tuning of their wettability all the way from superhydrophobic behavior to almost complete wetting - but these surfaces only work with high-surface-tension liquids.
Unfortunately, almost all organic liquids that are ubiquitous in human environment such as oils, solvents, detergents, etc. have fairly low surface tensions and thus readily wet even superhydrophobic surfaces. Researchers are now about to create surfaces that would extend superhydrophobic behavior to all liquids, no matter what the surface tension.
Contamination of superhydrophobic surfaces with low-surface-tension organic liquids is one of the leading reasons why superhydrophobic surfaces are not widely used in practical applications. If engineers were to succeed in creating a surface that repels any liquid the practical implications obviously would be substantial. Such materials would have the ultimate self-cleaning and self-decontaminating surfaces. Other applications include control of low-surface tension liquids in microfluidic and lab-on-chip devices, which often require organic solvents to operate.
"Solid-liquid surface systems can't always attain their minimum free energy state - sometimes the system is 'captured' in a state which is not a minimum
free energy state" Dr. Tom N. Krupenkin explains to Nanowerk. "Such states are called metastable. Unlike the minimum-free-energy state which is usually unique, a broad variety of metastable states with various properties can exist. In particular, for the case of low-surface tension liquids minimum-free-energy state is always a wetted state, but a metastable state can be a non-wetting state.
"My idea was to create a surface with a special type of nano-scale topography that would 'lock' the liquid in a metastable nonwetting state, thus preventing the surface from being wetted. Nanonails represent an example of such nano-topography. Moreover, we were able to electrically switch this metastable non-wetting state on and off, thus producing a surface that can be dynamically switched from superlyophobic (i.e. non-wettable state) to highly wettable hydrophilic state."
A nanonail-covered substrate in action. Droplets of two liquids with very different surface tensions, 72mN/m(water) and 21.8 mN/m (ethanol), sit next to each other on the 2-µm-pitch nanonail substrate. (Reprinted with permission from American Chemical Society)
Krupenkin, an Associate Professor in the Department of Mechanical Engineering at the University of Wisconsin-Madison, believes that his team, in collaboration with researchers from Bell Labs at Lucent Technologies, was the first to use metastable states to control wetting properties of solids.
Since there is a much wider range of potentially available metastable states than thermodynamically stable states, the approach proposed by Krupenkin and collaborators can greatly broaden the range of available control over the wetting behavior of solid surfaces.
Researchers call surfaces that extend superhydrophobic behavior to all liquids – no matter what their surface tension – superlyophobic (from the word lyophobic for 'solvent-fearing').
Krupenkin notes that in the initial state, with no voltage applied, these surfaces exhibit contact angles as high as 150° for a wide variety of liquids with surface tensions ranging from 21.8 mN/m (ethanol) to 72.0 mN/m (water). Upon application of an electrical voltage, contact angles of the liquids on these surfaces can be reduced to below 30°.
"Our proposed approach opens a pathway to a simple, material-independent method for creating electrically tunable superlyophobic surfaces and thereby extends many of the potentially attractive properties (high liquid droplet mobility, controllable chemical reaction initiation, tunable drag reduction, etc.)
usually associated with tunable superhydrophobic surfaces to a much broader range of materials and applications" says Krupenkin.
He points out that, in order to extend previously demonstrated, dynamically tunable superhydrophobic nanostructured surfaces into the superlyophobic domain, one needs to understand the reasons that prevent traditional superhydrophobic surfaces from being able to 'repel' low-surface tension liquids.
"When a droplet of a liquid is placed on a very rough surface, such as a fractal-like surface or a surface covered with a carpet of high-aspect-ratio micro- or nanoscale spikes, bumps, or posts, it assumes a state that minimizes its free energy" says Krupenkin. "This can be achieved by the droplet either wetting the rough surface and thus substantially increasing the total area of the liquid-solid interface or by minimizing the liquid-solid contact area by dewetting most of the surface."
Scanning electron microscopy (SEM) image of 2-µm-pitch nanonails. The nail head diameter is about 405 nm, the nail head thickness is about 125 nm, and the nanonail stem diameter is about 280 nm. (Reprinted with permission from American Chemical Society)
To create their superlyophobic surface, the researchers designed a special 3D surface topography that resembles arrays of nails. Each nanonail consists of a conductive silicon stem and a dielectric silicon oxide nail head. The resulting structure is covered with a thin conformal layer of low-surface-energy fluoropolymer.
Krupenkin says that during electrically induced transitions from the nonwetting to the wetting state, the presence of the conductive nail stem allows the researchers to generate high electrical field values in the immediate vicinity of the liquid-air interface. Unless high pressure is applied, the liquid has to stay on top of the nanonails and is unable to penetrate inside and wet the nanostructured layer.
"Obviously, the nanonail geometry is not the only structure that allows one to lock the liquid in the nonwetting state" says Krupenkin. "Essentially any geometry that features 'overhang' analogues to a nail head would produce similar results."
Expanding their work, the researchers still need to better understand the stability of these surfaces with respect to environmental factors such as temperature, mechanical damage, vibrations, etc.
"We would also want to extend this technique to a wider range of materials, most importantly plastics, with the goal of eventually producing practically usable superlyophobic films or coatings" says Krupenkin.