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Superamphiphobic Paper

June 10, 2013

Paper is one of the most common consumer materials. I can't write that it's the most common, since we each buy close to a hundred pounds of another material almost every week. If you haven't guessed, that's the gasoline for our automobiles, since a gallon of gasoline weighs about six pounds.

Although this falls short of the "pint's a pound" standard, hydrocarbon liquids have about the same density, so they weigh about the same, as the following table shows.

LiquidDensity  LiquidDensity
Hexane0.657  Octane0.701
Olive Oil0.703  Decane0.728
Gasoline0.739  Pentane0.755
Kerosene0.820  Toluene0.865
Turpentine0.871  Benzene0.876
Fuel oil0.893  Water1.000

Since paper is ubiquitous, and it has quite a few desirable properties, I've written quite a few articles about it in the past (Paper Chase, September 7, 2010, Paper Accelerometer, March 10, 2011, Crumpled Paper, October 25, 2011, and Crumpling, August 27, 2012). Although the demise of paper as a display medium has been predicted, it doesn't look as if that will happen anytime soon (Electronic Paper, November 4, 2010).

One of the more useful properties of paper is its ability to absorb liquid, and paper towels and facial tissues are the major paper products entering my home. I may buy a ream or two of printer paper every six months; but, in that case, absorbency is a disadvantage, since I spill my coffee far too often. Paper is sometimes coated to inhibit the absorption of water and other fluids.

It's possible to reduce water absorption of surfaces without the addition of a coating. A surface can be microstructured to exhibit the lotus effect, appropriately named after the particular surface microstructure of the water-repellent lotus leaf. The lotus leaf is Superhydrophobic; that is, its surface has excellent water repellent properties. The superhydrophobicity arises from microscopic surface protrusions that force a high contact angle for any droplet.

Leaf of an Indian Lotus, Nelumbo nucifera
Leaf of an Indian Lotus, Nelumbo nucifera. (Portion of a photograph taken in Kolkata, West Bengal, India, by J.M. Garg, via Wikimedia Commons.)

Superhydrophobicity has gained renewed interest, since nanoscale technology has facilitated rapid nanotexturing of surfaces. Five of my articles in 2012 were about superhydrophobicity (Tiny Droplets, March 6, 2012, Superhydrophobic Anti-Glare Glass, May 2, 2012, Icing-Resistant Surfaces, August 8, 2012, Droplet Logic, September 19, 2012, Bubbles and Boiling, September 24, 2012, and Super Condensing Surfaces, November 9, 2012)

It's time to add a new adjective to our portfolio of surface effects. If we replace the "hydro" part of superhydrophobic with "amphi," from the Greek adjective, αμφι, meaning "both," we get superamphiphobic. The both in this case refers to polar liquids, such as water, and non-polar liquids, such as oil. A superamphiphobic material will resist both water and oil; or, in other words, it will be both hydrophobic and oleophobic.

Scientists from the School of Chemical and Biomolecular Engineering and the Institute of Paper Science and Technology at the Georgia Institute of Technology have combined oxygen plasma etching and fluoropolymer deposition to create superamphiphobic paper.[1-2] They were able to achieve liquid drop contact angles greater than 150° for water, ethylene glycol, motor oil, and n-hexadecane.[1]

The paper is made by the creation of surface patterns on both the micrometer and nanometer size scales, followed by a thin fluoropolymer coating.[2] The starting materials are standard softwood and hardwood fibers used in papermaking, and the process is as follows:[2]
• The cellulose fibers are broken up through a mechanical grinding process.

• As in traditional papermaking, the fibers are then pressed in the presence of water and the water is removed.

• Butanol, which inhibits the hydrogen bonding between cellulose fibers, is used for additional processing. This allows better control of the cellulose fiber distribution than the water processing.

• The outer, amorphous layer of the paper is removed by an oxygen plasma etch. This exposes the crystalline cellulose nanofibrils.

• A thin fluoropolymer coating is applied.

The oxygen plasma etch uncovers smaller cellulose structures, and this adds a second level of surface texture needed for the lotus effect.

Dennis Hess and Lester LiMonitoring an oxygen plasma etch are Dennis Hess, a professor in the Georgia Tech School of Chemical and Biomolecular Engineering, and graduate research assistant Lester Li.

(Georgia Tech photograph by Gary Meek.)

Rigid surfaces have been textured by lithography to be superamphiphobic. Hydrophobicity is easiest to achieve, since water has a high surface tension. However, the lower surface tension oils need undercut features to cause re-entrant angles between the surface and droplets.[2] The superamphiphobic paper has been made in samples that are about four inches on a side. However, the process can be scaled up.[2]
Droplets on a superamphiphobic paper sample.Droplets of water, motor oil, ethylene glycol and n-hexadecane on a superamphiphobic paper sample.

(Georgia Tech photograph by Gary Meek.)

Dennis Hess, a professor in the Georgia Tech School of Chemical and Biomolecular Engineering, who teamed on this project with Lester Li, a graduate research assistant, and Victor Breedveld, an associate professor in the Georgia Tech School of Chemical and Biomolecular Engineering, had this to say.
"We believe this is the first time that a superamphiphobic surface - one that repels all fluids - has been created on a flexible, traditional and heterogeneous material like paper."[2]
One application for such paper is a "lab-on-a-sheet," an inexpensive biomedical diagnostic device in which liquid samples would flow along printed channels to combine with antigens and reagents. Says Hess,
"We have shown that we can do the operations necessary for a microfluidic device... We can move the droplet along a pattern, split the droplet and transfer the droplet from one piece of paper to another. We can do all of these operations on a two-dimensional surface."[2]

References:

  1. Lester Li, Victor Breedveld and Dennis W. Hess, "Design and Fabrication of Superamphiphobic Paper Surfaces," ACS Appl. Mater. Interfaces, Article ASAP, May 6, 2013, DOI: 10.1021/am401436m.
  2. John Toon, "Advanced Paper Could be Foundation for Inexpensive Biomedical and Diagnostic Devices," Georgia Tech Press Release, May 28, 2013. Also on the Georgia Tech School of Chemical and Biomolecular Engineering Web Site, and the Georgia Tech Institute of Paper Science and Technology Web Site.

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