Wetter Report: New Approach to Testing Surface Adhesion

May 11, 2007
Wetter Report: New Approach to Testing Surface Adhesion
Wetability gradient: Water sprayed on a glass slide coated with a nanostructured gradient wettability film using the new NIST technique illustrates the transition from (A) superhydrophobicity to (C) superhydrophilicity. The lower image shows the magnified image of the (A) hydrophobic to (B) transition wetting region. The pink dotted line indicates the border of the superhydrophobic region, and the yellow dotted region shows a hydrophobic 'sticky' region. Credit: NIST

With a nod to one of nature's best surface chemists—an obscure desert beetle—polymer scientists at the National Institute of Standards and Technology have devised a convenient way to construct test surfaces with a variable affinity for water, so that the same surface can range from superhydrophilic to superhydrophobic, and everything in between. Their technique, reported in a recent issue of the journal Langmuir, may be used for rapid evaluation of paints and other materials that need to stick to surfaces.

The NIST team developed a flexible technique, based on ultraviolet light and photosensitive materials, to mimic one of nature's cleverest feats of surface chemistry. The Stenocara beetle of Africa's Namib Desert is able to thrive in a habitat so parched that not even the morning fog will condense. All the beetle has to do is raise its warty-looking wing covers into the breeze.

Because the bumps are hydrophilic, or water-attracting, while the rest of the surface is hydrophobic, or water-repelling, the few water molecules that do strike the wing covers tend to get pushed uphill and collect on the bumps—where they eventually condense into artificial dewdrops that roll into the insect's mouth. The insect's trick is to use both surface structure and chemistry to create a surface that shifts rapidly from hydrophobic to hydrophilic.

The NIST researchers begin by coating the surface with a matrix of silica granules about 11 nanometers across. As with the beetle, whose wing covers are coated with organic particles about a thousand times larger, the spacing of the matrix provides a first, purely physical level of control over wettability: a water droplet placed atop the granules can sag only just so far into the gaps before it is stopped by surface tension.

The researchers then add a second level of control by coating the granules with a compound that changes their water affinity, in much the same way that a waxy substance makes some of the beetle's microparticles hydrophobic. This step in itself is not unique; other research groups have added such compounds to granular surfaces using electrochemical techniques. The NIST group's innovation is to use an optical technique that is much easier to modulate, and that can be carried out in air. They simply coat the granules with a photosensitive material, and expose it to ultraviolet light: the longer and more intense the exposure in a given area, the more hydrophilic that area becomes.

The new technique's most immediate application is for testing paints, adhesives and other coatings: instead of daubing the compounds on dozens of surfaces one by one, researchers can now spread them over a single surface that tests the entire range of wettability within the space of a few centimeters. Other applications also are possible, ranging from water collection in dry regions to open-air microchannel devices. Indeed, the same technique can be used to create surfaces that vary in their affinity for alcohol and many other small molecule liquids.

Citation: J.T. Han, S. Kim and A. Karim. UVO-tunable superhydrophobic to superhydrophilic wetting transition on biomimetic nanostructured surfaces. Langmuir 2007, 23, 2608-2614.

Source: National Institute of Standards and Technology

Explore further: United States, China team explore energy harvesting

Related Stories

Glitter cloud may serve as space mirror

21 hours ago

What does glitter have to do with finding stars and planets outside our solar system? Space telescopes may one day make use of glitter-like materials to help take images of new worlds, according to researchers ...

Recommended for you

United States, China team explore energy harvesting

Apr 18, 2015

Six authors have described their work in harvesting energy in a paper titled "Ultrathin, Rollable, Paper-Based Triboelectric Nanogenerator for Acoustic Energy Harvesting and Self-Powered Sound Recording." ...

The microscopic topography of ink on paper

Apr 14, 2015

A team of Finnish scientists has found a new way to examine the ancient art of putting ink to paper in unprecedented 3-D detail. The technique could improve scientists' understanding of how ink sticks to ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.