When a liquid film meets a solid surface, all heck breaks loose as film particles cling, drag, shift, lift upward and, eventually, settle. If, however, the particles don’t attach firmly enough, they may be prone to lifting off the surface material again in large swaths. Examples of this are when water bubbles up on a freshly waxed car or when paint blisters on a wall that was not prepared well in advance. Another is when the photoactive material used to coat a substrate destined to be turned into a microelectronic device bubbles up erratically, ruining the pattern later inscribed on the surface, thus creating a defect.
Many common items are in reality a type of film, including paint and photoresist materials, but also solder, ink, lubricants for hard drives and other mechanical equipment, stain repellent and antireflection coatings for lenses in everything from eyeglasses to laser optics.
“Understanding and controlling dewetting is important in a host of processes and materials,” said Ashutosh Sharma, chair professor at Indian Institute of Technology in Kanpur. Furthermore, according to Sharma, dewetting research is becoming more important because manufacturers are trying to reduce the thickness of thin-film coatings to the 10- to 1000-nm range. This is especially important in electronics and optoelectronics, where coatings composed of silver, gold and other precious metals are commonly used.
Researchers study the dewetting process to better understand the physics at the transition barriers between liquid films and solid (or another liquid) surfaces. Scientists at Indian Institute of Technology modify dewetting conditions to produce patterned arrays. Courtesy of Ashutosh Sharma.
Researchers at Nagoya University in Japan are investigating nanometer-thick coatings on solid surfaces, using a technique called ellipsometric microscopy to acquire sequential images of the dewetting process as it occurs. Typically, atomic force microscopy is used to visualize dewetting, but this technique can be slow. The Nagoya researchers, led by Kenji Fukuzawa, instead used ellipsometric microscopy, in which the subject is illuminated at a highly oblique angle and the polarization of the reflected light is measured and analyzed. They reported in the Oct. 21, 2008, issue of Langmuir that they obtained high-contrast images at a much faster 4 fps.
Application scientists and engineers are not limited solely to using precious metals for their films. Less expensive substances, such as the plastic polymers polymethyl methacrylate (PMMA) and polydimethylsiloxane, can be used in a number of applications. Many combinations of film and substrate materials are well understood, but not all of the underlying physics. For example, Sharma’s team is exploring how controlling dewetting of liquid bilayers on various surfaces can be used to form microchannels or microdot arrays with regularly patterned features.
Sharma also noted that there is a drive toward increased functionality: multiple layers of various liquids sandwiched on a substrate, newer substrate and film materials, and novel ways to handle high-speed coating processes, such as printing webs that move materials at hundreds of feet per second. Creating defect-free technologies such as high-efficiency solar panels or organic LED displays will depend on better knowledge of thin-film physics to assure fast and accurate manufacturing.
Beyond solid liquid
Dewetting isn’t limited to just liquid films on solid surfaces. Liquid-on-liquid interactions – think oil and water – have utility as well, especially in biology but also in applications such as liquid lenses and mirrors. When developing liquid-liquid systems, it isn’t enough to consider the properties of each liquid alone – the properties of the materials at the boundary between them are unique on many levels.
Furthermore, the reactions at the interfaces between liquids and solids need not be considered for films of liquid atop flat surfaces only. Researchers also are considering how dewetting affects the intricacies of protein folding. The way that proteins twist and unwind is a key part of biology, and there is some thought that the interplay of the molecules that make up a protein and the watery liquids in which they reside may affect how and when the proteins shift their shapes.
According to investigators at IBM’s Watson Research Center, protein folding is thought to be driven by the proteins’ innate hydrophobicity. When the various arms of a protein molecule engage a water molecule, they are repelled, bending them and folding the protein into a new form. A team led by Ruhong Zhou has been looking into the exact mechanisms of dewetting-based protein folding, with an eye toward using that knowledge to create a new generation of molecular-level water channels made of proteins.
No matter how they come to meet, whenever a liquid substance comes into contact with a solid (or liquid) surface is cause to ponder exactly what interactions are occurring. Such ponderings ultimately will lead to future investigations and the development of technologies that will change the way we touch the world – and it, us.