Wrinkles Measure Thin Films

A simple new experiment yields important information about the mechanical properties of thin films -- nanoscopically thin layers of material that are deposited onto a metal, ceramic or semiconductor base -- and this inexpensive way to measure material properties could impact industries such as biomedicine and nanoelectronics.

The research was performed at the University of Massachusetts at Amherst Materials Research Science and Engineering Center. Jiangshui Huang, Megan Juszkiewicz, Narayanan Menon, Wim de Jeu, Todd Emrick and Thomas Russell of UMass Amherst collaborated on the work with scientists from the Institute for Atomic and Molecular Physics in the Netherlands and the Universidad de Santiago in Chile.

The researchers said the findings impact a broad range of scientific disciplines and applications, from cosmetics to coatings, to micro- and nanoelectronics. Understanding the mechanical properties of thin films is essential to their performance and optimization.

Until now, determining the mechanical properties of these thin films was either an expensive and time-consuming endeavor, requiring powerful microscopes to view the films, or scientists examined composite structures and made uncertain assumptions. This new research will give scientists a simple way to access the material properties of most thin films, the team members said.

A starburst of wrinkles form in a thin-film material when a drop of water is placed on the film as it floats in water. These simple experiments, done with a pool of water in a Petri dish and a low-magnification microscope, give researchers insight into whether the material properties of ultrathin films differ from their properties in bulk quantities. (Image courtesy Jiangshui Huan, University of Massachusetts, Amherst)

"As we delve more into the nanotechnology, it becomes increasingly important to know if the material properties of ultrathin films differ from their properties in the bulk," said Russell, a program director in the Polymer Science and Engineering Department. "Every day we see examples where a material's dimensions can change its properties. Aluminum foil is flexible, whereas a bar of aluminum is not. But what happens when a film's thickness approaches molecular dimensions? These experiments give us a simple, inexpensive way to measure mechanical properties of films that are only tens of nanometers thick."

Russell and his colleagues use a low-power optical microscope to observe what happens when they place a tiny drop of water on a thin film of polystyrene -- a polymer typically used in hard plastics -- as it floats in a Petri dish of water. The "capillary tension" of the drop of water produces a starburst of wrinkles in the film, which they recorded with a digital camera. The number and length of the wrinkles are determined by the elasticity and thickness of the film.

In some other materials studied, the wrinkles in the ultrathin polymer films vanished with time, unlike the skin of a dried fruit or the crumpled hood of a car after an accident. This vanishing provides insight into the relaxation process of an ultrathin film by yielding information on the way polymer chains move in the highly confined geometry.

"These experiments give us a simple, inexpensive way to access the properties of such nanoscopically thin films that can be several tens of nanometers thick," said Menon.

The work is "very simple and very fundamental," said team member Emrick. "Yet it has implications the materials community will find very interesting. Polymer films are used worldwide under conditions where they withstand force and respond to it -- understanding this response is key to improving materials that span a broad range of applications from biomedical to cosmetic, and many others."

The research was funded by the National Science Foundation and appears in the Aug. 3 issue of the journal Science. For more information, visit: www.umass.edu