Wrinkles in nanoscale-sized films
An international team of scientists from Chile, the Netherlands and the U.S. has found a very simple way to measure the material properties of thin films having a thickness of only a dozen nanometers. The researchers just dropped water on thin film floating in a Petri dish. This causes wrinkles to appear on the ultrathin polymer films they've tested. And they found that the number and length of the wrinkles are determined by the elasticity and thickness of the film. In other words, they've found an easy way to discover the mechanical properties of films which might be used for applications such as cosmetics, coatings, and nanoelectronics.
You can see above pictures of four polystyrene films of varying thicknesses floating on the surface of water, each being wrinkled by water drops. "As the film is made thicker, the number of wrinkles N decreases (there are 111, 68, 49, and 31 wrinkles in these images), and the length of wrinkles L increases. L is defined as shown at top left, measured from the edge of the water droplet to the white circle. The scale varies between images, whereas the water droplets are approximately the same size." (Credit: University of Massachusetts, Amherst, via Science.
This research, which was funded by the National Science Foundation, was performed at the University of Massachusetts at Amherst Materials Research Science and Engineering Center (MRSEC) under the direction of Thomas Russell, a program director in the Polymer Science and Engineering Department.
Here are some quotes from Russell about the experiments. "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. Everyday 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?"
Here is a short explanation of the discovery. "Russell and his colleagues use a low-power optical microscope to observe what happens when they place a tiny drop of water on thin film 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. The number and length of the wrinkles are determined by the elasticity and thickness of the film."
For more information, this research work has been published in Science under the name "Capillary Wrinkling of Floating Thin Polymer Films" (Volume 317, Issue 5838, Pages 650-653, August 3, 2007). Here is a link to the abstract. "A freely floating polymer film, tens of nanometers in thickness, wrinkles under the capillary force exerted by a drop of water placed on its surface. The wrinkling pattern is characterized by the number and length of the wrinkles. The dependence of the number of wrinkles on the elastic properties of the film and on the capillary force exerted by the drop confirms recent theoretical predictions on the selection of a pattern with a well-defined length scale in the wrinkling instability. We combined scaling relations that were developed for the length of the wrinkles with those for the number of wrinkles to construct a metrology for measuring the elasticity and thickness of ultrathin films that relies on no more than a dish of fluid and a low-magnification microscope. We validated this method on polymer films modified by plasticizer. The relaxation of the wrinkles affords a simple method to study the viscoelastic response of ultrathin films."
Here are two additional links to the full text of the paper and to the figures that accompany it. The illustration above was extracted from this paper.
Sources: National Science Foundation news release, via EurekAlert!, August 7, 2007; and various websites
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