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Defining the thickness of polymer films by a drop of water

Place a drop of water on a floating thin polymer sheet and wrinkles will arise around the drop. You may see this even with a plain microscope. The amount of wrinkles and their lengths appear to be a straight measure for the elasticity and the thickness of the polymer sheet.

FOM-researcher Wim de Jeu and his fellow researchers from the United States of America and from Chile will announce this in Science, on August 3rd, 2007. The wrinkles arise, being influenced by the surface tension of the drop of water. Consequently, such a floating polymer sheet is a fine model for studying the behaviour of thin films in fluids. This combination is generally found in biological and synthetic soft materials.

The researchers used thin sheets (in academic jargon ‘films’) of polystyrene for their measurements, which they attached to a glass substratum. The thickness of the thin sheets, defined by means of x-ray reflectivity, varied from 31 to 233 nanometers. Then, they cut a circle 22.8 millimetres in diameter in each of the sheets and after that they dipped it into a Petri dish in distilled de-ionized water. In the Petri dish the surrounded part of the thin sheets came off and was floating on the water. As polystyrene is so-called ‘hydrophobic’, the surface tension flattens the polystyrene at the edge of the sheet. Thus, floating flat discs of polystyrene came into being. If the researchers then placed a droplet of water in the centre or pressed a needle, as a consequence of this disruption regular patterns of wrinkles arose that were facing outwards.

The wrinkled pattern appears to correspond nicely to the prognosis of a recently developed theory. This theory converts the surface tension of the drop of water in capillary forces that are affecting the polymer film. The researchers now combined the scaling relations that were developed for the length of the wrinkles with those for the number of wrinkles that will arise. This will produce a measuring standard to define the elasticity and thickness of very thin polymer sheets. In order to accomplish this, it appears that in actual practice, all that is necessary is a Petri scale with water, a plain microscope and a digital camera, because the patterns are extremely well visible at a small magnification. Next, they tested their method on polymer films that they had provided with plasticiser in order to vary the elasticity of the films. Even then the method produces reliable results to the thickness of the films.

The method that the researchers have been developing, just provides another large advantage. When they disrupt the film by placing a drop of water or pressing a needle, wrinkles will arise. In time the wrinkles will disappear, because the disruptions are spreading out all over the film. The film is ‘relaxing’ so to speak. Other research methods are applying the films to a firm substratum, which may lead to mechanical tensions in the sheets. These are disrupting the ‘relaxation’ of the film, which influences the measuring results. When the films are floating in or on a fluid, similar tensions may then ebb to the underlying fluid, after which the measurements will solely reproduce the process of the wrinkling.

Huub Eggen | alfa
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