Materials Science

A Wrinkle in Time and Space

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Science  10 Apr 2009:
Vol. 324, Issue 5924, pp. 149-151
DOI: 10.1126/science.324.5924.149d

If you squeeze together a supported piece of paper, or place a weight on a constrained plastic film, a wrinkled pattern will appear. The wrinkles tend to form perpendicular to the axis of the principal stress, so that it is possible to obtain concentric ring patterns (like an archery target) or spoke-like patterns (like a bicycle wheel) or combinations thereof. The correlation between film properties, applied stress, and the static wrinkle patterns that form are now well understood. Chung et al. examined the dynamics of the wrinkling process—specifically, by monitoring patterns that formed during toluene swelling of polystyrene that had been partially surface-cross-linked through exposure to ultraviolet light and ozone (UVO). For low UVO exposure times, spoke patterns emerged, which the authors attribute to Fickian diffusion of the toluene with a radial growth rate that scaled with the square root of time. Longer UVO exposure generated a thicker cross-linked layer, with the toluene diffusion limited by this barrier. For these films, an induction time was required before wrinkling occurred, and a linear concentration could develop within the rubbery parts of the film, causing a concentric ring pattern to form. In all cases, the initiation and focal points of the patterns appeared to be at surface defects. The authors tested this observation either by masking a portion of the film with glass beads before UVO exposure, to create an elevated area that was not cross-linked, or by indenting the film after UVO exposure to make a gap in the cross-linked surface: These sites proved to be the focal points for the wrinkling on toluene exposure. Because the wrinkling patterns reflect the solvent front and the diffusion kinetics, the authors envision that this technique could be used to easily extract diffusion coefficients for a range of solvent/polymer pairs. — MSL

Adv. Mater. 21, 1358 (2009).

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