Two regimes of stickiness

When you pull hard enough on two objects that are stuck together by an adhesive, they become unstuck. How that happens depends on the properties of the adhesive. A viscoelastic liquid deforms into thin fibrils as air penetrates the bulk of the adhesive, whereas an elastic solid can debond cleanly from the surface of one of the objects as air enters at the interface. Now, Julia Nase, Anke Lindner, and Costantino Creton of the École Supérieure de Physique et Chimie Industrielles in Paris have studied the debonding of adhesives with a range of viscous and elastic properties spanning those two extremes. To tune the properties, the researchers used polydimethylsiloxane with varying degrees of cross-linking among the polymers: The more cross-links, the more elastic the material. The bulk and interface mechanisms were distinguished by the patterns formed as air fingers penetrated between the surfaces, as shown in the figure. In the viscoelastic case, the characteristic size of the fingers decreased as the pulling speed increased; in the elastic case, the size was independent of the speed. Surprisingly, the researchers observed a sharp transition between the viscoelastic and elastic regimes, with no intermediate mechanism between bulk and interfacial debonding. (J. Nase, A. Lindner, C. Creton, Phys. Rev. Lett., in press.) — Johanna L. Miller