Smoothening creases on surfaces of strain-stiffening materials
When an elastic block (e.g., an elastomer or a soft tissue) is compressed to a critical strain, the smooth surface of the block forms creases, namely, localized regions of self-contact. Here we show how this instability behaves if the solid stiffens steeply. For a solid that stiffens steeply at large strains, as the compression increases, the surface is initially smooth, then forms creases, and finally becomes smooth again. For a solid that stiffens steeply at small strains, creases will never form and the surface remains smooth for all levels of compression. We also obtain the critical conditions for the onset of wrinkles. When the surface does become unstable, we find that creases always set in at a lower compression than wrinkles. Our findings may shed light in developing crease-resistant materials.
http://www.sciencedirect.com/science/article/pii/S0022509614002026
When a material is soft or the size of the material is small, the effect of surface energy on its deformation can be significant.The importance of surface energy on the deformation of a structure could be evaluated by the magnitude of a dimensionless number, called elastocapillary number: γ/μL, where γ is surface energy density, μ is shear modulus and L is the characteristic length of the structure. Many intriguing phenomena of surface energy induced deformation of even instabilities have been observed in different experiments. In this journal club, I want to initiate a discussion on how surface energy may affect mechanical instabilites in soft materials. In the following, I would like to use our recent work as exmaples. Any thoughts and comments on this topic are welcome.
Recently, we have studied the influence of surface energy on the creasing instability of an elastomer under uniaxial compression. In experiments we found that creases form by nucleation at preexisting defects and grow by channeling across the surface of the film. Surface energy provides a nucleation barrier and also resists channeling for finite values of the elastocapillary number. While the heterogeneous nucleation makes it difficult to characterize the critical strain for nucleation, the condition for channeling is well characterized and depends on the elastocapillary number. We further show that adhesion, rather than plastic deformation, is responsible for the dramatic hysteresis between the first and subsequent cycles of compression.
Our paper can be found in the following link. Some experimental videos have been put in the supplemental materials.
Another paper of us illustrates the influence of surface tension and streching limit of polymers on the snap-through instabilities of a cavity inside an elastomer. The link is given in the following.