Elastic response and wrinkling onset
Wrinkling in elastic films under stress is ubiquitous in nature. Among common systems exhibiting this phenomenon are our skin, textiles and more specifically thin coatings subjected to stresses mismatch. Apart from a fundamental interest, wrinkled membranes have found unexpected applications. For instance, in modern technology, pre-wrinkled conductive films are proposed as stretchable electrical contacts required in large-area electronics. In medicine, surgery has always involved cutting, thickening contraction and, in many cases, wrinkling of the skin. In cell biomechanics, wrinkled patterns produced by cells crawling onto elastic membranes (several nanometers thick), provided a useful tool to test living-cells locomotion.
On the one hand, we develop macroscopic experiments dedicated at the understanding of the relations between the applied forces and the resulting deformations. In a first macroscopic experiment , a stretched latex membrane is submitted to an axi-symmetric force at centre. Radial wrinkles appear as a consequence of a supercritical instability. The experiment shows that the length of the wrinkles is a slowly varying function of applied force, which challenges the sensitivity of cellular force measurements based on the direct measurement of the length. It thus suggests that a considerably more sensitive measurement can be obtained if the force is correlated directly to the amplitude instead.
On the other hand, we produce flat membranes from a polymeric liquid, similar to those used in biophysics experiments, and develop experimental techniques to characterize the membranes. First, I will describe how the membranes are produced and how characteristics, such as the membrane thickness, elastic constants, and tension, are obtained . Second, starting from a polymeric-fluid droplet, by vulcanization of the fluid free surface, curved elastic membranes, several nanometers thick and a few millimeters in diameter, which enclose a constant fluid volume, are produced. In an indentation type test, carried out by pushing the membrane along its normal by means of a microneedle, under some conditions, wrinkles are likely to appear around the contact region. Interestingly, we observe that the instability does not significantly alter the force-displacement relation: the relation between the force and the displacement remains linear and the associated stiffness is simply proportional to the tension of the membrane. In addition, we determine that the wrinkles develop when the stretching modulus of the membrane compares with its tension, which provides a reliable method to estimate the elastic constant . Results are contrasted with those from a second macroscopic experiment .
 J.-C. Géminard, R. Bernal, and F. Melo, Eur. Phys. J. E 15, 117-126 (2004).
 R. Bernal, C. Tassius, F. Melo, J.-C. Géminard, Appl. Phys. Lett. 90, 063903 (2007).
 R. Bernal, C. Tassius F. Melo J-C. Géminard, soumis à Soft Matter, 2010.
 Q. Puydt, stage M1, http://perso.ens-lyon.fr/jean-christophe.geminard/stages/qpuydt.pdf