Laboratoire Matière et Systèmes Complexes

Thickness effect in elastowetting

Menghua ZHAO, Laboratoire MSC

The wetting of liquids on soft materials such as elastomers has received a great deal of attention in the past decades. Many experiments were performed to gain insight into both the statics and the dynamics of wetting in such systems, but most neglected the effect of finite thickness of the gel sample. Here we report the results of a study of the thickness effect on both the statics and the dynamics of wetting. We vary systematically the thickness of slabs of silicone elastomers (Young modulus E = 3.6 kPa) from 10-2 to a few mm. First, we develop a quantitative Schlieren optics enabling us to directly observe the surface deformation after the deposition of a water droplet on time. We measure the out-of-plane deformation outside the droplet as a function of droplet size, gel thickness and elasticity. We identify a submicrometer-deep dimple, on the dry side of the droplet, that extends over a few mm away from the contact line. Second, we characterize the receding dynamics and we show that the dynamic contact angle, hence dissipation, depends in a non-trivial fashion on the thickness of the sample. We rationalize our experiments, with an analytical model accounting for the linear elastic response of the gel bulk as well as its surface tension. We find excellent agreement with experiments.

Integrated motions of molecular machines and motors : structure and dynamics

Giacomo MARIANI, Laboratoire MSC

Switchable functional molecules producing mechanical work constitute an active focus in nanotechnologies as they can be used for molecular-based devices and materials. The dynamic nature of mechanically interlocked molecules allows their components to undergo relative internal movements. However, the coupling of these components to transfer controlled motions from the molecular motors to the macroscopic scale (i.e. supramolecular self-assembled structures) is a challenge. Two different approaches have been followed to form contractile structures. In the first case, thousands of pH-responsive nanomachines capable of linear motions have been combined to synchronize their motion and therefore, to form fibers similar to what myofibrils do when packed in bundles in muscles. In the second case, light-driven rotary motors have been integrated as mechanically active reticulation units resulting in a light-controlled contractile supramolecular structure. In both the cases, a change on the molecular scale results in a contraction of the order of the hundreds nanometer on the supremolecular scale. Small angle neutron scattering and light scattering have been used to characterize the structure of the supramolecular aggregates before and after the induced structural changes and the dynamics of the contraction process at different length and time scales.