We investigate the dynamics of thin sheets freely expanding in air. The sheets are produced by impacting a drop onto a small target or onto a cushion of liquid nitrogen, in order to suppress any dissipation process. To disentangle the role of capillary, viscous and elastic forces in the dynamics of the sheets several materials are used, whose rheological characteristics are tuned over several orders of magnitude: viscous liquids, viscoelastic Maxwell fluids characterized by an elastic modulus, $G_0$, a relaxation time, $\tau$, and a zero-shear viscosity,$\eta_0=G_0 \tau$, and ultrasoft solids with elastic modulus as low as 10 Pa. For viscoelastic fluids, when$\tau$ is shorter than the typical lifetime of the sheet (~ 10 ms), the dynamics of the sheet is similar to that of Newtonian viscous liquids with equal zero-shear viscosity. In that case, the maximal expansion of the sheet, $d_{\rm{max}}$, decreases with $\eta_0$ and can be quantitatively accounted for by a model based on the viscous dissipation on the target. On the other hand, when $\tau$ is longer than the typical lifetime of the sheet, the behavior drastically differs. The sheet expansion is strongly enhanced as compared to that of viscous samples with comparable zero-shear viscosity, but is heterogeneous with the occurrence of cracks, revealing the elastic nature of the viscoelastic fluid. By contrast, sheets produced with ultrasoft solids expand but never break. Furthermore, we demonstrate that the surface tension of the soft elastic drops must be taken into account in order to successfully model their expansion. Overall, we have rationalized the spreading of sheets for viscous, viscoelastic and elastic materials by taking into account surface tension, elastic deformation and viscous dissipation.

We study experimentally the impact of ultra soft spherical gel balls of millimetric size d0 on a rigid substrate covered by a thin layer of liquid nitrogen to avoid viscous dissipation. The balls largely deform like a pancake at high impact velocities. We measure the maximally deformed size dmax and the the time needed to reach this maximal size after impact τmax, versus the impact velocity ui for various elastic moduli. We do the same type of experiments with liquid droplets of various surface tensions. The experiments reveal a universal scaling behavior of the maximum deformation dmax/d0 of both solid balls and liquid drops provided that both bulk and surface elasticity are properly taken into account. Moreover, we show that, in absence of viscous dissipation, the dynamics of the system can be understood as a conventional spring-mass system with a stiffness given by a combination of surface tension and bulk elasticity and a mass given by that of the ball (or drop); the deformation of the small ball (drop) during the impact linearly depends on the impact velocity, and the contact time scales as the period of this spring-mass system.

We investigate the dynamics of freely expanding liquid sheets prepared with fluids with different rheological properties, (i) viscous fluids with a zero-shear viscosity $\eta_0$ in the range $(1-1000) \, \mathrm{mPa.s}$ and (ii) viscoelastic Maxwell fluids whose elastic modulus, $G_0$, characteristic relaxation time, $\tau$, and zero-shear viscosity, $\eta_0=G_0 \tau$, can be tuned over several orders of magnitude. The sheets are produced by impacting a drop of fluid on a small cylindrical solid target. For viscoelastic fluids, we show that, when $\tau$ is shorter than the typical lifetime of the sheet ($\sim 10$ ms), the dynamics of the sheet is similar to that of Newtonian viscous liquids with equal zero-shear viscosity. In that case, for little viscous samples ($\eta_0 <\sim 30$ mPa.s), the maximal expansion of the sheet, $d_{\rm{max}}$, is independent of $\eta_0$, whereas for more viscous samples, $d_{\rm{max}}$ decreases as $\eta_0$ increases. We provide a simple model to account for the dependence of the maximal expansion of the sheet with the viscosity that is based on an energy balance between inertia, surface tension and viscous shear dissipation on the solid target, and which accounts well for our experimental data. By contrast, when $\tau$ is longer than the typical lifetime of the sheet, the behavior drastically differs. The sheet expansion is strongly enhanced as compared to that of viscous samples with comparable zero-shear viscosity, but is heterogeneous with the occurrence of cracks, revealing the elastic nature of the viscoelastic fluid.

We develop and thoroughly test a stress-controlled, parallel plates shear cell that can be coupled to an optical microscope or a small angle light scattering setup, for simultaneous investigation of the rheological properties and the microscopic structure of soft materials under an imposed shear stress. In order to minimize friction, the cell is based on an air bearing linear stage, the stress is applied through a contactless magnetic actuator, and the strain is measured through optical sensors. We discuss the contributions of inertia and of the small residual friction to the measured signal and demonstrate the performance of our device in both oscillating and step stress experiments on a variety of viscoelastic materials.

Conjugated polymer nanostructures have recently emerged as a new class of very active photocatalysts under solar light. Poly(3-hexylthiophene) (P3HT) is one of the commonly used conjugated polymers for photovoltaics applications. P3HT nanostructures synthesized in soft templates provided by hexagonal mesophases show high photocatalytic activity for degradation of pollutants under both UV and visible light. These photocatalysts are very stable even after repeated cycling. Addition of scavengers and mechanistic studies show that O2•−is the main radical responsible for degradation of phenol taken as a model pollutant. P3HT nanostructures can be easily deposited on flat supports such as quartz for photocatalytic applications avoiding a separation step by centrifugation. Most interestingly, the photocatalytic activityof these P3HT nanostructures is highly enhanced when they are supported on a solid surface opening new perspectives in photocatalytic reactors and self-cleaning surfaces.