A double rigidity transition rules the fate of drying colloidal drops.

The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previous works propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing.

Instabilities in freely expanding sheets of associating viscoelastic fluids.

We use the impact of drops on a small solid target as a tool to investigate the behavior of viscoelastic fluids under extreme deformation rates. We study two classes of transient networks: semidilute solutions of supramolecular polymers and suspensions of spherical oil droplets reversibly linked by polymers. The two types of samples display very similar linear viscoelastic properties, which can be described with a Maxwell fluid model, but contrasting nonlinear properties due to different network structures.

Viscoelasticity and elastocapillarity effects in the impact of drops on a repellent surface.

We investigate freely expanding viscoelastic sheets. The sheets are produced by the impact of drops on a quartz plate covered with a thin layer of liquid nitrogen that suppresses shear viscous dissipation as a result of the cold Leidenfrost effect. The time evolution of the sheet is simultaneously recorded from top and side views using high-speed cameras.

Spinning elastic beads: a route for simultaneous measurements of the shear modulus and the interfacial energy of soft materials.

Large deformations of soft elastic beads spinning at high angular velocity in a denser background fluid are investigated theoretically, numerically, and experimentally using millimeter-size polyacrylamide hydrogel particles introduced in a spinning drop tensiometer. We determine the equilibrium shapes of the beads from the competition between the centrifugal force and the restoring elastic and surface forces. Considering the beads as neo-Hookean up to large deformations, we show that their elastic modulus and interfacial energy constant can be simultaneously deduced from their equilibrium shape.

Impact of Beads and Drops on a Repellent Solid Surface: A Unified Description.

We investigate freely expanding sheets formed by ultrasoft gel beads, and liquid and viscoelastic drops, produced by the impact of the bead or drop on a silicon wafer covered with a thin layer of liquid nitrogen that suppresses viscous dissipation thanks to an inverse Leidenfrost effect. Our experiments show a unified behavior for the impact dynamics that holds for solids, liquids, and viscoelastic fluids and that we rationalize by properly taking into account elastocapillary effects. In this framework, the classical impact dynamics of solids and liquids, as far as viscous dissipation is negligible, appears as the asymptotic limits of a universal theoretical description.

Playing with Emulsion Formulation to Control the Perforation of a Freely Expanding Liquid Sheet.

A single-drop experiment based on the collision of one drop of liquid on a small solid target is used to produce liquid sheets that are visualized with a fast camera. Upon impact, the drop flattens into a sheet that is bounded by a thicker rim and radially expanding in air. Emulsion-based liquid sheets are destabilized through the nucleation of holes that perforate the sheet during its expansion.

Rearrangement Zone around a Crack Tip in a Double Self-Assembled Transient Network.

We investigate the nucleation and propagation of cracks in self-assembled viscoelastic fluids, which are made of surfactant micelles reversibly linked by telechelic polymers. The morphology of the micelles can be continuously tuned, from spherical to rodlike to wormlike, thus producing transient double networks when the micelles are sufficiently long and entangled and transient single networks otherwise. For a single network, we show that cracks nucleate when the sample deformation rate involved is comparable to the relaxation time scale of the network.

Bursting of Dilute Emulsion-Based Liquid Sheets Driven by a Marangoni Effect.

We study the destabilization mechanism of thin liquid sheets expanding in air and show that dilute oil-in-water emulsion-based sheets disintegrate through the nucleation and growth of holes that perforate the sheet. The velocity and thickness fields of the sheet outside the holes are not perturbed by holes, and hole opening follows the Taylor-Culick law. We find that a prehole, which widens and thins out the sheet with time, systematically precedes the hole nucleation.

Structural signature of a brittle-to-ductile transition in self-assembled networks.

We study the nonlinear rheology of a novel class of transient networks, made of surfactant micelles of tunable morphology reversibly linked by block copolymers. We couple rheology and time-resolved structural measurements, using synchrotron radiation, to characterize the highly nonlinear viscoelastic regime. We propose the fluctuations of the degree of alignment of the micelles under shear as a probe to identify a fracture process.

Microscopic mechanisms of the brittleness of viscoelastic fluids.

We show that a large class of viscoelastic fluids, i.e., transient networks, are brittle according to the Griffith's theory of solid fracture.

Nonlinear rheology of surfactant wormlike micelles bridged by telechelic polymers.

We have investigated the nonlinear rheology of a soft composite transient network made of a solution of surfactant wormlike micelles (WM) in the semidilute regime that are reversibly bridged by telechelic polymers. The samples are well described, in the linear regime, as two Maxwell fluids components blends, characterized by two markedly different characteristic times. The slow mode is mainly related to the transient network of entangled WM, and the fast mode to the network of telechelic chains.

Structure and rheological properties of model microemulsion networks filled with nanoparticles.

Model microemulsion networks of oil droplets stabilized by non-ionic surfactant and telechelic polymer C18 -PEO(10k)- C18 have been studied for two droplet-to-polymer size ratios. The rheological properties of the networks have been measured as a function of network connectivity and can be described in terms of simple percolation laws. The network structure has been characterised by Small Angle Neutron Scattering (SANS).

Copolymer-induced stabilizing effect of highly swollen hexagonal mesophases.

We show that small amounts of copolymer that decorate an oil/water interface can greatly enhance the stability of swollen surfactant hexagonal phases, comprising oil tubes regularly arranged in a water matrix. Both the radius of the tubes and the thickness of the aqueous channel between the tubes can be controlled independently over large ranges. Such soft composite materials offer a potential interest for the synthesis of mesoporous materials.

Quantitative analysis of lyotropic lamellar phases SANS patterns in powder oriented samples.

We have developed a detailed numerical method based on the Caillé model to fit Small Angle Neutron Scattering profiles of powder-oriented lyotropic lamellar phases. We thus obtain quantitative values for the Caillé parameter and the smectic penetration length from which we can derive the smectic compression modulus and the membrane mean bending modulus. Our method, applied to a surfactant lamellar phase system decorated by amphiphilic copolymers, provides excellent fits for any intermembrane spacing or membrane concentration over the entire q-range of the SANS experiments.

Bounded step superdiffusion in an oriented hexagonal phase.

Fluorescence recovery after pattern photobleaching is used to measure the self-diffusion of surfactant molecules, along cylinders and perpendicular to their main axis in an oriented hexagonal lyotropic phase. Unexpectedly, while the motion along cylinders is diffusive, a superdiffusive behavior is observed in the direction perpendicular to the cylinder axis. Moreover, varying the lattice parameter, we found that the perpendicular diffusion time is governed only by the number of cylinders to cross, providing experimental evidence for superdiffusion with a bounded step length.

Structure factor of polymers interacting via a short range repulsive potential: application to hairy wormlike micelles.

We use the random phase approximation to compute the structure factor S(q) of a solution of chains interacting through a soft and short range repulsive potential V. Above a threshold polymer concentration, whose magnitude is essentially controlled by the range of the potential, S(q) exhibits a peak whose position depends on the concentration. We take advantage of the close analogy between polymers and wormlike micelles and apply our model, using a Gaussian function for V, to quantitatively analyze experimental small angle neutron scattering profiles of solutions of hairy wormlike micelles.

Unusual Interfacial Phase Behavior of Two Nonmiscible Liquids in a Cylindrical Test Tube: Equilibrium Shapes and Stability of Axisymmetric Liquid Bridges under Gravity.

In this paper the unusual interfacial phase behavior of two nonmiscible fluids contained in a cylindrical glass test tube is reported. Water, which is the lighter phase, takes up the upper part of the tube, whereas the denser compound (a hydrofluorocarbon) is in the bottom. However, below some critical volume of water, the denser phase emerges at the air surface, by forming an axisymmetric liquid bridge through the aqueous phase.