Ty Phou

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

Abstract.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). A Reverse Monte Carlo (RMC) approach is used to demonstrate the interplay of attraction and repulsion induced by the copolymer. These model networks are then used as matrix for the incorporation of silica nanoparticles (R = 10 nm), individual dispersion being checked by scattering. A strong impact on the rheological properties is found for silica volume fractions up to 9%.

Gravity driven instability in elastic solid layers.

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Silica nanoparticles dispersed in a self-assembled viscoelastic matrix: structure, rheology, and comparison to reinforced elastomers

We demonstrate the instability of the free surface of a soft elastic solid facing downwards. Experiments are carried out using a gel of constant density ρ, shear modulus μ, put in a rigid cylindrical dish of depth h. When turned upside down, the free surface of the gel undergoes a normal outgoing acceleration g. It remains perfectly flat for ρgh/μ<α* with α*≃6, whereas a steady pattern spontaneously appears in the opposite case. This phenomenon results from the interplay between the gravitational energy and the elastic energy of deformation, which reduces the Rayleigh waves celerity and vanishes it at the threshold.

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

The effect of silica nanoparticles on transient microemulsion networks made of microemulsion droplets and telechelic copolymer molecules in water is studied, as a function of droplet size and concentration, amount of copolymer, and nanoparticle volume fraction. The phase diagram is found to be affected, and in particular the percolation threshold characterized by rheology is shifted upon addition of nanoparticles, suggesting participation of the particles in the network. This leads to a peculiar reinforcement behaviour in such microemulsion nanocomposites, the silica influencing both the modulus and the relaxation time. The reinforcement is modelled based on nanoparticles connected to the network via droplet adsorption. Contrast-variation small angle neutron scattering coupled to a reverse Monte Carlo approach is used to analyse the microstructure. The rather surprising intensity curves are shown to be in good agreement with the adsorption of droplets on the nanoparticle surface.

Capillarity driven instability of a soft solid.

Model self-assembled networks of telechelic polymer C18 - PEO(35k) - C18 in water have been studied. The rheology of such transient networks has been investigated as a function of polymer concentration, and a typical percolation law has been observed. The network structure has been characterised by Small Angle Neutron Scattering in D2O, where the interactions between micelles formed by the hydrophobic C18-stickers of the polymer give rise to a peak in the scattered intensity. These model networks have then been used as a matrix for the incorporation of silica nanoparticles (R = 10 nm), and we have checked individual dispersion by scattering using contrast variation. The rheological response of the networks is considerably modified by the presence of the silica nanoparticles, and in particular an interesting dependence of the relaxation time on silica concentration has been found. The analogy in reinforcement behaviour of such a self-assembled, viscoelastic, and aqueous system with model experiments of elastomers filled with nanoparticles is discussed by comparison to a silica-latex system.

Solid Drops: Large Capillary Deformations of Immersed Elastic Rods

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. A novel material-dependent characteristic velocity that includes both capillary and bulk elasticity emerges from this unified description of the physics of impact.