Binary c–T phase diagrams of organogelators in solvent are frequently simplified to two domains, gel and sol, even when the melting temperatures display two distinct regimes, an increase with T and a plateau. Herein, the c–T phase diagram of an organogelator in solvent is elucidated by rheology, DSC, optical microscopy, and transmitted light intensity measurements. We evidence a miscibility gap between the organogelator and the solvent above a threshold concentration, cL. In this domain the melting or the formation of the gel becomes a monotectic transformation, which explains why the corresponding temperatures are nonvariant above cL. As shown by further studies by variable temperature FTIR and NMR, different types of H-bonds drive both the liquid–liquid phase separation and the gelation.

The mechanical properties of amorphous solids such as glasses or gels are currently a topic of intense research, with implications in material science as well in fundamental condensed matter physics. At the macroscopic scale, a distinctive feature of these materials is the slow plastic deformation that is observed when they are subject to a step stress. Remarkably, this slow creep regime is often interrupted by the sudden failure of the material, with no macroscopic precursors.Recent works focus on the interplay between irreversible rearrangements at the microscopic level, resulting from an applied deformation or stress, and the macroscopic mechanical behavior. In fact, even though material failure is ubiquitous in our everyday life, the underlying microscopic mechanisms are still not well understood, mainly because the direct observation of its precursors at the particle level is experimentally very challenging in atomic or molecular materials.In this work, we study the microscopic dynamics of a model colloidal gel under load, by coupling a small angle light scattering apparatus to a custom stress-controlled shear cell. We find that the gel creep consists of three regimes. Initially, non-affine displacements grow linearly with strain. These non-affine dynamics are fully reversible upon removing the applied stress, and are associated to heterogeneity of the local gel elasticity. In the second regime, non-affine displacements grow much slower with strain, but are associated to irreversible rearrangements. In the third regime, a sharp acceleration of the dynamics at small length scale is observed. These rearrangements are a dynamic precursor of material failure; remarkably they occur thousands of seconds before the macroscopic yielding of the gel.

High-pressure polarized Raman spectra of vitreous silica are measured up to 8 GPa in a diamond-anvil cell atroom temperature. The combined use of either a nonpenetrating pressurizing medium—argon—or a penetrating one—helium, allows one to separate density from stress effects on the Raman frequencies. In the framework of a simple central force model, the results emphasize the distinct role played by the shrinkage of the intertetrahedral angle Si-O-Si and the force-constant stiffening during the compression. The polarization analysis further reveals the existence of an additional isotropic component in the high-frequency wing of the boson peak. The pressure dependence of the genuine boson peak frequency is found to be much weaker than previously reported and even goes through a minimum around 2 GPa in remarkable coincidence with the anomalous compressibility maximum of silica.

Brillouin spectroscopy is used to investigate the elastic properties of XAlSiO4 aluminosilicate glasses where X =Li, Na, K, Mg0.5, Ca0.5, Sr0.5, Ba0.5, and Zn0.5. The Brillouin frequency shifts obtained in two different scattering geometries allow the calculation of the refractive index, the two sound velocities and Poisson's ratio.Measurements of the mass density give in turn the elastic moduli and the Debye temperature.We find that the elastic properties scale with the atomic density of the glassy network or the charge-balancing cation field strengthwhile they negatively correlate with the glass transition temperature. Further, Poisson's ratio depends on the nature of the non-framework cations in this glass series.