Eni Kume and Laurence Noirez
In the conventional picture, the temperature of a liquid bath in the quiescent state is uniform down to thermal fluctuation length scales. Here we examine the impact of a low-frequency shear mechanical field (hertz) on the thermal equilibrium of polypropylene glycol and liquid water away from any phase transition confined between high-energy surfaces. We show the emergence of both cooling and heating shear waves of several tens of micrometers widths varying synchronously with the applied shear strain wave. The thermal wave is stable at low strain amplitude and low frequency while thermal harmonics develop by increasing the frequency or the strain amplitude. The liquid layer behaves as a dynamic thermoelastic medium challenging the extension of the fluctuation–dissipation theorem to nonequilibrium fluids. This view is in agreement with recent theoretical models predicting that liquids support shear elastic waves up to a finite propagation length scale of the order the thermal wave.
Marguerite Léang, Didier Lairez, Fabrice Cousin, Frédérique Giorgiutti-Dauphiné, Ludovic Pauchard, Lay-Theng Lee
During evaporative drying of a colloidal dispersion, the structural behavior at the air-dispersion interface is of particular relevance to the understanding of the consolidation mechanism and the final structural and mechanical properties of the porous media. The drying-interface constitutes the region of initial drying-stress that, when accumulated over a critical thickness, leads to crack formation. This work presents an experimental study of top-down drying of colloidal silica dispersions with three different sizes (radius 5, 8 and 13 nm). Using specular neutron reflectivity, we focus on the structural evolution at the free drying-front of the dispersion with a macroscopic drying surface, and demonstrate the existence of a thick concentrated surface layer induced by heterogeneous evaporation. The reflectivity profile contains a strong structure peak due to scattering from particles in the interfacial region, from which the interparticle distance is deduced. A notable advantage of these measurements is the direct extraction of the corresponding dispersion concentration from the critical total reflection edge, providing a straightforward access to a structure-concentration relation during the drying process. The bulk reservoir of this experimental configuration renders it possible to verify the evaporation-diffusion balance to construct the surface layer and also to check reversibility of particle ordering. We follow the structural evolution of this surface layer from a sol to a soft wet-gel that is the precursor of a fragile skin, and the onset of significant particle aggregation that precedes formation of the wet-crust. Separate complementary measurements on the structural evolution in the bulk dispersion are also carried out by small angle neutron scattering, where the particle concentration is also extracted directly from the experimental curves. The two sets of data reveal similar structural evolution with concentration at the interface and in the bulk, and an increase in the degree of ordering with particle size.
This article is dedicated to the memory of Loic Auvray
The results on Winsor phases, droplet and bicontinous microemulsions phases with polymer-grafted lipids studied by Small Angle Neutron Scattering (SANS) are reported below, together with the contrast variation techniques used to characterize the average curvature in the system. We have clearly shown that polymer-grafted lipids change the interaction between microemulsion droplets --it need not be just repulsive but could also be attractive. They induce structural changes or bring about complete phase changes as observed visually in the Winsor phases when added in sufficient amounts. In the bicontinous microemulsion phases, the polymer-grafted lipids decrease the persistence length, hence the bending rigidity, increase the apparent average thickness of the film, and cause a complex deformation of the film which brings about a negative curvature change at a semi-local scale. Contrary to the naive prediction that the polymer-grafted lipids should increase membrane rigidity our experiments show a decrease. This is a subtle effect caused by perhaps an indirect coupling between film curvature and concentration fluctuations.