CHARACTERIZATION OF NON-EQUILIBRIUMFLUCTUATIONS DURING FREE-DIFFUSION INHIGHLY-STRATIFIED SOLUTIONS OF GLYCEROL ANDWATER

We present two topics, both in the area of transport mechanisms (mass diffusion or heat transport) occurring in fluids. The first and main topic is a study of the non equilibrium fluctuations (NEF) that occur during isothermal free diffusion between two solutions of glycerol and water with various concentration differences. The nonlinearity of the system, determined by the strong stratification of the sample, requires the introduction of an interpretive model capable of characterizing the dependence of the correlation properties of the non-equilibrium fluctuations on the local thermophysical variables of the system. The proposed model makes it possible to characterize the dynamics of non-equilibrium fluctuations in the presence of a wide range of relaxation times determined by the strong stratification of the sample, unlike the cumulative methods commonly used in dynamic light scattering experiments, which work well in the presence of moderate dispersion of relaxation times. The second topic concerns the convective structures that appear in a thermophilic suspension of nanoparticles heated from below. Such a system presents a complex stability diagram, determined by the competition between the stabilizing flow of nanoparticles induced by thermophoresis and the destabilizing flow determined by thermal convection. We analyzed Rayleigh- Bénard convection in a suspension of highly thermophilic nanoparticles with a large negative separation ratio Ψ = −3.5 heated from below. We demonstrated the presence of transient states localized in the range of Rayleigh numbers 2200 < Ra < 3000. These states rotate rapidly around their axis and gradually decrease in size until the system returns to a purely conductive state. This second topic is not totally separate from the first one, since the study of the stability diagram is the first step toward studying the thermal and concentration fluctuations generated by heating a thermophilic colloidal suspension from below.