Wave vectors imaging of Rayleigh-Bénard convection in concentrated emulsions

Rayleigh-Bénard convection (RBC) is a thermal convective regime observed in fluids when heated from below, cooled from above, and subjected to a buoyancy force. Specifically, RBC is triggered when the temperature gradient overcomes a critical value. In this setup, simple fluids, such as water, were widely studied during the last century from theoretical, numerical, and experimental points of view. In contrast, the study of RBC in soft glassy materials (SGMs), such as foams and emulsions, has been poorly explored. SGMs show a nonlinear rheological response to an external force, behaving as solid for applied stress below a critical threshold (called yield stress) and flowing when this threshold is overcome. However, because of this viscoelastic nature, the RBC in SGM results is relevant in a wide variety of fields such as geophysics, astrophysics, meteorology, heat exchangers, building construction, and cooling systems. With respect to SGMs, the first important issue was characterization of the onset of thermal convection that is expected to be dramatically influenced by the presence of yield stress. Aim of this thesis is to realize a setup to address both the onset and the velocity fluctuations of the convective rolls. Two convective microfluidic cells have been realized, one for the lateral observations, the other designed to guarantee optical access from the top to be used under optical transmission microscopes. The stability of the thermal gradient has been addressed for both configurations. In parallel, a custom rotated microscope has been developed for lateral observations of RBC by using complementary algorithms that perform the Fourier transform of the difference of digital images of the emulsions acquired in time, useful to address the onset and the velocity of the convective rolls.