This work presents experiments for the stability study of oil-in-water emulsions at varying gravity conditions. A key factor influencing emulsions stability is droplet size which can be assessed by several techniques employing different physical principles. Experiments are performed during the 79th Parabolic Flight Campaign of European Space Agency (October 2022). A pulsatile miniature emulsification device is used to produce emulsions in ∼0 g conditions while using varying experimental parameters (oil volumetric fraction, surfactant concentration, pulsation frequency, number of strokes). The device resembles the Soft Matter Dynamics facility onboard the International Space Station whereas the examined parameters are also alike to Soft Matter Dynamics experiment. Optical measurements are employed to determine the droplet size distribution as well as, to observe coalescence events in the aqueous phase. Electrical measurements with a non-intrusive, on-line, impedance spectroscopy technique are carried out to track the evolution of oil volumetric fraction at the central region of the emulsification cell. As expected, in the absence of gravity, the buoyancy related phenomena are eliminated and, thus, phase gravitational separation between the two phases (creamy and aqueous) does not take place. Yet, the residual motion at the end of emulsification affects (a) the spatial homogeneity (distribution) of the two phases in the cell and (b) the capacity for coalescence of colliding droplets. In the absence of gravity, these two emulsion parameters are controlled solely by hydrodynamics and interfacial phenomena whereas on earth they are masked by buoyant phenomena. It is found that the oil fraction evolution curves resulting from electrical measurements are compatible with a bidisperse oil droplet size distribution. The characteristic sizes of the two droplet modes are estimated using theoretical arguments. Both the electrically volume-based determined droplet sizes and the number-based ones from high resolution image analysis are discussed in detail.
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