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Destabilisation of foamed emulsions, shaken or not
Foams are unstable and destined to disappear. The average bubble size grows through coarsening, driven by gas transfer from smaller bubbles to larger ones, and coalescence when the film between the bubbles ruptures. Understanding and controlling these mechanisms is a major challenge, in particular when the foams are made from viscoelastic fluids, such as pastes or gels.
We are interested in understanding the role of a viscoelastic continuous phase on foam destabilisation. We study the temporal evolution of foams made from concentrated emulsions. The behaviour of our foams is no longer dominated by capillary effects, as it is impacted by the rheological properties of the emulsion between the bubbles. We show that the yield stress of the continuous phase impacts both the foam dynamics and its structural evolution leading to spatially heterogeneous coarsening.
This ageing can be impacted by vibrations. Complex fluids are subject to mechanical shaking, for example during transport or homogenisation processes. This is why understanding their response to vibrations is important. We have carried out experiments on the impact of mechanical vibration on aqueous foams and foams made from yield-stress emulsions. Above a threshold amplitude we observe a catastrophic destabilisation of the foam, which is more dramatic in foams made from yield stress fluids. We rationalise their behaviour through a statistical model, combining classical foam coarsening and a bubble rupture model, where the probability to rupture increases with film size.