Three-dimensional foam flow resolved by fast X-ray tomographic microscopy

Adapting fast tomographic microscopy, we managed to capture the evolution of the local structure of the bubble network of a 3D foam flowing around a sphere. As for the 2D foam flow around a circular obstacle, we observed an axisymmetric velocity field with a recirculation zone, and indications of a negative wake downstream the obstacle. The bubble deformations, quantified by a shape tensor, are smaller than in 2D, due to a purely 3D feature: the azimuthal bubble shape variation. Moreover, we were able to detect plastic rearrangements, characterized by the neighbor swapping of four bubbles. Their spatial structure suggests that rearrangements are triggered when films faces get smaller than a characteristic area.