Relaxing in Foam
We report how aqueous foams lose their elasticity along two trajectories in the jamming phase diagram. With time, bubbles unjam due to coarsening. Rheology is measured over nearly six (five) decades in frequency (time); surprisingly, it is linear and well behaved at low frequencies. With shear, bubbles also unjam. Rheology is measured by a novel method in which a step strain is superposed on an otherwise steady flow; transient elasticity vanishes at the same strain rate at which successive bubble rearrangements merge together. Thus we connect the macroscopic rheology with the underlying microscopic bubble dynamics.