For many fluid flows, the state U=0 is quite boring, but for yield stress fluid flows this condition often describes a critical balance of yield stress to driving stress of the flow. One such flow involves the trapping of bubbles within a yield stress fluid, where the static flow requires the yield stress to overcome both buoyancy and interfacial stresses. We explain this balance from a theoretical perspective and illustrate how static stability varies with bubble shape. Next we move to the experimental realization of the same static balance, using Carbopol solutions in a vacuum chamber to control bubble size. Here we find that elastic deformations below the yield stress occur, allowing bubbles to change from spherical to elongated, before the onset of motion. In the last part of the talk we explore the static stability and motion of multiple bubbles, revealing interesting flow onset dynamics.
Accès Salle des séminaires FAST-LPTMS (Bât. 530, salle C.120, 1er)