Extreme Deformation of Capsules and Bubbles Flowing through a Localised Constriction

We explore experimentally the motion of deformable objects – bubbles and capsules – through narrow localised constrictions, with widths between 16 and 32% of the diameter of the object. Under constant volume-flux flow, both bubbles and capsules extend to a maximum length as their front passes through the constriction. Rapid contraction occurs as their rear accelerates towards the constriction, followed by relaxation upon exiting the constriction. We find that the large deformations imposed by narrowing constrictions and increasing flow rates highlight distinguishing features between bubbles and capsules, which reflect their distinct mechanics. These include tip-streaming of the rear of the bubble, buckling of the capsule upon contraction and wrinkling of the capsule membrane through excessive compressive strains. Finally, we present evidence of distinct modes of rupture for bubbles and capsule as the flow rate is further increased.