(110d) Pearling and Buckling of High Aspect Ratio Vesicles in Extensional Flows

When a high aspect ratio vesicle is placed in an extensional flow above a critical extension rate, the vesicle undergoes stretching transitions that are in many ways similar to the breakup of droplets.  One of these transitions, deemed ``pearling’’, is when the vesicle forms a series of beads in its central neck, reminiscent of the Rayleigh-Plateau instability.  In this talk, we describe the major physics behind vesicle pearling, and determine how it differs from the standard theories of drop breakup.  We perform boundary integral simulations of a vesicle under uniaxial extensional flow, where we approximate the phospholipid bilayer as an incompressible interface with bending energy.  We complement the simulations with simple analytic theories and scaling analysis.  The stability criterion we develop agrees well with in-vitro experiments, and differs from the standard Rayleigh-Plateau analysis due to the non-uniform tension induced by the flow on the membrane.  We conclude our talk by discussing the early time response of vesicles in compressional flows (e.g., uniaxial compression).   We find that vesicles undergo a variety of buckling/wrinkling instabilities, the physics of which we characterize using analytical theories.