(162f) The Instability of a Single Vesicle Under Extensional Flow At Zero Reynolds Number

When floppy vesicles are placed in extensional flows (planar or uniaxial), they undergo two unique sets of shape transitions that to our knowledge have not been observed for droplets.  At intermediate reduced volumes (i.e., intermediate particle aspect-ratio) and high shear rates, the vesicles stretch into an asymmetric dumbbell separated by a long, cylindrical thread.  At low reduced volumes (i.e., high particle aspect-ratio), pearling occurs, where the cylindrical thread forms a series of bulges with a characteristic size on the order of the cylinder’s radius.  In this talk, we will describe our efforts to elucidate the physical mechanisms behind these two seemingly-unrelated sets of instabilities.  We perform boundary integral simulations on single, fluid-filled particles whose interfacial dynamics are governed by a Helfrich energy (i.e., the membranes are inextensible with bending resistance).  With this model, we perform a linear stability analysis to determine the conditions under which the instabilities occur.  We also probe the modal structure of these shape transitions.  Lastly, we develop relatively simple analytical theories to characterize the stretching dynamics in the limit of high shear rate (characterized by a large bending Capillary number), or high particle aspect ratio.  We believe that this work will lend insight into the breakup and dynamics of many types of biological particles under strong tension (particularly those with soft membranes).