Description

When flexible vesicles are placed in an extensional flow (planar or uniaxial), they undergo a wide range of shape transitions. At intermediate aspect-ratios and high extension rates, a vesicle stretches into an asymmetric dumb-bell separated by a long, cylindrical thread. At high aspect-ratios, the vesicle extends indefinitely in a seemingly-symmetric fashion, in a manner similar to the breakup of liquid droplets. In this ``burst’’ phase, the vesicle may undergo “pearling” if the extension rate is above a critical value, i.e., the vesicle forms necklace-like structures in its central neck reminiscent of the Rayleigh-Plateau instability. In this discussion, we describe the mechanisms behind these shape transitions by solving the Stokes equations around a single, fluid-filled particle whose interfacial dynamics are governed by a Helfrich energy (i.e., the membrane is inextensible with bending resistance). We find that the shape transitions described above have their origins in a modified Rayleigh-Plateau analysis, even though the shapes look qualitatively different from each other. The stability criteria determined by our simulations and scaling analysis agree well with in-vitro, cross-slot microfluidic experiments (some of which we perform). In the last part of this discussion, we discuss the early time response of vesicles in uniaxial compressional flow. We find that vesicles undergo a variety of buckling/wrinkling instabilities, the physics of which we characterize using analytical theories. This study highlights the major differences between vesicle deformation and droplet breakup, the differences mostly being attributable to interface’s compressibility in the two systems.

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Shape transitions of vesicles in linear, hyperbolic flows: asymmetric dumbbells, pearling, and buckling

When flexible vesicles are placed in an extensional flow (planar or uniaxial), they undergo a wide range of shape transitions. At intermediate aspect-ratios and high extension rates, a vesicle stretches into an asymmetric dumb-bell separated by a long, cylindrical thread. At high aspect-ratios, the vesicle extends indefinitely in a seemingly-symmetric fashion, in a manner similar to the breakup of liquid droplets. In this ``burst’’ phase, the vesicle may undergo “pearling” if the extension rate is above a critical value, i.e., the vesicle forms necklace-like structures in its central neck reminiscent of the Rayleigh-Plateau instability. In this discussion, we describe the mechanisms behind these shape transitions by solving the Stokes equations around a single, fluid-filled particle whose interfacial dynamics are governed by a Helfrich energy (i.e., the membrane is inextensible with bending resistance). We find that the shape transitions described above have their origins in a modified Rayleigh-Plateau analysis, even though the shapes look qualitatively different from each other. The stability criteria determined by our simulations and scaling analysis agree well with in-vitro, cross-slot microfluidic experiments (some of which we perform). In the last part of this discussion, we discuss the early time response of vesicles in uniaxial compressional flow. We find that vesicles undergo a variety of buckling/wrinkling instabilities, the physics of which we characterize using analytical theories. This study highlights the major differences between vesicle deformation and droplet breakup, the differences mostly being attributable to interface’s compressibility in the two systems.