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Center for Nonlinear Studies |
Abstract: Dynamics of a single vesicle in shear and elongation flows is investigated experimentally. In a shear flow study of influence of thermal noise and a viscosity contrast on dynamics of vesicle in tank-treading motion reveals rather good agreement with theory. The surprisingly good agreement of the transition curve from tank-treading-to-tumbling regime with numerical simulations in 2D was found. A new type of unsteady motion at a large degree of vesicle deformability, called trembling, was discovered and described as follows: a vesicle trembles around the flow direction, while the vesicle shape strongly oscillates. The relaxation dynamics of vesicles subjected to a time-dependent elongation flow was studied. We observed and characterized a new instability, which results in the formation of higher order modes of the vesicle shape (wrinkles), after a switch in the direction of the velocity gradient. This surprising generation of membrane wrinkles can be explained by the appearance of a negative surface tension during the vesicle deflation, which tunes itself to alternating stress. Moreover, the formation of buds in the vesicle membrane was observed in the vicinity of the dynamical transition point. In a stationary elongation flow at sufficiently strong extensional rates an analog of coil-stretch transition well-known in dynamics of polymer molecules was discovered for a single vesicle. Its connection to the pearling instability was also established. Finally, we made the first attempt to relate dynamics of a single vesicle and vesicle interaction to a rheology of a vesicle suspension.
Host: Misha Chertkov