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Center for Nonlinear Studies |
Dynamics of deformable mesoscopic objects under hydrodynamic stresses determine rheology of many complex fluids, such as emulsions, suspensions of droplets or bubbles, solutions of vesicles, blood, biological fluids, etc. From a theoretical point of view this non-equilibrium problem is rather challenging due to the coupling between micro-scales of the object deformations and macro-scales of the flow, where the object shape is not given a priori but determined by interplay between flow, bending energy, and various physical constraints. A vesicle is an example of such deformable objects. Dynamics of a single vesicle in a general flow is investigated experimentally. Phase diagram of three dynamical of a vesicle is obtained experimentally in both shear and general flows. The new control parameter, the ratio of the vorticity to the strain rate ω/s, allows following an experimental path, which scans across the whole phase diagram with a single vesicle. Surprisingly, all three states and transitions between them are obtained on the same vesicle and at the same viscosity of inner and outer fluids. We reveal the physical nature of the key dynamical state, coined by us trembling, which shows up in intrinsic shape instability on each cycle resulted in periodical bursting of higher order harmonics depending on the value of the control parameter proportional to ω/s. Depending on the value of this control parameter the regions with the dominated second, third, and higher harmonics in the trembling dynamics are identified. Agreement with theory is discussed.
Host: Misha Chertkov, T-4