Dynamics of a single vesicle in various flows and hydrodynamics and rheology of concentrated solution of vesicles
    As continuation of our efforts to study single polymer dynamics in micro-devices and channels, e.g. mixing of very viscous fluids in micro-channels, we intend to study further coupling between local dynamics and hydrodynamics of vesicles, worm-like micelles, capsules etc in micro-flows. Interrelation between dynamics and statistics of conformations of constituents of a complex fluid, on one side, and hydrodynamic global properties of a flow of the complex fluid, on the other side, is particularly relevant to the hydrodynamics of solutions contained vesicles, worm-like micelles, capsules as well as large molecular weight polymer molecules. We study dynamics and statistics of a single constituent, such as a single vesicle in shear and elongation flows, together with measurements of global properties of the flow, such as viscosity, relaxation time and stresses. To observe dynamics of vesicles, a micro-channel is naturally used similar to our experiments with a single polymer molecule. Tank-treading, tumbling, and newly discovered by us trembling regimes of vesicle dynamics were studied in details and quantitatively compared with existing theoretical predictions and numerical simulations. It was realized that due to strong influence of thermal noise, large statistics is required in order to get reliable results on average values, such as the average inclination angle of vesicle in the tank-treading regime in a shear flow as a function of asphericity. The viscosity contrast as the control parameter was adjusted to study the phase diagram. Contrary to the theoretical predictions, the normalized shear rate turns out to be another control parameter particularly to determine the transition curve between tumbling and trembling regimes. Several theoretical groups are working now to explain the findings and map the phase diagram in the parameter space mentioned above. Studies of interaction and dynamics of vesicle in concentrated suspensions provide a firm basis to understand and describe hydrodynamics and rheology of such solutions. The latter was experimentally studied by us. New and unexpected vesicle dynamics was observed in an elongation flow, particularly in time-dependent regime. Here observation of higher order harmonics points out on negative surface tension that can be resulted from a delay in the vesicle response on a flow perturbation. The higher the elongation rate, the higher the contribution of large wave number modes. Spectacular coil-stretch-like transition in a vesicle configuration due to stress perturbation in the flow was detected and investigated. This experimental program on vesicle dynamics in various flows being finalized soon will initiated further theoretical studies and understanding of hydrodynamics of this complex suspension. Separately we are conducting similar studies with polymerized vesicles, the object similar to a vesicle but has intercalating polymer between two monolayers of amphiphilic molecules. These vesicles could have important applications, since being polymerized show transition to fluorescent state under external perturbations. The experimental progress in vesicle dynamics made in our laboratory during the last couple of years was possible due to extensive use of micro-fluidics and soft lithography. Micro-fluidics is developed very fast for the recent years. Besides obvious wide applications in biology and medicine it is interesting by its own due to new and unexpected manifestations of unusual hydrodynamic properties of complex fluids (such as elastic turbulence) combined with wall elasticity of pipes etc.