Browsing by Author "Abreu, David"

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    Vesicles in flow : role of thermal fluctuations
    (2014) Abreu, David; Seifert, Udo (Prof. Dr.)
    The present thesis deals with the dynamics of fluid vesicles in flow, with a particular focus on the role of thermal fluctuations. Vesicles are microscopic “bags” of liquid whose membrane consists of a very thin lipid bilayer. They are used in biological systems for intra- and inter-cellular communication. They also serve as pharmaceutical carriers. Moreover, they represent the simplest model system for more complex cells possessing a lipid membrane such as red blood cells. Therefore, studying the dynamics of vesicles in flow has a great biological and technological relevance. We are interested in so-called giant unilamellar vesicles (GUVs). Their size is of the order of 10 micrometers which is much larger than the thickness of their bilayer membrane (5 nanometers). A GUV can thus be modeled as a two-dimensional membrane enclosing a fluid and suspended in another fluid. The membrane is in a liquid phase at room temperature and is to very good approximation incompressible and impermeable to many ions, such that the volume and the surface area of a vesicle remain constant. The bending deformations of the membrane involve much lower energies than the stretching and shearing ones. They are therefore sufficient to predict the equilibrium shapes of vesicles under the two constraints mentioned above. Moreover, the membrane bending energy is around 10 to 50 kT, where kT is the typical thermal energy. Therefore, thermal fluctuations play an important role in the shape transitions of GUVs. In this work, we provide a theoretical analysis of the dynamics of GUVs in planar linear flows consisting of a rotational and an elongational component. Our main goal is to investigate the impact of thermal fluctuations on the different dynamical regimes. In order to derive analytical equations of motion, we consider either undeformable vesicles of ellipsoidal shape, or quasi-spherical vesicles corresponding to slightly deflated spheres. We solve these equations either analytically or numerically. We then compare our theoretical predictions to experiments to elucidate the various phenomena observed therein.