Supervisors: C. Marques and A. Schroder
We study in this dissertation the behavior of lipid bilayers under a viscous stress. We developed a new apparatus that measures the effect of a shear flow on the fluctuation pattern of giant vesicles. A classic micropipette suction device is placed in a thin gap where the flow is generated. Low alternative shear rates, typically in the range 10-4 à 10-2 s-1 , allow to apply a viscous stress without any optically visible deformation or displacement. Our experiments reveal a strong interaction between flow and membrane fluctuations. When a constant suction pressure is applied to the vesicle by the micropipette, the flow induces an increase of the visible membrane area, thus unfolding fluctuation modes hidden in the sub-optical range. Also, a classic suction experiment that allows to measure the curvature modulus of the membrane, leads here to smaller apparent modulus. These results, confirmed by optical interferometry experiments (RICM), are consistent with a scenario where the membrane fluctuations are unfold by shear stresses. They also imply that the constitutive Helfrich relation, connecting the membrane tension and the surface excess area consumed in the fluctuation modes, needs to be modified in the presence of the flow. We propose a new functional form of the Helfrich law, consistent with our results. In this thesis we also explored the effects on vesicle behavior of membrane modification by inclusion of macromolecules.