Tank-treading as a means of propulsion in viscous shear flows

The use of tank-treading as a means of propulsion for microswimmers in viscous shear flows is taken into account. We discuss the possibility of a vesicle to control the drift in an external shear flow, by locally varying the bending rigidity of its membrane. By analytical calculation in the quasi-spherical limit, the stationary shape and the orientation of the tank-treading vesicle in the external flow are determined, working to lowest order in the membrane inhomogeneity. The membrane inhomogeneity acts in the shape evolution equation as an additional force term, which can be used to balance the effect of the hydrodynamic stresses, thus allowing the vesicle to assume shapes and orientations that are impossible otherwise. The vesicle shapes and orientations required for migration transverse to the flow, together with the bending rigidity profiles leading to such shapes and orientations, are determined. Considering the variations in the concentration experienced during tank-treading, a simple model is presented, in which a vesicle is able to migrate up or down the gradient of a concentration field by stiffening or softening of its membrane.

ODD NUCLEI AND SHAPE PHASE TRANSITIONS: THE ROLE OF THE UNPAIRED FERMION

Shape phase transitions in even and odd systems are reviewed within the frameworks of the Interacting Boson Model(IBM) and the Interacting Boson Fermion Model(IBFM), respectively and compared with geometric models when available. We discuss, in particular, the case of an odd j = 3/2 particle coupled to an even-even boson core that undergoes a transition from the spherical limit U(5) to the γ-unstable limit O(6). Energy spectrum and electromagnetic transitions, in correspondence of the critical point, display behaviors qualitatively similar to those of the even core and they agree qualitatively with the model based on the E (5/4) boson-fermion symmetry. We describe then the UBF(5) to SUBF(3) transition when a fermion is allowed to occupy the orbits j = 1/2, 3/2, 5/2. The additional particle characterizes the properties at the critical points in finite quantum systems.