Vesicle adhesion in flow

The dynamic adhesion of lipid vesicles in shear flow is governed by the interplay of membrane-bending rigidity, membrane-substrate adhesion energy, and hydrodynamic stresses. While the equilibrium contact curvature is determined by the transversality condition, shear flow can additionally deform the adhered vesicle, drive translational motion, or, through hydrodynamic lift forces, promote detachment of the vesicle from the substrate. Here, we investigate adhered vesicles in shear flow subjected to a reversible change of the membrane-wall interactions. Using a stimuli-responsive poly(acrylic acid-co-cysteine) brush, whose adhesive properties are modulated by the controlled addition and removal of Cd $_{}^{2+}$ ions, and using reflection interference contrast microscopy, we track the membrane-substrate separation and vesicle shape throughout the transition. Using molecular dynamics simulations within the Helfrich framework, we provide microscopic insights into the deformation processes and systematically investigate the dependence on the shear-flow strength. Two parameters govern the dynamic response: (i) the balance between adhesion and membrane bending, and (ii) the ratio between viscous shear and bending stresses. In the strong-adhesion regime, a large rim of the contact zone between membrane and substrate and an increased density of wall springs lead to a significant increase in wall springs that must rupture per unit displacement. This effectively suppresses tank-treading motion. Reducing adhesion decreases the contact-zone rim and wall-spring density, enabling flow-driven translational transport. Simultaneously, shear flow induces a shape asymmetry, manifested as a forward tilt of the vesicle. As a consequence, the curvature at the preceding edge decreases, whereas the curvature at the receding edge increases. These results demonstrate that vesicle motion on polymer-brush-coated substrates can be reversibly controlled by chemical stimuli, providing a tunable platform to study adhesion dynamics under flow.