Lift force and dynamical unbinding of adhering vesicles under shear flow

We show that a vesicle under shear flow experiences a lift force of hydrodynamical origin. This force is of purely viscous nature, and it originates from upstream-downstream asymmetry. At a critical shear rate gamma similar or equal to 100-500 Hz the vesicle, initially making adhesion with a substrate, undergoes an unbinding transition in two nontrival steps. In the first step the lift force behaves as gamma(2), whereas in the second step as gamma. We present numerical and analytical results on this phenomenon. We discuss several implications.