Unbinding of vesicles under flow at a stagnation point

In the last decade, considerable efforts have been undertaken to measure, model and simulate the motion in Stokes flow of deformable particles such as capsules, vesicles and cells. They are motivated by cosmetic, pharmaceutical and agricultural industries and also by key biological involvements in blood rheology or leukocytes motion. We study the unbinding of a heavy vesicle interacting weakly with a flat substrate under a succion flow induced by a micropipette. The dynamics of unbinding depends mainly on the vesicle size, the flow rate and the contact area between vesicle and substrate, a quantity directly linked to the reduced volume (a measurement of the deflating). The range of applied hydrodynamic forces varies from sub-pN to one hundred picoNewton what covers the buoyancy force of our vesicles. Their detachments appear above a critical flow rate Uc depending on the characteristics of the studied vesicle. Uc varies linearly with the difference of densities and with the power minus two of the distance between the micropipette and the vesicle. At this threshold, the inverse of the rising time vanishes. Rv and Rc are the radii of the vesicle and of the circular contact area. Three kinds of behaviours are observed following if vesicles are quasi-spherical, slightly deflated and deflated. For Rv > Rc, Rc(t) varies very slowly during a lag period and after, Rc(t) falls to zero with a finite time. For Rv > Rc, the vesicle shape becomes a truncated egg with a smaller contact radius during a characteristic time. After, we recover the two phases of Rv > Rc. For Rv » Rc, the shape dynamics is more complex from egg and pear shapes to lipid tubes