Wrinkling, tumbling and swinging microcapsules in simple shear flow

In a series of experiments we examined the orientation behavior of non-spherical polysiloxane microcapsules in simple shear flow. At low shear rates we detected a "tumbling mode", which described the continuous rotation of the particles. In this regime, the orientation angle varied between −90° und +90°. With increasing shear rate a transition to "tank-treading" took place where the membrane rotated around the fluid core. Simultaneously to this membrane movement the deformation and orientation angle of the capsule oscillated, and this regime was denoted as "swinging mode". Between these two different deformation processes we detected an intermittent regime. For a couple of microcapsules we also observed shear-induced membrane wrinkling. Such folding phenomena can obviously be induced by the bending resistance of the surrounding shells or the presence of pre-stressed membranes.