Dynamics of red blood cells and vesicles in microchannels of oscillating width

We have studied the dynamics of red blood cells and fluid lipid vesicles in hydrodynamic flow fields created by microchannels with periodically varying channel width. For red blood cells we find a transition from a regime with oscillating tilt angle and fixed shape to a regime with oscillating shape with increasing flow velocity. We have determined the crossover to occur at a critical ratio L_y/v_m≈2.2 × 10 ^{− 3} s with channel width L_y and red blood cell velocity v_m. These oscillations are superposed by shape transitions from a discocyte to a slipper shape at low velocities and a slipper to parachute transition at high flow velocities.