Dynamics and shape transitions of red blood cells in time-dependent capillary flow*

Blood is mainly comprised of red blood cells (RBCs) that determine the unique flow properties of blood in the circulatory system. Their high deformability allows them to squeeze through microvessels much smaller than their equilibrium size. In microfluidic flows with channel dimensions similar to their size, RBCs exhibit characteristic shapes, such as croissants and slippers, depending on their confinement, velocity, and initial conditions. Although RBCs have been studied under steady flow conditions, knowledge about their flow behavior and shape transitions in unsteady flows remains vague.

In this study, we perform microfluidic experiments in combination with numerical simulations to examine single RBCs in time-dependent flow fields. We use a high-precision pressure device to generate an unsteady driving of the flow in the microchannels and track single cells along the channel flow direction in a comoving frame. Applying an increasing pressure ramp, we find that the transition from the croissant to the slipper shape is faster than the opposite shape transition at a decreasing pressure ramp. Further, we observe that slipper-shaped RBCs oscillate laterally while traveling through the microchannel. The frequency of these oscillations increases with the cell velocity and with the viscosity of the surrounding fluid. Our study aims to understand how the time scale of the flow couples with the characteristic time scale of single RBCs in capillaries.