“Cell Pinball”: What is the physics?

During the deformability test of Red Blood Cell (RBC) by utilizing a micro fluidic channel, we found an interesting phenomenon where some RBCs behave just like elastic pinball with the motion in the perpendicular direction with respect to the main flow line, while most of RBCs simply move along the main flow line. This phenomenon is called “Cell Pinball”. Through visualization, we found that the RBC being in Cell Pinball mode rotates around the perpendicular axis to the flow line and the rotating direction has one-to-one relationship with the moving direction without any exceptions. We also found that the rotating axis exists slightly behind the center of gravity with respect to the flow direction. This geometrical configuration makes an unstable condition for the cell under the fluid force, which eventually produces the motion perpendicular to the main flow line.