Aligning and flocking of ellipsoidal Quincke rollers by flipping

Active rods propelled along their long axis align their velocities and orientations simultaneously in collision. However, as the propulsion is perpendicular to the long axis, velocity alignment becomes dynamically difficult. Here, we show that ellipsoidal Quincke roller propelled along their short-axis (perpendicular to the long axis) can align their velocities by flipping and form flocking with nematic order. The flipping arises from the reversible transition between the static parallel spinless state and the spinning transversal state of ellipsoidal Quincke rollers. This is possible only near (above) the critical field where both the parallel spinless state and the spinning transversal spinning are metastable. The flipping-facilitated alignment offers an extra aligning mechanism for elongate active agents, and the resulting active liquid crystals serve a model system to explore the defect dynamics as the propulsion deviates from the local nematic orientation which has not been addressed yet.