  Memory can remarkably modify the collective behaviors of active particles. We
show that in a micellar fluid, Quincke particles driven by a square-wave
electric field exhibit a frequency-dependent memory. Upon increasing the
frequency, a memory of directions emerges whereas the activity of particles
decreases. As the activity is dominated by interaction, Quincke particles
aggregate and form dense clusters in which the memory of the direction is
further enhanced due to the stronger electric interactions. The
density-dependent memory and activity result in dynamic heterogeneity in
flocking and offer new opportunity for study of collective motions.
