Within the hydrodynamic description, we have found an event-by-event correlation between the emission time and the transverse momentum of final particles. Based on this, we propose an observable that measures the event-by-event fluctuations of the correlation between the event plane calculated for the particles below and above the mean-$p_t$. We show that this observable is very sensitive to the granularity of the initial conditions and argue that it may provide valuable information about the geometrical properties of the initial conditions in AA collisions.