  Eukaryotic cilia and flagella are chemo-mechanical oscillators capable of
generating long-range coordinated motions known as metachronal waves. Pair
synchronization is a fundamental requirement for these collective dynamics, but
it is generally not sufficient for collective phase-locking, chiefly due to the
effect of long-range interactions. Here we explore experimentally and
numerically a minimal model for a ciliated surface; hydrodynamically coupled
oscillators rotating above a no-slip plane. Increasing their distance from the
wall profoundly effects the global dynamics, due to variations in hydrodynamic
interaction range. The array undergoes a transition from a traveling wave to
either a steady chevron pattern or one punctuated by periodic phase defects.
Within the transition between these regimes the system displays behavior
reminiscent of chimera states.
