  Microscale fluid flows generated by ensembles of beating eukaryotic flagella
are crucial to fundamental processes such as development, motility and sensing.
Despite significant experimental and theoretical progress, the underlying
physical mechanisms behind this striking coordination remain unclear. Here, we
present a novel series of experiments in which the flagellar dynamics of two
micropipette-held somatic cells of Volvox carteri, with measurably different
intrinsic beating frequencies, are studied by high-speed imaging as a function
of their mutual separation and orientation. From analysis of beating time
series we find that the interflagellar coupling, which is constrained by the
lack of chemical and mechanical connections between the cells to be purely
hydrodynamical, exhibits a spatial dependence that is consistent with
theoretical predictions. At close spacings it produces robust synchrony which
can prevail for thousands of flagellar beats, while at increasing separations
this synchrony is systematically degraded by stochastic processes. Manipulation
of the relative flagellar orientation reveals the existence of both in-phase
and antiphase synchronized states, which is consistent with dynamical theories.
Through dynamic flagellar tracking with exquisite spatio-temporal precision, we
quantify changes in beating waveforms that result from altered coupling
configuration and distance of separation. The experimental results of this
study prove unequivocally that flagella coupled solely through a fluid medium
can achieve robust synchrony despite significant differences in their intrinsic
properties.
