  A new type of a levitating droplet clusters composed of often transforming
small aggregates of water droplets is described for the first time. Unlike
earlier observed droplet clusters controlled by aerodynamic forces, which
formed either an ordered hexagonal structure or a chain structure, the cluster
under consideration has a hierarchical organization. Small groups of closely
spaced or packed droplets with interactions controlled by the electrostatic
force are combined into larger structures controlled by aerodynamic forces.
Since charged droplets in the nucleus of the cluster do not have dynamically
stable configurations, droplet aggregates keep continuously restructuring.
However, droplets with lower charge in external layers of the cluster form a
stable hexagonal structure.
