  By revisiting the century-old problem of water bridge, we demonstrate that it
is in fact dynamic and comprises of two coaxial water currents that carry
different charges and flow in opposite directions. This spontaneous separation
is triggered by the different stages to construct the water bridge. Initially,
a flow is facilitated by the cone jet that is powered by H+ and flows out of
the positive-electrode beaker. An opposing cone-jet from negative beaker is
established later and forced to take the outer route. This spontaneous
arrangement of two-way flow is revealed by using fluorescein and carbon powder
as tracers, and the Particle Image Velocimetry, These two opposing flows are
found to carry non-equal flux that results in a net transport of water to the
negative beaker. We manage to estimate the flow speed and cross-sectional area
of these co-axial flows as a function of time and applied voltage. Note that
the water on the outer layer functions as a millimeter tube that confines and
interacts strongly with the flow inside. This provides a first natural and yet
counter example to the recently reported near-frictionless flow in an equally
miniatureized soft wall made from ferrofluid.
