  Nanobubbles in liquids are mysterious gaseous objects having exceptional
stability. They promise a wide range of applications but their production is
not well controlled and localized. Alternating polarity electrolysis of water
is a tool that can control production of bulk nanobubbles in space and time
without generating larger bubbles. Using the schlieren technique a detailed
three-dimensional structure of a dense cloud of nanobubbles above the
electrodes is visualized. It is demonstrated that the thermal effects produce
different schlieren pattern and have different dynamics. A localized volume
enriched with nanobubbles can be separated from the parent cloud and exists on
its own. This volume demonstrates buoyancy from which the concentration of
nanobubbles is estimated as 2x10^18 m^-3. This concentration is smaller than
that in the parent cloud. Dynamic light scattering shows that the average size
of nanobubbles during the process is 60-80 nm. The bubbles are observed 15
minutes after switching off the electrical pulses but their size is shifted to
larger values of about 250 nm. Thus, an efficient way to generate and control
nanobubbles is proposed.
