Spontaneous formation of bidirectional water flow in strong electric field

Water bridges, a nearly 130-year research topic in electrohydrodynamics, present a well-known yet unresolved problem in condensed matter physics. These structures can float polar substances against gravity in the shape of a bridge under strong electric fields. Recent interest has focused on their floating properties, internal structures, and flow characteristics. Although the mechanism of gravity resistance has been thoroughly described by Kelvin force density, the underlying structure and properties remain ambiguous, with no consensus between optical results and fluid experiments. In this study, we integrated several flow field tracking methods and concluded that the internal structure of water bridges features a unique spontaneously occurring bidirectional laminar flow and reproduced, through Multiphysics simulations, the possible origin of stratification during the transition from the initial formation of the water bridge to its stabilized state. We also address the issue of why there is a net flow for water bridges. While a reverse flow in electrolysis-like experimental setups is intuitive due to ion involvement, it does not guarantee the existence of a specific spatial flow field. Our results not only reveal new insights into water bridges but also enhance the understanding of the non-conservative nature of bidirectional flow.