For more than a century, it has been believed that all hydraulic jumps are
created due to gravity. However, we found that thin-film hydraulic jumps are
not induced by gravity. This study explores the initiation of thin-film
hydraulic jumps. For circular jumps produced by the normal impingement of a jet
onto a solid surface, we found that the jump is formed when surface tension and
viscous forces balance the momentum in the film and gravity plays no
significant role. Experiments show no dependence on the orientation of the
surface and a scaling relation balancing viscous forces and surface tension
collapses the experimental data. Experiments on thin film planar jumps in a
channel also show that the predominant balance is with surface tension,
although for the thickness of the films we studied gravity also played a role
in the jump formation. A theoretical analysis shows that the downstream
transport of surface tension energy is the previously neglected, critical
ingredient in these flows and that capillary waves play the role of gravity
waves in a traditional jump in demarcating the transition from the
supercritical to subcritical flow associated with these jumps.Commonwealth Scholarship Commission, EPSRC grant EP/K50375/
