Hydraulic jump in a flowing soap film

July 15, 2008
  • Soap film
  • 76A20
Joint work with S. Steers, J. Larkin, A. Prescott (University of Pittsburgh), T. Tran, G. Gioia, P. Chakraborty, G. Gioia, and N. Goldenfeld (University of Illinois, Urbana). A soap film flows vertically downward under gravity and in a steady state. At all lengths of the film, its thickness h(x) decreases as the distance x from the top reservoir increases. But then h(x) abruptly starts to increase and its downward flow velocity u(x) correspondingly decreases to very small value. To explain this nonmonotonic behavior in h(x) and u(x), it is necessary to invoke the film's elasticity; one has a type of Marangoni effect. The transition from subcritical flow speed to a supercritical one at the thickening point, is akin to the classical hydraulic jump. This transition will be explained, but other findings, also to be described, are not yet understood.