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flapping filaments in flowing soap film
(click on an image for a larger view)
We study here the dynamics of flexible filaments (silk threads) in a flowing soap film. For a single filament held at its upstream end and otherwise unconstrained, we found two distinct , stable dynamical states: a "stretched-straight" state, and a flapping state. The stretched-straight state is proven experimentally to be linearly stable. In this state, the filament is immobile and aligned in the flow direction, and sheds a narrow von Karman vortex street downstream from its free end. In the flapping state, the filament executes a sinuous motion with waves traveling downstream along its length in a manner akin to the flapping of a flag in the wind. We also study the side by side interaction between two coupled filaments. Four different dynamical states are observed, depending on the distance between the filaments: a steady state, an in-phase flapping state, an out-of-phase flapping state, and a decoupled flapping state. [Flexible filaments in a flowing soap film as a model for one-dimensional flags in a two-dimensional wind. by Zhang, Childress, Libchaber and Shelley, Nature, 408, 835, (2000)](*see "Publications" for most recent works).
The setup for the flowing soap film: the typical thickness of the soap film is 2-4 micron and the speed is about 200 cm/sec. We use the commercial soap "Dawn" and mix it with distilled water at a 1.5% concentration. The flow can run (on a good day) for hours without breaking. In this figure, H is the upper container, S the stopcock which regulates the flux (thus the flow speed), N the two nylon lines on which the soap film is formed as it flows, T four tension lines to separate the two nylon lines, L is the lower container and F the silk filament (exaggerated in size).
A stretched-straight state: The filament is stable under small disturbances. A vortex street similar to von Karman street is shed behind the free end. The filament is stationary. However, if the external disturbances are big enough, the filament will jump to a "flapping state" [see image below] . The flow is visualized with a low pressure sodium lamp whose light forms interference patterns that capture the flow fields qualitatively. The bifurcation from "straight" to "flapping" state can be shown experimentally to be subcritical.
The flapping state: the filament executes an oscillatory motion with a wave traveling downstream. The thin wake generated from the free end is now modified and displaced in concert with the flapping motion. This state is stable under finite disturbances, but it can be forced back to the "stretched-straight" state by holding it taut.
The free end of the filament executes a "figure-eight " motion due to the fact that the filament is inextensible and the flapping motion is a traveling wave moving along in the free stream direction.
Comparison of the wakes of (1) cylinder, (2) fish tail ,and (3) flexible filament. Notice that the vortex streets are significantly different. The cylinder leaves a typical von Karman vortex street. A swimming fish is believed to produce a "inverted von Karman" street.
Our laboratory is working extensively to visualize the wake of a swimming fish. We put live fish in the test section of water tunnels. Due to their instincts (rheotaxis), fish would orient themselves and swim against the flow. We are currently exploring different visualization techniques to make the wake visible. Our ultimate goal is to better understand how fish swim and why they are so efficient.
Two filaments are inserted side-by-side in the running soap film . The interaction is mediated through the fluid. Two phase-locked states are discovered. In one, both filaments are flapping in phase, and in the other the filaments flap 180 degrees out of phase. As the coupling distance is increased, each filament starts to flap independently. A "stretched-straight" state is also observed, where both filaments stay stationary (not shown).

main reference:
"Flexible filaments in a flowing soap film as a model for one-dimensional flags in a two-dimensional wind"
Zhang, Childress, Libchaber and Shelley Nature, 408, 835, (2000)
(full text, pdf - site or personal license required for Nature on-line)

Other references: click on an underlined title to view article
Physics : Silk and soap show why flags flap
Jonathan Trout
Nature, Science Update, Thurs 14 December 2000

Fluid dynamics: The physics of flapping
Nature, Highlights, 14 December 2000

Swimming in Flatsea Greg Huber
Nature, News and Views, vol 408, p777, Dec. 14, 2000

Flying the flag for fluid dynamics
PhysicsWeb, News: December 2000

Fun With Flat Fluids: Some very serious and sober experiments with giant soap films, courtesy of Shawn Carlson
Scientific American, Amateur Scientist

Silk and soap settle a century-old flap
P. Weiss
Science News, References & Sources, Week of Dec. 16, 2000 (Article in: Science News, Vol. 158, No. 25, Dec. 16, 2000, P. Weiss, p. 390 also see: Letters, vol. 159, No. 7, p99, February 17, 2001)

BBC radio, Science in Action,Presenter: Richard Black, Producer: Roland Pease, Dec 16, 2000

Flatternder Faden
Bild des Monats (Picture of the Month),
Spektrum Feb., 2001, p 27

Blowin' in the wind
Adrian Cho
New Scientist, This Week, 16 December 2000, p.15

All in a Flap
Physics World, Jan., 2001, P. 3

Fil battant
La Recherche No. 339, February 2001, p. 10
Much ado about flapping
Kathy A. Svitil
Discover Vol.22, No. 4, April 2001, p.16

Folha de S.Paulo
14.dec.2000, in Science section (Folha Ciencia)

How do fish swim?
Tony Phillips
American Mathematical Society Math in the Media, February 2001

Flag Day Newspaper Stories
Bruce T. Seeman
Newhouse News Service, July, 2001

Scientists remain engaged in a flag flap
Newark Star Ledger, June 13, 2001 Page A-2

Turbulent Heat Flow: Structures and Scaling
Leo P. Kadanoff, Physics Today, August, 2001, P. 34
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