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The Plateau–Rayleigh instability, often just called the Rayleigh instability, explains why and how a falling stream of fluid breaks up into smaller packets with the same volume but less surface area. It is related to the Rayleigh–Taylor instability and is part of a greater branch of fluid dynamics concerned with fluid thread breakup. This fluid instability is exploited in the design of a particular type of ink jet technology whereby a jet of liquid is perturbed into a steady stream of droplets.

The driving force of the Plateau–Rayleigh instability is that liquids, by virtue of their surface tensions, tend to minimize their surface area. A considerable amount of work has been done recently on the final pinching profile by attacking it with self similar solutions.[1][2]

History[edit]

The Plateau–Rayleigh instability is named for Joseph Plateau and Lord Rayleigh. In 1873, Plateau found experimentally that a vertically falling stream of water will break up into drops if its wavelength is greater than about 3.13 to 3.18 times its diameter.[3] Later, Rayleigh showed theoretically that a vertically falling column of non-viscous liquid with a circular cross-section should break up into drops if its wavelength exceeded its circumference.[4]

Theory[edit]

Intermediate stage of a jet breaking into drops. Radii of curvature in the axial direction are shown. Equation for the radius of the stream is \scriptstyle R\left( z \right) \;=\; R_0 \,+\, A_k \cos \left( kz \right), where \scriptstyle R_0 is the radius of the unperturbed stream, \scriptstyle A_k is the amplitude of the perturbation, \scriptstyle z is distance along the axis of the stream, and \scriptstyle k is the wave number

The explanation of this instability begins with the existence of tiny perturbations in the stream.[5][6] These are always present, no matter how smooth the stream is. If the perturbations are resolved into sinusoidal components, we find that some components grow with time while others decay with time. Among those that grow with time, some grow at faster rates than others. Whether a component decays or grows, and how fast it grows is entirely a function of its wave number (a measure of how many peaks and troughs per centimeter) and the radius of the original cylindrical stream. The diagram to the right shows an exaggeration of a single component.

By assuming that all possible components exist initially in roughly equal (but minuscule) amplitudes, the size of the final drops can be predicted by determining by wave number which component grows the fastest. As time progresses, it is the component whose growth rate is maximum that will come to dominate and will eventually be the one that pinches the stream into drops.[7]

Although a thorough understanding of how this happens requires a mathematical development (see references[5][7]), the diagram can provide a conceptual understanding. Observe the two bands shown girdling the stream—one at a peak and the other at a trough of the wave. At the trough, the radius of the stream is smaller, hence according to the Young–Laplace equation the pressure due to surface tension is increased. Likewise at the peak the radius of the stream is greater and, by the same reasoning, pressure due to surface tension is reduced. If this were the only effect, we would expect that the higher pressure in the trough would squeeze liquid into the lower pressure region in the peak. In this way we see how the wave grows in amplitude over time.

But the Young-Laplace equation is influenced by two separate radius components. In this case one is the radius, already discussed, of the stream itself. The other is the radius of curvature of the wave itself. The fitted arcs in the diagram show these at a peak and at a trough. Observe that the radius of curvature at the trough is, in fact, negative, meaning that, according to Young-Laplace, it actually decreases the pressure in the trough. Likewise the radius of curvature at the peak is positive and increases the pressure in that region. The effect of these components is opposite the effects of the radius of the stream itself.

The two effects, in general, do not exactly cancel. One of them will have greater magnitude than the other, depending upon wave number and the initial radius of the stream. When the wave number is such that the radius of curvature of the wave dominates that of the radius of the stream, such components will decay over time. When the effect of the radius of the stream dominates that of the curvature of the wave, such components grow exponentially with time.

When all the math is done, it is found that unstable components (that is, components that grow over time) are only those where the product of the wave number with the initial radius is less than unity (\scriptstyle kR_0 \;<\; 1). The component that grows the fastest is the one whose wave number satisfies the equation:[7]

 kR_0 \;\simeq\; 0.697

Examples[edit]

Water dripping from a faucet/tap[edit]

Water dropping from a tap.

A special case of this is the formation of small droplets when water is dripping from a faucet/tap. When a segment of water begins to separate from the faucet, a neck is formed and then stretched. If the diameter of the faucet is big enough, the neck doesn't get sucked back in, and it undergoes a Plateau–Rayleigh instability and collapses into a small droplet.

Urination[edit]

Further information: Urination

Another everyday example of Plateau–Rayleigh instability occurs in urination, particularly standing male urination.[8][9] The stream of urine experiences instability after about 15 cm (6 inches), breaking into droplets, which causes significant splash-back on impacting a surface. By contrast, if the stream contacts a surface while still in a stable state – such as by urinating directly against a urinal or wall – splash-back is almost completely eliminated.

Notes[edit]

  1. ^ a b Papageorgiou, D. T. (1995). "On the breakup of viscous liquid threads". Physics of Fluids 7 (7): 1529–1521. Bibcode:1995PhFl....7.1529P. doi:10.1063/1.868540.  edit
  2. ^ a b Eggers, J. (1997). "Nonlinear dynamics and breakup of free-surface flows". Reviews of Modern Physics 69 (3): 865. Bibcode:1997RvMP...69..865E. doi:10.1103/RevModPhys.69.865.  edit
  3. ^ Retardation of Plateau–Rayleigh Instability: A Distinguishing Characteristic Among Perfectly Wetting Fluids by John McCuan. Retrieved 1/19/2007.
  4. ^ See page 23 of this pdf Retrieved 1/19/2007.
  5. ^ a b Pierre-Gilles de Gennes; Françoise Brochard-Wyart; David Quéré (2002). Capillary and Wetting Phenomena — Drops, Bubbles, Pearls, Waves. Alex Reisinger (trans.). Springer. ISBN 0-387-00592-7. 
  6. ^ White, Harvey E. (1948). Modern College Physics. van Nostrand. ISBN 0-442-29401-8. 
  7. ^ a b c John W. M. Bush (May 2004). "MIT Lecture Notes on Surface Tension, lecture 5" (PDF). Massachusetts Institute of Technology. Retrieved April 1, 2007. 
  8. ^ Urinal Dynamics: a tactical guide, Splash Lab
  9. ^ University physicists study urine splash-back and offer best tactics for men (w/ Video), Bob Yirka, Phys.org, Nov 07, 2013

External links[edit]


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Plateau–Rayleigh_instability — Please support Wikipedia.
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13 news items

 
Wired
Sat, 30 Nov 2013 14:45:47 -0800

This effect is known as the Plateau–Rayleigh instability. If you look really closely, any stream of water has tiny irregularities — some bumps that are thicker, and slight necks that are thinner. Surface tension, which is the tendency of tiny water ...
 
Phys.Org
Tue, 03 Dec 2013 07:20:21 -0800

Not in this case, instead the fish uses a known fluid dynamics property called the Plateau–Rayleigh instability. This is where water moving as a stream brakes down into small pieces—small segments form and are eventually pinched off to create entirely ...
 
The Atlantic
Tue, 10 Dec 2013 09:57:38 -0800

The macro shots are brilliant as well; watch for ligaments of paint breaking into droplets due to the surface-tension-driven Plateau-Rayleigh instability. But two questions remain unresolved. One, what is a non-Newtonian fluid? There are complex ...

NEWS.com.au

Business Insider
Thu, 07 Nov 2013 15:40:46 -0800

According to Hurd, part of the messiness caused by male urination is due to a phenomenon called Plateau-Rayleigh instability, which causes streams of falling liquid to decompose into droplets. When a guy pees, the urine stream breaks into droplets ...
 
Daily Mail
Mon, 02 Dec 2013 09:11:01 -0800

... special way to ensure that the tail end of the jet of water is moving faster than the start of the stream. This effectively squeezes the jet into a smaller space and the front end of the jet widens slightly. Physicists recognise this as the Plateau ...

Salt Lake City Weekly

Salt Lake City Weekly
Mon, 11 Nov 2013 10:33:18 -0800

He said splashing is a problem because of the Plateau-Rayleigh instability, where a stream of liquid will break up into droplets. Truscott, along with physicist Randy Hurd, found that sitting on a toilet was the best option. Those who prefer urinals ...
 
Physics
Mon, 31 Jan 2011 07:45:07 -0800

(Top right) Plateau-Rayleigh instability that causes a fluid stream to break up into drops. (Bottom) Laboratory experiments mimicking a vertical counterflow in which oil is flowing from below and a mock fluid is injected at the top. (Left) Oil (light ...

Uproxx

Uproxx
Fri, 08 Nov 2013 11:20:54 -0800

They also discovered something likely few men have considered—that urine follows what is known as the Plateau-Rayleigh instability—where a pee stream breaks up into drops before striking something else. That's the worst thing that can happen, the ...
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