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Name: Daniel
Status: student
Grade: 9-12
Location: PA
Country: USA
Date: Spring 2014


Question:
When a continuous stream of water without any bubbles is falling from a faucet, eventually acceleration due to gravity will stretch the stream until it reaches a point where the stream begins to break up into droplets. The water's surface tension most likely affects when this break up occurs in the stream. However, does the tensile strength of water also factor into this situation? The methods about which I have read detailing the measurement of water's tensile strength involve creating acoustic cavitation bubbles in order to observe when they collapse. When they do collapse, the so called "acoustic cavitation threshold" is apparently a measure of the tensile strength. In this situation, though, I find it difficult to distinguish between the water's surface tension, which I would assume would exist on the inside of the cavitation bubble, and the water's actual tensile strength. Because of this, I am unsure of what force the acceleration of gravity must overcome in order to break a stream into droplets. If both surface tension and tensile strength affect the break up of the stream, which one has a greater effect?



Replies:
Hi Daniel,

Thanks for the question. I would not suggest (to my students) that water has a tensile strength. I would prefer to leave the concept of "tensile strength" to materials like steel and carbon fiber composites. Even though water is very simple (H2O), its properties are not well understood from a fundamental level. Many supercomputer simulations (done at Lawrence Livermore National Laboratory) have provided insight into how the properties of water arise from molecular interactions. Ultimately, the fact that water can undergo hydrogen bonding is the cause of all of the fluid properties (viscosity, surface tension, etc.) of water.

So, to answer your question, surface tension is the key factor that causes droplet formation. Stated another way, the water breaks into droplets since the total number (and strength) of hydrogen bonds is greatest when in the droplet configuration as compared to a cylindrical geometry.

I hope this helps. Thanks Jeff Grell


Hi Daniel,

I not sure what you have been reading, but there is a fundamental problem here. You spoke repetitively about the "tensile strength" of water. The problem is that, no liquid can exhibit tensile strength. One can easily see how tensile strength of a solid can be tested; you simply pull on both ends until the sample under test is ripped apart, and record the force needed to do this. But how can you pull on water?

So, I have to say that your question puzzles me!

Regards, Bob Wilson


Daniel,

The tensile strength of water is a measure of the amount of force that a can be exerted on the *leading edge* of a water column so that it breaks away from the main body. In a cavity-collapse experiment, the leading edge would be the walls of the cavity.

If you have a continuous stream, there is no "leading edge". In an ideal flow every section of the stream experiences the same force and the "thinning" of the stream is due a reduced amount of material in the stream, not to some part of the stream somehow going faster than the rest.

In a real flow (as opposed to an ideal flow) there will be a chaotic mix of amounts of material in the stream due to many factors: imperfect output from the source, random force from the surroundings (air) and so what causes the break in the stream is more of a measure of the rheological properties of the liquid (a general term covering many factors such as surface tension, cohesive forces, etc.). The Bernoulli principle is essentially at work here. The random constrictions in a real flow causing a change in the velocity of the material through the constriction and resulting in a change in the forces parallel and perpendicular to the flow direction.

Greg (Roberto Gregorius) Canisius College


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