# What fundamentally distinguishes cavitation and boiling as different phenomena?

Cavitation and boiling are names for phenomenon that both involve sudden appearance of bubbles of vapor within a liquid, and in both cases they happen when the local hydrostatic pressure is lower than the vapor pressure of the fluid, but that doesn't necessarily mean they are the same thing.

In this video of an electrical heating element in water, between 01:00 and 02:00 the sound produced by rapid bubble collapse gets louder and louder but there are few visible bubbles. Is the process that produces this sound considered boiling or cavitation? What is the distinction?

I've left a provisional answer to the related question on another SE site: How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier? I couldn't bring myself accept the answer on that question that starts with the claim, "cavitation is boiling."

Although they are related, what fundamentally distinguishes cavitation and boiling as different phenomena?

• This has nothing to do with engineering. – Wasabi Oct 23 '16 at 1:22
• @Wasabi Thanks for your comment. I see several questions here about cavitation. Engineers do more than calculate aspects of cavitation, engineers also try to understand cavitation. Clarifying the distinction between two related phenomena that engineers regularly deal with is certainly on-topic. Yes this is not another "how do I calculate..." question, but I think someone with good working knowledge of cavitation in an engineering and fluid-mechanics context will be able to offer an engineering answer. – uhoh Oct 23 '16 at 1:36

Mechanical engineer here, former US Navy nuke. The textbook definition of cavitation is, from my nuclear training:

"The formation and subsequent collapse of vapor bubbles as the suction pressure falls below and then rises above saturation pressure."

This definition is referring to the suction pressure as in a pump, but I would say more generally and apparently against most of the other posters here that cavitation refers more to the formation and subsequent collapse of vapor bubbles than it does about how those vapor bubbles occur.

Now, I understand that the cavitation effect generally happens (or is most discussed when it happens) in pumps and propellers, but it also happens in boiling water.

When you bring water to a boil, initially it is quiet and there are no bubbles. At some transition point (nucleate boiling), bubbles form on the bottom of the pan, break away, but collapse before they reach the surface. This kind of boiling (referred to as a simmer in culinary terms) can correctly be referred to as cavitation. This is also a very noisy phase in the boiling process - this is the "noisy" period in OP's video.

After cavitation comes (for cooking, at least) the final boiling phase, in which the bulk fluid boils and bubbles reach the surface of the water (departure from nucleate boiling). Despite boiling appearing to be more vigorous, this is actually much quieter because cavitation is no longer occurring.

Cavitation is the pinging sound a pot of water makes before a full boil. Once a full boil is achieved, steam bubbles reach the surface and the quality of the sound changes from a pinging to more of a gurgle.

All of that said, there has been a lot of talk on other posts about boiling being the application of heat and cavitation being about the reduction of pressure. Again, the reduction of pressure (below saturation pressure) is a cause of cavitation, but reducing pressure is not the definition of cavitation.

The term for creating vapor bubbles by reducing pressure is called flash distillation or flash evaporation. The term for creating vapor bubbles by increasing heat is called boiling.

The term cavitation refers to the formation and subsequent collapse of the vapor bubbles. Cavitation happens in pumps, in a pot of spaghetti water, in a submarine propeller, etc. It is not restricted to either mode of creation (pressure or heat). The video in OP's post shows cavitation during a boiling process.

## :EDIT:

I felt challenged by Air's comment to produce a source for the definition of cavitation that I provided here. The line I quoted above is as-memorized from about 15 years ago now. I have (on a bookshelf at home) a condensed technical handout of non-classified information we were given at the conclusion of the nuclear training courses for personal reference. In trying to find this manual online, I found a technical publications website that appears to reproduce some of the content we were taught in the nuclear power training program.

The first mechanical science volume has a section on cavitation that states,

If the pressure drop is large enough, or if the temperature is high enough, the pressure drop may be sufficient to cause the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where local pressure is greater than saturation pressure farther out the impeller vane, the vapor bubbles abruptly collapse. This process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitation.

(Emphasis added) The definition we were instructed to memorize (as I quoted at the top) is the condensed version of this statement for reproduction on exams.

Now, there is no source on this particular website, where the reference volumes are broken out by section, as to where this material originates, but at the top of the page is given the DOE document "DOE-HDBK-1018/1".

You can look that number up and find the document posted in full on the Department of Energy's website, where that passage can be found on page 12.

Further, regarding the comment about "industry not toeing the line of the US Navy," the copy hosted on the DOE website includes a foreword and an overview that state the material was prepared with input from the nuclear industry and is intended for use in training nuclear operators. So, maybe some industries do not use the definition of cavitation I have provided, but the nuclear industry does, and it seems like (from Bryon Wall's comment) that the chemical industry does as well.

• This should be the answer. All references to cavitation that I've encountered (chemical engineer in the process industry) refer to the collapse of a bubble. The bubble was nearly always formed by a reduction in pressure due to some device (e.g. pump or control valve) and then subsequent increase in pressure that collapses the bubble. For control valves, a distinction is made between flashing where bubbles are formed and cavitation where those bubbles subsequently collapse. The latter is destructive to a control valve; the former may impede control if not designed for. – Byron Wall Oct 26 '16 at 21:38
• Thank you for a no-nonsense answer that addresses the two phenomena head-on. It makes much more sense to me if the loud nose during "simmering" can indeed be called cavitation. The names of the phenomena refer more to what actually happens than to the conditions that lead up to them. – uhoh Oct 26 '16 at 23:45
• I would strongly advise taking a less dogmatic approach to terminology than is implied by this answer. It is an informed, expert answer and quite valuable in its own right, well worth an upvote, but it does not give the full story. In particular, the reference to "the textbook definition" in the face of an alternative definition given in an extremely credible textbook should give the reader pause. The whole of the professional world does not yet toe the line of the US Navy. – Air Oct 27 '16 at 16:06
• Actually i didnt say anything about wether or not cavitation happens in boiling water i said cavitation does not make boiling. but whatever. – joojaa Oct 27 '16 at 16:25

I think this is more about language than physics. The basic physical phenomenon - the phase change from liquid to gas when the vapor pressure equals the hydrostatic pressure in the fluid - is the same for boiling and cavitation.

In common (non-scientific) usage, "boiling" means heating the liquid until its vapor pressure is equal to the internal pressure of the fluid. In most "non-scientific" cases, the heating is done at (approximately) constant pressure with an interface between the liquid and a gas (e.g. water and air), and the vaporized liquid (steam) leaves the liquid and mixes with the gas, transferring heat from the liquid to the gas.

On the other hand "cavitation" is a local reduction of the pressure in the liquid, at (approximately) constant temperature. As with boiling, some of the liquid vaporizes when the liquid pressure equals the vapor pressure, but the vapor can't escape anywhere because the surrounding liquid is at a higher pressure. If a bubble of vapor starts to move through the fluid, it soon reaches a point where the fluid pressure is higher, and it collapses back into a liquid.

The sudden pressure waves in the liquid, which are created when the bubbles collapse, can cause damage to metal components like propellors, water turbines, etc.

• Thanks! I think we can't really separate engineering from language. If you take a text book or a publication and cross out "cavitation" and write "boiling" everywhere, will it still be correct from an engineering point of view, or would it be rejected as wrong? When we ask and answer questions in stackexchange, and when we write and read books and publish papers, we are using the valuable tool of language. Good use of language in engineering is essential for engineering to exist. Poor use of language causes errors, mistakes, and failures. – uhoh Oct 23 '16 at 1:04
• Forget "common (non-scientific) usage", can you find a technical, reviewed or at least well respected instance of boiling and cavitation being used interchangeably in engineering without distinction? – uhoh Oct 23 '16 at 1:10
• Thank you for your answer - I've had to adjust the wording of the question a bit because it was pointed out that I hand't actually stated a clear question, although you've obviously figured out exactly what I was trying to ask anyway! – uhoh Oct 23 '16 at 17:24
• In this video of an electrical heating element in water youtu.be/Lwk9Bi3j58o?t=105 between 01:00 and 02:00 the sound produced by rapid bubble collapse gets louder and louder but there are few visible bubbles. What is happening is similar to what you describe in your sentence, if the word "pressure" is simply changed to "temperature" - is this cavitation, or boiling? "If a bubble of vapor starts to move through the fluid, it soon reaches a point where the fluid pressure (temperature) is higher (lower), and it collapses back into a liquid." – uhoh Oct 25 '16 at 0:30
• We call water boiling when all of it is in the state of phase change. We do not call water that is locally hot enough for boiling, boiling. It boils when the whole mass is in this state, or atleast a substantial area boils. @uhoh cavitation is not causing water to boil too localized. – joojaa Oct 25 '16 at 2:18

The short answer is that cavitation and boiling both refer to a phase change from liquid to gas that causes bubbles to form, where cavitation is driven by a drop in pressure and boiling is driven by an increase in temperature. For a citation, see Cavitation and Bubble Dynamics, page 1:

A rough but useful way of distinguishing these two processes is to define cavitation as the process of nucleation in a liquid when the pressure falls below the vapor pressure, while boiling is the process of nucleation that ocurs [sic] when the temperature is raised above the saturated vapor/liquid temperature. Of course, from a basic physical point of view, there is little difference between the two processes... The differences in the two processes occur because of the different complicating factors that occur in a cavitating flow on the one hand and in the temperature gradients and wall effects that occur in boiling on the other hand.

If you want to know how this distinction can be useful, the full text of an older edition of the book is available via the Caltech library web site. Finding a more recent edition at the library shouldn't be difficult, considering the work has been cited nearly 3,000 times according to Google Scholar.

The long answer starts by noting that this quote doesn't pretend to give the only definitions of cavitation and boiling; it explicitly proposes one way of defining them as two processes that is "rough but useful." I expect Dr. Brennen would agree that there exist contexts in which other definitions are more useful.

In a very general sense, "cavitation" can mean the spontaneous appearance of cavities (also known as voids or bubbles) within a liquid. If you're researching how different materials or surface geometries promote or suppress nucleation, this might be a more useful definition to you than one that excludes heating.

In a more restrictive sense, "cavitation" can mean only that subset of the former that occurs at relatively constant temperature, in the presence of a solid interface, which later implodes and contributes to wear on mechanical components. If you're building a propulsion system for a submarine, this might be a more useful definition than either of the previous two.

The word "boiling" pre-dates modern thermodynamics so we shouldn't be surprised if it's hard to pin down. We typically think of boiling as a process involving bubbles but film boiling is an exception—clearly the folks researching what happens when you apply a ton of heat at a solid/liquid interface thought it was useful to put this phenomenon in the same category as nucleate boiling.

On the other hand, liquids are also said to "boil" in a vacuum (and here's a video of that, if you're curious—try to make out where the nucleation occurs!). Do you think the folks at NASA care whether boiling requires heat when they're working to mitigate risks associated with explosive decompression? I don't.

You gain very little by expecting, or giving the expectation of, objectively correct terminology. If you're doing some technical writing on the subject and intend to distinguish between cavitation and boiling, just make your definitions explicit. Do your due diligence to ensure that your definitions aren't a significant departure from consensus, or else build a very strong argument to support them.

• Hard to argue with a quote from a book, but that definition does not square with any of the language I've heard. What they call cavitation, I would call "flashing". After that, I agree your answer and think it's quite good. Only other caveat is that you don't necessarily need a surface to have cavitation, but that it becomes destructive if a surface is present. – Byron Wall Oct 26 '16 at 21:44
• @ByronWall Sure! We could be more explicit by saying that the implosion becomes destructive if a surface is present. Or more explicit still by adding "the force resulting from," and so on and so forth. If I'm arguing anything, it's that terminology represents tools to be selected, not laws to be obeyed. Back in high school, I remember arguing about whether it was more correct to say "potential energy" or "potential for energy"—boy, what a waste of time that was! Focusing on the terminology in that case was preventing me from overcoming a basic conceptual misconception I had about energy. – Air Oct 27 '16 at 16:56
• My comment was to say that you don't need a surface to have cavitation. This is a comment on your line In a more restrictive sense, "cavitation" can mean only that subset of the former that occurs at relatively constant temperature, *in the presence of a solid interface*. You can collapse a bubble without a surface (i.e. solid interface). – Byron Wall Oct 27 '16 at 22:32
• The larger point about language is that cavitation has a specific meaning in some contexts. If you go to a chemical plant and ask how a control valve is operating, there is a fundamental difference between bubbles being formed due to a drop in pressure (i.e. flashing) and those same bubbles collapsing due a subsequent pressure recovery (i.e. cavitation). Same is true of a distillation column or a flash drum. No one would say there is cavitation inside the vessel simply because a vapor (bubble) is being formed due to a decrease in pressure. That's flashing. – Byron Wall Oct 27 '16 at 22:44
• @ByronWall You're getting stuck in the weeds a bit here - those definitions are meant as more or less arbitrary examples, not as The Two Definitions Of Cavitation. – Air Oct 28 '16 at 15:32

I see, like me, you want a simple answer. In the kettle, the water is heated to boiling point around the element, but the surrounding water is not. Steam cannot exist at less than 100° C. at atmospheric pressure, so when the steam contacts the cooler water, it immediately condenses, leaving no cushioning effect, so it's like metal against metal.

Cavitation doesn't have to involve vaporization. A liquid like hydraulic fluid, if the pump inlet is restricted, vacuum bubbles form. Because there is no air in them to cushion the impact, again, it's like metal against metal. The pump sounds like it's crunching metal shavings. Even though it's a liquid, it impacts like metal & will fatigue metal parts. It also occurs if the flow is over a surface like the side of a ball & there is not enough pressure to keep it following the surface, or it flows off the edge of a surface like a propeller. The liquid is thrown off the surface and vacuum bubbles form, then collapse with no cushioning, giving a metallic crackling sound and eroding the propeller edges. It is even worse for submarines. It says "Here I am!" to the enemy. You can demonstrate the effect with a garden hose & a bucket of water. Take it up a flight of stairs. remove the fittings, and drape the hose over the handrail or hold it deep in the bucket and start syphoning the water, then slam your finger over the inlet. You'll here a faint metallic crack from inside the highest part of the hose as the water keeps going, then slams back against itself.

I'm a little confused, if you ask in relation about pump cavitation then my ans : cavitation emphasized on reverse boiling (when vapor back to liquid, the collapsing bubble). That's when destructive action done, surface impingement. Thats why impeller parts get damaged is near exit.

• A electrical heating element immersed in cool water will make a lot of noise due to tiny bubble formation and collapse which is almost invisible despite being quite audible. But it doesn't do (significant) damage because of the speed of the collapse. Is that also called cavitation? I think it might be, but the collapse is due to a thermal gradient, not a hydrostatic pressure gradient or change. – uhoh Oct 25 '16 at 0:17
• Nah, you lost the fundamental of cavitation: liquid boils also when surrounding pressure drops. – RainerJ Oct 25 '16 at 0:20
• Bubble collapsing when near exit, where pressure build up. Impingement by bubble collapse is at surface, while at electric heater the collapsing bubble not impingement to surface. – RainerJ Oct 25 '16 at 0:21
• The point is: Boiling happens when liquid vapor pressure (at current temperature) exceeds surrounding pressure. – RainerJ Oct 25 '16 at 0:23