Failure to recognise when an aerosol isn't a droplet 26

[LittleInch]

FacEngrPE dropped this in an obscure post and it reads very well.

Basically the whole epidemiological world though various dieses and viruses were spread by "droplets" which landed on surfaces and then infected people or were sneezed at you.

And a lot of times they are probably right.

But there was a magic 5 micron cut off between droplets and aerosols. Why? Read on

https://www.wired.com/story/the-tee...t-helped-covid-kill/?utm_source=pocket-newtab

So is this a disaster - Well you tell me.
But it goes to show that just because a lot of people write something, it doesn't mean that they originally got the wrong end of the stick and then the error repeats itself until it becomes fact.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RedSnake]

However, the info posted wasn't wrong with respect to the TOPIC.

If you refer to V/A as the topic.
I have to disagree not because there is something wrong with the statment.
Only because no droplets produced when coughing, sneezing or talking is actually a perfect sphere, they may have a lot of different forms.
Which means that the formula can't be used to really understand what happens in reality.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[1503-44]

At least none of you are talking about mass = volume, so we are making some progress.

RedSnake, I agree with you. The surface area affects heat transfer, not between droplets of different sizes as the heat flux across a unit area is constant and relative between drops of different sizes. As I have always said since the very beginning, it is the quantity of mass that any drop has and the heat needed to evaporate them that are the most important factors by far that determine the relative lifetimes of droplets of all size. Tugboat, ElectricPete, CompositePro and most other readers seem to grasp that concept, correct me if I misinterpreted, but I've still got some stragglers that want to continue to talk about mass=volume, constants, and temperature, rather than prove that mass and heat are not the prime drivers of the evaporation of spherical droplets. Quite silly IMO. If they could prove something else is driving evaporation rate amongst spheres, other than heat and mass, I would give them credibility, but they just want to lecture us in geometry and relative humidity. Those factors being equal for adjacent spheres of different sizes, they could not possibly be responsible for any observed variations.

At least none of you are talking about mass = volume, so we are making some progress.

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[RedSnake]

I am not sure if this is what 1503-44 is trying to say that the mass is constant but the volym increases with temperature

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[1503-44]

No. What I have always wanted to say was that size (volume and surface area) of a drop dont matter much. Evaporation rate is overwhelmingly driven by the mass of the droplets and the heat needed to evaporate them. A larger drop will always have a longer life than a smaller drop, because it will always require far more heat to evaporate it and that takes a longer and longer time to accumulate enough heat to do that as drops increase in size. The increased surface area of a larger drop is no help, because the mass of that drop has increased so many times more than surface area and it is the mass that must be heated for a drop to evaporate, not the surface area or volume. Others want to introduce density, say mass is equal to volume, since it has density and claim it was volume driving the show all along. Not true, but if conditions are right, i admit that you might get the same answer.

I think what I said was clear. Way up there on 27 May 21 14:42
"Spheres of all sizes have the same ratio of volume to surface area, however a smaller water sphere does not require so much heat, nor time to evaporate simply due to its lesser mass."

They are more content to try to give me lessons about constants, geometry , density and relative humidity and temperature in some twisted effort to prove that volume = mass, or something, and that I never said it was the mass that's driving the process. I will patiently wait until they prove otherwise.

Maybe they failed differential equations, where you can hold any variable constant while you look at the others. Maybe they want to prove something, Lord knows what. That big drops evaporate faster than smaller ones? Seems like a waste of time, but I've nothing else pressing at the moment.

So guys, what are your intentions? You trying to disprove my heat and mass theory or what? Get out your calculators. Heat and mass are mine. You work only with volume, surface area temperature and relative humidity. No. No. Density is a derived quantity.

..

 

[LionelHutz]

Actually, we're all waiting for the pedantic individual trolling the thread to actually explain how volume isn't closely related to mass, or how V/rA is the volume to surface area ratio. Remember, within the realm of the subject topic, which is indoor areas where people reside or work meaning the temperature and pressure don't vary much. We won't be holding our breath waiting, that's for sure.

 

[LionelHutz]

RedSnake - ya probably the argument is due to that curve. But, you won't find most of that curve within an indoor living area. Yes, droplets aren't perfectly round, but I think you'll find that the V/A ratio of most any shape of droplet changes with volume. We should all know how surface area and evaporation rate are related. anyone who doesn't shouldn't be on an engineering website pretending to be an engineer.

 

[Alistair_Heaton]

To be honest I think your both right from biology absolutely years ago.

Its the reason why marine life in artic waters is absolutely enormous and you don't get many small animals either for that matter.

There is I seem to remember a third variable which is something to do with air movement and stuff banging together. Which is why its meant to be impossible for fog to form when there is more than 4 knts of wind blowing.

Which comes round to Tugs point that ventilation and air movement in a room is a huge factor on things.

 

[TugboatEng]

I think 1503 is confusing rate and time. Evaporation rate is largely related to surface area. Evaporation time is largely related to mass.

 

[LionelHutz]

Maybe they failed differential equations, where you can hold any variable constant while you look at the others.

Also seems confused about what r represents in V/rA =3 if he thinks you vary volume and area while keeping the radius constant???

 

[Compositepro]

Airborne droplets are, in fact, almost perfectly spherical due to surface tension. This is why they are always modeled as spheres.

 

[1503-44]

No point in trying to communicate with people that don't read the thread.

Spheres of all sizes have the same ratio of volume to surface area, however a smaller water sphere does not require so much heat, nor time to evaporate simply due to its lesser mass."

Confusing rate and time? Really? WTF is wrong with this thread. You all on opiates or something?


If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[Eufalconimorph]

Spheres of all sizes have the same ratio of volume to surface area,

No, they don't. V/A is a function of r (radius, and thus size). A 5m sphere has a V/A of 1.666... A 5µm sphere has a V/A of 0.000001666... If you think these are the same number, something is seriously wrong with your ability to recognize different numbers.

This paper is likely of interest, as it provides some models for droplet evaporation. So is this paper.

Note that droplet diameter is non-linear with respect to time: the diameter shrinks faster the smaller the droplet gets. Volume, surface area, droplet temperature, air flow, relative humidity, etc, all apply. Ignoring these factors and focusing only on mass and heat will create incorrect results. For an actual droplet spray there will be complex interactions between the droplets, and you'll have to solve the Navier-Stokes equations to actually find the correct evaporation rates.

But to simplify: evaporation rate depends on the surface area, since evaporation only happens at the surface. Evaporation rate depends on the partial pressure of the liquid in the gas. Evaporation rate depends on the relative temperature of the liquid and the gas. Evaporation rate doesn't directly depend on the mass of the fluid, except insofar as that varies the surface area. Some other papers (this, and this) seem to indicate that droplet diameter dominates the other factors under normal room-temperature conditions, but I've not read either in detail yet.

 

[1503-44]

The only thing important is that the larger a sphere gets, all spheres, the mass inside increases at far greater rate than surface area. got it? R^3 vs R^2. dont take genius to get it.

In heating terms, a larger spheres needs a super flux of heat to warm the now much larger quantity of mass inside through its surface area. And Where's all that heat coming from. Flux is the same for each drop. Two spheres with the same heat flux, the smaller one is going to disappear first.

Are you really arguing that a bigger drop evaporates faster because its surface area is bigger than a smaller one? Dude! Change your meds. Read the thread. Its the mass. How many times do I have to say it. You don't heat volume. You heat mass. Less mass is going to disappear faster than a big one. If you believe your paper, you see it agrees, so why are you here?

If you think I am wrong about anything I say, then please do correct me, but please also refrain from condescending lectures to myself and others here, in the preschool level details of chem, math and physics. Thank you. I will try to do the same.

 

[moon161]

Airborne droplet are, in fact, almost perfectly spherical due to surface tension. This is why they are always modeled as spheres.

I'm gonna guess it's because the ratio of surface tension to body forces (weight, inertia) or directly related forces -the drag on a droplet at sedimentation velocity is also large.

 

[LionelHutz]

What Eufalconimorph posted.

You've been corrected multiple times, yet you keep insisting on claiming the V/A of a sphere is a constant because V/rA = 3, which is a completely nonsense formula and yet you wonder why others are annoyed.

I never wanted to talk about the surface area anyway. Nor the volume. I said it wasn't important every time.

simply due to its lesser mass

Not important and simply are both claiming it based on mass only, as in the evaporation rate is directly proportional to mass. That's not true either.

Sorry, But I'm way, way more correct claiming that the mass is highly dependent on the volume in regards to this subject than either of those claims.

Don't want to be treated like a pre-schooler? Don't act like one.

 

[moon161]

Are you really arguing that a bigger drop evaporates faster because its surface area is bigger than a smaller one? Dude! Change your meds. Read the thread. Its the mass. How many times do I have to say it. You don't heat volume. You heat mass. Less mass is going to disappear faster than a big one. If you believe your paper, you see it agrees, so why are you here?

Opposite. Evaporation is a surface interaction and the rate is limited by available surface. If I sneeze and expel at one end of possibility, a single 1 cm³ droplet or at the the other end, millions of micron range droplets, it's easy to imagine, at room temperature, which cm³ of water will evaporate first.

For the scope of the original discussion, virus transport near room and body temperature, the density of water doesn't change but 10 parts per million between 4 C and 40 C

 

[IRstuff]

R^3 vs R^2. dont take genius to get it.

RIGHT! so the ratio of volume to surface area is proportional to R! So, not a constant!

TTFN (ta ta for now)
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[LionelHutz]

Can you explain why the mass of a water droplet is not highly dependent on the volume of the droplet? Remember, within the context of this subject.

You called BS on this, even called me names because I suggested it, but you didn't explain why.

 

[Eufalconimorph]

Are you really arguing that a bigger drop evaporates faster because its surface area is bigger than a smaller one?

Exactly the opposite. Smaller droplets will evaporate more quickly. Larger droplets lose more mass faster, smaller droplets lose radius faster. Evaporation time is roughly linear with r^2: the time taken to evaporate to 0 radius decreases linearly as the surface area decreases.

 

[Compositepro]

A bigger drop does have a greater instantaneous evaporation rate due to higher surface area, but it will take longer to evaporate because of the greater volume to surface area ratio. This is why the volume to surface area ratio concept is so scientifically useful, and universally used. All particles with the same volume to surface area ratio will evaporate in the same amount of time or require the same amount of time to time to react chemically.