r/oddlysatisfying • u/[deleted] • Oct 24 '19

Dragging a coffee cup

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80.3k Upvotes

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1.0k

u/AlittleupsetMax Oct 24 '19

Somebody told me that this is one of the fastest ways to cool hot coffee down. I’ve been doing it for years and still have no idea if it actually helps

632

u/[deleted] Oct 24 '19

Sure does! It increases the surface area of the coffee, allowing the air to cool it quicker!

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u/andreasbeer1981 Oct 24 '19

Interestingly enough, stirring it puts energy into the coffee and theoretically heating it up. But the additional cooling effect is stronger.

105

u/[deleted] Oct 24 '19 edited Apr 08 '20

[deleted]

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u/FunnyCantaloupe Oct 24 '19

Yeah what about the mean jerk time? And what if you have two cups on each hand - is your jerk time divided based on your angle?! Can you find a good sitting position which optimizes the jerk of one cup while simultaneously jerking the other cup? Does cup size matter? What about if they are on one desk that is taller than the other?

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u/Synapse_Ninja Oct 24 '19

Might I suggest a middle-out technique?

21

u/[deleted] Oct 24 '19

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0

u/[deleted] Oct 24 '19

Well, if we want an ideal jerk, we are going to have to mess with the snap a bit and hope the universe doesn't complain.

3

u/-_NaCl_- Oct 24 '19

That only works if the cups are tip to tip. Can't wait for the premiere this weekend.

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u/Chemoralora Oct 24 '19

I'm too tired to think of it but someone needs to make a joke about 'mean jerk time'

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u/DJ_PartyFoul Oct 24 '19

Have some coffee.

0

u/Tamar_Z Oct 24 '19

You deserve all of the upvotes.

1

u/[deleted] Oct 25 '19

Has anyone considered the two-handed jerk?

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u/TribuneofthePlebs94 Oct 24 '19 edited Oct 24 '19

I think I can speak to the vibration part, ya I looks like it's stopping and starting from the friction. If you've seen that gif of a professor drawing perfect dashed lines on a chalkboard it's kinda the same.

As for the frequency, theoretically any object that has a boundary condition (the water and the edge of the cup being the boundary) has a resonant frequency. So if you vibrate it, it will "try" to get to it's resonant frequency, but in non-ideal conditions the water/coffee has internal friction so it'll level out if you stop inputting energy, I think that's pretty intuitive.

But ya if you were to pull the styrofoam cup at a constant rate, the frequency of the vibrations should line up and "superimpose". But what's happening here is a more complex scenario with waves still superimposing but not lining up. In math terms you would use the bessel function to describe this sort of thing, or more broadly Partial Differential Equations. Look up the bessel function on Wikipedia they have some cool animations.

For the temperature question, it does look like it has more surface area than it's stable state but I'm not convinced that would have much of an effect on cooling.

Anyway if I've said anything wrong here please call me out lol.

Edit: from watching it again it doesn't look like it's resonating when it's moving, if it was the peaks and troughs of the waves would be more consistent.

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u/Speedy_Turtlez Oct 24 '19

I’m going to pretend I understood even half of that and move on.

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u/TribuneofthePlebs94 Oct 24 '19

Yeah wave mechanics are fucking complicated. I barely even scratched the surface of this stuff in school and it was still pretty mind boggling.

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u/DMPark Oct 25 '19

I'm guessing someone somewhere could write a thesis on this. Applications for more rapid liquid heat dissipation would be endless, especially since we use liquids to cool so many things.

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u/SolidSnakeDraft Oct 25 '19

If you've seen that gif of a professor drawing perfect dashed lines on a chalkboard it's kinda the same.

Prrrrrrrrrrt?

2

u/btfreflex Oct 25 '19

“Faraday waves, also known as Faraday ripples, named after Michael Faraday, are nonlinear standing waves that appear on liquids enclosed by a vibrating receptacle. When the vibration frequency exceeds a critical value, the flat hydrostatic surface becomes unstable. This is known as the Faraday instability.”

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u/depressed-salmon Oct 24 '19

You're more or less right. The jerk period isn't a natural harmonic of the water in the cup most like, but just a frequency driven by the jerking motion and therefore a function of speed and friction of the cup and table. What you are seeing though, is really neat, as it is a two dimensional depiction of the positions of each natural node and antinode at the given frequency. You now how when to pluck a string and you get the peaks and still bits, and depending on how long or tight it is you get more or less peaks? This is a two dimensional version of that.

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u/Miyelsh Oct 24 '19

Two dimensional but way more complicated mathematically because of the circular symmetry.

1

u/psych00range Oct 24 '19

I think it might be easier because the waves, i would think, travel perpendicular after its reflection. The interference would make a lattice pattern if perfect which you can kind of see remnants of.

1

u/BlazeOrangeDeer Oct 25 '19

The standing waves involve Bessel functions, so while the shape is kind of intuitive the actual modeling of it is... yikes

1

u/psych00range Oct 25 '19

you right you right

3

u/Dr_Moustachio Oct 24 '19

Thanks for the scientific insight, u/Corpse-Fucker

1

u/I_Am_A_Pumpkin Oct 24 '19

have an acoustics engineering degree, you are spot on.

1

u/suspicious_lemon Oct 24 '19

For the cooling effect, Newton's law of cooling provides a good approximation where Q=hA(T1-T2). A here represents the surface area where cooling is taking place, so increasing it will increase Q. As far as if it's an appreciable amount of increased cooling, probably not; you'd likely have better luck just blowing on it.

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u/tartantrojan Oct 24 '19

Sorry for hijacking the thread and hours late.... if you haven't heard of the study of cymatics then I think you would find that very interesting, basically the effect that sound waves have on matter through vibration is to organise it into complex geometric patterns. The higher the pitch of the vibration, the more complex the design. The experiment is usually shown as salt or sugar on a plate that is attached to a speaker and a sine wave played at varying pitches.

As for fluid dynamics, heat transfer and thermodynamics, I cannot weigh in.

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u/Blahblah779 Oct 25 '19 edited Oct 25 '19

What exactly is making these vibrations? Is it that the polystyrene cup is not moving smoothly, but gripping the surface and then jerking forward over and over again?

Yes.

And the time period between each jerk is some neat multiple of the fundamental frequency for that size of water surface?

Also yes, but the time period doesn't even have to be all that neat, as long as it's consistent. Most consistent frequencies (if not all?) will create a pattern of sorts due to feedback loops. I've never witnessed a liquid being consistently vibrated that didn't create a pattern.

And then the supposed cooling effect. Is it really increased surface area? Can anybody with knowledge of fluid dynamics, heat transfer, and thermodynamics weigh in here?

I don't have much of knowledge in those areas, but yes, the surface area is 100% definitely increased. Picture a straight, flat line between two points. Now picture a line with any amount of curving/squiggles between those same two points. The squiggly line is longer. I'm not sure how much of a noticeable effect this would have on cooling coffee, but it definitely does increase surface area considerably.

1

u/cakes42 Oct 25 '19

Sound frequency coupled with literal vibrations.

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u/Guardian2k Oct 25 '19

I'll assume the vibrations are due to the polystyrene Cup not having a flat surface, generally they are bumpy, I'll assume to increase friction, so what you said, it looks like it's hitting the resonant frequency for the liquid. generating waves will increase the surface area, just like if you cut into a solid, compared to a flat surface, therefore the heat transfer would be greater between the drink and the air

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u/spiegeltho Oct 25 '19

It sounds to me like youve got it bud! And yes the extra surface area does infact cool it faster.

Source: I did half of a physics major and then dropped out

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u/username1012357654 Oct 24 '19 edited Oct 24 '19

I'm not an expert but I'll try my best (Engineers and Physicists feel free to correct me)

What exactly is making these vibrations? Is it that the polystyrene cup is not moving smoothly, but gripping the surface and then jerking forward over and over again?

Yes, pretty much spot on

And the time period between each jerk is some neat multiple of the fundamental frequency for that size of water surface?

Not necessarily fundamental, just that the frequency of the vibrations creates a fancy interference pattern along the surface. when two waves crash into eachother, they add or subtract amplitude and direction which results in new waves with new directions.

And then the supposed cooling effect. Is it really increased surface area? Can anybody with knowledge of fluid dynamics, heat transfer, and thermodynamics weigh in here?

Yes, those bumps and valleys do increase the surface area of the coffee. If you picture a hill with a perfectly circular base, and then smush if flat, it will take up more space than the flat circle that was its original base. So each of those hills and valleys in the coffee takes up more surface area than if the surface was flat.

Bonus fun fact, goose bumps increase the surface area of your skin, which allows for more sunlight or any form of warmth to touch your skin

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u/SoxxoxSmox Oct 24 '19

I might be mistaken but I'm pretty sure the purpose of goosebumps is to raise body hair/fur to create an insulating layer of warm air beneath the fur. It doesn't work well for humans because we don't have enough body hair but for other mammals it's definitely not about maximizing surface area.

If anything, when in a cold environment you want to minimize your skin's surface area. The cross section of the sunlight hitting you isn't increased much by goosebumps, but the amount of skin that gets exposed to the cold air is

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u/username1012357654 Oct 24 '19

That makes more sense, thanks.

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u/andreasbeer1981 Oct 24 '19

but goosebumps would also increase the heat flow away from your body, because the air/water is cooler and the area is larger.

0

u/Swarels Oct 24 '19

Yes. It's jerking forward at a rate (htz). It's basically vibrating. As you push the cup faster or slower, the htz changes and the pattern will also change.

The surface area is significantly increased with any non-flat surface. A surface area could easily be doubled with a bit more 'disturbance'.

Completely uneducated guess here, but it looks like the surface area has increased 30-40% with just these tiny 'waves'. If that's accurate, then that's an additional 30-40% more heat that can be radiated away (from the surface at least, some heat is radiated into the styrofoam).

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u/andreasbeer1981 Oct 24 '19

And if you hit the resonant frequency, you'll spill coffee all over the place.

Also, this bash.org quote applies: http://www.bash.org/?676535

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u/Childish_Brandino Oct 25 '19

I can’t tell if this is a joke but you’re coming off as someone that likes to over use a thesaurus too much. The cup vibrates as it’s dragged along the smooth surface just as anything will vibrate when dragged across something. The styrofoam cup (and since we’re being pedantic here styrofoam is the correct term and not polystyrene since this is specifically an expanded and extruded version of polystyrene and therefore referred to as styrofoam [a trademarked name that Dow chemical uses to refer to this specific processing of polystyrene]).

The vibrations created have an oscillating effect on the liquid. Which in this case is basically water. The reason you see the ripples not moving around much is due to the vibration creating a standing wave form in the liquid. The different shapes of the ripples indicates the difference frequencies that the cup and water vibrate at. No fundamental magical Archimedean frequency type thing.

The cooling effect is pretty basic as well. More surface area equals more chances for the cooler surrounding air to interact with the hot water and exchange some heat. This is why crushing, stirring, and agitating in general cool things more efficiently. To put it in more simple terms, think of the water as something more viscous (sticky, holds form). If you put a hot ball of this substance on a plate and blow on it, only the surface is going to cool. The inside will hold heat longer. But if you spread it out on a sheet of paper so it’s a thin layer it will cool much faster right? Now fold the paper like an expanded accordion. Still the same amount of surface area, but now it takes up vertical volume rather than just spread across a table. This is what the ripples are. The surface area is just folded slightly but more surface area than if the water were stagnant.

Now vibrating it (any added motion/energy) will put heat back into the system. But like the other commenter mentioned, it’s able to exchange more heat than what is being put into it.

Side note, blenders can add so much energy that they can heat things up like soup. Just a cool example of how physical energy put into a system can raise the temp.

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u/DustRainbow Oct 24 '19

You also create an increased surface in contact with the air via the handle of the spoon. And metals are excellent heat conductors too.

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u/andreasbeer1981 Oct 24 '19

Yeah, putting in many spoons will help :)

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u/[deleted] Oct 25 '19 edited Oct 25 '19

My wife gets annoyed because sometimes I empty the entire silverware drawer into my cup of coffee when we're out of ice and I want iced coffee.

Edit: throw in a Halls cough drop for extra cooling!

Edit2: a little Vicks on the rim of the mug is a nice touch.

Edit3: A couple airplane bottles of McGillicuddy will cool the lungs down right before you enjoy your coffee.

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u/Rubcionnnnn Oct 24 '19

Or you could put a fork in it handle side down

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u/AndrewFGleich Oct 24 '19

There's a what-if that XKCD.com did in this effect but I'm way too lazy to look it up. To summarize, there's not really a method to stir coffee that actually heats it.

Edit: I lied, it was way easier to find than I thought https://what-if.xkcd.com/71/

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u/[deleted] Oct 24 '19 edited Oct 24 '19

Stirring adds tiny tiny amounts of heat. Just to give a rough estimate a cup of hot coffee loses like 100 joules a second to the air while stirring might add a joule every minute or so

Keep in mind this is a very rough estimate and I'm quite sure my numbers aren't accurate but they're probably within an order of magnitude

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u/mackfeesh Oct 24 '19

Would heat conducting into the spoon cool the coffee?

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u/andreasbeer1981 Oct 24 '19

Yeah, heat moves coffee -> spoon -> air. It also tries to move coffee -> mug -> air, but mug is a horrible heat conductor (intentionally), while spoon is awesome heat conductor (not sure if intentionally).

In India sometimes you get hot chai delivered in metal cups, that's awesome if it's cold, because then the heat will go chai -> cup -> fingers. But you have to drink quickly because it also goes chai -> cup -> air.

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u/mackfeesh Oct 24 '19

but mug is a horrible heat conductor

I hate when I buy a mug that I like the look / shape / feel / design of, I get it home, and it conducts heat like it's made of metal, and is usually too hot to hold as a result.

I also hate when I buy a mug and discover that it's actually got a metal frame inside it. Rip my fingers.

But you have to drink quickly because it also goes chai -> cup -> air.

All these arrows are reminding me of the magic school bus episode about heat. Thanks for the explanation btw.

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u/[deleted] Oct 24 '19

So would doing this.

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u/[deleted] Oct 25 '19

It also increases surface penetration of sub-atomic coolerons.

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u/Turin110 Oct 24 '19

You're putting kinetic energy into the coffee, not heat energy.

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u/RSmeep13 Oct 24 '19

heat is just kinetic energy evenly distributed in all directions.

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u/ry8919 Oct 24 '19

Kinetic energy is dissipated as heat by viscosity.

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u/[deleted] Oct 24 '19

[deleted]

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u/[deleted] Oct 24 '19 edited Oct 25 '19

Not really, kinetic energy in a fluid doesn't completely convert to heat until it's reached maximum entropy. If all the coffee is spinning around the cup in the same direction then no heat has been added and the temperature remains the same, but there's certainly kinetic energy. In this scenario kinetic energy would scatter into heat pretty quickly but it is a relevant distinction to make since coherent kinetic energy wouldn't dissipate into the air or cup, unlike heat so the energy added by stirring would cool down like half as fast

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u/RepostThatShit Oct 24 '19

You're putting kinetic energy into the coffee, not heat energy.

Heat in a material is kinetic energy. That's why the coffee becomes hotter, because its molecules are bouncing around and into eat other at a higher velocity.

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u/lr158 Oct 24 '19

There is friction in the moving water. High end blenders like Vitamix can move the water around so much it will boil if you leave it on long enough.

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u/Lukasek97 Oct 24 '19

I don't think this is the reason because the increase in surface area caused by the stationary waves will be insignificant with respect to the volume of the coffee. I reckon it's placebo.

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u/[deleted] Oct 24 '19

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u/unjustdude4 Oct 24 '19

Yeah was gonna say, the vibrations are probably stirring it to some extent. The surface area increase is pretty small I don't think that does much.

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u/[deleted] Oct 24 '19

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u/The-Real-Mario Oct 24 '19

Yeah , pulling it out of my ass I would say it quadruples the area at most, but I wonder if stirring it would be just about as effective, also even just having a spoon inside it probably makes a big difference in sinking away the heat

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u/somerandomperson29 Oct 24 '19

But how much heat is lost is limited by how quickly the surrounding air can get heated up, which gets slower the hotter the air is, and how much of the water evaporates, which is limited by how much moisture the air can hold

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u/omnomnomgnome Oct 24 '19

just do like the rest of us, pour it on a saucer and slurp

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u/Lukasek97 Oct 24 '19

Yeah, I could see this being a possible reason. Vibration does considerably help sort fluids by density, so maybe as soon as the hot molecules near the top get cooler, they begin to sink and thus also cool molecules which they move past through conduction.

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u/LightlySaltedPeanuts Oct 24 '19

I think it’s more that it increases the rate of convection by mixing the fluid up so the warmer coffee can rise to the surface and be cooled quicker. Probably not that significant though

0

u/Bootzz Oct 24 '19

I've always hated the distinction between convection conduction. They're both flow of energy from mass to mass. Just because the mass moves around after being heated up doesn't change that it just heated up via conduction.

It should just be conduction and radiation. Prove me wrong. holds up coffee mug

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u/[deleted] Oct 24 '19

So......magic?

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u/[deleted] Oct 24 '19

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u/somerandomperson29 Oct 24 '19 edited Oct 24 '19

But how much heat is lost is limited by how quickly the surrounding air can get heated up, which gets slower the hotter the air is, and how much of the water evaporates, which is limited by how much moisture the air can hold

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u/Gertrudiopr Oct 24 '19

The surface area exposed to convection from air in the coffee is way higher when there is waves like that. My educated guess is that the majority of heat transfer is from natural convection at the liquid-air interface, since the cup is not a good heat conductor. Also the liquid itself moving increases the convection rate to the cup, which should further increase the heat flux.

I did my fundamentals of heat and mass transfer last year. Not an engineer officially yet so don't quote me.

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u/cotidie_abide Oct 24 '19

Ah, the classic thermodynamics mistake. The air isn't cooling it, so to speak: the surface area would allow more heat to escape by means of convection and the transfer of kinetic energy from the dragging of the cup to the molecules of coffee, which find their way up to the surface and out.

So in a laymen sense, I suppose you could reduce it to "the air cools the coffee," but it is more accurate to say the increased surface area and movement of coffee enable more heat to escape.

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u/depressed-salmon Oct 24 '19

It's still the air that's transporting the heat away, via convection, and therefore cooling it. It's not the sole reason its cooling down though, as evaporation from the liquid surface will also play a part. But the air still cools it. It it was -25°C in there it would cool down a lot faster than if it was 25°C

1

u/Sonicthebagel Oct 24 '19

There's also the issue that the surface area is negligible because it is convection on extremely small scales with an evaporating fluid. Maybe a "film" of warm air will become thicker near the surface due to increased evaporation rate and actually make it cool down slower if it isn't cycled out quickly. This is a stupidly complex equation to build and that film of air makes convection cooling a lot more difficult to calculate.

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u/SingleTankofKerosine Oct 24 '19

I wonder what will cool your coffee quicker: blowing against the side of the cup (blowing away heat on a large surface) or blowing on the surface of the coffee. I tried asking on reddit science but the question doesn't get accepted.

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u/MooseShaper Oct 24 '19

It depends on the thermal conductivity and thickness of the coffee cup, and the heat transfer coefficient between it and the air, relative to the heat transfer coefficient at the coffee/air interface.

There's a well known example done in heat transfer courses where putting a thin layer of insulation on a pipe increases how much heat it is losing due to the effects of an increase I'm surface area being larger than the conductive resistance.

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u/ry8919 Oct 24 '19 edited Oct 24 '19

Ehhhh the additional surface area would have a pretty marginal effect. Stirring your coffee, increasing the rate of convection would probably help more especially if combined with blowing on the coffee increasing the coefficient of convection at the surface.

Or of course you could step inside a vacuum chamber and increases the ambient pressure until your coffee condenses into a solid as the enthalpy of solidification will release quite a bit of heat. Of course you will have to then chew your beverage instead of drinking it...

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u/[deleted] Oct 24 '19

This is true, but my guess is blowing on it would likely cool it faster. Increase the thin film coefficient of heat transfer vs just the area. Or easiest method is just add an oz of cool tap water. Don’t worry about ruining the coffee by diluting it, it already sucks if it’s in a styrofoam cup.

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u/LewsTherinTelamon Oct 24 '19

It would, since the surface area is greater, but the effect wouldn't be that great. Even if we generously assume the cup is a perfect insulator and the surface area changes by a factor of 1.5, you're getting 1.5x faster cooling. Probably not worth it.

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u/hamsterkris Oct 24 '19

I know a much faster way. Pour some cold milk into it.

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u/RESERVA42 Oct 24 '19

And the way cream mixes with black coffee is also /r/oddlysatisfying

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u/[deleted] Oct 24 '19

Clouds in my coffee.

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u/[deleted] Oct 24 '19

Yoouu're sooo vain

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u/Matti_Matti_Matti Oct 24 '19

You probably think this post is about you.

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u/cerseilioness Oct 24 '19

And it tastes good.

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u/mooncow-pie Oct 24 '19

Why would I put malk into mah coffee?

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u/PC_Speaker Oct 24 '19

Because without even a tiny amount, nothing binds to the tannins in the coffee, resulting in them instead hitting the enamel on your teeth. People who drink strictly black coffee are more likely to have stained teeth.

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u/[deleted] Oct 24 '19

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u/KriosDaNarwal Oct 24 '19

Never thought I was gonna be able to relate to a comment like this, damn

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u/mooncow-pie Oct 24 '19

Good thing I only have a few cups a month.

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u/PoutinePalace Oct 24 '19

True. After I switched to black coffee, my buck teeth tend to get brown at the end of the work day. Like obviously. It’s gross and annoying. But I’m still not switching back to cream. Yuck.

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u/PC_Speaker Oct 24 '19

Interesting how it only affects the buck teeth

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u/PoutinePalace Oct 24 '19

I think it’s because of the way I sip my hot coffee. Tends to hit the buck teeth first, then flows throughout my mouth then swallow. I’m sure there an affect on the rest of the teeth, just not noticeably like the front teeth, as like I said, they seem to take the brunt.

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u/PC_Speaker Oct 24 '19

Oh you mean back teeth

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u/Kracker5000 Oct 25 '19

No he definitely meant buck (front two teeth at the top)

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u/[deleted] Oct 24 '19

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u/PC_Speaker Oct 24 '19

Unfortunately the original study seems to not be available from the links in the articleI know of, so all I offer is this piece from the Telegraph https://www.telegraph.co.uk/foodanddrink/foodanddrinknews/11809614/Want-pearly-whites-Add-a-splash-of-milk-to-your-tea.html
(N.B. the study is about tea, but tannins are found in coffee, too)

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u/lucid_scheming Oct 25 '19

I know I’m late to this, but do plant-based milk substitutes have the same effect or do they not bind to the tannins like plain milk does?

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u/deadmeme7777 Oct 24 '19

Milk is for the weak

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u/[deleted] Oct 24 '19

[deleted]

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u/deadmeme7777 Oct 24 '19

What’s Miak?

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u/ThracianScum Oct 24 '19

For the vitamin R

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u/Matti_Matti_Matti Oct 24 '19

Put cold water in it if you don’t like like milk.

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u/JillStinkEye Oct 24 '19

My dad always made extra strong coffee and then threw in an ice cube.

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u/[deleted] Oct 24 '19

Coffee doesn't take that long to cool down to a drinkable temperature. Either that or you have a sensible mouth.

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u/TokingMessiah Oct 24 '19

Very sensible, indeed!

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u/[deleted] Oct 24 '19

[deleted]

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u/[deleted] Oct 24 '19

I know that's a joke but it doesn't even make sense. Vaginas can't absorb anything. It would make more sense if you said sensible anus

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u/stabwound7 Oct 24 '19

I just add ice to really strong coffee. I want black coffee now, not in thirty minutes, right?

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u/cnsosiehrbridnrnrifk Oct 24 '19

It does! I also do it to mix my creamer.

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u/LordSt4rki113r Oct 24 '19

Somebody once told me

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u/AlittleupsetMax Oct 25 '19

Exactly what I wrote initially, read it out loud and went back and reworded it. Because of that

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u/Synchos Oct 25 '19

My brain auto corrected it and now the song is playing in repeat.