Frying Pan Trick: Newsletter Challenge (07/25/06)

Just a guess: the hotter pan generates enough steam on the underside of the drop, which 'levitates' it and somewhat 'insolates' it from the hot surface.

I think Jorrie is right. The drop must be heated so quickly that the bottom turns to steam before the heat can get to the rest of the drop, then the steam insulates the drop from the pan.

I think you're right, it is called the Leidenfrost effect. The hotter pan creates a barrier of water vapor between the drop and the hot surface. The steam acts as a barrier to heat transfer, keeping the drop from boiling away too fast. I think this is the same mechanism that allows you dip your hand (briefly) into a container of niquid nitrogen whithout freezing your hand off. It also may be at work when walking on hot coals- first you walk on wet grass to get the bottoms of your feet wet, then the steam barrier (and the higher heat capacity of water than that of your skin) protects you from getting burned.

Yes, this is the Leidenfrost effect. I've seen it used in even more extreme cases where people were able to plunge thier hand into molten lead and pull it out safely due to the layer of water vapor (coming from one's hand) protecting thier skin. I've even performed the effect by pouring liquid nitrogen all over my hands, arms, legs, head and even tounge in past physics demonstrations.

same way people can walk on hot coals...bernoulli...

Quite right; but at less than 212 degrees Fahrenheit, the water droplet should wet and spread out on the hot surface without being levitated.

You are right on the money!!! This was actually covered in American Scientist July-August 2006 issue on page 313 "Going Against the Flow"

Ancillary question: would the drop last more or less time in this condition (400 vs 200 F)?
I have my ideas, but being that I'm presenting it as a question, I'll refrain from presenting them.

The question says the drop at 200 F disappears almost instantly where the 400 F drop takes longer. Again, because a steam layer forms under the drop.

Do you believe the 'almost instantly' to be shorter than the 400F condition? It seems that you do, but I am not so sure. Think on it.

Here's what I'm thinking, when the 200F water starts splattering, the single drop becomes tens of tiny drops. The surface area as compared to volume is huge as compared to the single well behave drop at 400F. So I think that the efficiency of getting the heat to the 200 F pan drop is much greater than on the 400F pan. So I believe the 400F pan drop would last much longer, despite the higher temperature.

the 200 deg.F surface is far enough below the boiling point of water that it instantly wets the surface, thus quickly conducting heat from the surface and making it evaporate quickly. The 400 deg.F surface is far enough over the boiling point that drop forms an instant film of vapor between itself and the surface, which makes the primary mode of heating convection and radiation; therefore, it warms more slowly and the drop skitters around on the pan, losing size primarily to the continued formation of a layer of "insulating" vapor. Because the surface is irregular, the vapor's exhaust direction is always changing, so the drop moves erratically.

A liberal arts major's take on it...I agree with jmueller but have an additional thought. On the cooler frying pan, the evaporative pressure overwhelms the drop's surface tension and the drop loses its shape and the area of cantact with the frying pan is increased. On the hotter surface, the drop is insulated by the steam, the heat transfer is less efficient, and the evaporative pressure does not overwhelm the drop's surface tension. Thus, the drop retains its "drop" shape which has a lower surface area for heat transfer.

My post is slightly off-topic but this question did provide the opportunity to ask a question that I have pondered a few times - is 400˚F considered to be double 200˚F? This is purely on the basis that on other temperature scales it is not actually double. It is 204.4˚C to 93.3˚C (2.2 times), which is near enough I suppose, but in K it is 477.6 to 366.5 (1.3 times), which is not even close to double.

So yes a little off-topic but I would like to know what the general rule for that sort of statement is.

That's why it's better to cook an egg on a hotter frying pan. Otherwise it sticks to the pan.

Hey!!! Thats really smart... Never thought of it that way. Thanks - you made my day.

In the Ferenheight scale, which is what this question is posed in, increasing temperature from 200 to 400 degrees does not represent a doubling in temperature. This is beacuse the temperature scale does not start at 0. I'm not actually sure what 0 degrees ferenheight corresponds to physically, but in Celcius, 0 degrees corresponds to the freezing point of water (as I'm sure you know). In order to get an absolute scale of temperature, you need a temperature range in which 0 degrees corresponds to 0 temperature, or 0 thermal energy. Hence the Kelvin scale. 0 Kelvins means that the substance has absolutely no thermal energy. Doubling your temperature in Kelvins does correspond to a doubling of thermal energy... as far as I know.

So in this case, as you noted, the temperature has increased by an absolute factor of 1.3.

In statistics, they classify data into four categories: nominal, ordinal, interval, and ratio. Nominal data are named categories with no implicit order (e.g., male/female). Ordinal data are ordered categories (e.g., small/medium/large). Non-absolute temperature data are a classic example of interval data, which are numerical data with meaningful differences but not a meaningful ratio (e.g., 400 F gives a meaningful measure of how much hotter than 200 F the second pan is, but it is not twice as hot). Non-absolute temperature scales have arbitrary zero points, which makes them interval data, since converting from Fahrenheit to Celsius changes the temperature ratio from 2.0 to 2.18. Absolute temperatures, on the other hand, have a meaningful zero point (absolute zero), and thus do represent ratio data. Expressed in either Kelvin or Rankine, the ratio of the temperatures of the two pans are 1.303. So one could say the second pan had a 30% higher temperature in that case.

Wow! I had to get WAY DOWN in the replies posted to find someone who took issue with 400F being twice as hot as 200F! I was getting a little concerned about our scientific brethren and sistren.

Yes To double something you have to have 3 reference points a starting point the point you are going to double and the result.In this case you are changing your starting point 0 F=-17.8 C= 255.4 K

I love this story. It was my first science experiment as a kid, although I did not know it at the time. My mother had me scrub her pots and pans REALLY shiny, and the only cooking I was allowed to do by myself was making a hard-boiled egg. One day I forgot and all the water evaporated making a stinking mess, so after I cleaned the pot out very well, I made sure it was clean by evaporating some water in the pot and making sure it didn't leave a residue, or get stuck in a local goo spot. Eventually I put a tiny drop of water in the hot pot and watched it dance and sizzle. I was able to figure out that when it was a really small spherical drop, it would not even sizzle, but just slip all over on the steam film underneath it. I then figured out that the total time t, to evaporate a quantity Q of water had a minimum in it, which was about 3 drops big (about 0.1 ml in non-kiddie language). This was one of my greatest secrets: The rest of the world of course would consider it obvious that t(Q) was monotonically increasing, but I knew differently. This experience told me to always keep an eye on things in the limit of very big or very small, where things may become unusual, which is one of the reasons I am a Scientist today!

The drop of water does creat a steam bubbles around it. In the fire department the highter the pressure the more heat the water absorbs. the steam bubbles is pushed back as the drop goes throught the heated air. some expermintal nozzels went up to 500 psi and would put a bedroom out with a quick shot and maybe a half gallon of water compared to about 100 gallons of water. The steam peels of the drop thus the liquid absorbs the heat as the steam cannot absorb much more heat or as fast as the liquic, a side point is that in a car radiator steam does not absorb heat the liquid does.

Got a small problem with that, once water leaves a 500psi pressurised nozzle, it will be at atmospheric pressure. Maybe the velocity generated will cause the water to break up on impact into small droplets with a large surface area to absorb heat? Also, water in a car radiator is in a sealed pressurised system, with raised boiling point, so no steam.

You missed the point the steam is shed off because it is moving through the air thus leaving the drop open to absorb the heat. Has nothing to do with atmospherric pressure. That is why they use fog nozzels and not straight bore nozzels to allow more area of water exposed to heat then add pressure to shed off the steam to allow more heat, the idea behind it all is to cause little water damage as possible and it does work that way.

I have a gut feeling that a portion of the water droplet turns to steam and the drop takes off like a rocket. Unlike a rocket, it only rises a few millimeters and falls "back to earth" where it is reheated for another "sub-orbital" flight. Actually, the really important thing about this phenomenon is that your griddle is now at the correct temperature for making pancakes.

In order to exist in liquid form, albeit only momentarily, water must be at boiling point or below. In order to wet a surface, di-hydrogen oxide must be in (or change state to)liquid form. When contacting a pan at 200 degrees the drop can remain liquid until it evaporates--however briefly. A pan at 400 degrees--almost twice water's boiling point--cannot be wetted--or be cooled sufficiently by a mere drop of water--to permit a drop of water to evaporate (from liquid to gaseous) off (or should one say, on)its surface. Therefore, the second drop of water is destined to change states before it can wet the pan's surface. (The closest it can come to the pan--the most it can "come in contact"--is at the smallest possible area, a single (virtual) point. The drop shape that permits this single "almost-point-of-contact" is a sphere. The sphere that dances around above the pan until the (former) drop disappears into thin air.

This question has been pretty well answered but there are a few loose ends.

Someone said that the Fahrenheit scale was not based on anything, unlike the Celsius scale, which is based on the freezing and boiling points of water. Wrong! Fahrenheit scale, which is older than Celsius by the way (1709 vs. 1742), was officially based on 0 degrees being the temperature of a certain ice/ammonium chloride/water mixture bath. The 100 degree point was based on what Fahrenheit thought at the time to be the temperature of the human body, since the thermometers of that time were not very accurate. Some say that the 0 degree point was initially the coldest temperature that Fahrenheit could record in winter, but he did establish the standard in the laboratory as described above. Some also say that Fahrenheit may have been running a slight fever when he did his initial work, and so his temperature was slightly higher than normal. Anyway, as thermometers got better, we discovered that the average human body temperature is really less than that, but Fahrenheit had already established 32 degrees on his scale as the freezing point of water and 212 degrees on his scale being the boiling point, making a nice even 180 degrees between. Actually, 98.6 degrees is an exact conversion from 37 Celsius, whereas many people run closer to 98 degrees for their normal temp. There are actually six different versions of the story in the Wikipedia entry for "Fahrenheit". (See URL link below)

Can one say that 400 degrees F is "twice as hot" as 200? Depends on how you define "hot". If you mean the total heat content of an object at those two temperatures then, NO. If you mean "is the temperature double the number of Fahrenheit degrees above the standard ice/ammoni", then, YES!

Finally, I don't know about putting your hands into liquid nitrogen. I am not sure if gaseous Nitrogen would form the same protective barrier that water vapor (steam) does. I have heard the urban legend (actually told to me by my high school Physics teacher, probably just to make a point) that experienced steel foundry workers would "wash" their hands in molten steel to impress new apprentices, protected by a barrier of steam after soaking in water. Not sure I would want to take a chance like that!

http://en.wikipedia.org/wiki/Fahrenheit

As a boy my hobby was electronics. I didn't have a sponge to wipe my soldering iron tip but I learned that I could use my index finger and thumb to clean the tip of the iron. After a while my body learned to reflexively perspire on the pads of those two digits when I would do it. Today I have a sponge but I never lost that reflex. If I just thik about doing it the pads on my thumb and index finger instantly perspire!

When the frying pan is so hot that the bottom of the drop is turned to steam instantly this blocks further heat transfer to the water to some extent. The drop rides on the vapour cushion and becomes very mobile. At lesser temperature the water drop is heated up more uniformly and evaporates from its top surface.

The Leidenfrost Effect ("film boiling") is said to be the basis for being able to briefly put your hand into molten lead. But be warned by Jearl Walker's experience: the lead can be molten but not hot enough to produce film boiling; instead it produces severe burns. Walker wrote a book entitled "The Flying Circus of Physics" which is highly recommended.