Tag: boiling

Leidenfrost Collapse
When a droplet encounters a surface much hotter than its boiling point, it forms a thin layer of vapor that insulates the liquid from the surface. But this Leidenfrost effect can’t last forever. Eventually, the vapor layer destabilizes and the drop touches the surface, causing explosive boiling that destroys the drop.
To determine how the layer destabilizes, researchers simulated the breakdown. To their surprise, they found that inertial forces in the micron-thin vapor layer were critical for destabilization. The gas inertia caused reductions in pressure that pulled the liquid toward the surface. Usually at these small scales, we’d ignore inertial effects and focus instead on viscosity, but, for Leidenfrost drops, that simplification doesn’t work. (Image credit: L. Gledhill; research credit: D. Harvey and J. Burton)
November 6, 2023
Leidenfrost Explosion
When a water drop hits a surface that’s much hotter than its boiling point, part of it will vaporize immediately. Depending on the temperature, this Leidenfrost effect can be a relatively gentle process — or not. Here, the surface is so hot that the entire drop is boiling before it’s even finished spreading from impact. The vapor in contact with the surface is trying to escape, bubbling up so violently that it rips the original droplet into a spray of tiny droplets. (Video and image credit: L. Gledhill)
March 8, 2023
Tricking a Kettle
Electric kettles are designed to shut off when the water inside them boils. But what does that mean exactly? In this video, Steve Mould explores that question by trying to trick his kettles into boiling off ethanol, a liquid with a lower boiling temperature than water. Steve figures that, if a kettle is designed to shut off at 100 degrees Celsius (water’s boiling point), then it will overboil ethanol since its temperature will never get that high.
I’ll let you watch the video and see what happens…
(more…)
March 22, 2022
The Best of FYFD 2020
2020 was certainly a strange year, and I confess that I mostly want to congratulate all of us for making it through and then look forward to a better, happier, healthier 2021. But for tradition and posterity’s sake, here were your top FYFD posts of 2020:
There’s a good mix of topics here! A little bit of biophysics, some research, some phenomena, and some good, old-fashioned fluid dynamics.
If you enjoy FYFD, please remember that it’s primarily reader-supported. You can help support the site by becoming a patron, making a one-time donation, buying some merch, or simply by sharing on social media. Happy New Year!
(Image credits: catfish – Abyss Dive Center, owl – J. Usherwood et al., masks – It’s Okay to Be Smart, droplet – C. Kalelkar and H. Sai, boundary layer – J. Lienhard, bubble – A. Patsyk et al., boiling – S. Mould, ice – D. Spitzer, defects – The Lutetium Project, tadpoles – K. Schwenk and J. Phillips)
January 1, 2021
The Explosive Vaporization Derby
When pressurized, liquids can be superheated to temperatures well above their normal boiling point. When the pressure is released, the liquid will start boiling, sometimes explosively. In this video, researchers explore that dynamic by “racing” a series of liquids against one another. Each racer has been heated to a different temperature beyond the expected boiling point.
The clear winner is the liquid with the highest overheat; as explained in the latter part of the video, beyond a critical overheat temperature, vaporization waves in the fluid enhance the boiling, helping vaporization take place faster. (Video and image credit: K. Jing et al.)
August 12, 2020
Exploring Martian Mud Flows
When looking at Mars and other parts of our solar system, planetary scientists are faced with a critical question: if what I’m looking at is similar to something on Earth, did it form the same way it does here? In other words, if something on Mars looks like a terrestrial lava flow, is it actually made of igneous rock or something else?
To tackle this question, a team of researchers explored mud flows in a pressure chamber under both Earth-like and Martian conditions. They found that mud flowed quite freely on Earth, but with Martian temperatures and pressures, the flows resembled lava flows like those found in Hawaii or the Galapagos Islands.
On Mars, mud begins boiling once it reaches the low pressure of the surface. This boiling cools it, causing the outer layer of the mud to freeze into an increasingly viscous crust, which changes how the mud flows. In this regard, it’s very similar to cooling lava, even though the heat loss mechanisms are different. (Video and research credit: P. Brož et al.; image credit: N. Sharp; see also P. Brož; submitted by Kam-Yung Soh)
June 2, 2020
Why Compressed Air Cans Get Cold
Anyone who’s used a can of compressed air to clean their computer or keyboard knows that the can quickly gets quite cold to the touch. This Minute Physics video explores some of the thermodynamics behind that process. Henry first identifies a few explanations that don’t quite line up with observations, before focusing in on the contents of the can: 1,1-difluoroethane. Inside the sealed can, this chemical sits in an equilibrium of part-liquid, part-vapor. But when pressure is released by opening the nozzle, the liquid boils, generating extra vapor and cooling whatever remains in the reservoir.
Although it’s not a good explanation for the compressed air can’s cooling, the cooling of an expanding gas is very important in applications like supersonic wind tunnels. That first equation you see at 0:36 in the video (for isentropic adiabatic expansion) is key to what happens in a nozzle with supersonic flow. As the flow accelerates to supersonic speeds, its temperature drops dramatically. When I was in graduate school, we actually had to preheat our hypersonic wind tunnel (in pretty much the same way you would preheat your oven at home) before we ran at Mach 6 because otherwise the temperature inside the test section would drop so low that the oxygen would liquefy out of the air! (Image and video credit: Minute Physics)
April 6, 2020
Boiling Water Using Ice Water
Steve Mould demonstrates a neat thermodynamic trick in this video by using ice water to boil hot water. The key to understanding this is recognizing that the boiling point of water depends both on its temperature and its pressure.
Here’s the set-up (which, to be clear, neither he nor I recommend you try yourself): microwave some water in an open bottle until the water is hot enough to boil. Remove the bottle from the microwave and screw on the lid. At this point, you’ve confined any water vapor coming off the hot water, thereby raising the pressure inside the bottle. Even though it’s still quite hot, the water will stop visibly boiling.
Now pour ice water over the top of the bottle. Because water vapor has a lower heat capacity than liquid water, this will preferentially cool the vapor. As its temperature drops, its pressure will also drop. Liquid water boils at lower temperatures when the pressure is lower. (This is part of why cooking and baking instructions are quite different in Denver than they are in Miami.) When the internal pressure in the bottle drops, the remaining hot water will start to visibly boil. (Image and video credit: S. Mould)
February 19, 2020
Supercooling Thermodynamics
In the latest Gastrofiscia episode, Tippe Top Physics takes on thermodynamics and the complicated truth behind certain phase changes. Although we’re accustomed to thinking of water freezing at 0 degrees Celsius and boiling at 100 degrees Celsius, reality is more complex, and temperature is only one of the factors that goes into a change of phase. Pressure and purity also play an important role.
This is why it’s possible, for instance, to supercool purified water to below 0 degrees Celsius without freezing it. Liquid water needs a nucleus to serve as a seed for its freezing. Without dust or other impurities, it takes a lot of energy for water to spontaneously generate its own nucleus. Check out the full video to see how and why that’s so. (Image and video credit: Tippe Top Physics)
October 16, 2019
Boiling in Microgravity
In the playground of microgravity, every day processes can behave much differently. This photo comes from the RUBI experiment, the Reference mUltiscale Boiling Investigation, aboard the International Space Station. Freshly installed and switched on, the apparatus is now generating bubbles like this one. On the left, you see temperature sensors used to measure bubble temperatures. High-speed and infrared cameras are also part of the experiment.
The advantage of studying boiling in space is a lack of gravity that can mask or overwhelm subtler effects. It effectively slows down the process, making it easier to observe. And since boiling is such an important part of heat transfer in many manmade devices, it shows us how we have to adapt when operating in an environment where heat – and bubbles – don’t automatically rise. (Image credit: ESA; submitted by Kam-Yung Soh)
October 9, 2019