Water Baby: who came up with this immoral idea?
drop a few drops of water on a hot pot hot enough, and you will see them running around (mistakenly):
this is the Leighton Frost effect, and the rapidly forming steam layer on the interface holds up small water droplets. I won't go into details today. You can watch the video here: the simple and interesting experiment of walking the maze with small water droplets
is very popular, and there are many online videos about it. However, video bloggers are not satisfied with dripping water on a hot pot, and they will throw another thing into the pot, which is: does this video
if you throw these water-filled gel balls into a hot pot, the phenomenon will be even more lively: they will not only jump up, but also squeak! That sounds too bad. Hello. )
Let's click on the video and listen:
Scott Waitukaitis, a researcher at the University of Leiden, saw a video of Shuibao being executed in a hot pot on the Internet, and he was also interested: how on earth did this happen? The water droplets are quiet on the hot pot, why does the ball squeak?
in order to find out the problem, he and his colleagues decided to do a study themselves. They set up hot pots and high-speed cameras at 215 ℃ and recorded the whole process of this phenomenon in detail. They shot the video above. Not only did they shoot the video, but they also sent a paper for the water baby on the hot pot! And posted it in Natural Physics.
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so what happened in the middle? The power of the bounce of a gel ball is actually very similar to that of water droplets: after meeting a hot surface, the water gasifies quickly, providing the power to hold up. After all, more than 99% of this super absorbent gel ball is water. The researchers also call this phenomenon the "elastic Leighton Frost effect". After all, gel balls are not liquids, but elastic solids, so they move quite differently from water droplets.
through high-speed photography, the researchers found that when the gel ball came into contact with the hot pot, a narrow gap first appeared on the interface driven by water vapor. Unlike in the case of water droplets, the gap between the gel ball and the hot pot opens and closes repeatedly very quickly, with a frequency of 2000 to 3000 times per second. And this rapid vibration is also the source of the sound we hear.
Fast vibration on the interface between the gel ball and the hot pot, 5000 frames per second high-speed photography:
compared with the room temperature at 25 ℃:
it just bounces up very normally in the normal temperature pot, and the bounce is soon unsustainable.
in this process, elastic energy also accumulates in the gel ball, eventually pushing it to bounce. This kind of bounce will stabilize at a height of about 4 centimeters, and the process of vibration and bouncing can usually last 2-3 minutes. After that, gel balls can no longer keep bouncing because they tend to crack and break:
(two broken gel balls. )
so far, scientists' curiosity has been satisfied, and they are also reminded of the application of this thing: flexible robots are now fashionable, and many soft parts are made of hydrogel materials. can we drive them in a similar way?
so in the future, maybe people will hear the whining of flexible robots on the hot pot.
PS: this experiment can be tried at home, as long as you avoid burns and enjoy the wailing of the ball.
in addition, be careful not to let very young children come into contact with water babies, especially those without bubbles, accidental inhalation may block the trachea.
(Water Baby indicates that the heart is tired and wants to be quiet. )