快猫短视频

HERE鈥橲 the good news: scientists at Harvard University have created a beer
that retains its head for several hours. The bad news is that you have to
dissolve hexafluoroethane (C₂F₆) in your beer, which would not
please real-ale enthusiasts. 鈥淚t鈥檚 not harmful or anything,鈥� insists Sascha
Hilgenfeldt, now at the University of Twente in the Netherlands, although it is
a potent greenhouse gas.

As beer drinkers know, the head eventually collapses as the bubbles in it
burst, but Hilgenfeldt and colleagues Stephan Koehler and Howard Stone wanted to
understand why this happens. They reckoned that when the liquid separating the
bubbles drains into the beer, the bubbles are weakened and so become more likely
to burst. They also expected to find that foam disappears more quickly in a beer
with less gas dissolved in it, because bubbles are more widely separated and so
the drainage channels between the bubbles are wider.

To their surprise, varying the 鈥渇izziness鈥� of a soapy solution by changing
the amount of carbon dioxide dissolved in it did not affect how long the foam
lasted. But when the researchers used C₂F₆, the foam lasted
about 10 times as long and did depend on the amount of gas dissolved.

Hilgenfeldt says this disparity is caused by differences in the way the two
gases diffuse through the liquid separating the bubbles. The gas pressure is
higher in small bubbles, so gas diffuses out of them to larger, neighbouring
bubbles鈥攃reating larger drainage channels. When there is less CO₂
in the liquid, the bubbles are further apart and the CO₂ can鈥檛 diffuse
so quickly and stays trapped in small bubbles for longer. But the
C₂F₆
gas is not very soluble in beer and so can鈥檛 diffuse through the
liquid whatever the separation of the bubbles, so the more of it there is, the
better the head.

The new model will be useful for soft drinks producers and in industrial
processes that use foam, such as metal ore purification, Hilgenfeldt claims. But
he鈥檚 not sure beer makers will be lining up for C₂F₆.
鈥淕uinness already lasts a pretty long time because it has nitrogen dissolved in
it, which isn鈥檛 very soluble.鈥�