杏吧原创

While reducing red wine and olive oil in a flat frying pan on the stove, the mixture exploded with an audible pop, spraying the wine 鈥� but not the oil 鈥� up to 2 metres away from the pan. Significantly, the wine was not hot enough to scald me. I hadn鈥檛 stirred the pan for at least a minute, and the wine and oil had separated. What happened?

Several ideas about what went on here. If anyone feels the urge to find out for themselves which scenario is correct, please exercise extreme caution 鈥� Ed

鈥� I don鈥檛 know if chefs have their own term for this phenomenon, but chemists call it . It occurs when a liquid is heated above its boiling point but does not boil due to a lack of nucleation sites 鈥� scratches, sharp corners, or solid particles where bubbles form easily.

You can observe nucleation sites in a flute of champagne. They are the spots on the inside of the flute where bubbles are continuously forming before streaming upwards. They usually mark the location of a tiny scratch or a speck of dust: a very clean flute, with few or no nucleation sites, will keep your champagne fizzy much longer.

When there are no good nucleation sites in the frying pan, the temperature can rise well above the liquid鈥檚 normal boiling point, until it is so high that bubbles can form even without a nucleation site. Once a bubble forms, however, it acts as a nucleation site for the adjacent liquid. When such superheated liquid suddenly finds itself adjacent to a nucleation site, it boils explosively. This often happens in the chemistry lab because chemical glassware is so clean and defect-free that it often has no nucleation sites at all.

鈥淲hen a superheated liquid is suddenly next to a nucleation site, it boils explosively鈥�

To prevent bumping, a chemist might deliberately scratch the inside of a flask to create nucleation sites, or may add chemically inert 鈥渂oiling chips鈥� to a solution. In other cases, bumping is a necessary evil and the chemist tries to catch or deflect the resulting spray using a 鈥渂ump trap鈥� 鈥� usually in the form of a protective glass screen.

Nucleation is an important concept in many disciplines. In meteorology, for example, seeding clouds involves providing nucleation sites for the condensation of water vapour. And in metallurgy, the way that atoms crystallise into grains around nucleation sites greatly affects the strength and other properties of a metal or an alloy.

In the case mentioned above, my guess is that the wine in the frying pan bumped. It may be that oil stuck to the surface of the pan, creating a completely smooth surface, free of nucleation sites, or it may be that the pan was simply very clean and smooth. The wine was presumably hot enough to scald when it left the pan, but tiny droplets flying through the air cool rapidly. To prevent bumping in your kitchen, you might provide some nucleation sites, perhaps by dropping a sprig of rosemary into your oil and wine before heating it.

Ben Haller, Menlo Park, California, US

鈥� The mixture had not been disturbed and so had separated into oil on top and wine on the bottom, next to the heat. The alcohol in the wine would boil off well before the water in the wine could reach its boiling point and would bubble gradually through the layer of oil. Being heavier than air, the gaseous alcohol would sit on top of the oil, somewhat contained by the edges of the pan and mixing with the surrounding air to form an explosive mixture. After a while this mixture would overflow the pan and slip down into contact with the heat source, igniting it and the whole bubble of mixture above the oil. The resulting explosion would send a shock wave in all directions, causing the audible pop. The shock wave travelling downwards would hit the viscous oil layer and force it downwards too, pushing on the wine which would have nowhere to go except sideways, up the sides of the pan and out over the kitchen.

鈥淎s the gaseous alcohol ignited, the mixture above the oil in the pan would have ignited too鈥�

David Levien, Cambridge, UK

鈥� It looks like the red wine became superheated and flash-boiled explosively. Liquids start to boil when their vapour pressure equals the ambient pressure 鈥� in this case it was 1 atmosphere above the open pan. Usually the heat is released relatively smoothly but two-phase liquids, such as this mix of oil and wine, boil at a lower temperature than either liquid, or phase, would boil on its own. Each phase generates its own vapour pressure and the mixture boils when the combined vapour pressure reaches ambient pressure. However, if the mix is allowed to separate before getting hot, as this one did, the temperature of the phase below 鈥� the red wine 鈥� can then exceed the two-phase boiling point before the red wine has even started to boil. When the wine does boil, it causes remixing which results in a swift drop in boiling point and a rapid or instantaneous boil-off.

The wine was expelled rather than the oil because it provides almost all of the generated vapour. It was probably also hotter than the oil but hit your skin at a cool temperature because it was probably dispersed in a rapidly cooling aerosol by the force of its vapour expansion.

Paul Gladwell, Northwich, Cheshire, UK