🧅 揭示切洋葱流泪的物理机制：研究人员利用高速摄像技术，首次可视化并量化了切洋葱过程中液滴的释放过程。他们发现，洋葱释放的挥发性化学物质丙硫基亚磺酸是导致眼睛刺激和流泪的根源，而这些化学物质以液滴的形式被释放出来。

🔪 液滴释放的双阶段与能量来源：液滴的释放分为两个阶段：初期是快速的雾状喷射，随后是液态细丝 fragmentation 成大量液滴。研究发现，在刀刃与洋葱表皮初次接触时释放的液滴能量最为充沛，这表明了初始接触的力学特性是关键。

💡 刀具锋利度与切割速度的影响：出人意料的是，研究发现钝刀和更快的切割速度反而会产生更多具有更高动能的液滴。这与直觉相悖，表明更快的切割过程可能在短时间内释放更多受压的化学物质，并通过更剧烈的 fragmentation 形成更多液滴。

🧊 冷藏洋葱的意外效应：研究还发现，在切割前冷藏洋葱会增加液滴的数量，且这些液滴的速度与常温下相似。这一发现为减少切洋葱时的流泪提供了另一条思路，尽管其背后的具体机制仍需进一步探索。

🌱 未来研究方向：除了优化切割技巧以减少流泪，研究人员还计划深入研究不同品种洋葱对切割的反应，以及切割过程是否会影响沙门氏菌等空气传播病原体的扩散，从而为食品安全提供更多洞察。

Researchers in the US have studied the physics of how cutting onions can produce a tear-jerking reaction.

While it is known that volatile chemicals released from the onion – called propanethial S-oxide – irritate the nerves in the cornea to produce tears, how such chemical-laden droplets reach the eyes and whether they are influenced by the knife or cutting technique remain less clear.

To investigate, Sunghwan Jung from Cornell University and colleagues built a guillotine-like apparatus and used high-speed video to observe the droplets released from onions as they were cut by steel blades.

“No one had visualized or quantified this process,” Jung told Physics World. “That curiosity led us to explore the mechanics of droplet ejection during onion cutting using high-speed imaging and strain mapping.”

They found that droplets, which can reach up to 60 cm high, were released in two stages – the first being a fast mist-like outburst that was followed by threads of liquid fragmenting into many droplets.

The most energetic droplets were released during the initial contact between the blade and the onion’s skin.

When they began varying the sharpness of the blade and the cutting speed, they discovered that a greater number of droplets were released by blunter blades and faster cutting speeds.

“That was even more surprising,” notes Jung. “Blunter blades and faster cuts – up to 40 m/s – produced significantly more droplets with higher kinetic energy.”

Another surprise was that refrigerating the onions prior to cutting also produced an increased number of droplets of similar velocity, compared to unchilled vegetables.

So if you want to reduce chances of welling up when making dinner, sharpen your knives, cut slowly and perhaps don’t keep the bulbs in the fridge.

The researchers say there are many more layers to the work and now plan to study how different onion varieties respond to cutting as well as how cutting could influence the spread of airborne pathogens such as salmonella.

The post The physics behind why cutting onions makes us cry appeared first on Physics World.