Researchers Uncover Physics Behind Onion Tears with High-Speed Imaging

Researchers at Cornell University have delved into the complex physics behind why cutting onions results in tears. The study focuses on how volatile chemicals, particularly propanethial S-oxide, irritate the corneal nerves, triggering a tearful response. While this chemical reaction is well-known, the mechanisms by which these droplets reach the eyes and how factors like knife sharpness and cutting technique influence this process have remained less understood.

Experimental Approach and Findings

To explore these questions, lead researcher Sunghwan Jung and his team created an innovative guillotine-like apparatus. Using high-speed video technology, they captured the droplets ejected from onions during cutting. Jung expressed that “no one had visualized or quantified this process” before, prompting their investigation into the mechanics of droplet ejection.

The results revealed that droplets can reach heights of up to 60 cm. The release occurred in two distinct stages: an initial rapid mist-like outburst, followed by threads of liquid that fragmented into multiple droplets. The most energetic droplets were produced during the initial contact between the blade and the onion skin.

As the researchers varied the sharpness of the knife and the speed of cutting, they found that blunter blades and faster cutting speeds yielded significantly more droplets. Jung noted, “That was even more surprising,” highlighting that cutting at speeds up to 40 m/s with blunter knives resulted in droplets with higher kinetic energy.

Implications and Future Research

Another unexpected discovery was that refrigerating onions prior to cutting increased the number of droplets released, with similar velocities to those found in unchilled onions. This suggests that those looking to minimize tear production might consider using sharper knives and cutting at a slower pace, while also avoiding refrigerating the bulbs.

The researchers intend to expand their study to examine how different onion varieties react to cutting and how these cutting techniques may influence the spread of airborne pathogens, such as salmonella. These findings not only contribute to our understanding of the onion-cutting process but may also have wider implications for food safety and preparation.