Everyone has their own technique to avoid crying when cutting onions. Some people freeze the bulb. Others wear goggles. A few swear by cutting underwater. But scientists have now revealed a surprising truth: it’s not just the volatile chemicals wafting into your eyes. It’s a high-speed spray of microscopic onion droplets, propelled into the air the instant your blade cuts through the onion.

In a new study, researchers combined high-speed video, fluid dynamics, and food safety to show that slicing into an onion unleashes a rapid two-stage burst of juice-laden droplets. The violence of this eruption — and the tears it causes — depends heavily on how sharp your knife is and how fast you cut.

What happens when you cut an onion?

In the brief moment when the blade meets the onion flesh, something violent and invisible unfolds. Within milliseconds, a mist of microscopic droplets bursts from the onion’s surface. These droplets are launched at speeds of up to 40 meters per second, and they spread far beyond the cutting board. For decades, scientists have blamed enzymes and sulfur compounds for the stinging tears onions provoke. Turns out, these droplets were also important.

The experimental setup was straightforward. Researchers built a guillotine-like rig: a steel blade dropped onto half an onion while high-speed cameras recorded what happened. They recorded up to 20,000 frames per second, capturing every detail. To track the movement of onion juice, they developed custom particle-tracking tools and mapped the shape of the onion’s surface during cutting using image-correlation techniques.

They found that droplet ejection unfolds in two stages.

The first is a violent outburst. As the blade pierces the onion’s epidermis (the thin, tough outer layer) pressure builds in the soft interior. The inner tissue (called mesophyll) compresses, stores energy, and then ruptures. This initial burst is responsible for the fastest and most energetic droplets. These droplets shoot out nearly vertically from the blade’s surface.

Next comes the slower phase. Liquid ligaments — tiny threads of onion juice — stretch and fragment into smaller droplets, creating a lingering mist. Over just 20 milliseconds, thousands of droplets can emerge.

Their speeds decay with time, and the researchers found the largest and fastest droplets are ejected in the very first milliseconds. That’s when the onion’s built-up pressure is highest, and it’s also when most of the airborne material reaches eye level.

How to avoid crying when cutting onions? Just use a sharp knife

Researchers suspected that blade sharpness can influence how these droplets are created. They developed a mathematical model treating the onion’s skin as a stiff membrane over a soft foundation, like a trampoline. Then, they put it to the test.

They tried out blades of varying sharpness (measured down to microns using electron microscopy). They then compared how different blades performed.

What they found was striking. Dull blades (with tip radii around 13 microns) generated over 40 times more droplets than sharp ones (under 1 micron). These droplets were not only more numerous — they were also faster, larger, and carried more energy.

Simply put, sharpening your knife greatly reduces the number of irritant droplets. Sharpening doesn’t just make slicing easier — it fundamentally changes the physics of what happens inside the vegetable.

But this matters for far more than just crying. Onions, like many fresh vegetables, can carry bacteria such as Salmonella and Campylobacte, common causes of food-borne illness. And cutting them might spread these microbes further than we realize.

A 2022 study in Food Control found that cutting and washing areas in restaurants were frequently contaminated with Campylobacter. Another study in Foods observed a drop in contamination levels with distance from the cutting board. The new research helps explain why: cutting produces a literal spray of droplets capable of airborne travel.

The takeaway is clear, and surprisingly simple: sharpen your knives. Your eyes (and your health) will thank you.

The study was published in arXiv and has not yet been peer-reviewed.