Droplet outbursts from onion cutting

Cutting onions often leads to tear-inducing aerosol release, yet the underlying mechanics remain poorly understood. In this work, via high-speed characterizations, we show that droplet formation occurs via a two-stage process: an initial high-speed outburst followed by slower ligament fragmentation. By systematically varying blade sharpness and speed, we find that faster or blunter blades significantly increase both the number and energy of ejected droplets. Strain mapping reveals that the onion's tough epidermis acts as a barrier to fracture, enabling the underlying mesophyll to undergo significant compression before rupture. Developing a simplified bilayer model, we experimentally and theoretically capture the mechanism behind, with numerical calculations of the onion critical fracture forces matching independent Instron results. The work highlights the importance of blade sharpening routines to limiting ejected droplets infected with pathogens in the kitchen, which pack additional outburst energy due to vegetables' outer strong casings.