Droplet Outbursts from Onion Cutting

TL;DR

This study investigates the mechanics of droplet release during onion cutting, revealing how blade sharpness and cutting speed influence droplet quantity and energy, with implications for reducing pathogen spread in kitchens.

Contribution

It combines high-speed imaging, DIC, and modeling to elucidate the droplet formation process and how blade sharpness affects droplet ejection during onion cutting.

Findings

Faster or blunter blades increase droplet number and energy.

Sharpened blades reduce droplet ejection and velocity.

Onion's epidermis acts as a fracture barrier, affecting droplet dynamics.

Abstract

Cutting onions often leads to tear-inducing aerosol release in kitchen, yet the underlying mechanics of droplet generation remain poorly understood. In this work, we combine custom-developed high-speed particle tracking velocimetry (PTV) and digital image correlation (DIC) to visualize and quantify droplet ejection during onion cutting. We show that droplet formation occurs via a two-stage process: an initial high-speed ejection driven by internal pressurization of the onion first-layer, followed by slower ligament fragmentation in air. By systematically varying blade sharpness and cutting speed, we find that faster or blunter blades significantly increase both the number and energy of ejected droplets. Strain mapping via DIC reveals that the onion's tough epidermis acts as a barrier to fracture, enabling the underlying mesophyll to undergo significant compression before rupture, thereby increasing both the quantity and velocity of the resulting splashed droplets. Developing a scaling model and a simplified bi-layer model with a spring foundation, we experimentally and theoretically demonstrated how sharpened blades lead to not only fewer but also slower droplets. Numerical calculations accurately explain the onion critical fracture force obtained from independent Instron tests. 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.