A drop of liquid metal passes through a metal grate

Researchers at Seoul National University and Gachon University have created a liquid robot resembling a droplet in a protective shell. It squeezes between bars, picks up small particles, and carries them inside itself. The work was published in Science Advances on March 21, 2025.

Inside the robot is water, and outside is a dense layer of water-repellent particles. To create a durable shell, the scientists first covered a rectangular piece of ice with particles and then allowed it to melt. The supply of particles is sufficient to keep the surface covered even under severe deformation.

The droplets are millimeter-sized. In experiments, they were guided by an external ultrasonic emitter: they do not choose their own route. When encountering bars, the droplet splits and reunites, and sufficient pressure helps it pass through the gap. The video also includes slow-motion footage of compression tests.

Researchers are considering such designs for substance transport and soft robotics applications. Drug delivery inside the body remains a prospective application for now, as this is a laboratory experiment.