Seoul researchers built a liquid robot that squeezes through bars, splits and merges back into one

A robot made of water that squeezes through metal bars, splits in two and merges back into one, on camera and without CGI.

What the paper describes

Seoul National University and Gachon University published the “particle-armored liquid robot” in Science Advances on March 21, 2025. The design is a water droplet coated in unusually dense hydrophobic particles. The coating gives the droplet the structural stability of a solid while the water inside keeps the deformability of a liquid, which is the combination that makes the demonstrations possible.

In those demonstrations the robot passes through bars, engulfs and carries objects, moves across both water and solid surfaces, and survives heavy compression and drops. Two robots carrying different substances can merge, triggering a reaction inside the combined droplet. Movement is regulated with ultrasound, and the team is working on sound waves and electric fields for shape control.

The authors are Ho-Young Kim, Jeong-Yun Sun and Keunhwan Park, with Hyobin Jeon as first author.

The Terminator comparison, and its limits

The researchers make the pop-culture comparison themselves: the T-1000 from Terminator 2, the liquid machine that flows through obstacles and reassembles. The footage earns it. The differences are scale and purpose. This robot is millimeters wide, and its intended jobs are drug delivery and cell-level tasks inside the human body, not pursuit.

That purpose explains the feature list. Passing through narrow gaps, carrying a payload, surviving compression and merging with a second droplet to start a reaction are the operations a drug-delivery vehicle would need inside tissue. The ultrasound steering fits the same use, since it is a control signal that can reach inside a body without contact.

Why it matters for robotics

A conventional robot, rigid or soft, is built from parts with defined shapes. This design has no parts in that sense. It is a contained volume of liquid whose shape is whatever the task requires at the moment, held together by a particle shell rather than a frame.

The work is a 2025 paper, not a product, and the control methods beyond ultrasound are still in development by the team’s own account. But if shape control by sound waves and electric fields matures as intended, the next generation of small medical robots may have no joints at all, and the choice between rigid and liquid body plans inside the body becomes an engineering decision rather than a science-fiction premise.