Liquid robots become a reality, inspired by sea cucumbers

Produced by: Science Popularization China

Author: Keke (Material Designer)

Producer: China Science Expo

After the release of the movie "Terminator 2", the number one villain liquid metal robot T1000 left a very deep impression on the world. In the movie, T1000 not only has steel bones, but also can change its body shape at will, and even after turning into a pool of liquid, it can recondense into its original form.

Today, this once wild fantasy is gradually becoming a reality. On January 25, a research team from Sun Yat-sen University, Zhejiang University and Carnegie Mellon University in the United States announced their latest research results. They used a material called magnetically active phase change material (MPTM) to make a robot prototype that can switch between solid and liquid forms and can be controlled by magnetism to a certain extent.

In the public animation, the research team paid tribute to the famous scene in "Terminator 2" by crossing the cage, and at the same time showed people the magical properties of MPTM material. Come and see what's going on.

Terminator 2 poster

Image source: Wikipedia

Why is it difficult for materials to have the characteristics of "hardness and flexibility"

Common sense tells us that rigidity and the ability to deform freely cannot coexist at the same time. Indeed, for existing materials, it is difficult to have both. For example, materials with a hard appearance, such as metals, often have very high melting points. Although they can be melted into a free-flowing liquid at high temperatures, on the one hand, heating and melting requires considerable energy and time, and on the other hand, high-temperature molten metal can only stay in a special heat-resistant crucible, and has no other application value except for metal casting.

In other words, it is basically impossible for traditional materials to truly achieve "hardness and flexibility" like T1000.

However, there are some materials that have a certain degree of rigidity while maintaining a high degree of softness. For example, D3O, a very popular material in recent years, looks as soft as plasticine and can be shaped into any shape, but it can become very hard when subjected to a momentary impact (such as a hammer).

D3O actually uses the shear thickening effect. When some fluids are subjected to shear force (such as hammering), the viscosity will suddenly increase. It's like it looks like plasticine, but when you hit it suddenly, it's like hitting a large piece of carbon fiber.

Protective gloves made of D3O

Author: HecltoTrmel2020

Image source: Wikipedia

There are many species with similar characteristics in nature, such as the common marine creature sea cucumber. Although its body is soft, it can actually adjust its body hardness within a certain range. The principle behind this is to change the hardness of the interfibrillar matrix that makes up its body, so that the originally soft body has a certain load-bearing capacity and prevents various environmental factors from causing damage to its body.

Sea cucumber with a soft body

Author: François Michonneau Image source: Wikipedia

Previous materials of this type (including the body structure of sea cucumbers, of course) had a fatal problem, which is that they could not maintain their shape in a rigid state like structural materials such as aluminum alloys or steel, and they could not withstand the various mechanical loads such as tension, shear and torsion that general structural materials can withstand.

However, the team that completed this research has created a material that can easily switch between rigid and flexible states, and has excellent mechanical strength like metal in the rigid state. This is MPTM. It is reported that inspiration from sea cucumbers played a key role in the invention of MPTM.

MPTM's rigid-flexible conversion mechanism. Image source: Reference 1

Where is MPTM cattle?

MPTM is a composite material composed of metallic gallium and magnetic particles. Gallium is a low melting point metal that melts at 29.8°C, so it is a metal at normal room temperature (25°C), but it becomes liquid when the temperature rises slightly (such as the temperature of the palm of a person's hand).

Based on this special property, researchers have cleverly used the heating effect of alternating magnetic fields on magnetic particles in MPTM. Simply by controlling the magnetic field, the temperature of MPTM material can be raised to above the melting point of gallium, thus turning it into a liquid state. Afterwards, once it cools naturally, MPTM will turn into a solid with properties similar to metal.

Schematic diagram of MPTM in two states Image source: Reference 1

MPTM in solid state has mechanical properties similar to gallium metal, with a mechanical strength of 21MPa, and can withstand objects 30 times its own weight. Although it is still a bit fragile compared to general structural materials, it is fully practical as a general solid material. The most amazing thing is that due to the presence of magnetic particles, MPTM can be controlled by magnetic fields in both liquid and solid states, and can be moved by magnetic fields.

In the demonstration video, an MPTM figure trapped in a cage liquefies and then "escapes" from the cage, then returns to its original shape and "jumps up" on the spot, just like the T1000 itself. However, it must be pointed out that there is actually a figure-shaped mold under the cage, and the liquefied MPTM enters the mold and is recast into the figure shape after cooling. Even so, MPTM has still reached an unprecedented height.

GIF 1 The liquefied “jailbreak” process of the little robot Image source: Reference 1

Gif 2 The robot returns to its original shape after cooling down. Note the mold under the cage. Image source: Reference 1

The research team also provided a series of potential application scenarios, such as allowing MPTM to enter the body in solid form, then wrap the swallowed foreign matter while liquefying under the action of magnetic force, and then solidify and guide the MPTM carrying foreign matter out of the human body through magnetic field control. In addition, several demonstrations such as assembling circuits and components in a limited space and flowing into screw holes to fix objects were also impressive.

Image 3: Demonstration video of MPTM removing foreign matter from the body. Image source: Reference 1

Image 4: The MPTM robot successfully connected to the circuit. Image source: Reference 1

The MPTM material proposed by Chinese and American scientists has made new breakthroughs in the field of liquid robot research in terms of deformation, recovery and controllability. Although it still does not reach the practical level in terms of precise control, and can only be fully restored to its pre-liquefaction form with a little "ingenuity", it is still exciting to see the unrestrained imagination in science fiction movies come into reality. Are you looking forward to it?

Editor: Sun Chenyu

References:

1.Magnetoactive liquid-solid phase transitional matter
https://www.cell.com/matter/fulltext/S2590-2385(22)00693-2

2. Robot Liquefaction Prison Break: Chinese team realizes Terminator fantasy, inspired by sea cucumber
https://m.thepaper.cn/newsDetail_forward_21690166