What's so special about the robots that scientists were inspired to create by this fish?

Produced by: Science Popularization China

Author: Xiang Yangyang (Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences)

Producer: China Science Expo

"In our sandy land, when the tide is about to come, there are many jumping fish, and they all have two feet like frogs..." This is a rare thing in the countryside mentioned by the young Runtu in Mr. Lu Xun's essay "Hometown".

Learn from nature. The magical nature always inspires people and promotes the progress of science and technology. Can you imagine? There is a small gas analyzer in the cockpit of the spacecraft to detect the gas composition in the cabin. This is actually inspired by the disgusting flies; there is also a "radar pathfinder" developed based on the principle of bat ultrasonic locator; a crane hook designed to imitate animal claws, etc. These inventions are inseparable from bionics.

Image source: veer gallery

What kind of inspiration did the mudskippers give to scientists this time? What did the scientists create?

1. What kind of fish is mudskipper?

The frog-like jumping fish mentioned by Mr. Lu Xun is actually mudskipper, which is 50-90 mm long and widely distributed in Hong Kong, Taiwan and Southeast Asia. It is a small warm-temperate near-shore fish. It likes to live in the brackish and fresh waters of estuaries, harbors, mangrove areas, shallow waters along the coast, and mudflats with silt and sand bottom. It likes to live in burrows and is also called jumping fish, mud cow, mudflat tiger, etc. because of its jumping nature.

Image source: veer gallery

The round eyes of mudskippers are located on the top of their heads, which can detect food and predators from a distance. Unlike ordinary fish that live in water, they can not only breathe with gills in the water and swim happily, but also absorb oxygen through the capillary network distributed on the surface of the skin, the oral cavity, and the inner wall of the gill cavity, providing the necessary conditions for their survival on land. If you see a mudskipper staring blankly with its mouth open, it is most likely using this "alternative breathing method".

April to September every year is the most active season for mudskippers. If you are in a coastal city, you can often see them jumping happily in the shallows. However, it may be hard for you to imagine that they are not foraging. These "dancing" male mudskippers are actually attracting the attention of the opposite sex in order to complete the reproduction of the species.

The jumping behavior of mudskippers is due to the perfect coordination of their well-developed pectoral and pelvic fin muscles. When jumping, their fins are spread out like a small "sailfish". Of course, they can also use the fins on their abdomen to "swim" flexibly and quickly in the mud to avoid being predated by natural enemies.

2. Technological innovation brought by bionics

Natural biological organisms demonstrate unique abilities through autonomous feedback and effective motion strategies, such as earthworms crawling, fish swimming and jumping, etc., to better adapt to complex and changing living environments. The dynamic adaptability of biological organisms has inspired scientists to look for similar motion strategies to develop biomimetic robots. However, most actuators have single functions and limited application scenarios, which seriously affect their further development and utilization. Therefore, it is of great significance to develop responsive actuators suitable for multi-scenario applications and multi-functional drives.

Mudskippers have the unique characteristics of crawling on land, swimming in water, and jumping in mud. They are a kind of fish with multi-scenario adaptability and multifunctional driving behavior. Inspired by mudskippers, the Surface and Interface Research Team of the State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, developed an intelligent optical actuator that can respond to various environments.

Image source: Made by the author

3. Why can the mudskipper-like actuated membrane swim on the water surface under light?

Light provides unique advantages of wireless actuation and remote manipulation in a controllable and continuous manner. At the same time, photoactuators can work under different mechanisms, such as photochemistry, photothermal effect, photoelectric conversion, etc., which also makes the preparation of photoresponsive materials a very promising research field.

The driving film developed by the researchers has graphene oxide (GO) material with excellent photothermal effect, which can convert light energy into thermal energy. When light shines on the driving film floating on the water surface, the film will generate a certain temperature and transmit it to the surrounding liquid, causing the local water temperature to rise and thus affect the surface tension of the water.

Image source: Made by the author

Now, let's get back to the point. You must be eager to know what the connection is between temperature and the surface tension of water. It turns out that the surface tension of water at different temperatures is different. When the temperature of water increases, the surface tension decreases, and vice versa.

Therefore, after light shines on one side of the driving film, the liquid around the illuminated area heats up, causing the surface tension to decrease, but the surface tension of the liquid in the area without light does not change. Then the two sides of the film will generate a force due to the different surface tensions, driving the film to swim to the side with greater surface tension. Just like the tug-of-war we have all played, the rope will move to the side with greater force. This characteristic of water is also called the "Marangoni" effect by scientists.

4. How to control the direction of the mudskipper-like driving membrane?

Efficient and accurate control of the direction of the membrane's movement requires a lot of experimental attempts. As we all know, ships are usually pointed at both ends, which can effectively reduce the water resistance of the hull. Our surface swimming device also needs to be reasonably designed to achieve human-controllable operation.

Image source: Made by the author

Through experimental exploration, it was found that the triangular driving film can swim very quickly when the bottom edge is illuminated by light, but it has the defect of uncontrollable direction. It is obviously difficult for the triangular film to complete operations such as swimming in a straight line, turning, and self-rotation on the liquid surface.

The reason why the film's linear direction is uncontrollable is that the angle of the head is too sharp. Cutting it into a trapezoidal shape can make the film move in a straight line at the expense of a little speed. In this way, we only need to shine light on the center of the bottom edge to achieve linear motion of the film. Another advantage of this is that by illuminating the shorter bottom edge of the trapezoid, the film can also complete the backward operation.

After solving the problem of moving forward and backward, we should consider the turning problem of the film when it encounters a curve. How can the film change direction while swimming forward? By calculating the direction of the resultant force when the film turns, we can quickly design the location of the component force. Then, after cutting off a trapezoidal area on the basis of the trapezoid, various swimming behaviors can be achieved by irradiating specific parts.

5. Can the mudskipper-like driving membrane actually jump?

Under light stimulation, the actuator can jump from the liquid medium to the air in an extremely short response time (400 ms), with a maximum speed of 2 m/s and a height of up to 14.3 cm.

Image source: Made by the author

The reason for the jumping behavior of the light-driven device is actually very simple. As mentioned earlier, GO has a photothermal effect. When the film exists in a liquid medium, the light causes it to bend and deform, and the air hidden between the graphene oxide layers will be heated and overflow from it to form small bubbles. The number of these small bubbles will gradually increase and merge into large bubbles. When the pressure difference between the inside and outside of the large bubble reaches a certain value, the bubble will burst and generate a huge driving force to drive the film to suddenly jump from the liquid into the air.

Inspired by mudskippers, the researchers developed a driving film that can bend in the air, swim in the air/liquid interface, and jump in the liquid medium, just like mudskippers. This driver with multifunctional driving behavior and multi-scenario application characteristics is of great significance to the simple modular combination of light-responsive materials, and has certain reference and reference value for the further development of bionic drivers and their applications in the fields of micro-robots, sensors, and responsive motion.

The creatures in nature contain the greatest inspiration for inventions, and the development of science and technology is inseparable from people's learning from nature. With the accumulation of knowledge, human beings are becoming more and more capable of utilizing and transforming nature. Bionic technologies are gradually entering our lives and promoting social progress. Here, I call on you who love science to walk into nature with me, stop in the fragrant forest, find inspiration from nature, explore the mysteries of nature, and create new technologies.

References:

Yangyang Xiang, Bo Yu*, Feng Zhou*, et al., Toward a Multifunctional Light-Driven Biomimetic Mudskipper-Like Robot for Various Application Scenarios, ACS Applied Materials & Interfaces, 2022, 14(17), 20291–20302.

Original link: https://pubs.acs.org/doi/10.1021/acsami.2c03852