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In the natural world, there are numerous complex and exquisite patterns and forms:  clouds gathering and spreading, animal markings, and the plant’s leafy phyllosma. They convey the beauty of nature and stimulate the curiosity of scientists. In 1952, Alan Turing, the father of modern computers and artificial intelligence, came up with the conjecture that the forms of the part of the world of life that we see are essentially the result of an imbalanced chemical reaction.

More than 60 years later, Professor Zhang Lin’s laboratory at Zhejiang University’s School of Chemical Engineering and Bioengineering created a new type of water purification membrane. Its unusual surface topography is consistent with the Turing structure theory proposed by Turing. Compared with ordinary nanofiltration membranes, the Nanofiltration membrane of the Turing structure has a water permeability of 3-4 times.

The related paper “Polyamide membranes with nanoscale Turing structures for water purification” (DOI:10.1126/science.aar6308 with a nanoscale Turing structure for water purification) was published in the top scientific journal “Science” on May 4th. The first author is a doctoral student Tan Yi. A paper reviewer believes: “As far as I know, this is the first time that we have tried to fabricate a nanoscale Turing structure on a film.” Research on the Turing structure is progressing from the theoretical level to the application field for decades.

“Change face” nanofiltration membrane

Nanofiltration membrane is an important material widely used in deep water treatment, hard water softening, brackish water treatment and other fields, it can remove specific organic substances, pigments and salts in water. The manufacture of nanofiltration membranes is achieved mainly through interfacial polymerization: Piperazine and trimesoyl chloride are dissolved in water and oil, respectively, and the two small molecules are at the water-oil interface after contact with incompatible water and oil. Polymerization takes place, and within a few seconds, a layer of dense, dense polymer film about 100 nanometers thick can be formed.

Nanofiltration membrane sample

Zhang Lin, who has long been engaged in membrane separation technology research, is familiar with interfacial polymerization reactions. This method can be used not only for the manufacture of nanofiltration membranes but also for the manufacture of reverse osmosis membranes. However, the surface morphology of these two films is quite different: the nanofiltration membrane is smooth and flat, while the reverse osmosis membrane is rough and uneven. “Why there is such a difference. So far no one has explained it well.” Zhang Lin said.

Tan Zhen is also very confused about this phenomenon. One day, he went for a walk around Yongfu Temple near the school and encountered a spotted deer on the road. The round spots scattered on the spotted deer made him alive: He remembered Britain’s great science wizard Turing, thought of his paper “Chemical Basis of Morphogenesis,” and thought of the life world he mentioned in the paper. The shape and texture.

The genius Turing uses a “reaction-diffusion” equation to reveal the chemical nature of the natural form. After discussion, the research group proposed that the interfacial polymerization of nanofiltration membranes is a typical “reaction-diffusion” reaction. “Since this is the case, we can regulate the preparation of nanofiltration membranes based on Turing’s theory.” Zhang Lin introduced the study group tried to add different types of hydrophilic macromolecules in water: starch, polypyrrolidone, polyethylene glycol When the polyvinyl alcohol was tested, a periodic pattern of strips appeared on the surface of the nanofiltration membrane – the smooth nanofiltration membrane changed its face. Under the electron microscope, the scientists discovered that the surface of the membrane was criss-crossed. 20-nanometer-thick “pipeline” or “round bubble”. After testing, the water permeability of the membrane has doubled.

Two kinds of Turing nanofiltration membranes under the scanning electron microscope, the left side is a point structure, and the right side is a strip structure.

Chemical Edition Life Game

“The Chemical Basis of Morphogenesis” is a biochemistry paper published by Turing in 1952. Through a professional mathematics language, it tries to use simple mathematical formulas to explain the chemical mechanism behind complex and subtle life forms. For “Chemistry Edition of Life Game”.

In the paper, Turing established the famous “reaction-diffusion equation”, which states that there are a class of “morpho morphogens” in living organisms, one to promote the occurrence of the reaction, and the other to inhibit the reaction, after the reaction, one side reacts. diffusion. Under uniform conditions, this system will exhibit homogeneous system characteristics and will not produce patterns. However, the diffusion of the two systems will be different. When they reach a certain level, they will lead to system instability and eventually form a periodic complex pattern. Turing speculated that the pattern of zebras and other organisms may be the result of the reaction-diffusion system.

Turing structure of the living world

Since then, biologists have successively discovered Turing structures in different scales of life systems: black and yellow pigment cells on the surface of zebrafish interact, and stripe patterns appear on the body surface; gene expression process of mouse hair follicles There is a reaction-diffusion process in it, which determines the hair follicle spacing and affects the density distribution of mouse hair; the number of middle finger in the fin skeleton is also the result of several genes due to the Turing mechanism.

At the same time, experimental scientists have been looking for examples of Turing structures, but it is difficult. Academician Ouyang Yi analyzed in “Patch Pattern Dynamics in Reaction-diffusion Systems”: The difficulty is “to find a reaction system in which the diffusion rate of activators is much lower than that of inhibitors. But in most systems, the diffusion coefficient It is roughly equivalent.”

It was not until the 1990s that French scientists passed the “chlorite-iodine-malonic acid reaction,” and the first observation in a chemical experiment confirmed the Turing structure.

Turing structure on thin film

Does Zhang Lin’s group of researchers “make” complex textures on the surface of the nanofiltration membrane, does it belong to the Turing structure? If so, this may be the first time that people have created Turing structures in industrial products.

To give this answer, Zhang Lin’s task force must re-observe and analyze the process of membrane formation. The key to judging whether it belongs to the Turing reaction-diffusion equation is that the diffusion rates of the activators and inhibitors in the reaction must be orders of magnitude different.

After NMR experiments, the scientists found that the difference in the diffusion rate of piperazine and trimesoyl chloride was not sufficient to produce the Turing structure, and the addition of polyvinyl alcohol significantly decreased the diffusion rate of piperazine. It was polyvinyl alcohol that made piperazine slow down. In the process of interfacial polymerization, piperazine “dances” differently from trimesoyl chloride, and finally formed a nanometer-scale Turing structure. Filter membranes.

A schematic diagram of a nanofiltration membrane having a Turing structure.

“By adjusting the different concentrations of polyvinyl alcohol, we have different Turing structures such as tubular and blister.” Zhang Lin said that this research is the first time to extend the Turing structure to the application field to guide the water purification. Preparation of the membrane.

A nanofiltration membrane with a Turing structure has a larger water-permeable surface area.

With the water purification membrane of Turing structure, the water permeability greatly exceeds the water permeability limit of the nanofiltration membrane, and the water permeability efficiency is 3-4 times higher than that of the conventional nanofiltration membrane. This is because the surface texture of the Turing structure is uneven, giving the water purification membrane more permeable sites. “We have discovered the formation mechanism of the Turing Nanofiltration membrane. The next step will be to develop a water purification membrane with better performance,” said Zhang Lin. Compared with the traditional manufacturing process, this membrane preparation technology does not need any modification to the existing production line, and it can produce a water purification membrane with better performance, which will have a good application prospect.

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