Amolf Researcher Discovers Simple Method for Creating Regular Patterned Materials

Amolf researcher Christiaan Van Campenhout has found a simple method to create a material with a regular pattern of crystalline bands. This can help to easily and cost-effectively produce optics, electronics or sensors.

Microscopic patterns in A4 format. At first glance, this brown sheet of plastic (left) doesn’t look like anything special. But when you look at it under a microscope, you see an extremely regular pattern of microscopic stripes (right).

The pattern formed by the crystals is no coincidence. With a simple setup, the researchers can precisely control the width of the bands and their spacing.

The research is published in the journal Advanced Materials.

Van Campenhout conducts his research in the Self-organizing matter group led by Wim Noorduin, in collaboration with the Mechanical Materials group led by Martin van Hecke. The goal is to discover simple, nature-inspired methods to create such components. “In nature you can find regular patterns everywhere, from the stripes of a zebra to the patterns on a butterfly wing. We want to use a self-organizing, biologically inspired strategy to create high-tech materials. This research is a good step in that direction.”

In all its simplicity, the experiment and the results are remarkable. The plastic sheet looks rather plain with its brownish color. However, when a laser beam passes through it, it creates a pattern of dots on the other side: the result of the regular narrow bands of crystals embedded in the plastic, invisible to the naked eye. Such precise control over patterning for electronics typically requires expensive and complex techniques such as lithography.

Dipping instead of shrinking

The crystals in the pattern are created by a chemical reaction between a substance in a gel (which solidifies into a plastic sheet after the experiment) and a substance in a solution that diffuses into the gel. The formation of crystal bands in this so-called reaction – diffusion process was already known. Last year the researchers published an article in which they showed that they could create a regular band structure by slightly shrinking the gel. This observation got them thinking: couldn’t it be simpler than shrinking? “We noticed that the liquid level and the site of the reaction in the gel remained equidistant during shrinking. This leads to the regular band structure. We thought we could achieve the same result by gradually dipping the gel into the liquid instead of allowing the liquid to diffuse into the gel,” explains Van Campenhout. This straightforward set-up immediately worked, much to Van Campenhout’s great pleasure: “This is my favorite kind of research: don’t overanalyze but think: couldn’t it be simpler? And then it works.” The process was named R-DIP: reaction-diffusion driven immersion-controlled patterning.

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It soon became apparent that the distance between the tires depends on the rate at which the fluid level rises. The faster you dip the gel into the liquid, the closer the bands get together. Initially, the distance between the bands was 200 micrometers (0.2 mm) with a variation of only 6 micrometers. Currently, the smallest band gap is 7 micrometers. “For many interesting applications it is essential to have the bands even closer together, about 0.2 micrometres or less,” says Van Campenhout. The research will focus on achieving that goal in the near future.

Van Campenhout also showed that the method is scalable: it works with a sheet the size of an A4. “This suggests it is suitable for roll-to-roll manufacturing, a method already used for large-scale electronics manufacturing.” In addition, you can cover several sheets with tapes, each slightly twisted. “This allows us to create a polarizing film for items such as sunglasses and contact lenses.”

Ultrasensitive pressure sensor

Another application is an ultra-sensitive pressure sensor. Placing two layers parallel to each other creates a Moiré pattern, which changes when the layers are slightly pressed together. With the naked eye you can see a change when the gel is pressed 20 micrometres. In the near future, Van Campenhout will investigate whether he can change the composition of the tires through chemical modifications to make them more suitable for practical applications. This could be achieved with methods previously developed by Noorduin’s group to convert calcium carbonate into semiconductors. In addition, the films resemble photographic film, in which silver salts also contribute to color and contrast. “We are investigating whether we can apply the chemical knowledge from photography to develop films for other applications.”

2023-08-16 13:49:54
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