Perfect Particles Make 3D Structurally Coloured Objects

The particle theory of matter sits at the centre of the scientific methodology. Objects that appear uniform, solid and homogeneous at the everyday macroscopic scale are, when you investigate them with sufficient resolution and detail, in fact made up of particles (atoms, molecules and compounds) that are not the same entity as the object they form.

The particle theory explains several observable properties of matter, including its density, its ability to expand and contract in response to changes in temperature and applied force, and its behaviour when it undergoes phase changes such as melting or boiling. An assumption of the theory is that all of the particles in a material are perfectly spherical and of identical size. In their research at Saarland University (Germany) Professor of Polymer Chemistry Markus Gallei and his doctoral student Lukas Siegwardt set out to create ‘perfect particles’ with a hard centre and a soft shell to build 3D structurally coloured objects.

In August 2022’s edition, Holography News® reported on a group of MIT scientists who had printed large-scale images of flower bouquets onto an elastic material that transforms the colours and wavelengths of light that are reflected once it is stretched. The result of this photographic printing technique is that the images appear to change shades from warmer to cooler colours as the pliable film is deformed.

Methods to artificially fabricate materials that show structural colours have been around since 2001, but have been restricted to ultrathin films, fractions of a millimetre thick. ‘Conventionally, these materials have been processed in industrial presses or film rolling equipment, to produce thin polymer films that can change colour,’ explained Professor Gallei.

The colour of the film can be changed by numerous means, such as pulling the material, applying an electric voltage across it, changing the temperature or modifying the pH. ‘You can essentially control the colour of the material on demand,’ said Markus Gallei.

The two great advantages of these structural colours are that they are completely harmless – unlike many conventional dye pigments – and that they never fade.

Additionally, these materials are almost infinitely transformable, something that until recently was limited by the fact that they could only be produced as ultrathin films. If these materials could be shaped into 3D objects, they could be used in a wide range of applications, such as in anti-counterfeiting technology or as versatile measurement sensors. The particles can be manufactured so that they have highly specific properties, while also being easy to shape.

To understand the underlying chemistry, we need to go back to those ‘perfect particles’ of standard polymers such as polystyrene or poly(ethyl acrylates). These are commercially available as a white, tacky powder that is fed into an industrial press or, now more frequently, into a 3D printer.

‘During the printing process, the particles arrange themselves into regular patterns and these patterns will have different colours depending on the spacing between the particles,’ explained Markus Gallei.

The soft shells of the individual particles melt to create a flowable mass that surrounds the hard cores. Pulling on an object changes the distances between the individual core particles and the colour changes accordingly. The hard perfect particles move within the soft surrounding medium and arrange themselves into a new pattern. Gallei explained this molecular-level rearrangement as being like ‘squeezing honey from out between the individual particles.’ 

Changing the distances between these minute particles changes the way the material interacts with visible light, thus changing the colours we observe.

But preparing such materials for 3D printing involved a lot of lab work for Lukas Siegwardt. ‘I modified the material so that it could actually be printed. It took me months to find the right composition and the right recipes,’ says Siegwardt. There were two tough nuts to crack in the process. First, Siegwardt had to modify the flow properties of the powdery starting material so that the particles didn’t clog the printer’s nozzles and the material could be printed with as little residue as possible.

‘The second issue was the material’s thermal properties. In an industrial press, the starting material has to withstand about 120°C. But in a 3D printer, the material experiences temperatures of 140°C and sometimes as high as 200°C,’ said Siegwardt, explaining the demands placed on the material. ‘Many of the materials I tested during those months were simply not up to the job,’ he recalled.

The material demonstrates a chameleon-like nature as when it is pulled, the object’s colour changes progressively from red to blue. ‘So, you can see that this material already functions as a simple sensor that can react to tensile and compressive forces,’ explained Siegwardt.

The research is published under the title ‘Complex 3D‐Printed Mechanochromic Materials with Iridescent Structural Colors Based on Core–Shell Particles’, in Advanced Functional Materials (2023).