Crystalline materials, such as aluminium, consist of many small crystals. The boundaries between these crystals, known as grain boundaries, determine how strong a material is: the more grain boundaries, the stronger the material. Grain boundaries move through the material, but how this movement works had until now remained unclear. Roel Dullens, a physical chemist at Radboud University, and his team have now discovered a geometric mechanism that can predict this movement.
Colloids
The team studied grain boundaries in colloidal crystals. Colloids are substances whose particles are roughly between 1 nanometre and a few micrometres in size: milk, blood, and opal are examples. 'Colloids are therefore considerably larger than molecules and atoms, and as a result much easier to observe under a microscope,' says Dullens. 'They are also interesting because they behave in the same way as molecules and atoms in terms of phase transitions — from liquid to crystal, for instance — something Einstein already described in 1905.'
From liquid to crystal
Particles in a colloidal liquid move constantly back and forth due to Brownian motion, which is caused by collisions with the smaller molecules of the surrounding liquid. This motion effectively causes them to scan their environment, seek out the most favourable position, and arrange themselves on a lattice: a crystal structure. Because colloids are so large, they move relatively slowly, allowing researchers to observe, track, and manipulate individual particles.
Cooking – Looking - Tweezing
Roel Dullens: 'In our lab, chemists and physicists work together. This gives us the ability to create particles with very specific properties ourselves, and to build specialist microscopes to go with them.' The research team also makes use of an optical tweezer — a focused beam of light with which you can, in effect, pick up a particle. 'When you move the laser, the particle moves with it, so we can direct particles and observe what happens inside the crystal.'
Circle
Studying grain boundaries in a material is normally difficult, as they are connected in a network and influence one another's movements. The researchers therefore used a circular optical tweezer to create a single, circular grain boundary. 'That is important, because it has one curvature, perfectly defined,' explains Dullens. 'The only thing that matters then is the difference in orientation between the crystals on the inside and the outside.'
Once the desired difference in orientation is achieved by rotating with the optical tweezer, the grain boundary is released and subsequently shrinks. 'It shrinks because it seeks to use as little energy as possible. Interfaces always cost energy, and nature resists that.'
Hexagonal pattern
The researchers observed what the particles do as the grain boundary shrinks. They monitored the boundary move slowly through the crystal, with particles transitioning from one orientation to the other. By photographing this process and overlaying the images, it became clear which particles remained stationary and which moved: stationary particles appeared as a dot, moving particles as a streak.
The result was surprising: some particles remained entirely still as the grain boundary passed, whilst others moved around it in a kind of dance. The stationary particles arranged themselves in a hexagonal pattern.
