The secret to improving liquid crystal's mechanical performance

In a paper published in EPJ E, Patrick Oswald from the École Normale Supérieure of Lyon, France, and Lubor Lejček from the Czech Academy of Sciences have now theoretically calculated the static and dynamical properties of the Cottrell clouds, which form around edge dislocations in lamellar liquid crystals of the smectic A variety decorated with nanoparticles. This work could be important, for example, in the context of improving the lubricating performance of such liquid crystals. The Cottrell clouds are difficult to study in solid materials, and even more so when the dislocations are in motion. This is not the case in a smectic A liquid crystals doped with gold nanoparticles where the Cottrell clouds are visible under a simple optical microscope. In addition, the density of dislocations can be controlled experimentally in these materials, allowing the dislocation mobility to be directly measured.

A recent experiment showed that it decreases as the concentration of nanonoparticles increases. This leads to a hardening of the material, very similar to what is observed in metallic alloys. When the dislocations move slowly, the Cottrell clouds of nanoparticles are dragged by the dislocations, which decreases their mobility.

In this study, the authors demonstrate a formula previously used to approximate the mobility of dislocations in the presence of Cottrell clouds. They then perform a numerical simulation of the problem to study how the Cottrell cloud erodes when the dislocation moves at high speed.