Elongation of discotic liquid crystal strands and lubricant effects
Résumé
After a short review on the physics of pulled threads and their mechanical properties, the paper reports and discusses on the strand elongation of disordered columnar phases, hexagonal or lamello-columnar, of small molecules or polymers. The mechanical properties appear to be relevant to the length of the columns of molecules compared to the thread length, instead of the usual correlation length. When short, the column entanglement being taken into account, the strand exhibits rather fluid properties that may even look like nematic at a macroscopic scale. Then, the Plateau-Rayleigh instability soon breaks the thread. However, the hydrodynamic objects being the columns instead of the molecules, the viscosity is anomalously large. The observations show that the strands of columnar phases are made of filaments, or fibrils, that indeed are bundles of columns of molecules. They both explain the grooves and rings observed on the antenna or bamboo-like strand profiles. On pulling a strand, the elongation stress eventually exceeds the plasticity threshold, thus breaking columns and filaments. Cracks, more exactly, giant dislocations are thus formed. They change the strand thickness by steps of different birefringence colours. Interestingly, adding a solute may drastically change the effective viscosity of the columnar phase and its mechanical properties. Some solutes as alcanes, exhibit lubricant and detangling properties, while others as triphenylene, are quite anti-lubricant.
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