Past Seminars Seminários Já Decorridos 2014

Electric field induced switching and activity in nematic systems with interfaces

By: Matthew Blow
From: CFTC - Universidade de Lisboa
At: Instituto de Investigação Interdisciplinar, Anfiteatro
[2014-04-03] 14:00

Nematic liquid crystals are composed of rodlike molecules or colloidal objects that exhibit no ordering in their positions but have a tendency to align along a common axis. Examples include organic molecules such as 5CB and MBBA, and biological materials such as microtubule bundles, actin filaments and colonies of some microorganisms. The former are indispensable in the display industry due to their switchable light-modulating properties. The latter are often 'active', meaning that they convert a chemical energy into motion, and are key to many transport and motility processes in microbiology.

When a nematic fluid is in coexistence with an isotropic fluid and/or in contact with a solid substrate, its behaviour is determined by a delicate interplay between the bending rigidity of the orientational order, and orientational anchoring to the surfaces, and topological defects in the nematic order may play an import role. Using the lattice Boltzmann method to numerically solve the equations of nemato-hydrodynamics, we study two such systems. The first is a nematic liquid crystal filling a grooved substrate. We analyse the dynamical response of the system to an externally-applied electric field, with the aim of identifying switching transitions between different filled states. We identify the types of electric coupling required, the necessary magnitudes and orientations of the field, the transition dynamics, and the role played by the nematic-isotropic interface.

The second system is an active nematic fluid in phase-separation with an inert isotropic fluid. We show that when the nematic phase constitutes a high proportion of the fluid, the resulting 'turbulent' motion is driven by the creation and annihilation of pairs of defects with opposing topological charge. If the proportion of the isotropic material is increased, nematic regions have a tendency to become elongated, and solitary point defects, predominately of positive charge, are produced at the interface. We provide explanations for these effects in terms of the forces exerted by the active material at the interface.