Lecturer: Tomer Markovich, DAMTP, University of Cambridge and the Center for Theoretical Biological Physics, Rice University, Houston, TX
Abstract:
Molecular polar order usually originates from ferroelectric/magnetic interactions. This was realized in molecular dynamics simulations of interacting dipolar spheres in the last few decades but was experimentally seen only under extreme conditions. Hence, it was not thoroughly investigated as other liquid crystalline phases. Recently, it has been discovered that suspensions of magnetic platelets in nematic liquid crystals show polar order at room temperature. Furthermore, polar order is ubiquitous in biology, and can be seen in animal herds, cell migration, swimming bacteria, the cytoskeletal etc. The growing interest in biological active matter and these recent experimental advances call for further theoretical investigations of polar liquid crystals.
In the presence of external shear, polar particles tend to align with the shear flow at the Leslie angle, much like nematics, where the particles shape defines the shear alignment parameter that characterizes the Leslie angle. However, unlike the nematic director, the polar order parameter is not of fixed size. In this talk I will present our novel theory that introduces the shear elongation parameter. This often neglected parameter, give rise to new physics such as shear-induced first order transition and significantly changes the rheological properties of the fluid. Active fluids can further exhibit yield stress (positive/negative for pullers/pushers) for free and strong anchoring, even for weak activity. Pushers may also show a regime of negative apparent viscosity.