News & Seminars | HKUST Department of Physics

Abstract

Liquid crystals (LCs) represent a class of materials that exhibit long-range order in molecular orientations. LCs are widely used in display technology and its application has been extended to sensors, photonics, and many other fields in recent decades. When LCs are driven out of equilibrium through internal and external actuations, emerging patterns may arise and can lead to new opportunities in materials science and engineering. In this thesis, we discuss three different, yet related LC systems invovling LC flows driven by pressure or internal active stresses.

In the first system, we consider lyotropic chromonic liquid crystals (LCLCs), a special type of hierarchical material in which self-assembled molecular aggregates are responsible for the formation of liquid crystal phases. Thanks to its unusual material properties and bio compatibility, it has found wide applications including the formation of active nematic liquid crystals. Recent experiments have uncovered tumbling character of certain LCLCs. However, how tumbling behavior modifies structure and flow in driven and active nematics is poorly understood. Here, we rely on continuum simulation to study the interplay of extensile active stress and externally driven flow in a flow-tumbling nematic with a low twist modulus to mimic nematic LCLCs. We find that a spontaneous 1 transverse flow can be developed in a flow-tumbling active nematic confined to a hybrid alignment cell when it is in log-rolling mode at sufficiently high activities. The orientation of the total spontaneous flow is tunable by tuning the active stress. We further show that activity can suppress pressure-driven flow of a flow-tumbling nematic in a planar-anchoring cell but can also promote a transition of the director field under a pressure gradient in a homeotropicanchoring cell. Remarkably, we demonstrate that the frequency of unsteady director dynamics in a tumbling nematic under Couette flow is invariant against active stress when below a threshold activity but exhibits a discontinuous increase when above the threshold at which a complex, periodic spatiotemporal director pattern emerges. Taken together, our simulations reveal qualitative differences between flow-tumbling and flow-aligning active nematics and suggest potential applications of tumbling nematics in microfluidics.

In the second system, we study a new pattern emerged from pressure-driven flows of LCLC. To date, the spontaneous chirality emergence in achiral nematics was only observed in curved surface confinement at equilibrium state because of the saddle splay deformation. Surprisingly, we find the spontaneous emergence of chiral structures when an achiral lyotropic chromonic liquid crystal in the nematic phase relaxes from a high velocity flow to a steady-state lower velocity flow. The chirality results from a periodic double-twist deformation of the liquid crystal and leads to striking stripe patterns vertical to the flow direction. We analyse the director field and there are two candidate configurations, double splay and biaxial splay. Through the theoretical analysis, we show that the double-twist deformation is triggered at regions of biaxial-splay deformation which becomes unstable and the double splay becomes stable at which a sharp domain wall occurs. The transition from this biaxial-splay deformation to the double-twist deformation can be rationalized by the low Frank–Oseen elastic energy stored in double-twist deformations compared to that stored in biaxial-splay deformations.

In the last system, we study the emergent motion of topolgocial defects an active chiral nematic. In cholesteric (chiral) liquid crystals, a helical structure formed 2 naturally and the twist structure arranges along the pitch axis. However,The intrinsic chiral structure of the chiral material may lead to a unknown phenomenon in active system. Active nematics represent a range of dense active matter systems which can engender spontaneous flows and self-propelled topological defects. Two-dimensional (2D) active nematic theory and simulation have been successful in explaining many quasi-2D experiments in which self-propelled +1/2 defects are observed to move along their symmetry axis. However, many active liquid crystals are essentially chiral nematic, but their twist mode becomes irrelevant under the 2D assumption. However, in a 3D system, or quasi-2D system, the chirality works and play a vital role in either defect structure and defect dynamics. Here we use theory and simulation to examine a three-dimensional active chiral nematic confined to a thin-film, thus forming a quasi-2D system. The +1/2 defect structure is predicted symmetry-breaking between the splay-bend regions by our theoretical analysis. The asymmetric structure of the bend region near +1/2 defect is confirmed by the continuum simulation. We also predict that the self-propelled +1/2 disclination in a curved thin-film can break its mirror symmetry by moving circularly. The active force exerted on the +1/2 defect is inversely proportional to the local curvature. Our prediction is confirmed by hydrodynamic simulations of thin spherical-shell and thin cylindrical-shell systems. In the spherical-shell confinement, the four emerged +1/2 disclinations exhibit rich dynamics as a function of activity and chirality. As such, we have proposed a new symmetry-breaking scenario in which self-propelled defects in quasi-2D active nematics can acquire an active angular velocity, greatly enriching their dynamics for finer control and emerging applications.