Interplay of Active Stress and Driven Flow in Self-Assembled, Tumbling Active Nematics

Abstract

Lyotropic chromonic liquid crystals (LCLCs) are 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 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 homeotropic-anchoring 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.

Dynamic mode structure of active turbulence

“…Driven ambient flows are known to strongly modify the dynamics of active suspensions [37, 45, 46], leading to novel rheological [19, 20] and transport [37] properties. Pressure-driven flows of dense, swimming bacterial suspensions through microfluidic channels were recently observed to exhibit significant flow intermittency [37].…”

Flow-induced periodic chiral structures in an achiral nematic liquid crystal

“…To account for the weak surface anchoring strength of LCLC solutions on our surface-treated glass plates 60 , where the director can deviate from the initial surface anchoring condition in shear flow, we mimic finite surface anchoring conditions in our simulations that intrinsically have infinite surface anchoring conditions by assigning polar angles on the top and bottom walls of the microfluidic cell, θ b , t o p and θ b , b o t t o m , where θ b , b o t t o m = 180° − θ b , t o p . We then test various θ b , t o p and θ b , b o t t o m within the range of 45° to 90° to find the polar angles that correctly reflect the weak surface anchoring strength 61 .…”

“…By defining the strain rate D = ( ∇ u + (∇ u ) T )/2 and the vorticity Ω = ( ∇ u − (∇ u ) T )/2, we introduce an advection term , where ξ is a constant that depends on the molecular details of the liquid crystal. We use ξ = 0.6 for our tumbling nematic LCLC solutions 61 .…”

Capillary Flow Characterizations of Chiral Nematic Cellulose Nanocrystal Suspensions

“…A cylindrical region of radius 350 with axis along z -direction is selected to represent the LC and the rest of the grid become non-LC points which impose a degenerate planar anchoring condition to the flowing LC. Our simulation method has been validated by comparing to Leslie-Ericksen theory of nematodynamics , and also active nematic flow experiments. − The initial condition is a weakly perturbed uniform axial alignment. The perturbation is a 5-degree twist to ensure that the simulation is not trapped in the uniform ground state.…”