Structures and topological defects in pressure-driven lyotropic chromonic liquid crystals

Abstract

Lyotropic chromonic liquid crystals are water-based materials composed of self-assembled cylindrical aggregates. Their behavior under flow is poorly understood, and quantitatively resolving the optical retardance of the flowing liquid crystal has so far been limited by the imaging speed of current polarization-resolved imaging techniques. Here, we employ a single-shot quantitative polarization imaging method, termed polarized shearing interference microscopy, to quantify the spatial distribution and the dynamics of the structures emerging in nematic disodium cromoglycate solutions in a microfluidic channel. We show that pure-twist disclination loops nucleate in the bulk flow over a range of shear rates. These loops are elongated in the flow direction and exhibit a constant aspect ratio that is governed by the nonnegligible splay-bend anisotropy at the loop boundary. The size of the loops is set by the balance between nucleation forces and annihilation forces acting on the disclination. The fluctuations of the pure-twist disclination loops reflect the tumbling character of nematic disodium cromoglycate. Our study, including experiment, simulation, and scaling analysis, provides a comprehensive understanding of the structure and dynamics of pressure-driven lyotropic chromonic liquid crystals and might open new routes for using these materials to control assembly and flow of biological systems or particles in microfluidic devices.

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

“…Consequently, the directors escape from the yz plane, forming the so-called log-rolling state, trading splay-bend energy with twist energy. Log-rolling state has been studied in nematic DSCG solution experiments under different external flow conditions, where the directors are found pointing towards the vorticity direction at certain shear rates [48,49]. The stability of this state requires a low twist modulus [53], as is considered in our simulation.…”

“…Thus, it is convenient to use a scaled angular frequency ω/γ, which is a sole function of material constants (viscosity coefficients). For a nematic with very small twist modulus, the system can enter a logrolling state at certain shear rates, with directors rolling while pointing nearly normal to the shear plane [48,49,53]. Our perturbation analysis detailed in Appendix A.2 shows that the frequency of log-rolling mode is identical to the free tumbling frequency ω log-rolling = ω tumbling , indicating that the two types of unsteady dynamics found in flow-tumbling nematics are governed by the same intrinsic material-dependent time scale.…”

“…For a flow-tumbling nematic with α 3 > 0 and α 2 < 0, however, directors cannot settle into a stationary orientation and thereby tumble persistently. Recent experiments of nematic DSCG solutions have uncovered its tumbling character, manifested by the emergence of a log-rolling state at intermediate shear rates [48,49]. In this state, directors tend to rotate out of the shear plane and roll with respect to the vorticity direction, reminiscent of logs rolling on the ground [48].…”

Single-Shot Quantitative Polarization Imaging of Complex Birefringent Structure Dynamics

“…Lyotropic chromonic liquid crystals (LCLCs) are biocompatible water-based materials composed of self-assembled cylindrical aggregates. − They are promising candidates for controlling assembly of particles and biomaterials in microfluidic channels, − but their flow properties are currently poorly understood, − partly due to the lack of quantitative polarization imaging methods with fast enough imaging speeds that allow investigations of LCLCs at high flow rate . We apply PSIM to quantitatively image the flow of aqueous solutions of the LCLC disodium cromoglycate (DSCG) in a microfluidic channel.…”

“…We apply PSIM to quantitatively image the flow of aqueous solutions of the LCLC disodium cromoglycate (DSCG) in a microfluidic channel. The director field of the liquid crystal is three-dimensional (3D), varying across the channel thickness . Our two-dimensional (2D) technique provides the integrated retardance and orientation angle across the thickness, which can then be validated with and compared to simulations that can provide access to the 3D director field .…”