Control of active nematics

Thesis

Active matter describes systems, such as bacterial suspensions, cellular tissue or cytoskeleton biopolymers, that extract energy from their surroundings at the single-particle level and convert it into mechanical work. The resulting active work can be manifested in the form of self-propulsion and stress generation. The continuous injection of energy or activity may lead to complex, collective flow patterns and, for active materials composed of particles with nematic-like symmetry, this can produce orientational order and topological defects in the orientation field. These materials are commonly described as active nematics. An important theme in active nematic research is active turbulence, a steady-state in which hydrodynamic instabilities result in chaotic collective flows.

Using continuum simulations, we investigate how the surroundings can screen the hydrodynamics, which allows for the control of active nematics. First, by confining an active nematic between planar plates, we observe how point-like topological defects become topological disclination lines that eventually contort due to twist deformations driven by the active forces with increasing plate separation.

Another method to screen the hydrodynamics is through frictional damping of different substrates. We find the emergence of a laning state when the active nematic is subject to anisotropic friction. We show that the flow-aligning parameter, which determines reorientation to the self-generated shear flows, is important for the emergence of this flow state. We investigate the flow-aligning parameter further and uncover regimes where self-propelled defects are mutually bound. We also show that weak defect-defect ordering exists in active turbulence, and we demonstrate that defects can exhibit co-operative defect-defect interactions that span many defect pairs by varying global friction. Finally, we show that we can control the active material's defect, flow and concentration dynamics through position-dependent friction.

The work in this thesis will allow future research to extend knowledge on bulk active nematics to more complex biophysical systems.

Authors: Thijssen, K

• DOI: 10.5287/ora-wradpg4d6
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Hydrodynamics; Active matter; Active defects; Soft matter; Active nematics; Biological physics; Lattice-Boltzmann simulations
• Subjects: Biophysics; Computational fluid dynamics; Fluid mechanics; Soft condensed matter