arXiv subjects

Alexander J. H. Houston

Publications and source records attributed to Alexander J. H. Houston.

6 recordsLinked to original sources

arXiv2025

Heterogeneity-Induced Oscillations in Active Nematics

One of the defining features of active nematics is that above a critical activity the quiescent state becomes unstable to a distorted, flowing one. We show that spatial variations in activity can fundamentally change the nature of this instability, affecting the symmetry of the unstable mode and producing spontaneous oscillations. We analytically identify a dynamical system for the evolution of the odd and even director modes, with the leading-order coefficients dependent on the activity profile, allowing a quantitative connection between the spatially-heterogeneous activity and dynamics, which we verify numerically. In the context of constant gradients in activity, we determine a phase diagram for the active response and highlight how variation of the activity profile causes the oscillations to vary from almost harmonic to relaxational. Our results indicate a novel route to spatio-temporal structure in active nematics and suggest experiments on controllable light-activated systems.

cond-mat.soft↗

arXiv2025

The Role of Elastic Anisotropy in Active Nematics

We analyse the effect of anisotropy in elastic constants on the hydrodynamics of active nematics. Building on the multipole framework for a single elastic constant, we determine the leading effect of elastic anisotropy on the active response of generic distortions. The key findings are a new active torque, proportional to the anisotropy, in response to monopole distortions, and modifications to the propulsion of dipoles in both the direction of motion and changes in speed of up to 50\%. For point defects in two dimensions we find that, despite the large morphological changes in the director field, elastic anisotropy has only a minor impact on their hydrodynamics, with the self-propulsion speed of $+1/2$ defects lowered by less than 5\%. Finally, we determine the elastic torques exerted on defect pairs due to elastic anisotropy.

cond-mat.soft↗

arXiv2024

Spontaneous Flows and Quantum Analogies in Heterogeneous Active Nematic Films

Incorporating the inherent heterogeneity of living systems into models of active nematics is essential to provide a more realistic description of biological processes such as bacterial growth, cell dynamics and tissue development. Spontaneous flow of a confined active nematic is a fundamental feature of these systems, in which the role of heterogeneity has not yet been considered. We therefore determine the form of spontaneous flow transition for an active nematic film with heterogeneous activity, identifying a correspondence between the unstable director modes and solutions to Schrödinger's equation. We consider both activity gradients and steps between regions of distinct activity, finding that such variations can change the signature properties of the flow. The threshold activity required for the transition can be raised or lowered, the fluid flux can be reduced or reversed and interfaces in activity induce shear flows. In a biological context fluid flux influences the spread of nutrients while shear flows affect the behaviour of rheotactic microswimmers and can cause the deformation of biofilms. All the effects we identify are found to be strongly dependent on not simply the types of activity present in the film but also on how they are distributed.

cond-mat.soft↗

arXiv2023

Colloids in Two-Dimensional Active Nematics: Conformal Cogs and Controllable Spontaneous Rotation

A major challenge in the study of active systems is to harness their non-equilibrium dynamics into useful work. We address this by showing how to design colloids with controllable spontaneous propulsion or rotation when immersed in active nematics. This is illustrated for discs with tilted anchoring and chiral cogs, for which we determine the nematic director through conformal mappings. Our analysis identifies two regimes of behaviour for chiral cogs: orientation-dependent handedness and persistent active rotation. Finally, we provide design principles for active nematic colloids to achieve desired rotational dynamics.

cond-mat.soft↗

arXiv2023

Active Nematic Multipoles: Flow Responses and the Dynamics of Defects and Colloids

We introduce a general description of localised distortions in active nematics using the framework of active nematic multipoles. We give the Stokesian flows for arbitrary multipoles in terms of differentiation of a fundamental flow response and describe them explicitly up to quadrupole order. We also present the response in terms of the net active force and torque associated to the multipole. This allows the identification of the dipolar and quadrupolar distortions that generate self-propulsion and self-rotation respectively and serves as a guide for the design of arbitrary flow responses. Our results can be applied to both defect loops in three-dimensional active nematics and to systems with colloidal inclusions. They reveal the geometry-dependence of the self-dynamics of defect loops and provide insights into how colloids might be designed to achieve propulsive or rotational dynamics, and more generally for the extraction of work from active nematics. Finally, we extend our analysis also to two dimensions and to systems with chiral active stresses.

cond-mat.soft↗

arXiv2021

Defect Loops in Three-Dimensional Active Nematics as Active Multipoles

We develop a description of defect loops in three-dimensional active nematics based on a multipole expansion of the far-field director and show how this leads to a self-dynamics dependent on the loop's geometric type. The dipole term leads to active stresses that generate a global self-propulsion for splay and bend loops. The quadrupole moment is non-zero only for non-planar loops and generates a net `active torque', such that defect loops are both self-motile and self-orienting. Our analysis identifies right- and left-handed twist loops as the only force and torque free geometries, suggesting a mechanism for generating an excess of twist loops. Finally, we determine the Stokesian flows created by defect loops and describe qualitatively their hydrodynamics.

cond-mat.soft↗