Active nematics for mechanobiology: patterning and flows in living matter

Thesis

Living matter is characterized by its ability to consume chemical energy to grow or exert active forces on its surroundings. Biological tissues composed of anisotropic and active cells can develop long-range orientational order with nematic symmetry. The constant injection of energy at the cellular level leads to intricate collective flows within the tissue. In this thesis, we apply the theory of active nemato- hydrodynamics to explore the effects of cellular heterogeneity, chemical patterning, and mechanical confinement in living systems.

Using continuum simulations, we investigate a mixture of an active nematic fluid with a passive isotropic fluid. We demonstrate that the mixture spontaneously phase separates, forming micro-phases with distinct active and passive domains. This phase separation is dynamically driven by active forces at interfaces and can proceed even if it is energetically unfavourable. Introducing a second active species leads to complex alignment interactions between the two species. Mixtures of extensile and contractile nematics with elastic alignment show the strongest phase separation. Confining active nematic mixtures results in coherent spatial organization of the active component, with the resulting patterns depending on the orientation and flow fields inside the mixture, and the nematic orientation at the boundaries.

Flow patterns in an active nematic system are affected by external mechanical or chemical signals. Chemical patterns, in the form of Turing stripes, weaken and eventually dissolve in the presence of activity. Associating different stripes with extensile and contractile activities respectively can generate shearing flows inside each stripe, and long-time slipping of the stripes. On the other hand, confining active nematics with viscoelastic boundaries give rise to periodic flow reversals. This behaviour emerges from the interplay between the active forces driving fluid flow and the elastic restoring forces exerted by the confining walls.

Finally, we study breast cancer tumours inside an extracellular matrix as confined active matter. We show that active dynamics, rather than growth, dominate tumour behaviour. Our model accurately reproduces tumour morphologies obtained from clinical data and shows the effects of cancer clusters on fibre orientation in the extracellular matrix.

Overall, this thesis demonstrates how active matter physics can be a powerful tool to study biology. By exploring the interplay between active forces, mechanical properties, and environmental feedback, we highlight potential mechanisms for pattern formation and flows in living systems.

Authors: Bhattacharyya, S

• DOI: 10.5287/ora-n6raqzn9r
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English