Field Theories of Active Matter

I am a theoretical physicist specializing in emergent behavior in interacting many-particle systems. My work draws inspiration from self-organization and complex dynamics that arise in biological systems and in actively driven materials. In particular, I am interested in pattern formation arising in active mixtures. It is now well established that active mixtures evolve into dynamical states with signatures of broken detailed balance that are distinct from single-component systems. A key driver of this dynamical diversity is non-reciprocity, where effective interactions violate action-reaction symmetry. My research aims to explore and classify the large-scale structures that emerge in both scalar and polar active matter due to this key ingredient. In recent years, my collaborators and I have investigated the role of fluctuations, multi-specificity, and hydrodynamics in such non-reciprocal systems. 

Additionally, we find that non-reciprocity plays an important role in determining the manner in which chirality manifests itself in collective dynamics. Chirality, whether intrinsic to active particles or emerging due to non-reciprocity, presents features that are unique in systems that are far from equilibrium. We have explored how chiral effects comes alive in the presence of non-reciprocity and its role in multi-species systems. Recently, we have become interested in the role of memory in living systems. Collections of particles where the past history of evolution influences present behaviour belongs in the general class of non-Hermitian systems.

 

Memory as activity

We explore the concept of memory in scalar active matter, focusing on the collective dynamics of particles whose interactions depend on their evolutionary history rather than solely on their current configuration. At the coarse-grained level, the dynamics of the number density of these active particles receives contributions from two free energies whose effects are not additive due to the incorporation of memory. We demonstrate the pattern-forming ability of the model using time-delayed interactions with one or more characteristic times, showing the emergence of spirals, travelling waves with defects, and irregular chaotic behaviour in the dynamics of a single conserved scalar field.

Chirality and non-reciprocity
 

Mixtures of chiral active particles provide a minimal setting where nonreciprocity and chirality intertwine to shape collective dynamics. Nonreciprocal interactions can generate and transfer chirality across scales even in theories formulated solely in terms of scalar number densities. At the coarse-grained level, chirality manifests as localized domains (called charges) with a finite curl of the interspecies nonreciprocal current. These chiral charges actively redistribute particles, strongly modifying the underlying nonreciprocal dynamics. Their interplay gives rise to distinct nonequilibrium steady states, including phase separation with persistent edge currents and a spatiotemporally disordered phase combining chiral and nonreciprocal signatures. Our results establish nonreciprocity as a generic mechanism for generating chiral dynamics in scalar mixtures.