Prof Ramin Golestanian

Abstract:

In the context of self-assembly, where complex structures can be assembled from smaller units, it is desirable to devise strategies toward disassembly and reassembly processes that reuse the constituent parts. A non-reciprocal multifarious self-organization strategy has been recently introduced and shown to have the capacity to exhibit this complex property. In this work, we study the model using continuous-time Gillespie simulations and compare the results against discrete-time Monte Carlo simulations investigated previously. Furthermore, using the continuous-time simulations, we explore important features in our system, namely, the nucleation time and interface growth velocity, which comprise the timescale of shape-shifting. We develop analytical calculations for the associated timescales and compare the results to those measured in simulations, allowing us to pin down the key mechanisms behind the observed timescales at different parameter values.

Abstract:

The collective chasing dynamics of nonreciprocally coupled densities leads to stable traveling waves which can be mapped to a model for emergent flocking. In this Letter, we couple the nonreciprocal Cahn-Hilliard model to a fluid to minimally describe scalar active mixtures in a suspension, with the aim to explore the stability of the waves, i.e., the emergent flock in the presence of self-generated fluid flows. We show that the emergent polarity is linearly unstable to perturbations for a specific sign of the active stress recalling instabilities of orientational order in a fluid. Using numerical simulations, we find, however, that nonreciprocity stabilizes the waves against the linear instability in a large region of the phase space.

Abstract:

Breakdown of time-reversal symmetry is a defining property of nonequilibrium systems, such as active matter, which is composed of units that consume energy. We employ a formalism that allows us to derive a class of identities associated with the time-reversal transformation in nonequilibrium field theories, in the spirit of Ward-Takahashi identities. We present a generalization of the fluctuation dissipation theorem valid for active systems as a particular realization of such an identity, and consider its implications and applications for a range of active field theories. The field theoretical toolbox developed here helps to quantify the degree of nonequilibrium activity of complex systems exhibiting collective behavior.

Abstract:

Recent results demonstrate how deviations from equilibrium fluctuation–dissipation theorem can be quantified for active field theories by deriving exact fluctuations dissipation relations that involve the entropy production [M. K. Johnsrud and R. Golestanian, ]. Here we develop and employ diagrammatic tools to perform perturbative calculations for a paradigmatic active field theory, the nonreciprocal Cahn-Hilliard (NRCH) model. We obtain analytical results, which serve as an illustration of how to implement the recently developed framework to active field theories, and help to illuminate the specific nonequilibrium characteristics of the NRCH field theory.

Abstract:

Non-reciprocal interactions between scalar fields that represent the concentrations of two active species are known to break the parity and time-reversal (PT) symmetries of the equilibrium state, as manifested in the emergence of travelling waves. We explore the notion of nonlinear non-reciprocity and consider a model in which the non-reciprocal interactions can depend on the local values of the scalar fields in such a way that the non-reciprocity can change sign. For generic cases where such couplings exist, we observe the emergence of spatiotemporal chaos in the steady-state. We associate this chaotic behaviour with a local restoration of PT symmetry in fluctuating spatial domains, which leads to the coexistence of oscillating densities and phase-separated droplets that are spontaneously created and annihilated. We uncover that this phenomenon, which we denote as effervescence, can exist as a dynamical steady-state in large parts of the parameter space in two different incarnations, as characterised by the presence or absence of an accompanying travelling wave.