Title and Abstract

Talks :

Venkata Sathish Akella

The Narrow Escape Problem of a Chiral Active Particle (CAP): An Optimal Scheme

"We report a simulation study on the narrow escape kinetics of a Chiral Active Particle (CAP) confined to a circular domain with a narrow escape opening. The study’s main objective is to optimize the CAP’s escape chances as a function of the relevant parameters, such as translational and rotational speeds of the CAP, domain size, etc. We identified three regimes in the escape kinetics namely the noise-dominated regime, the optimal regime, and the chiral activity-dominated regime. In particular, the optimal regime is characterized by an escape scheme that involves a direct passage to the domain boundary at first and then a unidirectional drift along the boundary towards the exit. Furthermore, we propose a non-dimensionalization approach to optimize the escape performance across microorganisms with varying motile characteristics. Additionally, we explore the influence of the translational and rotational noise on the CAP’s escape kinetics.”

Shahsi Thutupalli



Abhik Basu

Mobility-induced order in active XY spins on a substrate


Madan Rao

Mechanically excitable active epithelia

Living tissues exhibit mechanical and shape deformations that often manifest as sustained flows or excitable waves/pulses arising from an interplay between active cell autonomous stresses, cell shape deformations, biochemical signalling and tissue geometry. A vivid example of this is during the morphogenesis of the posterior endoderm in the Drosophila embryo. We present a covariant active hydrodynamic description of a thick deformable epithelial monolayer of cells, that maintains individual cellular identity through the constraints of cell impermeability and incompressibility. We show that cell-autonomous myosin-dependent contractile stresses can activate myosin in neighbouring cells via mechano-chemical signaling involving the Rho kinase (ROCK) pathway, giving rise to an effective ``advection-diffusion'' in the dynamics of myosin density. This leads to a curvature gradient induced polarized movement and compression of the tissue when presented with an asymmetric profile of myosin, consistent with the polarized flows observed during early Drosophila morphogenesis. A linear stability analysis reveals the existence of travelling wave solutions, whose speed is set by contractility. The interplay between contractility and elasticity (and ``diffusion'' coming from biochemical signalling), gives rise to travelling pulses -- we provide a systematic analysis of its nucleation, movement and shape, including a boundary layer analysis establishing the existence of homoclinic orbits. Apart from capturing the essential physics of the posterior midgut invagination, our study provides a general framework to analyse mechanical excitability in a variety of flat and curved epithelia.

Urna Basu

Activity reservoirs and energy transport in one-dimension

The transport properties of an extended system driven by active reservoirs is an issue of paramount importance, which remains virtually unexplored. We address this issue in the context of energy transport between two active reservoirs connected by a chain of harmonic oscillators. The active reservoirs are modeled by a harmonic chain of overdamped run-and-tumble particles, the tumbling time-scales being a measure of the activity of the reservoir. These reservoirs satisfy a modified fluctuation-dissipation relation, which we illustrate by exactly computing the effective noise and dissipation kernels. We study the energy transport through a chain of harmonic oscillators driven by two active reservoirs of different activities and show that the stationary energy current shows remarkable features like negative differential conductivity and a non-trivial direction reversal. We also find nontrivial spatiotemporal velocity correlations in the stationary state, which distinguish this activity-driven nonequilibrium state from the usual thermally driven systems.

Pinaki Chaudhuri

Active breathing: a micro-meso analysis


Raghunath Chelakkot

Dynamic patterns due to follower activity in particle chains.

One striking feature of active matter is the emergence of complex, collective patterns from simple rules at the microscopic level. It has been shown that connected chains of self-propelled particles, one such simple rule, give rise to follower forces, leading to various dynamical states. Here, we show various such dynamical patterns observed in active chains under various external constraints.

Vijayakumar Chikkadi

Nonequilibrium assembly of colloidal particles in active liquids

The growing interest in the non-equilibrium assembly of colloidal particles in active liquids is driven by the motivation to create novel structures endowed with tunable properties, unattainable within the confines of equilibrium systems. Here, we present an experimental investigation of the assembly of passive colloids dispersed in active liquids of E. coli that are conducted near a solid-liquid interface. The interactions between chiral swimmers and the colloids result in the formation of colloidal clusters with persistent rotations. These clusters grow by increasing the density of colloids and eventually percolating the system. The investigation of cluster size distribution, average cluster size, and correlation length and their associated exponents in the vicinity of percolation density reveals features reminiscent of random percolation transition observed in two-dimensional equilibrium systems. We highlight the role of the chiral swimmers on the nature of the percolation transition, offering a fresh perspective on the non-equilibrium structures of colloidal assemblies in active liquids.

Vijayakumar Krishnamurthy

Physics of asymmetric cytokinetic ingression


Subir K. Das

Kinetics of Phase Transitions in Systems of Aligning Active Particles

Problems associated with phase transitions and universality have been receiving much attention from the active matter community. In this talk, I will discuss a few such issues related to the kinetics of phase transitions, viz., domain growth and aging phenomena, in systems having aligning interactions. In addition to showing the scaling picture in the thermodynamically large system limit, I will also present results on the finite-size scaling. For active matters, the latter has specific importance, by considering the fact that the systems are typically finite in size. Towards the end, I will discuss how the considered interaction may lead to the (by now) well-known Mpemba effect that has become a topic of much general interest in recent times.

Anupam Kundu

Active dynamics at microscopic level

Active particles perform self propulsion. They exhibit interesting phenomena dissimilar to those shown by passive particles. While at the collective level they display exotic phenomena, at the microscopic level also they exhibit interesting features. In this talk I will describe some of our recent results concerning microscopic dynamics of many active particles in single file configuration, which in some cases involve interesting crossover and universal features.

Shradha Mishra

Micro-swimmers Unveil Novel Dynamical Phase in Active Nematics

Active nematics has shown interesting dynamical and steady state properties: large-scale dynamic structures and collective flows, multi-spatial temporal dynamics etc. for pure active nematics. Whereas it also exhibits interesting dynamical and steady state features in the presence of external agents. Here, we focus on the properties of active nematics when the foreign agents are polar in nature. The example of such systems can be visualised as a collection of active apolar cells, where microswimmers are present as impurities. In this work we performed an extensive numerical study and showed that by varying the motility of microswimmers the background active nematics show an attractive dynamical state. Where the system evolves to a new dynamical rotating phase with the presence of vortices on the macroscopic scales, this phase appears at the cost of globally ordered active nematics. The motility of the microswmmers is responsible for such phase and this phase appears for intermediate range of motility, where microswimmers start to develop local clusters and coherent motion. Whereas the vortex state weakens when clustering of microswimmer increases for large motility. Hence our study provides a new interesting dynamical phase in active nematics, which can be completely tuned by controlling the motility of microswimmers and can be used for differentiating a variety of pathogens in apolar cells.

Pradeep Kumar Mohanty

Interactive active matter

Motile particles with excluded volume interaction exhibit motility induced phase separation transition. We find that attaractive interaction does not help rather hinder the cluster formation process - we explore this on a lattice model of hardcore run and tumble particles.

Sudipto Muhuri

Effect of contact inhibition locomotion on confined cellular organization

Experiments performed using micro-patterned one dimensional collision assays have allowed a precise quantitative analysis of the collective manifestation of contact inhibition locomotion (CIL) wherein, individual migrating cells reorient their direction of motion when they come in contact with other cells. Inspired by these experiments, we present a discrete, minimal 1D Active spin model that mimics the CIL interaction between cells in one dimensional channels. We analyze the emergent collective behaviour of migrating cells in such confined geometries, as well as the sensitivity of the emergent patterns to driving forces that couple to cell motion.

Amitabha Nandi

Kinesin-mediated axonal transport of vesicles


Arnab Pal

Anomalous relaxation in active systems


Raja Paul

Flocking transition in discretized Vicsek model

We present a discretized Vicsek model (DVM) with self-propelled particles orienting in q possible directions in two dimensions. The DVM is an off-lattice flocking model similar to the active clock model yet the dynamical rules of particle alignment and movement are formulated according to the Vicsek model. The DVM shows a transition from macrophase to microphase separation of the coexistence region as q is increased at a moderate noise strength. At small q and noise strength, a configuration of multiple clusters with different polarization is observed which does not exhibit any long-range order. The ordered liquid phase appears at small noise and relatively large q is found to be metastable subjected to a perturbation. Overall the study summarizes that the dynamical rules have a profound influence on the broad features of the flocking phase.

Subhajit Paul

Dynamical Crossovers and Correlations in a Linear Chain of Active Particles

We explore the dynamics of a tracer in an active particle harmonic chain, investigating the influence of interactions. Our analysis involves calculating mean-squared displacements (MSD) and space-time correlations through Green's function techniques and numerical simulations. Depending on chain characteristics, i.e., different time scales determined by interaction stiffness and persistence of activity, tagged-particle MSD exhibit ballistic, diffusive, and single-file diffusion (SFD) scaling over time, with crossovers explained by our analytic expressions. Our results reveal transitions in bulk particle displacement distributions from an early-time bimodal to late-time Gaussian, passing through regimes of unimodal distributions with finite support and negative excess kurtosis and longer-tailed distributions with positive excess kurtosis. The distributions exhibit data collapse, aligning with ballistic, diffusive, and SFD scaling in appropriate time regimes. However, at much longer times, the distributions become Gaussian. Finally, we derive expressions for steady- state static and dynamic two-point displacement correlations, consistent with simulations and converging to equilibrium results for small persistence. Additionally, the two-time stretch correlation extends to longer separation at later times, while the autocorrelation for the bulk particle shows diffusive scaling beyond the persistence time.

Prasad Perlekar

Concentration wave turbulence in polar active matter

We present a new instability mechanism in polar active suspensions, which is driven by the complex interplay of inertia, motility, and concentration fluctuations. Using high-resolution numerical simulations, we perform a detailed exploration of the parameter space and find the presence of concentration waves. In the appropriate parameter range, we observe concentration-wave turbulence.

Punyabrata Pradhan

Time-dependent properties of run-and-tumble particle

We characterize collective transport in hardcore run-and-tumble particles (RTPs) by calculating two transport coefficients - the bulk-diffusion coefficient and the mobility - for arbitrary density and tumbling rate. We study two minimal models of RTPs: Model I is a standard version of hardcore RTPs, whereas model II is a long-ranged lattice gas (LLG) with hardcore exclusion - an analytically tractable variant of model I. We calculate the density- and tumbling-rate-dependent transport coefficients analytically for model II and numerically for model I through an efficient Monte Carlo algorithm; notably, both models have qualitatively similar features. In the strong-persistence regime, the fascinating interplay between persistence and interaction is quantified in terms of two length scales: Persistence length and a “mean free path”, which is simply a measure of the average gap (empty stretch) size in the hopping direction. Notably, for finite density and tumbling rate, the transport coefficients for RTPs do not have any singularity.

Pramod Pullarkat

Actin dynamics and force generation in membrane nanotubes

This talk will cover experiments on active responses of membrane nano-tubes drawn out of axons using optical tweezers. The tubes ehibit contractile load and fail cycles which require invation of the tube by dynamic actin filaments. We show that the contractility is independent of myosin-II activity, and possibly that of other myosins as well. A new mechanism is proposed for this active force generation process and a simple theoretical model will also be discussed.

Sabyasachi Rakshit

Dynamics of autonomous chiral active GUVs


Kabir Ramola

Current Fluctuations in Interacting and Non-interacting Active Particle Systems

We study the fluctuations of the integrated density current across the origin up to time $T$ in one dimensional systems of non-interacting as well as interacting active particles. For non-interacting particles, we focus on the case of zero diffusion and study the differences between annealed and quenched initial conditions. We show that for the case of particles initiated with an initial bias in the positive direction, the fluctuations of the current at short times display a surprising difference: $T$ versus $T^2$ behaviours respectively, with a $\sqrt{T}$ behaviour emerging at large times. For the interacting case, we explore a lattice model of active particles with hard-core interactions that is amenable to an exact description within a fluctuating hydrodynamics framework. For the case of uniform initial profiles, we show that the second cumulant of the integrated current displays three regimes: an initial $\sqrt{T}$ rise with a coefficient given by the symmetric simple exclusion process, a cross-over regime where the effects of activity increase the fluctuations, and a large time $\sqrt{T}$ behavior. In the limit of zero diffusion for the interacting system, we show that the fluctuations once again exhibit a $T^2$ behavior at short times. Finally, we show that the results for non-interacting active particles are recovered for low densities.

Arnab Saha

Microscopic Heat Engines: From Passive to Active

Developing efficient and powerful artificial machines at micro- and nano- scales is a challenging task. The challenge is primarily posed by ambient fluctuations. The performance of such machines depends crucially on the fluctuations. However, naturally available biological machines operating at small scales can perform complex tasks (such as transporting cargo inside cells) quite efficiently. This motivates the study of such systems under the framework of stochastic thermodynamics. In this talk, some of the recent results in this topic will be discussed focussing on microscopic heat engines with thermal as well as athermal (active) fluctuations.

Anirban Sain

Active chiral flows during cell division

Material flow in the acto-myosin cortex of a cell, during cell division, has been found to be chiral in nature. It has been attributed to active chiral torques generated in the actomyosin cortex. Here we explore possible signature of such chirality during the growth of the intra-cellular membrane partition which physically divides the cell into two compartments.

Ramanujam Srinivasan

Assembly of bacterial cytokinetic ring proceeds via a helical intermediate and is driven by treadmiling and lateral association of filaments

Cytokinesis is the crucial step of physically partitioning the daughter cells after the duplication and segregation of the genetic material. Bacteria assemble a cytokinetic ring called a Z-ring formed the tubulin homologue, FtsZ. The precise mechanism by which the ring is assembled remains to be completely understood. Traditional mutational approaches have yielded a deep but is limited by the lethality of the FtsZ mutations in bacteria. We had earlier used a eukaryotic heterologous host Schizosaccharomyces pombe, to study FtsZ ring assembly and showed that the ring assembles by spooling a linear filament. We have further utilized the system to screen for mutants defective in Z-ring assembly and identified several mutants. Specifically, two mutants that are trapped in a helical conformation provide insights into the Z-ring assembly process. Using a combination of cell biological, biochemical and genetic approaches, we show that the mutants trapped in helical conformations are defective in nucleotide hydrolysis and are defective in lateral association of filaments and ring compaction. Together with physical modelling of ring assembly using a coarse-grained theoretical model of a treadmilling FtsZ filament, our results suggest that Z-ring assembly is driven by torsional stress coupled to treadmilling and lateral association among filaments.

Sumesh Thampi

Spontaneous flow transitions in confined active nematics

Active nematics is a model fluid that captures the hydrodynamic behaviour of several active systems such as bacterial suspensions, vibrated granular rods and mixture of motor proteins and cytoskeletal elements. When confined in a channel, active nematics exhibit spontaneous flow transitions. The flow transition occurs beyond a critical activity or at sufficiently large width of the channel such that the activity in the system can overcome the elastic and viscous forces that arises from confinement as illustrated in experiments, theory and computer simulations. This picture provides a basic understanding of the behaviour of active fluid flows and explains the origin of interesting flow patterns such as dancing flows and active turbulence. In several biological systems, the confinement may be corrugated and need not be rigid. This talk describes the investigation of effect of undulated surfaces and elastic confinement on spontaneous flow transitions exhibited by active nematics.

Fernando Peruani

Challenges in Active Matter

Our current theoretical understanding of active matter is based on two paradigmatic mechanisms. (i) The so-called velocity alignment mechanism that exploits the analogy to spin systems to explain self-organized patterns in active systems and constitutes the cornerstone of Vectorial Active Matter. And (ii), motility-induced phase separation, a central element in Scalar Active Matter, that makes use of the analogy between classical and active phase separation by assuming an effective coupling between local density and local particle speed. It is worth noting that these two mechanisms are the results of theoretical speculations -- exploiting analogies with classical physical models -- formulated before the realization of specifically designed, quantitative, active-matter experiments. Several fundamental questions need to be addressed. Are these mechanisms robust? What are the limitations of descriptions based on these concepts? And finally, are there alternative mechanisms that produce similar macroscopic patterns? In this talk, we will visit the foundation of Active Matter Theory and address these issues.

P. B. Sunil Kumar

Vesicle morphology changes through active membrane recycling

Polymers composed of active structural units, capable of converting chemical energy to mechanical work, are abundant in living organisms and can also be synthesized in the laboratory. Understanding how fluid flows affect the dynamics and deformations of these active filaments is crucial for comprehending various biological processes. In this presentation, I will discuss the emergent dynamics exhibited by a system of active flexible filaments and their response to external flow. Firstly, I will delve into the interplay between the filament's activity, flexibility, and hydrodynamic interactions, which gives rise to a multitude of novel dynamic behaviours. Notably, we observe the formation of knots and links, nonequilibrium transitions from isotropic to nematic phases, as well as the emergence of motile defects. With the aim to understand the dynamics of such active filaments in the crowded environment of a biological cell, we have developed a dry active polymer model that retains the essential characteristics of its wet counterpart. In the second part of this talk we will explore the dynamics of such an active filament in the presence of a shear flow, uncovering additional complexities in the system.

Saroj Kumar Nandi

The glassy dynamics in confluent epithelial monolayers

The glassy properties of confluent epithelial monolayers are crucial for several biological processes, such as wound healing, embryogenesis, cancer progression, etc. These systems also extend the scope and extent of the as-yet mysterious physics of glass transition. In recent discoveries, we found that the confluent systems have an unusual glassy dynamics exhibiting both sub- and super-Arrhenius relaxation. Furthermore, the static and dynamic properties strongly correlate in the sub-Arrhenius regime, which seems to be particularly suited for the much-celebrated mode-coupling theory of glass. In this talk, I will discuss the glassy properties of these systems and argue that the results are promising for a deeper understanding of the mechanism of glassy dynamics.

Posters :

Rajeev Kapri

Unzipping of DNA in an active environment


Chamkor Singh

Dynamics of passive droplet in active turbulence


Joydeep Das

Nonlinearity-induced pattern formation in active fluids

Pattern formation is ubiquitous in nature. It is available everywhere from ripples of sands, hydrodynamical systems, chemical systems, biological systems, combustion theory and even in the structure of galaxies. In this work, I will be discussing the emergence of patterns in an active fluid medium and the appearance of various types of spatiotemporal structures like Chimera states.

Subhashree Subhrasmita Khuntia

Exploring auto-chemotactic random walk: Single and collective behavior

Active matter, comprises systems that are out of equilibrium driven at individual scale by their internal energy or their local environment. Auto-chemotaxis represent one such active process where each individual modifies the chemical field in their locality and this further drives the motion of the system. We studied the trajectory of a single particle where the dynamics is governed by its chemical environment and vice versa. The phase diagram in the deposition and the evaporation rate of the chemical shows a structural transition from an extended coil state to a collapsed globule state. We developed a mean field theory following Keller-Segel model for chemotaxis and this shows good agreement with the simulation results. Further we incorporate many particles in our system and observe emergent phenomena evolving from the indirect interaction among them via their local chemical environment.

Rajneesh Kumar



Subhransu Sekhar Mishra

Flocking in Active Matter


Manish Patel

Re-entrant transitions in inertial dynamics of active Brownian particles

The poster investigates the inertial effect on the dynamics of active Brownian particles (ABP) and their long-time behavior. While numerous studies have explored the over damped dynamics of ABP, presuming that the late-time behaviors would be independent of inertia, this study of inertial ABP challenges that view. Our theoretical approach allows us to write the precise time evolution of any dynamics variable in arbitrary d-dimensions. The moment's calculation allows one to write the observables such as diffusivity, kinetic temperature, and pressure. While diffusivity was found to be independent of inertia, the kinetic temperature and pressure highly depend on inertia, even in the asymptotic limit. The steady-state velocity distribution shows a re- entrant crossover from 'passive' Gaussian to 'active' non-Gaussian as a function of inertia and activity.

Anweshika Pattanayak

Exact computation of moments for chiral active particles

Chiral active matter has been a very fertile ground of research in the world of biology and physics since the last few decades. In addition to the self-propulsion, these particles are acted upon by a torque that give rise to a circular trajectory in 2-d and a helical trajectory in 3-d. We can explore a huge number of examples in the nature of such particles: from E coli bacteria to spermatozoa. In vitro experiments, performed with the artificial models of chiral active particles, also produced fascinating results. Although chiral active particles have been the subject of scientific studies for a long time now, in most of the theoretical studies, the focus is mostly on the 2-d chiral active particles, while their 3-d counterparts are mostly ignored. We look for the analytical expressions of the exact time-dependent moments associated with the motion exhibited by a single chiral active Brownian particle both in 2-d and 3-d and verify them with the result obtained by simulating the Langevin dynamics of the particle.

Sandip Roy

Role of defects in an in-silico motility assay set-up

In our study, we construct and analyze a computational model to describe the dynamics of a single passive semiflexible polymer as it glides on a bed of motor proteins, in the presence of "motility defects''. The system consists of an extensible semiflexible filament moving in two dimensions in a motility assay of motor proteins explicitly represented as active harmonic linkers. Their heads bind stochastically to polymer segments within a capture radius and extend along the filament in a directed fashion before detaching. Both extension and detachment rates are load-dependent and generate an active drive on the filament. In this work, we demonstrate that "motility defects'' that impede the motion of the filament give rise to varied dynamics of the filament. While a line defect generates periodic flagellar motion, a point defect results in spiral conformations of the polymer. We show explicitly how the attachment/detachment dynamics and active velocity of the motor proteins affect the polymer dynamics in the presence of these defects.

Arpan Sinha

How does reciprocity impact ordering and phase separation in active nematics?

Active nematics undergo spontaneous symmetry breaking and show phase separation instability. Within the prevailing notion that macroscopic properties depend only on symmetries and conservation laws, different microscopic models are used out of convenience. Here, we test this notion carefully by analyzing three different microscopic models of apolar active nematics. They share the same symmetry but differ in implementing reciprocal or non-reciprocal interactions, including a Vicsek-like implementation. We show how such subtle differences in microscopic realization determine if the ordering transition is continuous or first order. Despite the difference in the type of phase transition, all three models exhibit fluctuation-dominated phase separation and quasi-long-range order in the nematic phase.

Tanmoy Chakraborty

Anomalous collective diffusion of interacting run-and-tumble particles

We characterize the collective motion of interacting run-and-tumble particles (RTPs) by calculating the bulk-diffusion coefficient in two minimal model systems, for arbitrary density ρ and tumbling rate γ, and offer a generic mechanism to account for the early-time anomalous relaxations. In the strong-persistence limit of γ → 0, the fascinating interplay between persistence and interaction is quantified in terms of two length scales: “mean free path” and persistence length lp = v/γ, with v being the self-propulsion speed. Indeed, we show that the bulk-diffusion coefficient has a scaling form D(ρ, γ) =D_0 F (ρv/γ), where D_0 is proportional to the diffusivity of noninteracting particles; the scaling function F (ψ) is calculated analytically for one model and numerically for the other. In this limit, we find that the bulk-diffusion coefficient varies as a power law in a wide range of densities: D ∝ ρ−α, with the exponent α gradually crossing over from α = 2 at high densities to α = 0 at low densities. As a result, the density relaxation is governed by a nonlinear diffusion equation with anomalous spatiotemporal scaling that can be ballistic in certain regimes. Our arguments, as demonstrated in simulations, are rather generic, being independent of dimensions and microscopic details.

Shubhendu Shekhar Khali

Dynamics of Active Scalar Baths: Unveiling Equilibrium-Like Properties in the Presence of Inertial Mass

In our study, we considered a homogeneous fluid of interacting active Brownian particles (ABPs) and ask the question to what extent this fluid can be considered as an active scalar bath analogous to its equilibrium counterpart governed by the Einstein relation. We investigated how the interplay between activity (Pe) and inertial mass influences the non- equilibrium behavior of the system in terms of velocity distribution, kinetic temperature, diffusivity, and mobility. As the activity (Pe) increases, the departure from equilibrium becomes more pronounced. However, with increased inertial mass, the system shows a crossover to equilibrium-like properties. This includes the transition of non-Gaussian velocity distributions, fore-aft asymmetry in pair distributions, and the restoration of the Einstein fluctuation-dissipation relation.

Sudipta Pattanayak

Cluster formation, order, and aggregation in Active Matter: different classes of active phase separation

In active systems, phase separation can be induced by a variety of mechanisms. In the Vicsek model, velocity alignment mediates the formation of clusters and phase separation in the form of polar bands. In self-propelled disk (SPD) systems, repulsive interactions lead to what has been called as motility-induced phase separation. For self-propelled rods, torques and forces lead to a phase separation process distinct from the one observed in the Vicsek model or SPD. Here, combining simulations and mean-field calculations, we demonstrate how the interplay of attraction and velocity-alignment interaction leads to a variety of phase separation processes and unveils two novel types of condensation classes.

Amir Shee

Collective States in Dense Chiral Active Systems

Across scales, from molecules to tissues, dense biological systems can exhibit collective dynamics, such as flocking and other forms of spatiotemporal order, driven by the interplay between activity and elasticity. In this talk, I will describe how the intrinsic individual chirality of densely packed active Brownian agents can lead to the emergence of a variety of states, including collective rotating mesoscopic order. I will present two analytical approaches (one based on normal modes and the other on continuum elasticity) that provide a comprehensive understanding of the states observed in the systems and match very well our simulations. Our findings suggest that collective rotating states may generically appear in natural and artificial dense chiral active systems.

Keywords: Chirality, Mesoscopic-range Order, Dense Active Systems. This project was made possible through the support of Grant 62213 from the John Templeton Foundation.

Amit Kumar

Size, shape, and fluctuations of condensates in non-equilibrium liquid-liquid phase separation

Equilibrium phase separation, in the absence of chemical reactions, leads at long times, to a condensate of system size due to the interfacial tension of smaller-sized domains. In contrast, additional long-range (Coulomb) interaction competing with interfacial tension are known to stabilize the condensate size at intermediate length scales. Examples of such tension long-range interacting (TLR) systems are – Rayleigh instability of charged liquids, block copolymer melts, binary solvent with antagonistic salt, and biomolecular condensates. In the later case, the chemical reactions (production and degradation of proteins, RNA molecule, etc.) involve an input of energy (activity) and are coupled to the equilibrium aspects of phase separation. In such non-equilibrium phase separation, the slow chemical kinetics of the constituents act antagonistically to fast molecular diffusion (Ostwald ripening) and leads to a non-equilibrium steady state. For first-order chemical kinetics, the non-equilibrium term maps to a Coulomb interaction in the effective free energy. In the mean-field limit, for infinite periodic systems, the transition between various morphologies (sphere, cylinder, lamellar, etc.) depends on the relative concentrations of the solute-rich and the solute-poor domains. An important finding of our theory is that for finite (but very large) systems with lamellar microstructure, the sample aspect ratio enter the system free energy and the steady-state domain size. While the lamellar phase is locally stable, its restoring force to undulations is related to the curvature of the undulations and does not depend on the extra area of the layer (effectively zero tension) for needle-like and periodic systems.

Purnendu Pathak

Mpemba-like Effects during Phase Transitions in an Active Matter System

Mpemba Effect (ME) is a counterintuitive fact stating that a hotter sample of water freezes faster than a colder one, when quenched to a subzero temperature. There has been a surge in interest in the study of ME. Similar effects have been observed in other systems like granular fluids, spin glasses, colloidal systems, pure ferromagnets and active Brownian systems. We investigate its presence in an aligning active matter system. The system we consider is the well-known Vicsek model that exhibits clustering in density field as well as an order-disorder transition in the velocity field, following quenches from states of high noise strengths, having random orientations of the velocity vectors, to a state point that corresponds to a low enough noise strength. We obtain compelling evidence for the presence of ME in such a simple system. We also demonstrate, how the spatial correlation in the initial states influences the effect.

Debayan Jana

Logarithmic or algebraic: roughening of an active Kardar-Parisi-Zhang surface

The Kardar-Parisi-Zhang (KPZ) equation sets the universality class for growing and roughening of nonequilibrium surfaces without any conservation law and nonlocal effects. We argue here that the KPZ equation can be generalised by including a symmetry- permitted nonlocal nonlinear term of active origin that is of the same order as the one included in the KPZ equation. Including this term, the 2D active KPZ equation is stable in some parameter regimes, in which the interface conformation fluctuations exhibit sub- logarithmic or super-logarithmic roughness, with nonuniversal exponents, giving positional generalised quasi-long-ranged order. For other parameter choices, the model is unstable, suggesting a perturbatively inaccessible algebraically rough interface or positional short- ranged order. Our model should serve as a paradigmatic nonlocal growth equation.

Sourav Pal

Availability, storage capacity, and diffusion: Stationary states of an asymmetric exclusion process connected to two reservoirs

We explore how the interplay of finite availability, carrying capacity of particles at different parts of a spatially extended system and particle diffusion between them control the steady state currents and density profiles in a one-dimensional current- carrying channel connecting the different parts of the system. To study this, we construct a minimal model consisting of two particle reservoirs of finite carrying capacities connected by a totally asymmetric simple exclusion process (TASEP). In addition to particle transport via TASEP between the reservoirs, the latter can also directly exchange particles via Langmuir kinetics-like processes, modeling particle diffusion between them that can maintain a steady current in the system. We investigate the steady state density profiles and the associated particle currents in the TASEP lane. The resulting phases and the phase diagrams are quite different from an open TASEP, and are characterised by the model parameters defining particle exchanges between the TASEP and the reservoirs, direct particle exchanges between the reservoirs, and the filling fraction of the particles that determines the total resources available. These parameters can be tuned to make the density on the TASEP lane globally uniform or piecewise continuous, and can make the two reservoirs preferentially populated or depopulated.

Vijit Ganguly

Actin driven collective vesicle transport through endocytosis

During endocytocis in the cell, vesicles are internalised at the cell membrane and transported actively towards the cell interior. We develop a hydrodynamic active gel theory to understand this collective inward flow of vesicles. We predict how the vesicle velocity field may depend on a steady state radial distribution of polymerized actin in the cell.

Partha Sarathi Mondal

Microswimmers produce dynamical phase in Active Nematics

Active nematics has shown interesting dynamical and steady state properties: large-scale dynamic structures and collective flows, multi-spatial temporal dynamics etc. for pure active nematics. Whereas it also exhibits interesting dynamical and steady state features in the presence of external agents \cite{}. Here, we focus on the properties of active nematics when the foreign agents are polar in nature. The example of such systems can be visualised as collection of active apolar cells, where microswimmers are present as impurities. In this work we performed the extensive numerical study and showed that by varying the motility of microswimmers the background active nematics show an attractive dynamical state. Where system evolves to a new dynamical phase with the presence of swirl type structures on the macroscopic scales, this phase appears at the cost of globally ordered active nematics. The motility of the microswmmers is responsible for such phase and this phase appears for intermediate range of motility, where microswimmers starts to develop local clusters and coherent motion. Whereas the swirled structures weakens when clustering of microswimmer increases for large motility. We also proposed that the presence of microswimmers clusters amplify the bending instability is active nematics and responsible for such dynamical phase. The dynamics of such phase is solely tuned by the controlling the motility of microswimmers and can be used for differentiating variety of pathogens in apolar cells.

Pratikshya Jena

Polarised crowd in motion: Insights into statistical and dynamical behavior

Collection of active self-propelling agents have tendency to show interesting statistical and dynamical properties. Such chracteristics are more significant in the context of human crowd. In the present study, we develop a minimal computational model to mimic the crowd in a marathon race. We aim to examine the influence of frontliners on crowd dynamics by comparing the simulated races with and without their presence. The primary outcome of our study revealed that the local velocity and density of the participants exhibit a wave pattern similar to what is observed in actual races. The traveling wave in the crowd consistently propagates with a constant speed, irrespective of the system size under consideration. The dynamic of participants in the longitudinal direction mainly contributes to the velocity fluctuation and the fluctuation in the transverse direction is suppressed. In the absence of frontliners, the fluctuations in density and velocity weakens without significantly influencing the other statistical and dynamical characteristics of the crowd. It is also observed that the density wave travels faster than the velocity wave. Through this work, we aim to enhance our understanding of crowd motion, which can inform the development of effective crowd management strategies and contribute to the successful control of such events.

Puneet Pareek

Re-entrance of Relaxation Dynamics in Dense Assemblies of Persistent Particles

Dense glassy active matter is similar to its passive counterpart for less persistence time, but as persistence time increases, the relaxation dynamics show non-monotonic behavior in which the relaxation first decreases and then increases. The poster will try to unravel the interesting physics behind this phenomenon.

Rasmita Sahoo

Bacterial biofilms: Shear mediated structural ordering and dynamics

Bacterial biofilms are colonies of living bacteria within an extracellular matrix composed mainly of polysaccharides and proteins secreted by the cells themselves. This structure protects the bacteria from outside chemical attack making them drug resistant and hence responsible for recurrent bacterial infections in the host organisms. On the other hand, they are also very useful in water treatment, waste sequestration and are ubiquitous in environmental ecology. Although there have been extensive studies of biofilms from a biochemical perspective, so far, there is very little understanding of their physical structure, dynamics and mechanical properties. Study of physical properties of biofilms is crucial for devising methods for tailoring their structure and functions. In the current project, we intend to use concepts and techniques from soft matter physics treating the biofilm as a complex fluid in which the bacteria are modeled as soft colloids dispersed in a polymeric matrix. We intend to elucidate the role of different interactions between bacteria mediated by polymeric substances in pre-biofilm and biofilm states. Using rheology and rheo-microscopy as the tools I am studying their dynamics and phase behavior under shear from a diluted system to the concentrated phase.