Talks

Microbial communities exhibit complex behaviors driven by species interactions and individual characteristics. In this study, we delve into the dynamics of a mixed bacterial population comprising two distinct species with different morphology and motility aspects. Employing agent-based modeling and computer simulations, we analyze the impacts of size ratios and packing fractions on dispersal patterns, aggregate formation, clustering, and spatial ordering. Notably, we find that motility and anisotropy of elongated bacteria significantly influence the distribution and spatial organization of nonmotile spherical species. Passive spherical cells display superdiffusive behavior, particularly at larger size ratios in the ballistic regime. As the size ratio increases, clustering of passive cells is observed, accompanied by enhanced alignment and closer packing of active cells in the presence of higher passive cell area fractions. Additionally, we identify the pivotal role of passive cell area fraction in influencing the response of active cells toward nematicity, with its dependence on size ratio. These findings shed light on the significance of morphology and motility in shaping the collective behavior of microbial communities.

https://sites.google.com/view/pushpita-ghosh/home

Abstract

A many-body interacting system of classical kicked rotor serves as a prototypical model for studying Floquet heating dynamics. Having established the fact that this system exhibits a long-lived prethermal phase with quasi-conserved average Hamiltonian before entering into the chaotic heating regime, we use spatio-temporal fluctuation correlation of kinetic energy as a two-point observable to probe the above dynamic phases. We remarkably find the diffusive transport of fluctuation in the prethermal regime suggesting a novel underlying hydrodynamic picture in a generalized Gibbs ensemble with a definite temperature that depends on the driving parameter and the initial conditions. On the other hand, the heating regime is characterized by a diffusive growth of kinetic energy where the correlation is sharply localized around the fluctuation center for all time. Consequently, we attribute non-diffusive and non-localize structure of correlation to the crossover regime, connecting

the prethermal phase to the heating phase, where the kinetic energy displays a complicated growth structure. We understand these numerical findings using the notion of relative phase matching where prethermal phase (heating regime) refers to an effectively coupled (isolated) nature of the rotors. We exploit the statistical uncorrelated nature of the angles of the rotors in the heating regime to find the analytical form of the correlator that mimics our numerical results in a convincing way.

Ref: Phys. Rev. B 104, 075161 (2021)

https://www.bits-pilani.ac.in/hyderabad/tanay-nag/

Abstract

Active matter systems - such as a suspension of microorganisms or synthetic microswimmers - dissipate energy and maintain exciting collective dynamics. This talk will discuss three distinct experimental active matter systems. The first part of the talk will be about the emergence of rigidity from phoretic self-interactions in an active polymer. Then, I will explain the experimental phenomena of freezing colloids by heating one of them. I will also show that a frozen state can be converted to an orbiting state under suitable conditions. Finally, I will describe a method to trap microparticles outside a laser beam using phoretic and hydrodynamic interactions.

https://rajeshrinet.github.io/

Abstract

Large groups of active cilia collectively beat in a fluid medium as metachronal waves, essential for some microorganisms motility and for flow generation in mucociliary clearance. Several models can predict the emergence of metachronal waves, but what controls the properties of metachronal waves is still unclear. We investigate numerically a simple model for cilia in the presence of noise on regular lattices in one- and two-dimensions. We characterize the wave using spatial correlation and the frequency of collective beating. Our results clearly show that the viscosity of the fluid medium does not affect the wavelength; the activity of the cilia does. These numerical results are supported by a dimensional analysis, which is expected to be robust against the model for active force generation, unless surrounding fluid influences the cilia activity. This result might have significance in understanding paramecium locomotion and mucociliary clearance diseases.

https://arxiv.org/abs/2310.16770

https://scholar.google.com/citations?user=OIpe7NYAAAAJ&hl=en

Abstract

The kinetics of phase separation in segregating fluid mixtures has been extensively studied from the perspectives of physics, materials science, and engineering. Various regimes characterized by a time-dependent length scale for the domain growth have been predicted. However, a much richer physics is expected when the constituent components are not simple fluids but contain living particles that self-propel or the mixture is confined in geometrical structure. In this talk, we will discuss our recent findings on the complex demixing behavior of such systems. 

https://sites.google.com/view/softcondmat/home?authuser=0

Abstract

The outcome of a game between players, the victory or the defeat, is unpredictable. While said outcome can depend on various aspects, a player’s skill is the prominent determining factor. In any game, the skill is evaluated in terms of their ratings through a specific type of rating system. The given rating system is assumed to assign higher ratings to the better-skilled player. The Elo rating system has been widely used in games such as tennis, soccer, chess, etc. However, a theoretical and computational framework to connect the skills of players with the rating of players assigned under the Elo rating system remains unexplored. In this study, we investigate the rating dynamics of players in two-player games by combining skill-dependent outcomes of games with the Elo rating system using theoretical and computational tools of non-equilibrium statistical mechanics. We analyse the evolution of the average rating as a function of the number of games for different skills of players and find a systematic, approximate relationship between skill and rating under specific circumstances. The qualitative behaviour of the rating evolution predicted in our study is consistent with the existing data on the rating growth of chess and draught players.

https://sites.google.com/view/deepak-bhat/home

Abstract

In this work, we study a system of bosonic Rydberg atoms in an optical lattice. Here each lattice site supports a ground state and a Rydberg excited state. The atoms can spontaneously decay from the excited state to the ground state. The system is characterized by the Rydberg interaction that makes the doubly excited Rydberg state almost excluded from the system dynamics. There is an on-site interaction among different bosons occupying the ground state of the same lattice site. The dissipative dynamics of the system is probed, and the mean population differences between the ground state and the excited state is studied as a function of time. We observe that the lattice emerges as two alternating sublattices in terms of the population distribution. As the on-site interaction is varied, one can have (i) a uniform region (the population distribution is

same for both the sublattices), (ii) a non-uniform oscillatory region (the population difference between the ground state and excited state oscillates with time, and is different for the two sublattices), and (iii) a non-uniform non-oscillatory region (the population difference is fixed but has a different value for alternating sublattices). The origin of these phases are explained using nonlinear dynamics, and also by studying the spatial correlations. 

https://www.caluniv.ac.in/academic/Physics/Raka-D.pdf

Abstract

We demonstrate that stability and chaotic-transport features of paradigmatic nonequilibrium many-body systems, i.e., periodically kicked and interacting particles, can deviate significantly from the expected ones of full instability and normal chaotic diffusion for arbitrarily strong chaos, arbitrary number of particles, and different interaction cases. 

We rigorously show that under the latter general conditions there exist fully stable orbits, accelerator-mode (AM) fixed points, performing ballistic motion in momentum. It is numerically shown that an “isolated chaotic zone” (ICZ), separated from the rest of the chaotic phase space, remains localized around an AM fixed point for long times even when this point is partially stable in only a few phase-space directions and despite the fact that Kolmogorov-Arnol’d-Moser tori are not isolating. The time evolution of the mean kinetic energy of an initial ensemble containing an ICZ exhibits superdiffusion instead of normal chaotic diffusion.

https://journals.aps.org/pre/abstract/10.1103/PhysRevE.102.062120

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.105.184302

https://scholar.google.co.in/citations?hl=en&user=RJhUov4AAAAJ&view_op=list_works&sortby=pubdate

Abstract

Berry phase interference arguments that underlie the theory of deconfined quantum criticality (DQC) for spin-1/2 antiferromagnets have also been invoked to allow for continuous transitions in spin-1 magnets including a Néel to (columnar) valence bond solid (cVBS) transition. We provide a microscopic model realization of this transition on the square lattice consisting of Heisenberg exchange (JH) and biquadratic exchange (JB) that favor a Néel phase, and a designed Q-term (QB) interaction which favors a cVBS through large-scale quantum Monte Carlo (QMC) simulations. For JH=0, this model is equivalent to the SU(3) JQ model with a Néel-cVBS transition that has been argued to be DQC through QMC. Upon turning on JH which brings down the symmetry to SU(2), we find multiple signatures -- a single critical point, high quality collapse of correlation ratios and order parameters, "U(1)-symmetric" cVBS histograms and lack of double-peak in order parameter histograms for largest sizes studied near the critical point -- that are highly suggestive of a continuous transition scenario. However, Binder analysis finds negative dips that grow sub-extensively that we interpret as these transitions rather being pseudocritical. This along with recent results on spin-1/2 models suggests that deconfined pseudocriticality is the more generic scenario.

https://arxiv.org/abs/2307.14254

https://www.phy.iitb.ac.in/en/content/sumiran-pujari 

Abstract

Amorphous solids relax via slow molecular rearrangement induced by thermal fluctuations or applied stress. Microscopic structural measures that can predict the events leading to relaxation have remained elusive.

We introduce an order parameter derived from the mean-field caging potential experienced by the particles due to their neighbors. This order parameter is based on density functional theory and measures the susceptibility to particle rearrangements. It is effective in identifying weak or defect-like regions in disordered systems. Using experiments on dense colloidal suspensions, we demonstrate a strong correlation between the order parameter and the events leading to the structural relaxation of colloidal glass. The order parameter reliably identifies soft regions where plastic rearrangements occur due to thermal fluctuations or applied shear. Our study paves the way for a structural understanding of the relaxation of a wide range of amorphous solids, from suspensions to metallic glasses.

https://arxiv.org/abs/2403.02517

https://sites.google.com/site/vchikkadi/home?authuser=0 

Abstract

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The system of Active Brownian particles has been used as a simplified theoretical model to study a large class of living and non-living active matter.  The ability of Active Brownian particles to undergo a motility-induced phase separation (MIPS) even without any adhesive forces is well known. Detailed studies of such systems have concluded that their phase behaviour is relatively simple, with only homogeneous and MIPS states. However, we show that the phase behaviour of such systems is richer than what was previously conceived. More specifically, we observe another transition at high motility, where the particles form a percolated cluster. This transition follows all the characteristics of a standard percolation transition. In the second part of the talk, we show that the aggregates of such particles on walls undergo a morphological transition with a change in wall interactions. This transition is strikingly similar to the wetting-dewetting transitions in equilibrium systems. 

Abstract

Stochastic thermodynamics and associated fluctuation theorems have long been of interest to the statistical mechanics community. The studies in this domain have mostly focused on the calculation of the distribution functions of various thermodynamic variables, such as work, heat and entropy for microscopic systems. In this talk, I will discuss our work on this topic, specifically, the calculation of force distribution for a polymer that is being stretched at a constant velocity [1], the work distribution of an elastic dumbbell model of a polymer in an elongational flow [2] and finally, the work and heat distributions of a colloid in a non-linear flow that also experiences an external Ornstein-Uhlenbeck noise [3]. All of these distributions obey the fluctuation theorem individually and also have implications for the other thermodynamic quantities.

https://journals.aps.org/pre/abstract/10.1103/PhysRevE.109.014111 

https://sites.google.com/iiserb.ac.in/cebp/home 

Abstract

Profile: https://scholar.google.co.uk/citations?user=rFIfXJMAAAAJ&hl=en 

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