arXiv subjects

Dipanjan Chakraborty

Publications and source records attributed to Dipanjan Chakraborty.

At least 19 recordsLinked to original sources

arXiv2026

Acoustic dip in the effective temperature of hot Brownian motion

A hot Brownian particle is driven by thermal fluctuations from a nonuniformly heated solvent and therefore obeys a fluctuation-dissipation relation with a frequency-dependent effective temperature. Existing theories assume an incompressible solvent. We show that finite sound speed qualitatively changes this effective temperature in the kinetic regime. Solving the compressible fluctuating-hydrodynamic problem for a heated sphere, we find that the effective noise temperature develops a pronounced acoustic dip when the sound wavelength becomes comparable to the particle radius. The dip occurs because weakly attenuated longitudinal modes carry mechanical energy away from the heated surface before it is dissipated, so that the dissipation is weighted by colder regions of the temperature field. Its position is set by $aω/c=\mathcal{O}(1)$, while its depth is controlled by the dimensionless viscous-acoustic attenuation length $a_c/a$. The result identifies an acoustic window in hot Brownian motion in which compressibility lowers, rather than raises, the kinetic noise temperature.

cond-mat.soft↗

arXiv2026

Asymmetry-controlled resonant transport in a Brownian flashing ratchet

We investigate directed transport in a one-dimensional Brownian flashing ratchet with a piecewise-linear asymmetric periodic potential. Numerical solutions of the Fokker--Planck equation and Brownian dynamics simulations reveal a nonmonotonic dependence of the stationary current on the switching frequency, with a resonant maximum whose position depends on the potential asymmetry $δ$ and barrier height $Δ$. For moderate asymmetry, $|δ|<0.5$, the current obeys a scaling form that separates the dependence on the potential parameters from a common frequency dependence, resulting in a data collapse upon appropriate scaling of the current and frequency. The current amplitude varies linearly with $δ$, while the resonance frequency follows $ν(δ,Δ)=ν_0(Δ)/ [1-b_0\, δ^2]$. We interpret the resulting $(1-δ^2)^{-1}$ scaling in terms of coupled relaxation along the two branches of the asymmetric potential, which provides a physical basis for the observed dependence of the resonant frequency on the potential asymmetry.

cond-mat.stat-mech↗

arXiv2026

Independent Control of Transport and Order in a Ratcheted Colloidal Suspension

We study directed transport in a two-dimensional suspension of repulsively interacting colloids driven by a stochastic asymmetric piecewise-linear flashing ratchet using large-scale molecular dynamics simulations. The driving frequency and the ratchet asymmetry offer two independent ways of controlling the particle current, but they affect the suspension differently. At fixed asymmetry, the current shows a resonance with ratcheting frequency that is set by the collective relaxation dynamics of the interacting particles. The resulting increase in transport is accompanied by defect-mediated structural changes, showing density-dependent hexatic and solid-like states, with larger currents generally associated with weaker ordering. By contrast, at fixed frequency, changing the ratchet asymmetry mainly alters the strength of the directed bias and can significantly enhance the current while leaving the hexatic order largely unchanged. Near the equilibrium hexatic-melting regime, this makes it possible to generate substantial directed currents without strongly disrupting sixfold orientational order. These results show that frequency tuning couples transport to structural reorganization, whereas asymmetry tuning primarily controls transport leaving the structure largely unaltered, providing distinct and complementary routes for manipulating transport and order in driven colloidal suspensions.

cond-mat.soft↗

arXiv2025

Finite size effect in the persistence probability of the Edwards-Wilkinson model of surface growth and effect of non-linearity

The dynamical evolution of the surface height is controlled by either a linear or a nonlinear Langevin equation, depending on the underlying microscopic dynamics, and is often done theoretically using stochastic coarse-grained growth equations. The persistence probability $p(t)$ of stochastic models of surface growth that are constrained by a finite system size is examined in this work. We focus on the linear Edwards-Wilkinson model (EW) and the nonlinear Kardar-Parisi-Zhang model, two specific models of surface growth. The persistence exponents in the continuum version of these two models have been widely investigated. Krug et al.[Phys. Rev. E , 56:2702-2712, (1997)] and Kallabis et al. [EPL (Europhysics Letters) , 45(1):20, 1999] had shown that, the steady-state persistence exponents for both these models are related to the growth exponent $β$ as $θ=1-β$. It is numerically found that the values of persistence exponents for both these models are close to the analytically predicted values. While the results of the continuum equations of the surface growth are well known, we focus to study the persistence probability expressions for discrete models with a finite size effect. In this article, we have investigated the persistence probabilities for the linear Edwards-Wilkinson(EW) model and for the non-linear Kardar-Parisi-Zhang(KPZ) model of surface growth on a finite one-dimensional lattice. The interesting phenomenon which is found in this case is that the known scenario of $p(t)$ of the following algebraic decay vanishes as we introduce a finite system size.

cond-mat.stat-mech↗

arXiv2022

Comparison of Popular Video Conferencing Apps Using Client-side Measurements on Different Backhaul Networks

Video conferencing platforms have been appropriated during the COVID-19 pandemic for different purposes, including classroom teaching. However, the platforms are not designed for many of these objectives. When users, like educationists, select a platform, it is unclear which platform will perform better given the same network and hardware resources to meet the required Quality of Experience (QoE). Similarly, when developers design a new video conferencing platform, they do not have clear guidelines for making design choices given the QoE requirements. In this paper, we provide a set of networks and systems measurements, and quantitative user studies to measure the performance of video conferencing apps in terms of both, Quality of Service (QoS) and QoE. Using those metrics, we measure the performance of Google Meet, Microsoft Teams, and Zoom, which are three popular platforms in education and business. We find a substantial difference in how the three apps treat video and audio streams. We see that their choice of treatment affects their consumption of hardware resources. Our quantitative user studies confirm the findings of our quantitative measurements. While each platform has its benefits, we find that no app is ideal. A user can choose a suitable platform depending on which of the following, audio, video, or network bandwidth, CPU, or memory are more important.

cs.MM↗

arXiv2022

Effect of inertia on the evasion and pursuit dynamics of prey swarms and the emergence of an optimal mass ratio for the predator-prey arms race

We show, based on a theoretical model, how inertia plays a pivotal role in the survival dynamics of a prey swarm while chased by a predator. With the varying mass of the prey and predator, diverse escape patterns emerge, such as circling, chasing, maneuvering, dividing into subgroups, and merging into a unitary group, similar to the escape trajectories observed in nature. Moreover, we find a transition from non-survival to survival of the prey swarm with increasing predator mass. The transition regime is also sensitive to the variation in prey mass. Further, the analysis of the prey group survival as a function of predator-to-prey mass ratio unveils the existence of three distinct regimes: (i) frequent chase and capture leading to the non-survival of the prey swarm, (ii) an intermediate regime where competition between pursuit and capture occurs, resembling an arms race, and (iii) the survival regime without the capture of prey. Interestingly, our study demonstrates the existence of a favourable predator-prey mass ratio for efficient predation, which corroborates with the field studies.

physics.bio-ph↗

arXiv2022

Persistence of an active asymmetric rigid Brownian particle in two dimensions

We have studied the persistence probability $p(t)$ of an active Brownian particle with shape asymmetry in two dimensions. The persistence probability is defined as the the probability of a stochastic variable that has not changed it's sign in the fixed given time interval. We have investigated two cases: diffusion of a free active particle and that of harmonically trapped particle. In our earlier work, \emph{Ghosh et. al.}, Journal of Chemical Physics, \textbf{152},174901, (2020), we had shown that $p(t)$ can be used to determine translational and the rotational diffusion constant of an asymmetric shape particle. The method has the advantage that the measurement of the rotational motion of the an-isotropic particle is not required. In this paper, we extend the study to an active an-isotropic particle and show how the persistence probability of an an-isotropic particle is modified in the presence of a propulsion velocity. Further, we validate our analytical expression against the measured persistence probability from the numerical simulations of single particle Langevin dynamics and test whether the method proposed in our earlier work can distinguish between an active and a passive an-isotropic particle.

cond-mat.stat-mech↗

arXiv2022

ReGNL: Rapid Prediction of GDP during Disruptive Events using Nightlights

Policy makers often make decisions based on parameters such as GDP, unemployment rate, industrial output, etc. The primary methods to obtain or even estimate such information are resource intensive and time consuming. In order to make timely and well-informed decisions, it is imperative to be able to come up with proxies for these parameters which can be sampled quickly and efficiently, especially during disruptive events, like the COVID-19 pandemic. Recently, there has been a lot of focus on using remote sensing data for this purpose. The data has become cheaper to collect compared to surveys, and can be available in real time. In this work, we present Regional GDP NightLight (ReGNL), a neural network based model which is trained on a custom dataset of historical nightlights and GDP data along with the geographical coordinates of a place, and estimates the GDP of the place, given the other parameters. Taking the case of 50 US states, we find that ReGNL is disruption-agnostic and is able to predict the GDP for both normal years (2019) and for years with a disruptive event (2020). ReGNL outperforms timeseries ARIMA methods for prediction, even during the pandemic. Following from our findings, we make a case for building infrastructures to collect and make available granular data, especially in resource-poor geographies, so that these can be leveraged for policy making during disruptive events.

cs.LG↗

arXiv2021

Evaluating Trust in the Context of Conversational Information Systems for new users of the Internet

Most online information sources are text-based and in Western Languages like English. However, many new and first time users of the Internet are in contexts with low English proficiency and are unable to access vital information online. Several researchers have focused on building conversational information systems over voice for this demographic, and also highlighted the importance of building trust towards the information source. In this work we develop four versions of a voice based chat-bot on the Google Assistant platform in which we vary the gender, friendliness and personalisation of the bot. We find that the users rank the female version of the bot with more personalisations over the others; however when rating the bots individually, the ratings depend on the ability of the bot to understand the users' spoken query and respond accurately.

cs.HC↗

arXiv2021

Coarse Graining Nonisothermal Microswimmer Suspensions

We investigate coarse-grained models of suspended self-thermophoretic microswimmers. Upon heating, the Janus spheres, with hemispheres made of different materials, induce a heterogeneous local solvent temperature that causes the self-phoretic particle propulsion. Starting from atomistic molecular dynamics simulations, we verify the coarse-grained description of the fluid in terms of a local molecular temperature field, and its role for the particle's thermophoretic self-propulsion and hot Brownian motion. The latter is governed by effective nonequilibrium temperatures, which are measured from simulations by confining the particle position and orientation. They are theoretically shown to remain relevant for any further spatial coarse-graining towards a hydrodynamic description of the entire suspension as a homogeneous complex fluid.

cond-mat.soft↗

arXiv2021

Continuous demixing transition of binary liquids: finite-size scaling from the analysis of sub-systems

A binary liquid near its consolute point exhibits critical fluctuations of the local composition; the diverging correlation length has always challenged simulations. The method of choice for the calculation of critical points in the phase diagram is a scaling analysis of finite-size corrections, based on a sequence of widely different system sizes. Here, we discuss an alternative using cubic sub-systems of one large simulation as facilitated by modern, massively parallel hardware. We exemplify the method for a symmetric binary liquid at critical composition and compare different routes to the critical temperature: (1) fitting the critical divergences of the correlation length and the susceptibility encoded in the composition structure factor of the whole system, (2) testing data collapse and scaling of moments of the composition fluctuations in sub-volumes, and (3) applying the cumulant intersection criterion to the sub-systems. For the last route, two difficulties arise: sub-volumes are open systems with free boundary conditions, for which no precise estimate of the critical Binder cumulant $U_c$ is available. Second, the periodic boundaries of the simulation box interfere with the sub-volumes, which we resolve by a two-parameter finite-size scaling. The implied modification to the data analysis restores the common intersection point, and we estimate $U_c=0.201 \pm 0.001$, universal for cubic Ising-like systems with free boundaries. Confluent corrections to scaling, which arise for small sub-system sizes, are quantified at leading order and our data for the critical susceptibility are compatible with the universal correction exponent $ω\approx 0.83$.

cond-mat.stat-mech↗

arXiv2020

Two step melting of the Weeks-Chandler-Anderson system in two dimensions

We present a detailed numerical simulation study of a two dimensional system of particles interacting via the Weeks-Chandler-Anderson potential, the repulsive part of the Lennard-Jones potential. With reduction of density, the system shows a two-step melting: a continuous melting from solid to hexatic phase, followed by a a first order melting of hexatic to liquid. The solid-hexatic melting is consistent with the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario and shows dislocation unbinding. The first order melting of hexatic to fluid phase, on the other hand, is dominated by formation of string of defects at the hexatic-fluid interfaces.

cond-mat.soft↗

arXiv2020

Persistence in Brownian motion of an ellipsoidal particle in two dimensions

We investigate the persistence probability $p(t)$ of the position of a Brownian particle with shape asymmetry in two dimensions. The persistence probability is defined as the probability that a stochastic variable has not changed it's sign in the given time interval. We explicitly consider two cases -- diffusion of a free particle and that of harmonically trapped particle. The later is particularly relevant in experiments which uses trapping and tracking techniques to measure the displacements. We provide analytical expressions of $p(t)$ for both the scenarios and show that in the absence of the shape asymmetry the results reduce to the case of an isotropic particle. The analytical expressions of $p(t)$ are further validated against numerical simulation of the underlying overdamped dynamics. We also illustrate that $p(t)$ can be a measure to determine the shape asymmetry of a colloid and the translational and rotational diffusivities can be estimated from the measured persistence probability. The advantage of this method is that it does not require the tracking of the orientation of the particle.

cond-mat.soft↗

arXiv2019

Structure-dynamics relationship in ratcheted colloids: Resonance melting, dislocations, and defect clusters

We consider a two dimensional colloidal dispersion of soft-core particles driven by a one dimensional stochastic flashing ratchet that induces a time averaged directed particle current through the system. It undergoes a non-equilibrium melting transition as the directed current approaches a maximum associated with a resonance of the ratcheting frequency with the relaxation frequency of the system. We use extensive molecular dynamics simulations to present a detailed phase diagram in the ratcheting rate-mean density plane. With the help of numerically calculated structure factor, solid and hexatic order parameters, and pair correlation functions, we show that the non-equilibrium melting is a continuous transition from a quasi-long ranged ordered solid to a hexatic phase. The transition is mediated by the unbinding of dislocations, and formation of compact and string-like defect clusters.

cond-mat.soft↗

arXiv2019

Survival chances of a prey swarm: how the cooperative interaction range affects the outcome

A swarm of preys when attacked by a predator is known to rely on their cooperative interactions to escape. Understanding such interactions of collectively moving preys and the emerging patterns of their escape trajectories still remain elusive. In this paper, we investigate how the range of cooperative interactions within a prey group affects the survival chances of the group while chased by a predator. As observed in nature, the interaction range of preys may vary due to their vision, age, or even physical structure. Based on a simple theoretical prey-predator model, here, we show that an optimality criterion for the survival can be established on the interaction range of preys. Very short range or long range interactions are shown to be inefficient for the escape mechanism. Interestingly, for an intermediate range of interaction, survival probability of the prey group is found to be maximum. Our analysis also shows that the nature of the escape trajectories strongly depends on the range of interactions between preys and corroborates with the naturally observed escape patterns. Moreover, we find that the optimal survival regime depends on the prey group size and also on the predator strength.

physics.bio-ph↗

arXiv2018

Orientational dynamics of a heated Janus particle

Using large scale molecular dynamics simulations we study the orientational dynamics of a heated Janus particle which exhibits self-propulsion. The asymmetry in the microscopic interaction of the colloid with the solvent is implemented by choosing different wetting parameters for the two halves of the sphere. Consequently a gradient in temperature is created across the poles of the sphere due to different interfacial resistance across the solid-fluid boundary in the two hemispheres. It is this self-created temperature gradient which leads to a self-propulsion along the direction of the symmetry axis. In this article, we look at the orientational dynamics of such a system, as well as the subsequent enhancement of the translational diffusivity at late times. The orientational correlation of the symmetry axis is measured from the simulation and provides a direct access to the rotational diffusion constant. The heating leads to an increase in the rotational diffusivity of the colloid. We quantify this increase in rotational diffusion against the temperature difference across the poles of the Janus sphere as well as the average surface temperature difference from the ambient fluid.We show that later quantification is better and results in a data collapse for different choices of the microscopic interaction. The average propulsion velocity is also measured for different choices of the wetting parameter. The directionality of self-propulsion changes depending on the microscopic interaction. We show that whenever the attractive interaction of the colloid with the solvent is switched off, the phoretic mobility changes sign. Further, the propulsion velocity is zero for heating below a certain threshold value. Finally, we combine the measured propulsion velocity and the rotational diffusion time to estimate the enhancement in the long time diffusion coefficient of the particle.

cond-mat.soft↗

arXiv2018

Hot Microswimmers

Hot microswimmers are self-propelled Brownian particles that exploit local heating for their directed self-thermophoretic motion. We provide a pedagogical overview of the key physical mechanisms underlying this promising new technology. It covers the hydrodynamics of swimming, thermophoresis and -osmosis, hot Brownian motion, force-free steering, and dedicated experimental and simulation tools to analyze hot Brownian swimmers.

cond-mat.soft↗

arXiv2018

The effective temperature for the thermal fluctuations in hot Brownian motion

We revisit the effective parameter description of hot Brownian motion -- a scenario where a colloidal particle is kept at an elevated temperature than the ambient fluid. Due to the time scale separation between heat diffusion and particle motion, a stationary halo of hot fluid is carried along with the particle, resulting in a spatially varying comoving temperature and viscosity profile. The resultant Brownian motion in the overdamped limit can be well described by a Langevin equation with effective parameters such as effective temperature $T_{\rm HBM}$ and friction coefficient $ζ_{\rm HBM}$ that quantifies the thermal fluctuations and the diffusivity of the particle. These parameters can exactly be calculated using the framework of fluctuating hydrodynamics. Additionally, it was also observed that configurational and the kinetic degrees of freedom admits to different effective temperatures, $T^{\mathbf{x}}_{\rm HBM}$ and $T^{\mathbf{v}}_{\rm HBM}$, respectively, with the former predicted accurately from fluctuating hydrodynamics. A more rigorous calculation by Falasco et. al. Physical Review E , 90, $032131(2014)$ extends the overdamped description to a generalized Langevin equation where the effective temperature becomes frequency dependent and consequently, for any temperature measurement from a Brownian trajectory requires the knowledge of this frequency dependence. We use this framework to expand on this earlier work and look at the first order correction to the effective temperature. The effective temperature is calculated from the weighted average of the temperature field with the dissipation function. Further, we provide a closed form analytical result for effective temperature in the small as well high frequency limit and using this we determine the kinetic temperature from the generalized Langevin equation and the Wiener-Khinchine theorem.

cond-mat.soft↗