arXiv subjects

Debasish Saha

Publications and source records attributed to Debasish Saha.

12 recordsLinked to original sources

arXiv2025

Non-Markovian escape under stochastic resetting

Stochastic resetting is a powerful strategy known to optimize target-search processes at microscopic scales. While its effects on Markovian systems are well understood, its influence on memory-driven systems, such as in viscoelastic baths, has not been adequately investigated. In this work, we study the first-passage properties of escape for a harmonically trapped particle in a non-Markovian environment under stochastic resetting. We employ a complete renewal approach and find that the characteristic non-exponential heavy tail of the first-passage time (FPT) distribution becomes exponential when resetting is introduced. We further find that optimal resetting is achievable at a lower reset rate when the dynamics are weakly correlated; however, for stronger correlations, the process needs to be reset more frequently. Therefore, resetting in memory-driven dynamics can be used as an effective control strategy to initiate faster escape, thereby regulating efficient transport mechanisms in complex chemical and biomolecular environments that follow non-Markovian dynamics.

cond-mat.stat-mech↗

arXiv2024

Cybloids $-$ Creation and Control of Cybernetic Colloids

Colloids play an important role in fundamental science as well as in nature and technology. They have had a strong impact on the fundamental understanding of statistical physics. For example, colloids have helped to obtain a better understanding of collective phenomena, ranging from phase transitions and glass formation to the swarming of active Brownian particles. Yet the success of colloidal systems hinges crucially on the specific physical and chemical properties of the colloidal particles, i.e. particles with the appropriate characteristics must be available. Here we present an idea to create particles with freely selectable properties. The properties might depend, for example, on the presence of other particles (hence mimicking specific pair or many-body interactions), previous configurations (hence introducing some memory or feedback), or a directional bias (hence changing the dynamics). Without directly interfering with the sample, each particle is fully controlled and can receive external commands through a predefined algorithm that can take into account any input parameters. This is realized with computer-controlled colloids, which we term cybloids - short for cybernetic colloids. The potential of cybloids is illustrated by programming a time-delayed external potential acting on a single colloid and interaction potentials for many colloids. Both an attractive harmonic potential and an annular potential are implemented. For a single particle, this programming can cause subdiffusive behavior or lend activity. For many colloids, the programmed interaction potential allows to select a crystal structure at wish. Beyond these examples, we discuss further opportunities which cybloids offer.

cond-mat.soft↗

arXiv2023

Work distribution of a colloid in an elongational flow field and under Ornstein-Uhlenbeck noise

The study of thermodynamic properties of microscopic systems, such as a colloid in a fluid, has been of great interest to researchers since the discovery of the fluctuation theorem and associated laws of stochastic thermodynamics. However, most of these studies confine themselves to systems where effective fluctuations acting on the colloid are in the form of delta-correlated Gaussian white noise (GWN). In this study, instead, we look into the work distribution function when a colloid trapped in a harmonic potential moves from one position to another in a fluid medium with an elongational flow field where the effective fluctuations are given by the Ornstein-Uhlenbeck (OU) noise, a type of coloured noise. We use path integrals to calculate this distribution function and compare and contrast its properties to the case with GWN. We find that the work distribution function turns out to be non-Gaussian as a result of the elongational flow field, but continues to obey the fluctuation theorem in both types of noise. Further, we also look into the effects of the various system parameters on the behaviour of work fluctuations and find that although the distribution tends to broaden with increasing noise intensity, increased correlation in fluctuations acts to oppose this effect. Additionally, the system is found to consume heat from the surroundings at early times and dissipate it into the media at later times. This study, therefore, is a step towards gaining a better understanding of the thermodynamic properties of colloidal systems under non-linear complex flows that also display correlated fluctuations.

cond-mat.soft↗

arXiv2022

Swift J1728.9-3613 is a black hole X-ray binary: spectral and timing study using NICER

We study different timing and spectral properties of the new Galactic X-ray transient Swift J1728.9-3613 using NICER and Swift, discovered by the Burst Alert Telescope (BAT) on the Neil Gehrels Swift Observatory. The source went through multiple transitions to different spectral states during the outburst, and the complete evolution created a q-shaped track in the hardness intensity diagram. A partial hysteresis is also observed in the RMS-intensity diagram, which is another well-defined phenomenon of black hole transients. In SIMS, power density spectra were dominated by broadband noise components, and two type B QPOs were detected. We have fitted 1-10 keV energy spectra obtained from NICER observations that were performed during the outburst, and the temporal evolution of spectral parameters were studied. On MJD 58584.69, a small-scale reflare happened, and we observed that the spectral index decreased to a much lower value associated with finite changes in other spectral parameters also, and the 1-10 keV averaged flux also increased. We observed that the innermost radius of the accretion disc was almost constant during the soft state, which corresponds to the Innermost Stable Circular Orbit (ISCO). We have measured the lower limit of mass of the compact object to be approximately 4.6 M, considering a non-spinning black hole binary system, by fitting 1-10 keV NICER spectra with the diskbb component. The soft-to-hard transition occurred when the bolometric luminosity was 0.01 times the Eddington luminosity. Based on our combined study of the evolution of the timing and spectral properties, we conclude that the new source Swift J1728.9-3613 is a black hole X-ray binary.

astro-ph.HE↗

arXiv2022

Pattern selection in radial displacements of a confined aging viscoelastic fluid

Intricate fluid displacement patterns, arising from the unstable growth of interfacial perturbations, can be driven by fluid viscoelasticity and surface tension. A soft glassy suspension ages, $i.e.$ its mechanical moduli evolve with time, due to the spontaneous formation of suspension microstructures. The shear and time-dependent rheology of an aging suspension can be exploited to generate a wide variety of interfacial patterns during its displacement by a Newtonian fluid. Using video imaging, we report a rich array of interfacial pattern morphologies: dense viscous, dendritic, viscoelastic fracture, flower-shaped, jagged and stable, during the miscible and immiscible displacements of an aging colloidal clay suspension by Newtonian fluids injected into a radial quasi-two-dimensional geometry at different flow rates. We propose a new parameter, the areal ratio, which we define as the fully-developed pattern area normalized by the area of the smallest circle enclosing it. We show that the natural logarithms of the areal ratios uniquely identify the distinct pattern morphologies, such that each pattern can be segregated in a three-dimensional phase diagram spanned by the suspension aging time, the displacing fluid flow rate, and interfacial tension. Besides being of fundamental interest, our results are useful in predicting and controlling the growth of interfaces during fluid displacements.

cond-mat.soft↗

arXiv2021

Multi-wavelength observation of MAXI J1348$-$630 during the outburst in 2019

We study the multi-wavelength spectral properties of the black hole X-ray binary MAXI J1348$-$630 using quasi-simultaneous $\textit{ALMA}$, $\textit{NICER}$, and $\textit{Swift}$ observations during the decay phase of the January 2019 outburst. In millimeter wavelengths, radio continuum emissions in the frequency range of 89.56$-$351.44~GHz are measured. We found that the flux densities at millimeter wavelength varied between 12.18 mJy and 18.47 mJy with spectral index ($\alpha $) of $0.28\pm 0.02$. The broadband spectrum suggests that the source was accompanied by weak synchrotron emission from the compact jets. Broadband spectral study indicates that MAXI J1348--630 falls in the regime of ``radio-quiet'' during the decay phase of the outburst. The $\textit{NICER}$ spectrum is fitted by a combined model of disk blackbody component $(\textit{diskbb})$ along with a comptonization component $(\textit{simpl})$ which explains the power-law continuum caused by the thermal Comptonisation of soft disk photons in a hot gas of electrons. The $\textit{NICER}$ spectrum is dominated by the comptonised components during the decay phase of the outburst close to the hard state of the source. We have investigated the correlation between X-ray and radio luminosity using quasi-simultaneous $\textit{ALMA}$ and $\textit{NICER}$ data to understand the source nature by locating the source in the $L_{X}$-$L_{R}$ diagram. The correlation study of radio/X-ray luminosity suggests that MAXI J1348--630 did not follow the well-known track for black holes and it is a new member of a restricted group of sources.

astro-ph.HE↗

arXiv2020

Quantifying the destructuring of a thixotropic colloidal suspension using falling ball viscometry

The settling dynamics of falling spheres inside a Laponite suspension is studied. Laponite is a colloidal synthetic clay that shows physical aging in aqueous suspension due to the spontaneous evolution of inter-particle electrostatic interactions. In our experiments, millimeter-sized steel balls are dropped in aqueous Laponite suspensions of different ages (i.e., time elapsed since sample preparation). The motion of the falling balls are captured using a high-speed camera and the velocities of their centroids are estimated from the images. Interestingly, we observe that balls of larger diameters fail to achieve terminal velocity over the entire duration of the experiment. We propose a mathematical model that accounts for rapid structural changes (expected to be induced by the falling ball) in Laponite suspensions whose aging time scales are much slower than the time of fall of the ball. For a range of ball sizes and Laponite suspension ages, our model correctly predicts the time-dependence of the ball velocity. Furthermore, fits to our model allow us to estimate the rates of destructuring of the thixotropic suspensions due to the passage of the falling ball.

cond-mat.soft↗

arXiv2017

Effects of polydispersity on the glass transition dynamics of aqueous suspensions of soft spherical colloidal particles

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) particles of a nearly constant swelling ratio and with polydispersity indices (PDIs) varying over a wide range (7.4% - 48.9%) are synthesized to study the effects of polydispersity on the dynamics of suspensions of soft PNIPAM colloidal particles. The PNIPAM particles are characterized using dynamic light scattering (DLS) and scanning electron microscopy (SEM). The zero shear viscosity ($η_{0}$) data of these colloidal suspensions, estimated from rheometric experiments as a function of the effective volume fraction $ϕ_{eff}$ of the suspensions, increases with increase in $ϕ_{eff}$ and shows a dramatic increase at $ϕ_{eff}=ϕ_{0}$. The data for $η_{0}$ as a function of $ϕ_{eff}$ fits well to the Vogel-Fulcher-Tammann (VFT) equation. It is observed that increasing PDIs results in increasingly fragile supercooled liquid-like behavior, with the parameter $ϕ_{0}$, extracted from the fits to the VFT equation, shifting towards higher $ϕ_{eff}$. The observed increase in fragility is attributed to the prevalence of dynamical heterogeneities (DHs) in these polydisperse suspensions, while the simultaneous shift in $ϕ_{0}$ is ascribed to the decoupling of the dynamics of the smallest and largest particles. Finally, it is observed that the intrinsic nonlinearity of these suspensions, estimated at the third harmonic near $ϕ_{0}$ in Fourier transform oscillatory rheological experiments, increases with increase in PDIs. Our results are in agreement with theoretical predictions and simulation results for polydisperse hard sphere colloidal glasses and clearly demonstrate that jammed suspensions of polydisperse colloidal particles can be effectively fluidized with increasing PDIs.

cond-mat.soft↗

arXiv2016

Study of dynamical heterogeneities in aging colloidal nanoclay suspensions

An aqueous suspension of the synthetic clay Laponite undergoes a transition from a liquid-like ergodic state to a glass-like nonergodic arrested state. In an observation that closely resembles the dynamical slowdown observed in supercooled liquids, the phenomenon of kinetic arrest in Laponite suspensions is accompanied by a growth in the $α$-relaxation time with increasing sample aging time, $t_{w}$. The ubiquitous dynamic slowdown and fragile behavior observed in glass forming liquids approaching the glass transition is typically ascribed to the growth in the size of distinct dynamical heterogeneities. In this article, we present the characterization of the dynamical heterogeneities in aging colloidal Laponite clay systems by invoking the three-point dynamic susceptibility formalism. The average time-dependent two-point intensity autocorrelation and its sensitivity to the control parameter $t_{w}$ are probed in dynamic light scattering experiments. Distributions of relaxation time scales deduced from Kohlrausch-Williams-Watts equation widen with increasing $t_{w}$ signifying the heterogeneous dynamic slowdown. A suitable formalism to calculate three-point correlation function is employed for aging colloidal suspension where the main control parameter is $t_{w}$. The calculated three-point dynamic susceptibility exhibits a peak, with the peak height increasing with evolving $t_{w}$. The number of dynamically correlated particles is seen to initially increase with increasing $t_{w}$ at a fast rate, before eventually slowing down close to the non-ergodic transition point.This observation is in agreement with reports on supercooled liquids. Our study confirms the growth of dynamical heterogeneities in suspensions of Laponite, thereby shedding new light on the fragile supercooled liquid-like dynamics of aging suspensions of these anisotropic, charged, colloidal clay nanoparticles.

cond-mat.soft↗

arXiv2015

Characteristics of the secondary relaxation process in soft colloidal suspensions

A universal secondary relaxation process, known as the Johari-Goldstein (JG) $β$-relaxation process, appears in glass formers. It involves all parts of the molecule and is particularly important in glassy systems because of its very close relationship with the $α$-relaxation process. However, the absence of a J-G $β$-relaxation mode in colloidal glasses raises questions regarding its universality. In the present work, we study the microscopic relaxation processes in Laponite suspensions, a model soft glassy material, by dynamic light scattering (DLS) experiments. $α$ and $β$-relaxation timescales are estimated from the autocorrelation functions obtained by DLS measurements for Laponite suspensions with different concentrations, salt concentrations and temperatures. Our experimental results suggest that the $β$-relaxation process in Laponite suspensions involves all parts of the constituent Laponite particle. The ergodicity breaking time is also seen to be correlated with the characteristic time of the $β$-relaxation process for all Laponite concentrations, salt concentrations and temperatures. The width of the primary relaxation process is observed to be correlated with the secondary relaxation time. The secondary relaxation time is also very sensitive to the concentration of Laponite. We measure primitive relaxation timescales from the $α$-relaxation time and the stretching exponent ($β$) by applying the coupling model for highly correlated systems. The order of magnitude of the primitive relaxation time is very close to the secondary relaxation time. These observations indicate the presence of a J-G $β$-relaxation mode for soft colloidal suspensions of Laponite.

cond-mat.soft↗

arXiv2015

Kinetics of the glass transition of fragile soft colloidal suspensions

Microscopic relaxation timescales are estimated from the autocorrelation functions obtained by dynamic light scattering experiments for Laponite suspensions with different concentrations ($C_{L}$), added salt concentrations ($C_{S}$) and temperatures ($T$). It has been shown in an earlier work [Soft Matter, 10, 3292-3300 (2014)] that the evolutions of relaxation timescales of colloidal glasses can be compared with molecular glass formers by mapping the waiting time ($t_{w}$) of the former with the inverse of thermodynamic temperature ($1/T$) of the latter. In this work, the fragility parameter $D$, which signifies the deviation from Arrhenius behavior, is obtained from fits to the time evolutions of the structural relaxation timescales. For the Laponite suspensions studied in this work, $D$ is seen to be independent of $C_{L}$ and $C_{S}$, but is weakly dependent on $T$. Interestingly, the behavior of $D$ corroborates the behavior of fragility in molecular glass formers with respect to equivalent variables. Furthermore, the stretching exponent $β$, which quantifies the width $w$ of the spectrum of structural relaxation timescales is seen to depend on $t_{w}$. A hypothetical Kauzmann time $t_{k}$, analogous to the Kauzmann temperature for molecular glasses, is defined as the timescale at which $w$ diverges. Corresponding to the Vogel temperature defined for molecular glasses, a hypothetical Vogel time $t^{\infty}_α$ is also defined as the time at which the structural relaxation time diverges. Interestingly, a correlation is observed between $t_{k}$ and $t^{\infty}_α$, which is remarkably similar to that known for fragile molecular glass formers. A coupling model that accounts for the $t_{w}$-dependence of the stretching exponent is used to analyse and explain the observed correlation between $t_{k}$ and $t^{\infty}_α$.

cond-mat.soft↗

arXiv2013

Investigation of the dynamical slowing down process in soft glassy colloidal suspensions: comparisons with supercooled liquids

The primary and secondary relaxation timescales of aging colloidal suspensions of Laponite are estimated from intensity autocorrelation functions obtained in dynamic light scattering (DLS) experiments. The dynamical slowing down of these relaxation processes are compared with observations in fragile supercooled liquids by establishing a one-to-one mapping between the waiting time since filtration of a Laponite suspension and the inverse of the temperature of a supercooled liquid that is rapidly quenched towards its glass transition temperature. New timescales, such as the Vogel time and the Kauzmann time, are extracted to describe the phenomenon of dynamical arrest in Laponite suspensions. In results that are strongly reminiscent of those extracted from supercooled liquids approaching their glass transitions, it is demonstrated that the Vogel time calculated for each Laponite concentration is approximately equal to the Kauzmann time, and that a strong coupling exists between the primary and secondary relaxation processes of aging Laponite suspensions. Furthermore, the experimental data presented here clearly demonstrates the self-similar nature of the aging dynamics of Laponite suspensions within a range of sample concentrations.

cond-mat.soft↗