arXiv subjects

Shuvojit Paul

Publications and source records attributed to Shuvojit Paul.

15 recordsLinked to original sources

arXiv2025

A stochastic heat engine driven using a nonlinear protocol

A colloidal particle confined in a time-dependent optical trap can function as a microscopic heat engine, with optimization strategies playing a crucial role in enhancing its performance. In this study, we numerically investigate a Stirling heat engine operating in both passive and active environments using a protocol inspired by the Engineered Swift Equilibration (ESE) method. This approach differs from the standard process and focuses on enhancing engine efficiency, particularly at short time scales. We analyze various fluctuating parameters throughout the cycle to validate the robustness of the engine, and demonstrate a significant enhancement in performance compared to conventional Stirling engines. Most crucially, we observe that the nonlinear protocol can even transform a heat-pump-like operation into a genuine heat engine under strong activity, thereby surpassing bounds imposed on efficiency by high-temperature and quasi-static conditions. Finally, the proposed protocol is designed with experimental feasibility in mind, making it a promising framework for the practical realization of efficient microscopic heat engines.

cond-mat.stat-mech↗

arXiv2025

Decoding active force fluctuations from spatial trajectories of active systems

Mesoscopic active systems exhibit various unique behaviours - absent in passive systems - due to the forces generated by the corresponding constituents by converting their available free energies. However, estimating these forces - which are also stochastic and remain intertwined with the thermal noise - is especially non-trivial. Here, we introduce a technique to extract such fluctuating active forces acting on a passive particle immersed in an active bath with high statistical accuracy by filtering out the related thermal noise. We first test the efficacy of our method under numerical scenarios with different types of activity, and then apply it to the experimental trajectories of a microscopic particle (optically) trapped inside an active bath consisting of motile \textit{E.Coli.} bacteria. We believe that our simple yet powerful approach, which appears agnostic to the nature of the active force, should enable accurate measurement of force dynamics in living matter and potentially allow direct but reliable estimation of key thermodynamic parameters such as heat, work, and entropy production.

cond-mat.soft↗

arXiv2025

Simultaneous active and diffusive behaviour of asymmetric microclusters in a photophoretic trap

Active and diffusive motion in Brownian particles are regularly observed in fluidic environments, albeit at different time scales. Here, we experimentally study the dynamics of highly asymmetric microclusters trapped in air employing photophoretic forces generated from a loosely focused laser beam, where the trapped particles display active and diffusive dynamics simultaneously in orthogonal spatial directions. Thus, particle motion in the longitudinal direction ($z$) is enslaved to irregular kicks that naturally arise from an interplay of gravitational and photophoretic forces. This leads to a bimodal nature of the probability distribution function with a near-ballistic scaling of mean-squared displacement in the $z$ direction demonstrating active like dynamics, while the dynamics along the transverse ($x$) direction displays diffusive behaviour with a strong dependence on the motion along $z$. To explain these unique characteristics, we developed a 2D-Langevin model of a confined elliptic particle experiencing an additional stochastic force along $z$ to account for the arbitrary jumps. The numerical results show excellent qualitative agreement with the experimental observations. Our findings should pave the way for the design of high-efficiency Brownian engines in air, besides stimulating new research in the emerging field of photophoretic trapping.

physics.optics↗

arXiv2025

Irreversibility of mesoscopic processes with hydrodynamic interactions

Optically confined colloidal particles, when placed in close proximity, form a dissipatively coupled system through hydrodynamic interactions. The role of such interactions influencing irreversibility and energy dissipation in out-of-equilibrium systems is often not well deciphered. Here, we demonstrate - through the estimation of the entropy production rate - that the nonequilibrium features of the system with such interactions vary depending on the nature of external driving, and importantly, on the level of coarse-graining. Crucially, we show that coarse-graining reverses the dependence of the measured entropy production rate on the strength of the hydrodynamic interactions. Furthermore, we clarify that such interactions do not violate energy balance at the level of individual trajectories, as was believed earlier. Our results highlight a previously unnoticed effect of coarse-graining in nonequilibrium systems, and have implications for the inference of entropy production in experimental contexts.

cond-mat.stat-mech↗

arXiv2023

Enhanced directionality of active processes in a viscoelastic bath

Active fluctuations are known to play a significant role in the intracellular transport of passive objects. However, the effect of viscoelasticity of the environment in shaping such processes is relatively less understood. Here, with a minimal experiment using a driven colloid in a viscoelastic bath, we show that viscoelasticity significantly increases the mean injected power to the passive object ($\sim 50\%$ compared to a viscous medium), for the same strength of the external driving. Additionally, we observe a notable reduction in negative work fluctuations across a wide range of driving amplitudes. These findings collectively suggest an enhanced directionality in driven processes within a viscoelastic bath, which we attribute to the emergence of interactions between the colloid and the viscoelastic medium.

cond-mat.stat-mech↗

arXiv2022

Force generation in confined active fluids: The role of microstructure

We experimentally determine the force exerted by a bath of active particles onto a passive probe as a function of its distance to a wall and compare it to the measured averaged density distribution of active particles around the probe. Within the framework of an active stress, we demonstrate that both quantities are - up to a factor - directly related to each other. Our results are in excellent agreement with a minimal numerical model and confirm a general and system-independent relationship between the microstructure of active particles and transmitted forces.

cond-mat.soft↗

arXiv2021

Single-shot wideband active mircorheology using multiple-sinusoid modulated Optical Tweezers

We employ multiple sinusoid modulated optical tweezers to measure the frequency dependent rheological parameters of a linear viscoelastic fluid over five decades of frequency in a single shot, hitherto not achieved using active microrheology alone. Thus, we spatially modulate a trapped probe particle embedded in a fluid medium with a combination of a square wave - which is by definition a superposition of odd sinusoidal harmonics - and a linear superposition of multiple sinusoids at a wideband frequency range, with complete control over the amplitude, frequency and relative phase of the modulating signals. For the latter, we selectively excite the particle by larger amplitudes at high frequencies where the particle response is low, thereby enabling wideband active microrheology with large signal-to-noise. This mitigates the principal issue associated with conventional active microrheology - which is low bandwidth - and also renders our method better in terms of signal to noise, and faster compared to passive microrheology. We determine the complex viscoelastic parameters of the fluid by extracting the phase response (relative to input excitation) of the probe from the experimentally recorded time series data of the probe displacement, and employing well-known theoretical correlations thereafter. We test the efficacy of our method by studying a linear viscoelastic media (polyacrylamide-water solution) at different concentrations, and find good agreement of the measured fluid parameters with known literature values.

cond-mat.soft↗

arXiv2019

Single-shot wideband active microrheology of viscoelastic fluids using pulse-scanned optical tweezers

We present a fast active microrheology technique exploring the phase response of a microscopic probe particle trapped in a linear viscoelastic fluid using optical tweezers under an external perturbation. Thus, we experimentally determine the cumulative response of the probe to an entire repertoire of sinusoidal excitations simultaneously by applying a spatial square pulse as an excitation to the trapped probe. The square pulse naturally contains the fundamental sinusoidal frequency component and higher odd harmonics, so that we measure the phase response of the probe over a very wide frequency band in a single shot. We then determine the responses to individual harmonics using a lock-in algorithm, and compare the phase shifts to those obtained theoretically by solving the equation of motion of the probe particle confined in a harmonic potential in the fluid in the presence of a sinusoidal perturbation. We go on to relate the phase response of the probe to the complex shear modulus $G^{*}(ω)$, and proceed to verify our technique in a mixture of polyacrylamide and water, which we compare with known values in literature and obtain very good agreement. Our method ensures that any drifts in time are almost entirely ruled out from the data, with the added advantage of high speed and ease of use.

physics.app-ph↗

arXiv2018

Active microrheology to determine viscoelastic parameters of Stokes-Oldroyd B fluids using optical tweezers

We use active microrheology to determine the frequency dependent moduli of a linear viscoelastic fluid in terms of the polymer time constant ($λ$), and the polymer ($μ_p$) and solvent viscosity ($μ_s$), respectively. We measure these parameters from the response function of an optically trapped Brownian probe in the fluid under an external perturbation, and at different dilutions of the viscoelastic component in the fluid. This is an improvement over bulk microrheology measurements in viscoelastic Stokes-Oldroyd B fluids which determine the complex elastic modulus $G(ω)$ of the fluid, but do not, however, reveal the characteristics of the polymer chains and the Newtonian solvent of the complex fluid individually. In a recent work [Paul \textit{et al}., 2018 J. Phys. Condens. Matter \textbf{30} 345101], we linearized the Stokes-Oldroyd B fluid model and thereby explicitly formulated the frequency dependent moduli in terms of ($μ_p$) and ($μ_s$), which we now extend to account for an external sinusoidal force applied to the probe particle. We measure $λ$, $μ_p$, and $μ_s$ experimentally, and compare with existing the $λ$ values in the literature for the same fluid at some of the dilution levels, and obtain good agreement. Further, we use these parameters to calculate the complex elastic modulus of the fluid again at certain dilutions and verify successfully with existing data. This establishes our method as an alternate approach in the active microrheology of complex fluids which should reveal information about the composition of such fluids in significantly greater detail and high signal to noise.

cond-mat.soft↗

arXiv2018

A quantitative analysis of the emergence of memory in the viscously coupled dynamics of colloids

We provide a quantitative description of the evolution of memory from the apparently random Markovian dynamics of a pair of optically trapped colloidal microparticles in water. The particles are trapped in very close proximity of each other so that the resultant hydrodynamic interactions lead to non-Markovian signatures manifested by the double exponential auto-correlation function for the Brownian motion of each particle. In connection with the emergence of memory in this system, we quantify the storage of energy and demonstrate that a pair of Markovian particles - confined in individual optical traps in a viscous fluid - can be described in the framework of a single Brownian particle in a viscoelastic medium. We define and quantify the equivalent storage and loss moduli of the two-particle system, and show experimentally that the memory effects reduce with increasing particle separation and increase with a skewed stiffness ratio between the traps.

cond-mat.soft↗

arXiv2018

Dynamics of hydrodynamically coupled Brownian harmonic oscillators in a Maxwell fluid

Recently, many interesting features of the hydrodynamically coupled motions of the Brownian particles in a viscous fluid have been reported which are impossible for the uncoupled motions of the similar particles. However, it is expected that those physics in a viscoelastic fluid is much more interesting due to the presence of the additional frequency dependent elasticity of the medium. Thus, a theory describing the equilibrium dynamics of two hydrodynamically coupled Brownian harmonic oscillators in a viscoelastic Maxwell fluid has been derived which appears with new and impressive aspects. Initially, the response functions have been calculated and then the fluctuation-dissipation theorem has been used to calculate the correlation functions between the coloured noises present on the concerned particles placed in a Maxwell fluid due to the thermal motions of the fluid molecules. These correlation functions appear to be in a linear relationship with the delta-correlated noises in a viscous fluid. Consequently, this reduces the statistical description of a simple viscoelastic fluid to the statistical representation for an extended dynamical system subjected to delta-correlated random forces. Thereupon, the auto and cross-correlation functions in the time domain and frequency domain and the mean-square displacement functions of the particles have been calculated which are perfectly consistent with their corresponding established forms in a viscous fluid and emerge with exceptional characteristics.

cond-mat.soft↗

arXiv2018

Microscopic approach to understand Brownian dynamics in viscoelastic fluid

We present an entirely microscopic formulation of viscoleasticity of a fluid starting from the microscopic Stokes-Oldroyd B Model assuming instantaneous hydrodynamic friction, and show that linearization leads to a form for the frequency dependent viscosity that can be directly applied to the Langevin equation. Interestingly, the calculated expression of viscosity can be directly mapped to the Jeffreys' model which is essentially macroscopic in nature with the bulk viscoelasticity of the fluid being characterized by a complex elastic modulus G(ω). Further, we demonstrate that the concerned Green's function is same as that in an incompressible, low Reynold's number Newtonian fluid with the simple incorporation of frequency dependence in the viscosity term. We proceed to evaluate the trajectory of a free Brownian particle in a viscoelastic environment using our formalism and calculate parameters such as the power spectral density, the autocorrelation function and the mean-square displacement, which we then extend to the particle confined in a harmonic potential in the fluid.

physics.flu-dyn↗

arXiv2018

Two-point active microrheology in a viscous medium exploiting a motional resonance excited in dual-trap optical tweezers

Two-point microrheology measurements from widely separated colloidal particles approach the bulk viscosity of the host medium more reliably than corresponding single point measurements. In addition, active microrheology offers the advantage of enhanced signal to noise over passive techniques. Recently, we reported the observation of a motional resonance induced in a probe particle in dual-trap optical tweezers when the control particle was driven externally [Paul et al. Phys. Rev. E {\bf 96}, 050102(R) (2017)]. We now demonstrate that the amplitude and phase characteristics of the motional resonance can be used as a sensitive tool for active two-point microrheology to measure the viscosity of a viscous fluid. Thus, we measure the viscosity of viscous liquids from both the amplitude and phase response of the resonance, and demonstrate that the zero-crossing of the phase response of the probe particle with respect to the external drive is superior compared to the amplitude response in measuring viscosity at large particle separations. We compare our viscosity measurements with that using a commercial rheometer and obtain an agreement $\sim1\%$. The method can be extended to viscoelastic material where the frequency dependence of the resonance may provide further accuracy for active microrheological measurements.

physics.app-ph↗

arXiv2017

Direct verification of the fluctuation-dissipation relation in viscously coupled oscillators

The fluctuation-dissipation relation, a central result in non-equilibrium statistical physics, relates equilibrium fluctuations in a system to its linear response to external forces. Here we provide a direct experimental verification of this relation for viscously coupled oscillators, as realized by a pair of optically trapped colloidal particles. A theoretical analysis, in which interactions mediated by slow viscous flow are represented by non-local friction tensors, matches experimental results and reveals a frequency maximum in the amplitude of the mutual response which is a sensitive function of the trap stiffnesses and the friction tensors. This allows for its location and width to be tuned and suggests the utility of the trap setup for accurate two-point microrheology.

cond-mat.soft↗

arXiv2017

Fast Bayesian inference of optical trap stiffness and particle diffusion

Bayesian inference provides a principled way of estimating the parameters of a stochastic process that is observed discretely in time. The overdamped Brownian motion of a particle confined in an optical trap is generally modelled by the Ornstein-Uhlenbeck process and can be observed directly in experiment. Here we present Bayesian methods for inferring the parameters of this process, the trap stiffness and the particle diffusion coefficient, that use exact likelihoods and sufficient statistics to arrive at simple expressions for the maximum a posteriori estimates. This obviates the need for Monte Carlo sampling and yields methods that are both fast and accurate. We apply these to experimental data and demonstrate their advantage over commonly used non-Bayesian fitting methods.

physics.data-an↗