arXiv subjects

Aloke Kumar

Publications and source records attributed to Aloke Kumar.

At least 19 recordsLinked to original sources

arXiv2026

Influence of Rotational Diffusion on Macromolecular Self-Assembly Kinetics

Macromolecular self-assembly underlies a plethora of biological processes and provides a versatile route for fabricating functional soft materials. The kinetics of self-assembly in solution are inherently stochastic and are fundamentally governed by the interplay of translational and rotational diffusion of the constituent macromolecules. While most computational studies model macromolecules as patchy spherical colloids, thereby neglecting the influence of polymer architecture and internal conformational dynamics, the role of these factors in macromolecular self-assembly kinetics remains poorly understood. Here, we investigate the self-assembly of two patchy macromolecules with different architectures, namely linear chains and star polymers with four and seven arms. The hydrodynamic radii of the macromolecules are chosen to be nearly identical, thereby matching their translational diffusion coefficients and thus isolating the influence of rotational diffusion on the self-assembly process. The binding probability of the patchy macromolecules is found to depend strongly on their internal architecture. Furthermore, reactive path density analysis reveals that self-assembly pathways are influenced by the rotational diffusion coefficient of the individual macromolecules. Overall, this study establishes a bridge between the equilibrium dynamics of macromolecules and their self-assembly kinetics, highlighting the importance of polymer internal architecture in the process of self-assembly.

cond-mat.soft↗

arXiv2025

Elasto-viscous regime in coalescence of viscoelastic droplets

We report a regime transition in the coalescence of concentrated polymeric droplets in a pendant-pendant configuration. While Newtonian droplet coalescence has been extensively studied with distinct identification of viscous and inertial regimes, the presence of polymers introduces additional regimes governed by elasticity and molecular relaxation effects. The coalescence process is typically characterized by the neck radius, $R$, of the liquid bridge connecting the two droplets, following a power-law relation with time: $R=at^{b}$. Most of the existing studies, including Newtonian and non-Newtonian fluids, report a unique value of $b$ for a given fluid. In contrast, our findings reveal that elasticity induces a temporal transition from one $b$ values to another, marking a shift in the coalescence regime. In particular, our measured $b$ value falls in the sub-Newtonian regime, highlighting the role of elasticity in governing the dynamics. We conducted two-dimensional simulations using a volume-of-fluid framework with the exponential Phan-Thien-Tanner model, which quantitatively reproduced Newtonian benchmarks and accurately captured viscoelasticity induced neck growth in close agreement with experiments. Furthermore, we determined the curvature experimentally, as the assumptions typically employed in the literature to approximate axial curvature are not universally valid.

physics.flu-dyn↗

arXiv2024

Re-orientational dynamics of ring polymers in dilute solutions

Advances in controlled polymerization have enabled the synthesis of mechanically interlocked polymers like molecular knots and linear[n]catenane. These aesthetic macromolecules with unique topological constraints in the form of mechanical bonds are well known for their fascinating transport and rheological properties in the development of molecular machines and in knotted protein dynamics in biological applications. The diffusion dynamics of such macromolecular structures with large internal degrees of freedom are generally studied by using an equivalent size parameter, i.e., hydrodynamic radius, defined using Zimm theory. Although diffusion rates are expected to depend strongly on the molecular topological constraints in macromolecules, their explicit effects on translational and reorientational dynamics are still unknown. Here, we perform an in silico study on the diffusion dynamics of seven topologically distinct polymer chains in the limit of infinite dilution using multi-particle collision dynamics. The modeled polymers are linear, ring, linear[2]catenane, trefoil knot, linear[3]catenane, cyclic[3]catenane, and Borromean ring. The molecular weights of these macromolecules are selected such that the resulting hydrodynamic radius is approximately equal to each other. We show that while the translational diffusion coefficients of these topologically distinct polymer chains are approximately equal to each other in agreement with the Zimm theory, there are significant differences among the values of the corresponding rotational diffusion coefficients. We show that the presence of mechanical bonds in the polymer chains slows down the rotational diffusion significantly, thus suggesting the role of molecular topology on reaction kinetics of macromolecules.

cond-mat.soft↗

arXiv2024

The SAP-1 Payload: A Technology Demonstration for Space-Based Microbiology Experiments

The SSPACE Astrobiology Payload (SAP) series, starting with the SAP-1 project is designed to conduct in-situ microbiology experiments in low earth orbit. This payload series aims to understand the behaviour of microbial organisms in space, particularly those critical for human health, and the corresponding effects due to microgravity and solar/galactic radiation. SAP-1 focuses on studying Bacillus clausii and Bacillus coagulans, bacteria beneficial to humans. It aims to provide a space laboratory for astrobiology experiments under microgravity conditions. The hardware developed for these experiments is indigenous and tailored to meet the unique requirements of autonomous microbiology experiments by controlling pressure, temperature, and nutrition flow to bacteria. A rotating platform, which forms the core design, is innovatively utilised to regulate the flow and mixing of nutrients with dormant bacteria. The technology demonstration models developed at SSPACE have yielded promising results, with ongoing efforts to refine, adapt for space conditions, and prepare for integration with nanosatellites or space modules. The anticipated payload will be compact, approximately 1U in size (10cm x 10cm x 10cm), consume less than 5W power, and offer flexibility for various microbiological studies.

astro-ph.IM↗

arXiv2024

Elasticity affects the shock-induced aerobreakup of a polymeric droplet

Boger fluids are viscoelastic liquids having constant viscosity for a broad range of shear rates. They are commonly used to separate the effects of liquid elasticity from viscosity in any experiment. We present an experimental study on the shock-induced aerobreakup of a Boger fluid droplet in the Shear-induced entrainment (SIE) and catastrophic breakup regime (Weber number ranging from ~ 800 to 5000). The results are compared with the aerobreakup of a Newtonian droplet having similar viscosity, and with shear-thinning droplets. The study aims to identify the role of liquid elasticity without the added complexity of simultaneous shear-thinning behavior. It is observed that at the early stages of droplet breakup, liquid elasticity plays an insignificant role, and all the fluids show similar behavior. However, during the late stages, the impact of liquid elasticity becomes dominant, which results in a markedly different morphology of the fragmenting liquid mass compared to a Newtonian droplet.

physics.flu-dyn↗

arXiv2023

Jamming modulates coalescence dynamics of shear-thickening colloidal droplets

Recent investigations into coalescence dynamics of complex fluid droplets revealed the existence of sub-Newtonian behaviour for polymeric fluids (elastic and shear thinning). We hypothesize that such delayed coalescence or sub-Newtonian coalescence dynamics may be extended to the general class of shear thickening fluids. To investigate this droplets of aqueous corn-starch suspensions were chosen and its coalescence in sessile pendant configuration was probed by high-speed real time imaging. Temporal evolution of the neck (growth) during coalescence was quantified as a function of suspended particle weight fraction \phi_w. The necking behaviour was found to evolve as the power-law relation $R=at^b$ where R is neck radius with exponent $\b\le0.5$ implying it is a subset of the generic sub-Newtonian coalescence. Second significant delay in the coalescence dynamics is observed for particle fractions beyond the jamming fraction {\ \phi}_w>\ \phi_J\geq0.35}. Our proposed theoretical model captures this delay implicitly through altered suspension viscosity stemming from increased particle content.

cond-mat.soft↗

arXiv2023

Insights into bubble droplet interactions in evaporating polymeric droplets

Polymer droplets subjected to a heated environment have significance in several fields ranging from spray drying and powder formation to surface coating. In the present work, we investigate the evaporation of a high viscoelastic modulus aqueous polymeric droplet in an acoustically levitated environment. Depending on the laser irradiation intensity, we observe nucleation of a bubble in the dilute regime of polymer concentration, contrary to the previously observed bubble nucleation in a semi-dilute entangled regime for low viscoelastic modulus polymer droplets. After the bubble nucleation, a quasi steady bubble growth occurs depending on the laser irradiation intensity and concentrations. Our scaling analysis reveals that bubble growth follows Plesset-Zwick criteria independent of the viscoelastic properties of the polymer solution. Further, we establish that the onset of bubble growth has an inverse nonlinear dependence on the laser irradiation intensity. At high concentrations and laser irradiation intensities, we report the expansion and collapse of polymer membrane without rupture, indicating the formation of an interfacial skin with significant strength. The droplet oscillations are primarily driven by the presence of multiple bubbles and, to some extent, by the rotational motion of the droplet. Finally, depending on the nature of bubble growth, different types of precipitate form contrary to the different modes of atomization observed in low viscoelastic modulus polymer droplets.

cond-mat.soft↗

arXiv2023

Aerodynamic bag breakup of a polymeric droplet

The aerodynamic breakup of a polymeric droplet in the bag breakup regime is investigated experimentally and compared with the result of the Newtonian droplet. To understand the effect of liquid elasticity, the Weber number is kept fixed ($\approx$ 12.5) while the elasticity number is varied in the range of $\sim 10^{-4}-10^{-2}$. Experiments are performed by allowing a liquid droplet to fall in a horizontal, continuously flowing air stream. It is observed that the initial deformation dynamics of a polymeric droplet is similar to the Newtonian droplet. However, in the later stages, the actual fragmentation of liquid mass is resisted by the presence of polymers. Depending upon the liquid elasticity, fragmentation can be completely inhibited in the timescale of experimental observation. We provide a framework to study this problem, identify the stages where the role of liquid elasticity can be neglected and where it must be considered, and finally, establish a criterion that governs the occurrence or the absence of fragmentation in a specified time period.

physics.flu-dyn↗

arXiv2023

A dynamic fluid landscape mediates the spread of bacteria

Microbial interactions regulate their spread and survival in competitive environments. It is not clear if the physical parameters of the environment regulate the outcome of these interactions. In this work, we show that the opportunistic pathogen Pseudomonas aeruginosa occupies a larger area on the substratum in the presence of yeast such as Cryptococcus neoformans , than without it. At the microscopic level, bacterial cells show an enhanced activity in the vicinity of yeast cells. We observe this behaviour even when the live yeast cells are replaced with heat-killed cells or with spherical glass beads of similar morphology, which suggests that the observed behaviour is not specific to the biology of microbes. Upon careful investigation, we find that a fluid pool is formed around yeast cells which facilitates the swimming of the flagellated P. aeruginosa , causing their enhanced motility. Using mathematical modeling we demonstrate how this local enhancement of bacterial motility leads to the enhanced spread observed at the level of the plate. We find that the dynamics of the fluid landscape around the bacteria, mediated by the growing yeast lawn, affects the spreading. For instance, when the yeast lawn grows faster, a bacterial colony prefers a lower initial loading of yeast cells for optimum enhancement in the spread. We confirm our predictions using Candida albicans and C. neoformans, at different initial compositions. In summary, our work shows the importance of considering the dynamically changing physical environment while studying bacterial motility in complex environments.

q-bio.CB↗

arXiv2023

Synthetic space bricks from lunar and martian regolith via sintering

The prospect of establishing extra-terrestrial habitats using in situ resource utilization (ISRU) constitutes a long-term goal of multiple space agencies around the world. In this work, we investigate sintering as a potential route for making building blocks -- termed synthetic space bricks -- using \emph{in situ} regolith material. By systematically investigating sintering parameters using a numerical lattice model, coupled with experimental observations and post sintering characterization, we propose a process protocol for two lunar -- lunar highland simulant (LHS) and lunar mare dust simulant (LMS) -- and one martian (martian global simulant, MGS) simulants. The resulting bricks demonstrate compressive strengths of upto 45 MPa under uniaxial loading, depending on the simulant used. These strengths are much greater than those typically mandated for structural applications under reduced gravity. We infer microscale sintering mechanisms at the individual particle level indirectly, by measuring temporal evolution exponents of sample dimensions during sintering. For all three simulants, volume diffusion appears to be the primary mechanism for particle coalescence. Our results clearly make a strong case for the use of sintering as a potentially scalable method for consolidating regolith into brick-like structures for load-bearing applications in extra-terrestrial settings.

physics.app-ph↗

arXiv2023

Diffusion dynamics of star-shaped macromolecules in dilute solutions

Polymer chains dissolved in a solvent take random conformations due to large internal degrees of freedom and are characterized geometrically by their average shape and size. The diffusive dynamics of such large macromolecules play an indispensable role in a plethora of engineering applications. The influence of the size of the polymer chain on its diffusion is well studied, whereas the same cannot be said for the shape of the polymer chain. In the present work, the influence of shape on the center-of-mass diffusion of the star-shaped chains in solution is investigated using Multi-particle Collision Dynamics. Star-shaped chains of varying degrees of functionality are modeled in a good solvent at infinite dilution. The radius of gyration($R_g$) of the star-shaped chains follows a functionality-independent scaling law with the chain length($N$), $R_g \sim N^{\nu}$, where $\nu \sim 0.627$. The shape of the polymer chains is calibrated by relative shape anisotropy. Highly anisotropic star-shaped polymer chains are found to have a faster rate of diffusion along the translational direction due to a slower rate of rotational diffusion when the radius of gyration of the polymer chains is maintained constant.

cond-mat.soft↗

arXiv2023

Shock induced atomisation of a liquid metal droplet

The present study uses Galinstan as a test fluid to investigate the shock-induced aerobreakup of a liquid metal droplet in a high Weber number regime (We ~ 400 - 8000). Atomization dynamics is examined for three test environments: oxidizing (Galinstan-air), inert (Galinstan-nitrogen), and conventional fluids (DI water-air). Due to the readily oxidizing nature of liquid metals, their atomization in an industrial scale system is generally carried in inert atmosphere conditions. However, no previous study has considered gas-induced secondary atomization of liquid metals in inert conditions. Due to experimental challenges associated with molten metals, laboratory scale models are generally tested for conventional fluids like DI water, liquid fuels, etc. The translation of results obtained from conventional fluid to liquid metal atomization is rarely explored. Here a direct multi-scale spatial and temporal comparison is provided between the atomization dynamics of conventional fluid and liquid metals under oxidizing and inert conditions. The liquid metal droplet undergoes breakup through Shear-Induced Entrainment (SIE) mode for the studied range of Weber number values. The prevailing mechanism is explained based on the relative dominance of droplet deformation and KH wave formation. The study provides quantitative and qualitative similarities for the three test cases and explains the differences in morphology of fragmenting secondary droplets in the oxidizing test case (Galinstan-air) due to rapid oxidation of the fragmenting ligaments. A phenomenological framework is postulated for predicting the morphology of secondary droplets. The formation of flake-like secondary droplets in the Galinstan air test case is based on the oxidation rate of liquid metals and the properties of the oxide layer formed on the atomizing ligament surface.

physics.flu-dyn↗

arXiv2022

Depth from Defocus Technique Applied to Unsteady Shock-Drop Secondary atomization

The two-sensor depth from defocus (DFD) technique for the measurement of drop sizes in a spray is further developed to achieve higher spatial and temporal resolution, to improve estimates of size and number concentration, and to provide additional guidelines for the calibration and design of the optical system for a specific application. The technique and these improvements are demonstrated using the case of secondary atomization when a shock wave interacts with a single drop. This is an application in which both high spatially and temporally resolved number density and size distributions of secondary droplets generated in the wake of the original drop are necessary.

physics.flu-dyn↗

arXiv2022

Newtonian coalescence in colloidal and non-colloidal suspensions

Coalescence event in pendant and sessile drop is distinguished by the formation and evolution of the liquid bridge created upon singular contact. The bridge radius, $R$, is known to evolve as $R\sim t^b$, with power-law exponent, $b$, signifying the dominant governing forces. In this work, we experimentally explore the phenomenon in sub-classes of complex fluids namely, colloidal and non-colloidal suspensions that have particle hydrodynamic interactions as origin of viscoelasticity. Our observations suggest that such fluids have flow dependent thinning response with finite elasticity in shear flows but negligible in extensional flows. Based on these, the study extends the Newtonian universality of $b=0.5$ to these thinning fluids. Further we fortify these observations through a theoretical model developed by employing Ostwald-de Waele constitutive law. Finally, we utilize this theoretical model to inspect the existence of arrested coalescence in generalized Newtonian fluids.

physics.flu-dyn↗

arXiv2022

Sub-Newtonian coalescence in polymeric fluids

We present a theoretical framework for capturing the coalescence of a pendant drop with a sessile drop in polymeric fluids. The framework is based on the unification of various constitutive laws under high Weissenberg creeping flow limit. Our results suggests that the phenomenon comes under a new regime namely, the sub-Newtonian regime followed by the limiting case of arrested coalescence with the arrest angle $\theta_{arrest}\propto Ec^{-1/2}$, where $Ec$ is the Elasto-capillary number. Further, we propose a new time scale $T^*$ integrating the continuum variable $Ec$ and the macromolecular parameter $N_e$, the entanglement density to describe the liquid neck evolution. Finally, we validate the framework with high speed imaging experiments performed across different molecular weights of Poly(ethylene oxide) (PEO).

physics.flu-dyn↗

arXiv2022

Contact angle hysteresis can modulate the Newtonian rod climbing effect

The present work investigates the role of contact angle hysteresis at the liquid-liquid-solid interface (LLS) on the rod climbing effect of two immiscible Newtonian liquids using experimental and numerical approaches. Experiments revealed that the final steady state contact angle, $\theta_{w}$ at the LLS interface varies with the rod rotation speed, $\omega$. For the present system, $\theta_{w}$ changes from $\sim$69$^{\circ}$ to $\sim$83$^{\circ}$ when the state of the rod is changed from static condition to rotating at 3.3 Hz. With further increase in $\omega$, the $\theta_{w}$ exceeds 90$^{\circ}$ which cannot be observed experimentally. It is inferred from the simulations that the input value of $\theta_{w}$ saturates and attains a constant value of $\sim$120$^{\circ}$ for $\omega>$ 5 Hz. Using numerical simulations, we demonstrate that this contact angle hysteresis must be considered for the correct prediction of the Newtonian rod climbing effect. Using the appropriate values of the contact angle in the boundary condition, an excellent quantitative match between the experiments and simulations is obtained in terms of: the climbing height, the threshold rod rotation speed for onset of climbing, and the shape of liquid-liquid interface. This resolves the discrepancy between the experiments and simulations in the existing literature where a constant value of the contact angle has been used for all speeds of rod rotation.

physics.flu-dyn↗

arXiv2022

Porosity governs failure in bioconsolidated space bricks

Understanding the mechanical response and failure of consolidated extra-terrestrial soils requires analyses of the interactions between propagating cracks the material's inherent pore structure. In this work, we investigate the fracture behaviour of lunar soil simulant consolidated using microbially induced calcite precipitation (MICP). We develop a numerical framework, based on a lattice network with local beam elements, to simulate the nucleation, propagation, branching and merging of multiple cracks within the sample. Our simulations capture the effects of local pores on crack paths as well as provides a means to predict the behaviour of samples with varying global porosity and/or uncertainties in local material stiffness. We identify multiple statistical lattice parameters that encode signatures of single or multiple crack growth events. Our results reveal the complexities involved in the fracture process with porous brittle solids and may easily be adapted to understand failure mechanisms and micro/macro crack evolution in other consolidated structures.

cond-mat.mtrl-sci↗

arXiv2022

Shock induced aerobreakup of a polymeric droplet

Droplet atomization through aerobreakup is omnipresent in various natural and industrial processes. Atomization of Newtonian droplets is a well-studied area; however, non-Newtonian droplets have received less attention despite their frequent encounters. By subjecting polymeric droplets of different concentrations to the induced airflow behind a moving shock wave, we explore the role of elasticity in modulating the aerobreakup of viscoelastic droplets. Three distinct modes of aerobreakup are identified for a wide range of Weber number ($\sim 10^2-10^4$) and Elasticity number ($\sim 10^{-4}-10^2$) variation; these modes are: vibrational, shear-induced entrainment and catastrophic breakup mode. Each mode is described as a three stage process. Stage-I is the droplet deformation, stage-II is the appearance and growth of hydrodynamic instabilities, and stage-III is the evolution of liquid mass morphology. It is observed that elasticity plays an insignificant role in the first two stages, but a dominant role in the final stage. The results are described with the support of adequate mathematical analysis.

physics.flu-dyn↗