arXiv subjects

Durbar Roy

Publications and source records attributed to Durbar Roy.

11 recordsLinked to original sources

arXiv2025

Preliminary experimental investigation on the interaction of a subaqueous dune like granular structure with a turbulent open channel flow

We study the interaction of a subaqueous dune like granular structure with a turbulent open channel flow experimentally using optical diagnostics in the Shields and Froude parameter space ($0.05{\leq}{\theta}{\leq}0.32$, $0.1{\leq}Fr{\leq}0.4$). Interactions between the turbulent flow and the granular structure give rise to transient erosion-deposition dynamics leading to various types of particle transport. The subaqueous structures in the channel bed evolves due to shear-stress-induced erosion, gravity-driven deposition, and subsequent particle transport. We study the centroid motion and the granular structure shape evolution. At lower end of our ${\theta}-Fr$ parameter space, we observe no erosion and the structure remains at rest. We show that the critical Shields number (${\theta}_{cr}$) is of the order of $0.1$ beyond which erosion starts to occur. At intermediate values of ${\theta}$ and $Fr$ we observe slow erosion, resulting in a rigid body motion of the granular structure without significant shape deformation. Higher values of ${\theta}$ and $Fr$ causes vortex formation at the upstream of the dune resulting in stronger erosion, rapid shape deformation and relatively higher translation velocity of the centroid.

cond-mat.soft↗

arXiv2024

Insights into Transient Dynamics of Bacteria Laden Liquid Bridges

We study evaporation and precipitate formation mechanics of bacteria-laden liquid bridge using experimental and theoretical analysis. Aqueous suspension of motile and non-motile Salmonella Typhimurium and Pseudomonas aeruginosa typically found in contaminated food and water were used in liquid bridge configuration between hydrophilic substrates. Using inverse logarithmic evaporation flux model, we study volume regression for cylindrical/catenoid volume models with confinement distance as a parameter. For all confinement distances, the regression is linear on normalizing both volume and time as in the case of pure sessile drop. However, in normalized volume and dimensional time space, we observe non-linearities as the evaporation time scales non linearly with the confinement distance. The non-linearities were captured using the catenoid model. The catenoid model conforms to the experimental volume regression data at all confinement distances, and the transient liquid bridge interface evolution profile at high confinement distance. We also study the precipitate pattern and bacterial distribution using micro/nano characterization techniques. We show the average precipitate pattern for both sessile and higher confinement distance resembles coffee ring type deposits although the underlying bacterial distribution differs. For lower confinement, we observe pattern resulting from a combination of coffee ring effect, stick-slick motion, and thin film instability. The reduction in confinement distance causes an altered bacterial agglomeration, resulting in a multi-pattern network instead of a single circumferential edge deposition. We show the aerial size of motile bacteria increases with decreasing confinement, whereas the size for non-motile bacteria remains constant in the precipitate.

cond-mat.soft↗

arXiv2024

Insights into the mechanics of pure and bacteria-laden sessile whole blood droplet evaporation

We study the mechanics of evaporation and precipitate formation in pure and bacteria-laden sessile whole blood droplets in the context of disease diagnostics. Using experimental and theoretical analysis, we show evaporation process has three stages based on evaporation rate. In the first stage, edge evaporation results in a gelated contact line along the periphery through sol-gel phase transition. The intermediate stage consists of gelated front propagating radially inwards due to capillary flow and droplet height regression in pinned mode, forming a wet-gel phase. We unearthed that the gelation of the entire droplet occurs in the second stage, and the wet-gel formed contains trace amount of water. In the final slowest stage, wet-gel transforms into dry-gel, leading to desiccation-induced stress forming diverse crack patterns in the precipitate. Slow evaporation in the final stage is quantitatively measured using evaporation of trace water and associated transient delamination of the precipitate. Using axisymmetric lubrication approximation, we compute the transient droplet height profile and the erythrocytes concentration for the first two stages of evaporation. We show that the precipitate thickness profile computed from the theoretical analysis conforms to the optical profilometry measurements. We show that the drop evaporation rate and final dried residue pattern do not change appreciably within the parameter variation of the bacterial concentration typically found in bacterial infection of living organisms. However, at exceedingly high bacterial concentrations, the cracks formed in the coronal region deviate from the typical radial cracks found in lower concentrations.

physics.flu-dyn↗

arXiv2024

Evaporation of bacteria-laden surrogate respiratory fluid droplets: On a hydrophilic substrate versus contact-free environment confers differential bacterial infectivity

The transmission of viruses/ bacteria cause infection predominantly via aerosols. The transmission mechanism of respiratory diseases is complex, including direct or indirect contact, large droplet, and airborne routes apart from close contact transmission. With this pretext, we have investigated two modes of droplet evaporation to understand its significance in airborne disease transmission; a droplet in a contact-free environment, which evaporates and forms droplet nuclei, and a droplet on a hydrophilic substrate (fomite). The study examines mass transport, the deposition pattern of bacteria in the precipitates, and their survival and virulence. The osmotic pressure increases with the salt concentration, inactivating the bacteria embedded in the precipitates with accelerated evaporation. Further, the bacteria's degree of survival and enhanced pathogenicity are compared for both evaporation modes. The striking differences in pathogenicity are attributed to the evaporation rate, oxygen availability, and reactive oxygen species (ROS) generation.

physics.bio-ph↗

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

Future Research Perspective on the Interfacial Physics of Non-Invasive Glaucoma Testing in Pathogen Transmission from the Eyes

Non-contact Tonometry (NCT) is a non-invasive ophthalmologic technique to measure intraocular pressure (IOP) using an air puff for routine glaucoma testing. Although IOP measurement using NCT has been perfected over many years, various phenomenological aspects of interfacial physics, fluid structure interaction, waves on corneal surface, and pathogen transmission routes to name a few are inherently unexplored. Research investigating the interdisciplinary physics of the ocular biointerface and of the NCT procedure is sparse and hence remains to be explored in sufficient depth. In this perspective piece, we introduce NCT and propose future research prospects that can be undertaken for a better understanding of the various hydrodynamic processes that occur during NCT from a pathogen transmission viewpoint. In particular, the research directions include the characterization and measurement of the incoming air puff, understanding the complex fluid-solid interactions occurring between the air puff and the human eye for measuring IOP, investigating the various waves that form and travel; tear film breakup and subsequent droplet formation mechanisms at various spatiotemporal length scales. Further, from ocular disease transmission perspective, the disintegration of the tear film into droplets and aerosols poses a potential pathogen transmission route during NCT for pathogens residing in nasolacrimal and nasopharynx pathways. Adequate precautions by opthalmologist and medical practioners are therefore necessary to conduct the IOP measurements in a clinically safer way to prevent the risk associated with pathogen transmission from ocular diseases like conjunctivitis, keratitis and COVID-19 during the NCT procedure.

physics.med-ph↗

arXiv2023

Insights into air cushion dynamics during drop impact on heated substrate at low impact energy

We study the air layer dynamics beneath a drop impinging a heated surface at low impact energy using high-speed reflection interferometry imaging and theoretical analysis. The air film has been subdivided into two distinct disjoint regions, the central dimple and the peripheral disc. We decipher that a gaussian profile can approximate the dynamic shape evolution of the central air dimple. We further observe that the dimple geometry is a function of impact energy and its dependence on surface temperature is relatively weak. The air layer rupture time and rupture radius increases with increase in substrate temperature. We characterize the air layer profile as a 2D Knudsen field and show that a unified treatment, including continuum and non-continuum mechanics, is required to comprehend the air layer dynamics coherently. The airflow dynamics in the central dimple region falls within the purview of continuum stokes regime. In contrast, the peripheral air disc falls within the non-continuum (gas kinetic effects) slip flow and transition regime characterized by a high Knudsen number. However, the initial average air disc expansion dynamics could be understood in terms of stokes approximation. In non-continuum regimes of the peripheral air disc, we discover intriguing asymmetric interface perturbations. The asymmetric wetting of the substrate initiates at the edge of the peripheral disc region.These perturbative structures cause asymmetric wetting/contact between the droplet and the substrate. Due to the asymptotic effects of capillary and van der Waals interaction in the disc region, the sub-micron spatial structures can exist at short time scales.

physics.flu-dyn↗

arXiv2022

On the mechanics of droplet surface crater during impact on immiscible viscous liquid pool

We study drop impacts on immiscible viscous liquid pool and investigate the formation of droplet surface craters using experimental and theoretical analysis. We attribute the formation of air craters to the rapid deceleration of the droplet due to viscous drag force. The droplet response to the external impulsive decelerating force induces oscillatory modes on the surface exposed to the air forming capillary waves that superimpose to form air craters of various shapes and sizes. We introduce a non-dimensional parameter ($Γ$), that is, the ratio of drag force to the capillary force acting on the droplet. We show that $Γ$ is directly proportional to the capillary number. We show that droplets forming air craters of significant depths have $Γ>1$. Further, we demonstrate that Legendre polynomials can locally approximate the central air crater jet profile. We also decipher that the air crater response time scale ($T$) varies as the square root of impact Weber number ($T{\sim}We^{1/2}$). Further, we generalize the local droplet response with a global response model for low-impact energies based on an eigenvalue problem. We represent the penetrating drop as a constrained Rayleigh drop problem with a dynamic contact line. The air-water interface dynamics is analyzed using an inviscid droplet deformation model for small deformation amplitudes. The local and global droplet response theory conforms with each other and depicts that the deformation profiles could be represented as a linear superposition of eigenmodes in Legendre polynomial basis. We unearth that the droplet response in an immiscible impact system differs from the miscible impact systems due to the presence of such a dynamic contact line.

physics.flu-dyn↗

arXiv2022

Droplet impact on immiscible liquid pool: Multi-scale dynamics of entrapped air cushion at short timescales

We have detected unique hydrodynamic topology in thin air film surrounding the central air dimple formed during drop impact on an immiscible liquid pool. The pattern resembles spinodal and finger-like structures typically found in various thin condensed matter systems. However, similar structures in thin entrapped gas films during drop impacts on solids or liquids have not been reported to date. The thickness profile and the associated dewetting dynamics in the entrapped air layer are investigated experimentally and theoretically using high-speed reflection interferometric imaging and linear stability analysis. We attribute the formation of multiscale thickness perturbations, associated ruptures, and finger-like protrusions in the draining air film as a combined artifact of thin-film and Saffman-Taylor instabilities. The characteristic length scales depend on the impact Weber number, the ratio of the liquid pool to droplet viscosity, and the ratio of air-water to air-oil surface tension.

physics.flu-dyn↗

arXiv2021

On the Thermofluidics of a Steady Laminar Jet Impacting on a Rotating Hot Plate: An ab-initio Scaling Perspective

We provide an ab-initio scaling analysis for liquid film thickness and Nusselt number of a steady laminar jet impacting a rotating heated plate. The limiting scaling regimes incorporating the evaporative effects along the liquid-vapor interface have been probed. The dependence of liquid film thickness and Nusselt number on Reynolds, Rossby, and Prandtl number have been unearthed using scaling analysis of the integral and differential form of the continuity, Navier-Stokes and energy equation in a cylindrical coordinate system. Boundary layer analysis has been used to discover a critical length that plays a significant role in understanding the effect of evaporation on hydrodynamic, thermal boundary layer thicknesses, and subsequently Nusselt number. The evaporative effects on liquid film thickness become increasingly important after a certain critical radius. The scaling laws derived were compared with existing experimental data available in the literature, and the trends predicted were consistent.

physics.flu-dyn↗

arXiv2020

Dynamics of Droplet Generation from Corneal Tear Film during Non-contact Eye Procedure in the Context of COVID-19

Non-invasive medical diagnostics demonstrate a propensity for droplet generation and should be studied to devise risk mitigation strategies against the spread of the SARS-CoV-2 virus. We investigate the air-puff tonometry, which uses a short-timed air-puff to applanate the human eye in a bid to detect the early onset of glaucoma by measuring the intraocular pressure. The air-puff consists of a vortex trailed by a high-speed jet. High-speed imaging of the eye during a typical tonometry measurement reveals a sequence of events starting with the interaction between the tear layer and the air puff leading to an initial sheet ejection. It is immediately followed by the trailing jet applanating the central corneal section, causing capillary waves to form and interact with the highly 3D transient expanding sheet. Such interaction with the capillary waves and the surrounding airfield due to the trailing jet causes the expanding sheet to undergo bag breakup, finger formation by Rayleigh Taylor instability and further break up into subsequent droplets by Rayleigh Plateau instability, which eventually splashes onto nearby objects, potentially forming fomites or aerosols which can lead to infections. The complex spatiotemporal phenomenon is carefully documented by rigorous experiments and corroborated using comprehensive theoretical analyses.

physics.med-ph↗