arXiv subjects

A. K. Sen

Publications and source records attributed to A. K. Sen.

At least 19 recordsLinked to original sources

arXiv2026

Confinement-induced evolution and breakup of viscoelastic filaments in microfluidic coflows

Viscoelastic filament thinning is classically described by elastocapillary dynamics in extensional flows, yet in confined microchannels the combined effects of wall-induced shear, elasticity, and capillarity remain poorly understood. Here, we experimentally investigate the breakup of a shear-thinning viscoelastic liquid coflowing with an immiscible Newtonian fluid in a rectangular microchannel, focusing on the formation, stretching, instability, and breakup of the thin filament connecting the primary droplet to the upstream liquid. Four regimes: stable coflow, squeezing, dripping, and jetting are identified and mapped using the capillary numbers of the dispersed and continuous phases and an elastocapillary parameter. Although elasticity weakly affects the onset of primary droplet formation, it strongly alters later filament dynamics by delaying capillary breakup and stabilising long-lived filaments. Scaling analyses based on capillary, viscous, and elastic force balances predict the primary droplet size, critical filament thickness at instability onset, maximum filament length, and critical jet length. Particle tracking shows that confinement creates a non-uniform wall-induced shear field along the inclined filament, producing spatial variations in interfacial velocity and initiating the first bead-on-a-string instability at the location of maximum shear. A Rayleigh-Plateau analysis incorporating an effective viscosity derived from the Oldroyd-B model predicts the instability wavelength and growth rate to the correct order of magnitude. These results show that confined viscoelastic breakup is governed not solely by classical elastocapillary thinning, but by a coupled wall-shear-elasticity mechanism controlling filament stretching, instability, and secondary droplet formation, thereby providing a predictive framework for filament-mediated breakup in confined viscoelastic multiphase flows.

physics.flu-dyn↗

arXiv2026

Viscoelastic control of acoustic particle migration and trapping in microchannels

Particle migration and trapping in ultrasonically actuated microscale flows arise from the competition between acoustic radiation forces and streaming-induced drag. While these mechanisms are well understood in Newtonian fluids, the role of fluid viscoelasticity in governing particle dynamics remains largely unexplored. Here, we investigate particle transport and trapping in a viscoelastic fluid within an ultrasonically excited microchannel under the combined action of acoustic streaming and radiation forces. Using a perturbation framework, we solve the continuity, momentum and constitutive equations for an Oldroyd-B fluid to obtain the oscillatory acoustic field and the resulting steady streaming flows in the bulk and near-wall boundary layers. Acoustic radiation forces, incorporated through a semi-analytical model, drives particle migration, while streaming-induced drag can oppose, alter or suppress trapping. We show that particle trajectories and equilibrium trapping locations are governed primarily by the Deborah number ($De$) and viscous diffusion number ($Dv$). At high $Dv$, increasing $De$ shifts the trapping location from the bulk region to the channel wall, pressure nodal line, channel centre or ultrasound symmetry line. We further determine the critical particle size governing the transition between radiation-dominated and streaming-dominated regimes as a function of $De$ and $Dv$. The critical particle size can become significantly smaller than that in a Newtonian fluid, enabling effective manipulation of submicron particles and overcoming a key limitation of conventional acoustofluidics. These results demonstrate how viscoelasticity fundamentally modifies acoustophoretic transport and establish new mechanisms for tunable particle migration and trapping in complex fluids.

physics.flu-dyn↗

arXiv2026

Ultrasound-controlled stream splitting in a microfluidic coflow

Precise control of multiphase microfluidic flows underpins applications ranging from chemical processing to biomedical diagnostics. We investigate the response of a liquid--liquid coflow in a rectangular microchannel to an externally applied standing acoustic field. Acoustic excitation destabilizes an otherwise stable interface, giving rise to a sequence of reversible interfacial regimes: waviness, splitting, relocation, and stream-droplet breakup. Remarkably, a distinct splitting regime emerges, where a continuous stream partially splits into droplets at tunable locations while retaining a thin residual stream. Unlike conventional droplet breakup, this regime avoids complete disruption of the main flow, enables droplet generation at high capillary numbers, and allows spatial control over droplet formation. Extending across a broad range of capillary numbers, we examine how variations in flow conditions and applied acoustic power influence these regimes. Combining experiments, numerical simulations, and theoretical scaling, we elucidate the mechanisms governing this droplet generation mode and the associated regime transitions. Systematic measurements show that droplet size and residual stream thickness are governed primarily by hydrodynamic parameters, whereas the acoustic field controls the onset and spatial location of the breakup. These results establish a simple avenue for stream splitting and drop generation on-demand in a microfluidic coflow, opening new possibilities for spatially programmable manipulation of multiphase flows.

physics.flu-dyn↗

arXiv2026

Fluid viscoelasticity controls acoustic streaming via shear waves

Control of acoustic streaming can significantly impact fluid and particle transport in microfluidics. We report enhancement, suppression, and reversal of acoustic streaming inside a rectangular microchannel by controlling the fluid viscoelastic properties. Our study reveals that the streaming regimes depend on Deborah number ($De$) and viscous diffusion number ($Dv$), expressed in terms of a Streaming Coefficient ($C_s$). We find streaming is enhanced when $C_s>1$, suppressed for $0\leq C_s\leq1$, and reversed when $C_s<0$. We explain the regimes in terms of the interplay between the Reynolds and viscoelastic stresses that collectively drive fluid motion. Remarkably, we discover the role of viscoelastic shear waves in acoustic streaming transition characterized by the ratio of acoustic attenuation length and shear wavelength. We gain deeper insight into the streaming transition by examining energy dynamics in terms of the loss and storage moduli. Our study may find applications in acousto-microfluidics systems for particle handling and fluid pumping/mixing.

physics.flu-dyn↗

arXiv2025

Deflection of Light due to Kerr Sen Black Hole in Heterotic String Theory using Material Medium Approach

The deflection of light in the gravitational field of a massive body can be analyzed through diverse theoretical approaches. The null geodesic approach is commonly employed to calculate light deflection within strong and weak field limits. Alternatively, several studies have explored the gravitational deflection of light using the material medium approach. For a static, non-rotating spherical mass, the deflection in a Schwarzschild field can be determined by expressing the metric in an isotropic form and evaluating the refractive index to trace the light ray's trajectory. In this study, we extend the above-mentioned approach to the Kerr-Sen black hole spacetime in heterotic string theory, a solution representing a rotating, charged solution in heterotic string theory. The frame-dragging effects inherent to the Kerr-Sen geometry are incorporated to compute the velocity of light rays, enabling the derivation of the refractive index in this field. Considering the far-field approximation, we calculate the deflection of light in the Kerr-Sen spacetime and compare our results with those obtained for the Kerr and Schwarzschild black hole solution in GR.

gr-qc↗

arXiv2024

Transformation equation for frames undergoing non-uniform acceleration such as SHM and rotational motion

Lorentz transformation equations provide us a set of relations between the spacetime coordinates as observed from two different inertial frames. In case, one of the frames is moving with a uniform rectilinear acceleration we have Rindler's transformation equations under such a situation. In the present work, we extend the Rindler's equations to a situation where we have in general non-uniform acceleration. After that we consider the non-inertial frame to undergo simple harmonic motion (SHM) and as a second case we consider the non-inertial frame to move uniformly along a circle. This set of transformation equations will have applications in various branches of Physics and in general in Astrophysics.

gr-qc↗

arXiv2024

The extension of Schwarzschild line element to include uniformly accelerated mass

In this paper we analyze the spacetime geometry due to a Schwarzschild object having uniform accelerated motion. In the beginning, we investigate the gravitational field due to a uniformly moving Schwarzschild object and obtain the spacetime line element for such an object. After analyzing the necessary limiting conditions, the obtained line element is found to be consistent. Next, we extend our work to a uniformly accelerated Schwarzschild object. In that case, we obtain the spacetime line element for both when the acceleration is along the $X-$direction and when the acceleration is in an arbitrary direction on the $XY$ plane (in the later case $X$ may be some third reference direction, which may be useful for future work). The limiting conditions of those line elements have been examined. Such work will have relevant applications in astrophysics when we calculate the geodesic equations of test particles in the gravitational field of objects having uniform accelerated motion.

gr-qc↗

arXiv2022

The space-time line element for static ellipsoidal objects

In this paper, we solved the Einstein's field equation and obtained a line element for static, ellipsoidal objects characterized by the linear eccentricity ($η$) instead of quadrupole parameter ($q$). This line element recovers the Schwarzschild line element when $η$ is zero. In addition to that it also reduces to the Schwarzschild line element, if we neglect terms of the order of $r^{-2}$ or higher which are present within the expressions for metric elements for large distances. Furthermore, as the ellipsoidal character of the derived line element is maintained by the linear eccentricity ($η$), which is an easily measurable parameter, this line element could be more suitable for various analytical as well as observational studies.

gr-qc↗

arXiv2021

Deflection of light due to spheroidal oblate static objects

Deflection of light due to massive objects was predicted by Einstein in his General Theory of Relativity. This deflection of light has been calculated by many researchers in past, for spherically symmetric objects. But, in reality, most of these gravitating objects are not spherical instead they are ellipsoidal ( oblate) in shape. The objective of the present work is to study theoretically the effect of this ellipticity on the trajectory of a light ray. Here, we obtain a converging series expression for the deflection of a light ray due to an ellipsoidal gravitating object, characterised by an ellipticity parameter. As a boundary condition, by setting the ellipticity parameter to be equal to zero, we get back the same expression for deflection as due to Schwarzschild object. It is also found that the additional contribution in deflection angle due to this ellipticity though small, but could be typically higher than the similar contribution caused by the rotation of a celestial object. Therefore for a precise estimate of the deflection due to a celestial object, the calculations presented here would be useful.

gr-qc↗

arXiv2018

The Variation of Photon Speed with Photon Frequency in Quantum Gravity

In the present work, an expression for Planck Mass or Planck Energy is derived by equating the Compton wavelength with the gravitational radius of the Kerr rotating body. Using the modified photon energy-momentum dispersion relation, the variation of the photon propagation speed with photon frequency is derived. It is found that, the photon propagation speed, depends on the frequency of the photon, the rotation parameter of the Kerr rotating body and also on the polarization state of the photon. Quantum gravity effect could be seen from the derived results for the photon propagation speed.

gr-qc↗

arXiv2017

Deflection of light ray due to a charged body using Material Medium Approach

The gravitational deflection of light ray is an important prediction of General Theory of Relativity. In this paper we develop analytical expression of the deflection of light ray without any weak field approximation due to a charged gravitational body represented by Reissner_Nordström (RN) and Janis-Newman-Winicour (JNW) space time geometry, using material medium approach. It is concluded that although both the geometries represent the charged, non-rotating, spherically symmetric gravitating body, but the effect of charge on the gravitational deflection is just opposite to each other. The gravitational deflection decreases with charge in the RN geometry and increases with charge in the JNW geometry. The calculations obtained here are compared with other methods done by different authors. The formalism is applied to an arbitrary selected pulsar PSRB1937+21 as a gravitating body, as a test case.

gr-qc↗

arXiv2017

Gravitational Redshift in Kerr-Newman Geometry Using Gravity's Rainbow

Gravitational redshift is generally reported by most of the authors without considering the influence of the energy of the test particle using various spacetime geometries such as Schwarzschild, Reissner-Nordstrom, Kerr and Kerr-Newman geometries for static, charged static, rotating and charged rotating objects respectively. In the present work, the general expression for the energy dependent gravitational redshift is derived for charged rotating body using the Kerr-Newman geometry along with the energy dependent gravity's rainbow function. It is found that the gravitational redshift is influenced by the energy of the source or emitter. One may obtain greater correction in the value of gravitational redshift, using the high energy photons. Knowing the value of gravitational redshift from a high energy sources such as Gamma-ray Bursters (GRB), one may obtain the idea of upper bounds on the dimensionless rainbow function parameter ($ξ$). Also there may be a possibility to introduce a new physical scale of the order of $\fracξ{E_{Pl}}$.

physics.gen-ph↗

arXiv2016

The effect of gravitation on the polarization state of a light ray

In the present work the rotation of polarization vector due to the gravitational field of a rotating body has been derived, from the general expression of Maxwell's equation in the curved space-time. Considering the far field approximation (i.e impact parameter is greater than the Schwarzschild radius and rotation parameter), the amount of rotation of polarization vector as a function of impact parameter has been obtained for a rotating body (considering Kerr geometry). Present work shows that, the rotation of polarization vector can not be observed in case of Schwarzschild geometry. This work also calculates the effect, considering prograde and retrograde orbit for the light ray. Although the present work demonstrates the effect of rotation of polarization vector for electromagnetic wave (light ray), but it confirms that there would be no net polarization of electromagnetic wave due to the curved space-time geometry.

gr-qc↗

arXiv2016

The effect of porosity of dust particles on polarization and color with special reference to comets

Cosmic dusts are mostly responsible for polarization of the light that we ob- serve from astrophysical objects. They also lead to color-extinction, thermal re- emission and other scattering related phenomena. Dusts are made of small particles which are characterised by their size (radius), composition (matter), and structure (morphology, including porosity). In the present work, we address the question of the role of the dust particle porosity on light polarization and color, using Discrete Dipole Approximation (DDA) light scattering code. To answer this question, we developed an algorithm to generate solid particles of arbitrary values of porosity. In brief, the model considers a given homogeneous structure made of touching dipoles. The dipoles are randomly removed one by one, such that the remaining structure remains connected. We stop the removal process when the desired poros- ity is obtained. Then we study the optical properties of the porous particle. That way, we show how the proper value of the porosity affects the polarization and color of the light scattered by these porous particles. In addition to polarization, porosity has important effects on photometric color. Considering an important application, we emphasize the possible role of the porosity of the cometary dust particles on polarization and color of the light scattered by cometary coma.

astro-ph.SR↗

arXiv2016

Light deflection in Kerr field for off-equatorial source

Deflection angle for a light ray travelling in the equatorial plane of a rotating Kerr mass has been already calculated by various investigators. Considering the light ray to be travelling only slightly above the equatorial plane, calculations have been made for such a ray for its deflection angle. In this paper, we calculate deflection angles for the light ray at various heights, which are small compared to the impact parameter and derive corresponding analytical expressions for deflection angle.

gr-qc↗

arXiv2016

Gravitational redshift from rotating body having intense magnetic field

It is well known fact that gravitational field can alter the space-time structure and gravitational redshift is its one example. Electromagnetic field can also alter the space-time similar to gravitational field. So electromagnetic field can give rise to an additional effect on gravitational redshift. There are many objects in nature, like neutron stars, magnetars etc which have high amount of rotation and magnetic field. In the present paper we will derive the expression of gravitational redshift from rotating body having intense magnetic field by using the action function of the electromagnetic fields.

gr-qc↗

arXiv2016

Frame-Dragging from Charged Rotating Body

In the present paper, we have considered the three parameters: mass, charge and rotation to discuss their combined effect on frame dragging for a charged rotating body. If we consider the ray of light which is emitted radially outward from a rotating body then the frame dragging shows a periodic nature with respect to coordinate $ϕ$ (azimuthal angle). It has been found that the value of frame dragging obtains a maximum at, $ ϕ=\fracπ{2}$ and a minimum at $ ϕ=\frac{3 π}{2}$.

gr-qc↗

arXiv2016

The trajectory of light ray under Kerr-Taub-NUT space time

According to General Relativity, there are three factors namely mass, rotation and charge that can influence the path of light ray. Many authors showed that there is another factor which can influence the path of light ray namely gravitomagnetism. Here we discuss the effect of a rotating body with non-zero (Kerr- Taub-NUT) magnetic field on the motion of light ray. We use the null geodesic of photon method and obtain the deflection angle of light ray for such a body up to fourth order term in the equatorial plane. Our calculation shows that magnetism has a noticeable effect on the path of light ray. If we set the magnetism equal to zero, our expression of bending angle reduces to the Kerr bending angle. However, we get non-zero bending angle for a hypothetical mass less, magnetic body.

gr-qc↗