arXiv subjects

Gaurav Tomar

Publications and source records attributed to Gaurav Tomar.

At least 19 recordsLinked to original sources

arXiv2026

Celestial probes of dark matter electromagnetic interactions

We investigate the neutrino flux from the Sun and Earth, as well as the heating of Jupiter and white dwarfs, induced by the annihilation of spin-$1/2$ dark matter particles captured through their electromagnetic interactions with nuclei. Using current data, we derive constraints on the electromagnetic moments and compare them with existing bounds from direct dark matter searches, laboratory experiments, and astrophysical observations. We find that celestial bodies provide leading constraints in several regions of parameter space: the Earth is particularly sensitive to millicharged dark matter, while massive white dwarfs provide powerful probes of the magnetic dipole moment, charge radius, and anapole moment in regions where direct searches lose sensitivity. We illustrate the implications of these constraints in simplified models of Dirac and Majorana dark matter with radiatively generated electromagnetic moments, as we discuss the complementarity between capture in celestial bodies and other dark matter searches.

hep-ph↗

arXiv2026

Probing Sub-GeV Dark Matter with the Migdal Effect at JUNO

We discuss the sensitivity of the JUNO neutrino detector to the Migdal ionization signal triggered by nuclear scattering events produced by sub-GeV weakly interacting massive particles (WIMPs). Exploiting JUNO's large target mass and the annual modulation effect we find that the aggregate rate from many independent and indistinguishable WIMP events in JUNO's liquid scintillator can be isolated from the total dark rate of the photomultipliers, potentially providing for spin-dependent interactions a world-leading sensitivity across the sub-GeV mass range.

hep-ph↗

arXiv2026

Deformation dynamics of Oldroyd B drop in alternating electric field

The deformation of viscoelastic drops in alternating electric fields is relevant to electrohydrodynamic applications such as microfluidics, inkjet printing, and drop manipulation. We investigate the dynamics of a neutrally buoyant Oldroyd-B drop subjected to a uniform alternating electric field using asymptotic analysis and direct numerical simulations with the open-source solver Basilisk. An analytical solution is derived for small deformation and weak elasticity for an Oldroyd-B drop suspended in an Oldroyd-B medium, with both fluids modeled as leaky dielectrics under axisymmetric Stokes flow. Depending on the conductivity and permittivity ratios, six electrohydrodynamic regions are identified, characterized by distinct deformation modes, flow directions, and nonlinear responses; while some exhibit stable spheroidal deformation, others undergo pointed-tip, multi-lobed, or oblate breakup beyond a critical electric capillary number. Across high-, intermediate-, and low-frequency regimes, the deformation oscillates at twice the applied field frequency, with its mean and amplitude governed by the field frequency and viscoelasticity. For cases that produce prolate deformation under a steady field, the drop remains prolate at high frequency, exhibits brief oblate excursions at intermediate frequency, and undergoes large-amplitude oscillations between near-spherical and highly deformed states at low frequency. The mean deformation varies monotonically or non-monotonically with $De$, depending on frequency and electrical property ratios. For cases producing oblate deformation under a steady field, the drop remains oblate at high frequency, develops dimples at intermediate frequency, and breaks up at low frequency for sufficiently large $De$ and $Ca_E$; the mean deformation increases monotonically with $De$.

physics.flu-dyn↗

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↗

arXiv2026

Universality of Bubble Coalescence in Electrolytic Media

Bubble coalescence phenomenon in electrolytic media finds applications in technologies from mineral flotation to electrochemical energy conversion. However, the underlying governing physics still remains unresolved, with longstanding disagreement over the extent to which Marangoni stresses affect the coalescence time by modulating the interfacial mobility. Here, we show that the thin film morphology governs drainage more strongly than the interfacial boundary conditions. We demonstrate experimentally that thin film drainage during bubble coalescence proceeds through three distinct regimes. An initial visco-capillary stage that exhibits a power-law thinning, followed by an exponential decrease in film thickness with time induced by rim stabilisation. The final regime is governed by disjoining pressure and is marked by an exponential relaxation of the film to the equilibrium thickness. We show that, irrespective of the electrolyte type and concentration, film evolution exhibits universal behavior by collapsing onto a single curve when rescaled with the characteristic film thickness and time scale, demonstrating that electrolyte effects act only to renormalize timescales rather than alter the underlying dynamics.

cond-mat.soft↗

arXiv2026

On the deformation of a shear thinning viscoelastic drop in a steady electric field

The deformation of viscoelastic drops under electric fields plays a crucial role in applications such as microfluidics, inkjet printing, and electrohydrodynamic manipulation of complex fluids. This study examines the deformation and breakup dynamics of a linear Phan-Thien-Tanner (LPTT) drop subjected to a uniform electric field using numerical simulations performed with the open-source solver Basilisk. Representative combinations of conductivity ratio ($\sigma_r$) and permittivity ratio ($\epsilon_r$) are chosen from six characteristic regions of the ($\sigma_r$, $\epsilon_r$) phase space, $PR_A^+$, $PR_B^+$, $PR_A^-$, $PR_B^-$, $OB^+$, and $OB^-$. In regions where the first- and second-order deformation coefficients have the same sign ($PR_A^-$, $PR_B^-$, $OB^+$), the LPTT drops exhibit deformation dynamics that negligibley deviate from the Newtonian behavior. In the $PR_A^+$ region, drops deform into prolate spheroidal shapes below a critical electric capillary number and transition to stable multi-lobed shapes or breakup beyond this threshold. Increasing elasticity of drop opposes the deformation, thereby reducing deformation and increasing critical $Ca_E$ with the Deborah number ($De$). In the $PR_B^+$ region, drops form prolate shapes below critical $Ca_E$ and develop conical ends above it. The steady-state deformation exhibits a non-monotonic dependence on $De$, increasing at low $De$ and decreasing at higher values. A similar non-monotonic variation is also observed in critical $Ca_E$. In the $OB^-$ region, LPTT drops attain oblate shapes below critical $Ca_E$ and undergo breakup beyond it. The deformation magnitude shows a non-monotonic variation with $De$, increasing initially and decreasing at higher elasticity.

physics.flu-dyn↗

arXiv2026

On Large Deformations of Oldroyd-B Drops in a Steady Electric Field

The deformation of viscoelastic drops under electric fields is central to applications in microfluidics, inkjet printing, and electrohydrodynamic manipulation of complex fluids. This study investigates the dynamics of an Oldroyd-B drop subjected to a uniform electric field using numerical simulations performed with the open-source solver Basilisk. Representative pairs of conductivity ratio ($\sigma_r$) and permittivity ratio ($\epsilon_r$) are selected from six regions ($PR_A^+$, $PR_B^+$, $PR_A^-$, $PR_B^-$, $OB^+$, and $OB^-$) of the $(\sigma_r, \epsilon_r)$ phase space. In regions where the first- and second-order deformation coefficients share the same sign ($PR_A^-$, $PR_B^-$, $OB^+$), deviations from Newtonian behavior are negligible. In $PR_A^+$, drops develop multi-lobed shapes above a critical electric capillary number, with elasticity reducing deformation and increasing the critical $Ca_E$ with Deborah number ($De$). In $PR_B^+$, drops form shapes with conical ends above the critical $Ca_E$, while steady-state deformation decreases with $De$ below this threshold, and critical $Ca_E$ shows non-monotonic variation. At high $Ca_E$ and $De$, transient deformation exhibits maxima and minima before reaching steady state, with occasional oscillations between spheroidal and pointed shapes. In $OB^-$, drops deform to oblate shapes and breakup above a critical $Ca_E$, with deformation magnitude increasing and critical $Ca_E$ decreasing with $De$; at low $Ca_E$ and high $De$, dimpling and positional oscillations are observed. These results elucidate viscoelastic-electric interactions and provide guidance for controlling drop behavior in practical applications.

physics.flu-dyn↗

arXiv2025

WimPyC: an extension module of WimPyDD for the calculation of WIMP capture in celestial bodies

We introduce WimPyC, a Python code for the calculation of the capture rate of Weakly Interacting Massive Particles (WIMPs) by celestial bodies through nuclear scattering in the optically thin regime. WimPyC is an extension of the WimPyDD code, that calculates WIMP-nucleus scattering signals in direct detection (DD) experiments, and allows to combine DD and capture in celestial bodies in virtually any scenario within the framework of Galilean-invariant non-relativistic effective theory (NREFT), including inelastic scattering, an arbitrary WIMP spin and a generic WIMP velocity distribution in the Galactic halo. WimPyDD and WimPyC are suitable for both top-down approaches, where the interaction operators of a high-energy physics model are matched to those of the NREFT, and to bottom-up studies, where the Wilson coefficients of the NREFT are explored in a model-independent way and/or where the velocity distribution is written in terms of a superposition of streams taken as free parameters. As in the case of WimPyDD WimPyC exploits the factorization of the three main components that enter in the calculation of the capture rate: i) the Wilson coefficients that encode the dependence of the signals on the ultraviolet completion of the effective theory; ii) a response function that depends on the nuclear physics; iii) the halo function that depends on the WIMP velocity distribution. In WimPyC these three components are calculated and stored separately for later interpolation and combined together only as the last step of the signal evaluation procedure. This makes the phenomenological study of the capture rate with WimPyC transparent and improves computational speed.

hep-ph↗

arXiv2025

Effect of viscoelasticity on electrohydrodynamic drop deformation

The impact of viscoelasticity on drop deformation in the presence of an electric field is investigated using both analytical and numerical methods. The study focuses on two configurations: a viscoelastic drop suspended in a Newtonian fluid and a Newtonian drop suspended in a viscoelastic medium. Oldroyd-B constitutive equation is employed to model constant viscosity viscoelasticity. Effect of Deborah number (ratio of polymer relaxation time to convective time scale) on drop deformation is studied and explained by examining the electric, elastic and viscous stresses at the interface. For small deformations, we apply the method of domain perturbations, and show that the viscoelastic properties of the drop significantly influence its deformation more than when the surrounding fluid is viscoelastic. Numerical computations are performed using a finite volume framework for larger drop deformations. The transient dynamics of the drops show distinct oscillatory patterns before eventually stabilizing at a steady deformation value. We observe a trend of decreased deformation in both configurations as the Deborah number increases. Relative magnitude of normal and tangential stresses plays a crucial role in drop deformation.

physics.flu-dyn↗

arXiv2025

Investigating sub-MeV dark matter annihilation to neutrinos using direct detection experiments

Dark matter (DM) could self-annihilate into neutrinos in dense regions of the Universe. We consider the resulting flux of neutrinos from the Milky Way DM halo and derive exclusion limits on the annihilation cross-section using XENONnT electron recoil data. Assuming a $J$-factor independent of the annihilation cross-section, we find leading limits for DM masses below $\mathcal{O}$(MeV). Self-annihilating DM affects the DM halo via dissolution, introducing a cross-section dependency on the halo profile and thus the $J$-factor. We discuss such a situation in more detail, finding that the signal rate is below the experimental sensitivity of XENONnT, leaving the annihilation cross-section unconstrained.

hep-ph↗

arXiv2025

Role of Transient Dynamics in Dripping-Jetting Transition in Newtonian Fluids

Dripping dynamics has been well studied over the past century and forms a classic example of chaotic system in physics. With an increase in the inlet flow rate, periodic droplet formation from a faucet becomes chaotic in terms of the droplet size and the length of the liquid column at the time of pinch-off. With a further increase in the flow rate, dripping regime transitions into jetting regime where the liquid column length is much longer than that observed in the dripping case. In general, dripping faucet is seen as a long time behavior of the system at fixed control parameters. In the steady state condition, different nonlinear behaviors such as periodic and chaotic formation of droplets are observed in the dripping and jetting regimes. It is known that dripping faucet shows chaotic dripping regime before jetting regime ensues. At a critical inlet velocity, $U_{m-d_j}$, we note that dripping to jetting transition occurs after several droplets have formed in the dripping regime. The transition behaviour can be characterized by the time evolution of the liquid jet length $L$ and droplet size $D_p$. Solution to slender jet equation show that the dripping-jetting transition region is a function of the fluid properties. Further, we show that perturbations in the inlet velocity can significantly modify the transient behavior of the dripping to jetting regime transition.

physics.flu-dyn↗

arXiv2025

Weakly nonlinear analysis of particle-laden Rayleigh-Bénard convection

We investigate the effect of inertial particles on Rayleigh-Bénard convection using weakly nonlinear stability analysis. In the presence of nonlinear effects, we study the limiting value of growth of instabilities by deriving a cubic Landau equation. An Euler-Euler/two-fluid formulation is being used to describe the flow instabilities in particle-laden Rayleigh-Bénard convection. The nonlinear results are presented near the critical point (bifurcation point) for water droplets in the dry air system. It is found that supercritical bifurcation is the only type of bifurcation beyond the critical point. Interaction of settling particles with the flow and the Reynolds stress or distortion terms emerge due to the nonlinear self-interaction of fundamental modes, breaking down the top-bottom symmetry of the secondary flow structures. In addition to the distortion functions, the nonlinear interaction of fundamental modes generates higher harmonics, leading to the tendency of preferential concentration of uniformly distributed particles, which is completely absent in the linear stability analysis. It is shown that in the presence of thermal energy coupling between the fluid and particles, the difference between the horizontally averaged heat flux at the hot and cold surface is equal to the net sensible heat flux advected by the particles. The difference between the heat fluxes at hot and cold surfaces is increased with an increase in particle concentration.

physics.flu-dyn↗

arXiv2025

Probing Dark Matter Electromagnetic Properties in Direct Detection Experiments

Astronomical and cosmological observations indicate that dark matter should interact very weakly with the electromagnetic radiation. Nevertheless, the existence of such interactions is not precluded by observations nor by theoretical considerations. A promising approach to probe the dark matter electromagnetic properties is through the search of photon-mediated dark matter-nucleus interactions in direct detection experiments. In this paper we present a simple methodology to calculate the scattering rate in a direct detection experiment for given values of the dark matter electric charge, charge radius, electric- and magnetic- dipole moments and anapole moment. In our work we include contributions to the scattering from nuclear recoils and from the Migdal effect. We finally apply this formalism to determine exclusion limits on the five electromagnetic interactions using data from XENON1T, LZ, PICO-60 and DS50 experiments, and we discuss the implications for a simplified dark matter model with t-channel mediators.

hep-ph↗

arXiv2025

Low-mass constraints on WIMP effective models of inelastic scattering using the Migdal effect

We use the Migdal effect to extend to low masses the bounds on each of the effective couplings of the non-relativistic effective field theory of a WIMP of mass $m_χ$ and spin 1/2 that interacts inelastically with nuclei by either upscattering to a heavier state with mass splitting $δ>0$ or by downscattering to a lighter state with $δ<0$. In order to do so we perform a systematic analysis of the Migdal bounds in the $m_χ-δ$ parameter space comparing them to those from nuclear recoil searches. The Migdal effect allows to significantly extend to low WIMP masses the nuclear recoil bounds for $δ<0$. In this case the bounds are driven by XENON1T, except when $δ$ is vanishing or very small, when, depending on the WIMP-nucleus interaction, in the lower end of the $m_χ$ range either DS50 or SuperCDMS are more constraining. On the other hand, when $δ>0$ and the WIMP particle upscatters to a heavier state nuclear recoil bounds are stronger than those from the Migdal effect.

hep-ph↗

arXiv2024

Centre mode instability of a dilute particle-laden swirling jet in a swirl flow combustor

Linear stability of a locally parallel annular swirling jet laden with particles in a swirl flow combustor is considered. At low Stokes numbers, the eigenspectra of the particle-laden jet with uniform particle concentration shows three unstable modes namely centre, sinuous and varicose modes. As the Stokes number is increased to unity, the growth rates of the centre and shear layer modes reduces compared to that of the unladen swirling jet. The magnitude of the velocity eigenmodes peaks in the vortex core and decays radially outward. The variation in particle concentration occurs mostly in the vortex core and almost none in the shear layer. The strength of flow reversal at the jet centreline is given by the backflow parameter. An increase in the backflow parameter increases the growth rate of the centre mode. Non-uniformity in the base-state particle concentration is introduced using a Gaussian function varying in the radial direction and a reduction in the growth rate of the centre mode is seen compared to the uniform particle concentration profile. When the location of the peak of the base-state particle concentration profile is inside the vortex core, the centre modes are stable. Linearized vorticity budget analysis reveals that this is accompanied by a decrease in the net generation of perturbation vorticity in the axial direction and increased radial and azimuthal perturbation vorticity.

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

Reemergence of Trampolining in a Leidenfrost Droplet

The levitating Leidenfrost (LF) state of a droplet on a heated substrate is often accompanied by fascinating behaviors such as star-shaped deformations, self-propulsion, bouncing, and trampolining. These behaviors arise due to the vapor flow instabilities at the liquid-vapor interface beneath the droplet at sizes typically comparable to the capillary length scale of the liquid. Here, we report on the spontaneous bouncing, trampolining, and hovering behavior of an unconstrained LF water droplet. We observe that a droplet exhibits intermittent increase in bouncing height at specific radii and subsequent reduction in the height of bounce leading to a quiescent LF state. The reemergence of the trampolining behavior from the quiescent hovering state without any external forcing is observed at sizes as low as 0.1 times the capillary length. We attribute the droplet bouncing behavior to the dynamics of vapor flow beneath the LF droplet. We propose that the trampolining behavior of the droplet at specific radii is triggered by subharmonic and harmonic excitation of the liquid-vapor interface. We attribute the intermittent trampolining events to the change in the natural frequency of the droplet and the vapor layer due to evaporative mass loss. This proposed mechanism of resonance-driven trampolining of LF droplets is observed to be applicable for different liquids irrespective of the initial volume and substrate temperatures, thus indicating a universality of the behavior.

physics.flu-dyn↗

arXiv2024

Inviscid stability of compressible flows past compliant surfaces

The classical theorems of inviscid stability have been extended for compressible flows past compliant surfaces. We consider normal modes imposed on a plane parallel compressible flow past compliant walls modelled as spring-backed plates and analyze the inviscid equations to derive the theorems. We show that the generalised inflection point criteria of compressible rigid wall flows is modified for flows past dissipative compliant walls. Theorems on the bounds for the wave-speed for unstable modes in the inviscid limit are derived. These are similar to the ones for incompressible compliant wall flows, but are different from compressible rigid wall flows. A new criterion for existence of neutral modes with wave-speeds outside the range of minimum and maximum base velocities is derived for compressible flows past non-dissipative compliant walls. We show that in external compressible flows, neutral modes without a critical point can exist even with dissipative compliant walls, which is not the case in the incompressible limit.

physics.flu-dyn↗