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Debasish Das

Publications and source records attributed to Debasish Das.

At least 37 records · Page 2Linked to original sources
arXiv2019

Review of Dark Matter

In this review of Dark Matter we review dark matter as sterile neutrinos, fermions, with their present and possibly future detection via neutrino Oscillations. We review the creation of Dark Matter via interactions with the Dark Energy (quintesence) field. We also review bosons as dark matter, discussing a proposed search for dark photons. Since photons are vector bosons, if dark photons exist at least part of dark matter are vector bosons. Ongoing experimental detection of Dark Matter is reviewed.

physics.gen-ph↗
arXiv2019

Active particles powered by Quincke rotation in a bulk fluid

Dielectric particles suspended in a weakly conducting fluid are known to spontaneously start rotating under the action of a sufficiently strong uniform DC electric field due to the Quincke rotation instability. This rotation can be converted into translation when the particles are placed near a surface providing useful model systems for active matter. Using a combination of numerical simulations and theoretical models, we demonstrate that it is possible to convert this spontaneous Quincke rotation into spontaneous translation in a plane perpendicular to the electric field in the absence of surfaces by relying on geometrical asymmetry instead.

cond-mat.soft↗
arXiv2019

Self-organization of swimmers drives long-range fluid transport in bacterial colonies

Bacteria commonly live in structured communities that affect human health and influence ecological systems. Heterogeneous populations, such as motile and non-motile populations, often coexist in bacteria communities. Motile subpopulations in microbial communities are believed to be important to dispersal, quest for food, and material transport. However, except in circumstances where motile cells drive colony expansion (e.g. bacterial swarming), the physiological functions of motile subpopulations in bacterial communities are largely unclear. Here we discovered that motile cells in routinely cultured sessile colonies of peritrichously flagellated bacteria can self-organize into two adjacent, centimeter-scale motile rings surrounding the entire colony. The motile rings arise from spontaneous segregation of a homogeneous swimmer suspension that mimics a phase separation; the process is mediated by intercellular interactions and shear-induced depletion. As a result of this self-organization, cells drive fluid flows to circulate around the colony at a constant peak speed of approximately 30 microns per second, providing a stable and high-speed avenue for directed material transport at the macroscopic scale. These findings present a unique form of bacterial self-organization that influences population structure and material distribution in bacterial communities.

physics.bio-ph↗
arXiv2018

HBT radii : Comparative studies on collision systems and beam energies

Two-particle Hanbury-Brown-Twiss (HBT) interferometry is an important probe for understanding the space-time structure of particle emission sources in high energy heavy ion collisions. We present the comparative studies of HBT radii in Pb+Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 17.3 GeV with Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 19.6 GeV. To further our understanding for this specific energy regime we also compare the HBT radii for Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 19.6 GeV with Cu+Cu collisions at $\sqrt{s_{\rm{NN}}}$ = 22.4 GeV. We have found interesting similarity in the $R_{\rm out}/R_{\rm side}$ ratio with $m_{\rm T}$ across the collision systems while comparing the data for this specific energy zone which is interesting as it acts as a bridge from SPS energy regime to the RHIC energy domain.

nucl-ex↗
arXiv2018

Computing the motor torque of Escherichia coli

The rotary motor of bacteria is a natural nano-technological marvel that enables cell locomotion by powering the rotation of semi-rigid helical flagellar filaments in fluid environments. It is well known that the motor operates essentially at constant torque in counter-clockwise direction but past work have reported a large range of values of this torque. Focusing on Escherichia coli cells that are swimming and cells that are stuck on a glass surface for which all geometrical and environmental parameters are known (Darnton et al., J. Bacteriology, 2007, 189, 1756-1764), we use two validated numerical methods to compute the value of the motor torque consistent with experiments. Specifically, we use (and compare) a numerical method based on the boundary integral representation of Stokes flow and also develop a hybrid method combining boundary element and slender body theory to model the cell body and flagellar filament, respectively. Using measured rotation speed of the motor, our computations predict a value of the motor torque in the range 440 pNnm to 829 pNnm, depending critically on the distance between the flagellar filaments and the nearby surface.

physics.bio-ph↗
arXiv2018

Swimming of peritrichous bacteria is enabled by an elastohydrodynamic instability

Peritrichously-flagellated bacteria, such as Escherichia coli, self-propel in fluids by using specialised motors to rotate multiple helical filaments. The rotation of each motor is transmitted to a short flexible segment called the hook which in turn transmits it to a flagellar filament, enabling swimming of the whole cell. Since multiple motors are spatially distributed on the body of the organism, one would expect the propulsive forces from the filaments to push against each other leading to negligible swimming. We use a combination of computations and theory to show that the swimming of multi-flagellated bacteria is enabled by an elastohydrodynamic bending instability occurring for hooks more flexible than a critical threshold. Using past measurements of hook bending stiffness, we demonstrate how the design of real bacteria allows them to be safely on the side of this instability that promotes systematic swimming.

physics.bio-ph↗
arXiv2018

Helical micropumps near surfaces

Recent experiments proposed to use confined bacteria in order to generate flows near surfaces. We develop a mathematical and a computational model of this fluid transport using a linear superposition of fundamental flow singularities. The rotation of a helical bacterial flagellum induces both a force and a torque on the surrounding fluid, both of which lead to a net flow along the surface. The combined flow is in general directed at an angle to the axis of the flagellar filament. The optimal pumping is thus achieved when bacteria are tilted with respect to the direction in which one wants to move the fluid, in good agreement with experimental results. We further investigate the optimal helical shapes to be used as micropumps near surfaces and show that bacterial flagella are nearly optimal, a result which could be relevant to the expansion of bacterial swarms.

physics.bio-ph↗
arXiv2017

Heavy quark state production in Pb-Pb collisions at $\sqrt{s_{NN}}$=5.02 TeV

We estimate differential rapidity cross sections for $J/Ψ$, $Ψ(2S)$, $Υ(1S)$, $Υ(2S)$, and $Υ(3S)$ production via Pb-Pb collisions at proton-proton energy $\equiv \sqrt{s_{pp}}$ =5.02 TeV. For the $Ψ(2S)$, $Υ(3S)$ states we use the mixed heavy quark hyrid theory, with these states being approximately 50\% standard and 50\% hybrid charmonium, bottomonium meson states. This is an extension of previous work on heavy quark state production via Cu-Cu and Au-Au collisions at $\sqrt{s_{pp}}$=200 GeV.

hep-ph↗
arXiv2016

Electrohydrodynamics of viscous drops in strong electric fields: Numerical simulations

Weakly conducting dielectric liquid drops suspended in another dielectric liquid and subject to an applied uniform electric field exhibit a wide range of dynamical behaviors contingent on field strength and material properties. These phenomena are best described by the Melcher-Taylor leaky dielectric model, which hypothesizes charge accumulation on the drop-fluid interface and prescribes a balance between charge relaxation, the jump in Ohmic currents from the bulk and charge convection by the interfacial fluid flow. Most previous numerical simulations based on this model have either neglected interfacial charge convection or restricted themselves to axisymmetric drops. In this work, we develop a three-dimensional boundary element method for the complete leaky dielectric model to systematically study the deformation and dynamics of liquid drops in electric fields. The inclusion of charge convection in our simulations permits us to investigate drops in the Quincke regime, in which experiments have demonstrated a symmetry-breaking bifurcation leading to steady electrorotation. Our simulation results show excellent agreement with existing experimental data and small-deformation theories.

physics.flu-dyn↗
arXiv2016

A nonlinear small-deformation theory for transient droplet electrohydrodynamics

The deformation of a viscous liquid droplet suspended in another liquid and subject to an applied electric field is a classic multiphase flow problem best described by the Melcher-Taylor leaky dielectric model. The main assumption of the model is that any net charge in the system is concentrated on the interface between the two liquids as a result of the jump in Ohmic currents from the bulk. Upon application of the field, the drop can either attain a steady prolate or oblate shape with toroidal circulating flows both inside and outside arising from tangential stresses on the interface due to action of the field on the surface charge distribution. Since the pioneering work of \cite{taylor1966}, there have been numerous computational and theoretical studies to predict the deformations measured in experiments. Most existing theoretical models, however, have either neglected transient charge relaxation or nonlinear charge convection by the interfacial flow. In this work, we develop a novel small-deformation theory accurate to second order in electric capillary number ${O}(Ca_E^2)$ for the complete Melcher-Taylor model that includes transient charge relaxation, charge convection by the flow, as well as transient shape deformation. The main result of the paper is the derivation of coupled evolution equations for the induced electric multipoles and for the shape functions describing the deformations on the basis of spherical harmonics. Our results, which are consistent with previous models in the appropriate limits, show excellent agreement with fully nonlinear numerical simulations based on an axisymmetric boundary-element formulation and with existing experimental data in the small-deformation regime.

physics.flu-dyn↗
arXiv2016

Review of QCD, Quark-Gluon Plasma, Heavy Quark Hybrids, and Heavy Quark State production in p-p and A-A collisions

This is a review of the Quantum Chromodynamics Cosmological Phase Transitions, the Quark-Gluon Plasma, the production of heavy quark states via p-p collisions and RHIC (Relativistic Heavy Ion Collisions) using the mixed hybrid theory for the $Ψ(2S)$ and $Υ(3S)$ states; and the possible detection of the Quark-Gluon Plasma via heavy quark production using RHIC. Recent research on fragmentation for the production of D mesons is reviewed, as is future theoretical and experimental research on the Collins and Sivers fragmentation functions for pions produced in polarized p-p collisions.

hep-ph↗
arXiv2015

Emergent vortices in populations of colloidal rollers

Coherent vortical motion has been reported in a wide variety of populations including living organisms (bacteria, fishes, human crowds) and synthetic active matter (shaken grains, mixtures of biopolymers), yet a unified description of the formation and structure of this pattern remains lacking. Here we report the self-organization of motile colloids into a macroscopic steadily rotating vortex. Combining physical experiments and numerical simulations, we elucidate this collective behavior. We demonstrate that the emergent-vortex structure lives on the verge of a phase separation, and single out the very constituents responsible for this state of polar active matter. Building on this observation, we establish a continuum theory and lay out a strong foundation for the description of vortical collective motion in a broad class of motile populations constrained by geometrical boundaries.

cond-mat.soft↗