Sandip Ghosal - Academia.edu (original) (raw)

Papers by Sandip Ghosal

Journal of Fluid Mechanics, 2021

Electrophoresis of a tightly fitting sphere of radius aaa along the centreline of a liquid-filled... more Electrophoresis of a tightly fitting sphere of radius aaa along the centreline of a liquid-filled circular cylinder of radius RRR is studied for a gap width h_0=R−allah_0=R-a\ll ah0=R−alla . We assume a Debye length kappa−1llh0\kappa ^{-1}\ll h_0kappa−1llh_0 , so that surface conductivity is negligible for zeta potentials typically seen in experiments, and the Smoluchowski slip velocity is imposed as a boundary condition at the solid surfaces. The pressure difference between the front and rear of the sphere is determined. If the cylinder has finite length LLL , this pressure difference causes an additional volumetric flow of liquid along the cylinder, increasing the electrophoretic velocity of the sphere, and an analytic prediction for this increase is found when LggRL\gg RLggR . If NNN identical, well-spaced spheres are present, the electrophoretic velocity of the spheres increases with NNN , in agreement with the experiments of Misiunas & Keyser (Phys. Rev. Lett., vol. 122, 2019, 214501).

Biomicrofluidics, 2019

The resistive pulse method based on measuring the ion current trace as a biomolecule passing thro... more The resistive pulse method based on measuring the ion current trace as a biomolecule passing through a nanopore has become an important tool in biotechnology for characterizing molecules. A detailed physical understanding of the translocation process is essential if one is to extract the relevant molecular properties from the current signal. In this Perspective, we review some recent progress in our understanding of hydrodynamic flow and transport through nanometer sized pores. We assume that the problems of interest can be addressed through the use of the continuum version of the equations of hydrodynamic and ion transport. Thus, our discussion is restricted to pores of diameter greater than about ten nanometers: such pores are usually synthetic. We address the fundamental nanopore hydrodynamics and ion transport mechanisms and review the wealth of observed phenomena due to these mechanisms. We also suggest future ionic circuits that can be synthesized from different ionic modules ...

Physics of Fluids, 2014

Electrically generated flows around a thin dielectric plate pierced by a cylindrical hole are com... more Electrically generated flows around a thin dielectric plate pierced by a cylindrical hole are computed numerically. The geometry represents that of a single nanopore in a membrane. When the membrane is uncharged, flow is due solely to induced charge electroosmosis, and eddies are generated by the high fields at the corners of the nanopore. These eddies meet at stagnation points. If the geometry is chosen correctly, the stagnation points merge to form a single stagnation point at which four streamlines cross at a point and eight eddies meet.

Journal of Fluid Mechanics, 2014

Electro-osmotic pumping of fluid through a nanopore that traverses an insulating membrane is cons... more Electro-osmotic pumping of fluid through a nanopore that traverses an insulating membrane is considered. The density of surface charge on the membrane is assumed to be uniform and sufficiently low for the Poisson–Boltzmann equation to be linearized. The reciprocal theorem gives the flow rate generated by an applied weak electric field, expressed as an integral over the fluid volume. For a circular hole in a membrane of zero thickness, an analytical result is possible up to quadrature. For a membrane of arbitrary thickness, the full Poisson–Nernst–Planck–Stokes system of equations is solved numerically using a finite volume method. The numerical solution agrees with the standard analytical result for electro-osmotic flux through a long cylindrical pore when the membrane thickness is large compared to the hole diameter. When the membrane thickness is small, the flow rate agrees with that calculated using the reciprocal theorem.

Nature Communications, 2017

In nanopore sensing, changes in ionic current are used to analyse single molecules in solution. T... more In nanopore sensing, changes in ionic current are used to analyse single molecules in solution. The translocation dynamics of polyelectrolytes is of particular interest given potential applications such as DNA sequencing. In this paper, we determine how the dynamics of voltage driven DNA translocation can be affected by the nanopore geometry and hence the available configurational space for the DNA. Using the inherent geometrical asymmetry of a conically shaped nanopore, we examine how DNA dynamics depends on the directionality of transport. The total translocation time of DNA when exiting the extended conical confinement is significantly larger compared to the configuration where the DNA enters the pore from the open reservoir. By using specially designed DNA molecules with positional markers, we demonstrate that the translocation velocity progressively increases as the DNA exits from confinement. We show that a hydrodynamic model can account for these observations.

Open Journal of Fluid Dynamics, 2012

Langmuir : the ACS journal of surfaces and colloids, Jan 12, 2014

A theoretical model of electroosmosis through a circular pore of radius a that traverses a membra... more A theoretical model of electroosmosis through a circular pore of radius a that traverses a membrane of thickness h is investigated. Both the cylindrical surface of the pore and the outer surfaces of the membrane are charged. When h ≫ a, end effects are negligible, and the results of full numerical computations of electroosmosis in an infinite pore agree with theory. When h = 0, end effects dominate, and computations again agree with analysis. For intermediate values of h/a, an approximate analysis that combines these two limiting cases captures the main features of computational results when the Debye length κ(-1) is small compared with the pore radius a. However, the approximate analysis fails when κ(-1) ≫ a, when the charge cloud due to the charged cylindrical walls of the pore spills out of the ends of the pore, and the electroosmotic flow is reduced. When this spilling out is included in the analysis, agreement with computation is restored.

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2010

The problem of the one-dimensional electro-diffusion of ions in a strong binary electrolyte is co... more The problem of the one-dimensional electro-diffusion of ions in a strong binary electrolyte is considered. The mathematical description, known as the Poisson–Nernst–Planck (PNP) system, consists of a diffusion equation for each species augmented by transport owing to a self-consistent electrostatic field determined by the Poisson equation. This description is also relevant to other important problems in physics, such as electron and hole diffusion across semiconductor junctions and the diffusion of ions in plasmas. If concentrations do not vary appreciably over distances of the order of the Debye length, the Poisson equation can be replaced by the condition of local charge neutrality first introduced by Planck. It can then be shown that both species diffuse at the same rate with a common diffusivity that is intermediate between that of the slow and fast species (ambipolar diffusion). Here, we derive a more general theory by exploiting the ratio of the Debye length to a characteristi...

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2012

Physics of Fluids, 1997

Turbulent wakes are known to develop self-similarly sufficiently far downstream from obstacles th... more Turbulent wakes are known to develop self-similarly sufficiently far downstream from obstacles that generate them. It has long been assumed that the spreading rate of the wake in the self-similar regime is independent of the details of the body generating the wake, being dependent only on the total drag (or momentum deficit). This assumption seems to be in contradiction with some recent experiments. In this study we attempt to complement these experimental investigations through a numerical study of a time-developing wake. A numerical study has the advantage of eliminating many of the uncontrolled factors present in experiments and allowing precise control of initial conditions. Large-eddy simulations employing the recently developed dynamic localization model are used to extend previous results from direct numerical simulations. The large-eddy simulation results are compared to the direct numerical simulation database, wherever such comparisons are feasible, as a check of the metho...

Physical Review Letters, 2007

Journal of Fluid Mechanics, 2003

Journal of Fluid Mechanics, 2000

In non-premixed turbulent combustion the reactive zone is localized at the stoichiometric surface... more In non-premixed turbulent combustion the reactive zone is localized at the stoichiometric surfaces of the mixture and may be locally approximated by a diffusion flame. Experiments and numerical simulations reveal a characteristic structure at the edge of such a two-dimensional diffusion flame. This ‘triple flame’ or ‘edge flame’ consists of a curved flame front followed by a trailing edge that constitutes the body of the diffusion flame. Triple flames are also observed at the edge of a lifted laminar diffusion flame near the exit of burners. The speed of propagation of the triple flame determines such important properties as the rate of increase of the flame surface in non-premixed combustion and the lift-off distance in lifted flames at burners. This paper presents an approximate theory of triple flames based on an approximation of the flame shape by a parabolic profile, for large activation energy and low but finite heat release. The parabolic flame path approximation is a heurist...