Bulletin of the American Physical Society, Nov 19, 2006
The transport of a species, in a microfluidic capillary of arbitrary but axially-invariant cross-... more The transport of a species, in a microfluidic capillary of arbitrary but axially-invariant cross-sectional shape in the presence of an adsorption-desorption reaction on the capillary wall is studied, motivated by applications to capillary electrophoresis and open-tubular capillary electrochromatography. An asymptotic approach based on the long time limit is adopted. Numerical simulations are conducted for (a) an axially homogeneous flow with arbitrary crosssectional variation of the axial velocity and (b) an electroosmotic flow rendered temporally and axially variable by the adsorbed species concentration on the wall. The predictions from asymptotic theory are tested against numerical results obtained from the full three-dimensional solution of the governing equations.
Proceedings of The Royal Society A: Mathematical, Physical and Engineering Sciences, Mar 1, 2017
The screened Coulomb interaction between a pair of infinite parallel planes with spatially varyin... more The screened Coulomb interaction between a pair of infinite parallel planes with spatially varying surface charge is considered in the limit of small electrical potentials for arbitrary Debye lengths. A simple expression for the disjoining pressure is derived in terms of a two-dimensional integral in Fourier space. The integral is evaluated for periodic and random charge distributions and the disjoining pressure is expressed as a sum over Fourier-Bloch reciprocal lattice vectors or in terms of an integral involving the autocorrelation function, respectively. The force between planes with a finite area of uniform charge, a model for the DLVO interaction between finite surfaces, is also calculated. It is shown that the overspill of the charge cloud beyond the region immediately between the charged areas results in a reduction of the disjoining pressure, as reported by us recently in the long Debye length limit for planes of finite width.
Single molecule experiments on bacteriophages show an exponential scaling for the dependence of m... more Single molecule experiments on bacteriophages show an exponential scaling for the dependence of mobility on the length of DNA within the capsid. It has been suggested that this could be due to the "capstan mechanism"-the exponential amplification of friction forces that result when a rope is wound around a cylinder as in a ship's capstan. Here we describe a desktop experiment that illustrates the effect. Though our model phage is a million times larger, it exhibits the same scaling observed in single molecule experiments.
Bulletin of the American Physical Society, Nov 25, 2013
and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a... more and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a solid-state membrane under an applied voltage. The ion distribution near the surface of the membrane arises due to the combined effect of the intrinsic surface charge as well as concentration polarization due to the applied field. It gives rise to an electric pressure that drives hydrodynamic flow in the vicinity of the pore. There is a net hydrodynamic flow through the nanopore due to the asymmetry in the Debye layer induced by the membrane surface charge. The qualitative behavior is similar to that observed in a previous study using molecular dynamic simulations. The flow strength is a strongly nonlinear function of the applied field. Combination of electrophoretic and hydrodynamic effects can lead to ion selectivity in terms of valences and this could have some practical applications in separations.
Bulletin of the American Physical Society, Nov 24, 2014
flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density ... more flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density is considered. In the limit where the surface charge is small and the applied electric field weak, the reciprocal theorem is used to derive an expression for the electroosmotic flux through the pore up to quadratures over the fluid volume. Thus, an "electroosmotic conductance" (the fluid flux per unit applied voltage) may be defined in analogy to the corresponding electrical conductance of a hole in an insulating membrane immersed in a uniform conductor. In the limit when the membrane is thick compared to the pore diameter, the usual result for the electroosmotic conductance through long cylindrical channels (which varies inversely as the membrane thickness) is recovered. The electroosmotic conductance is shown to approach a finite value for an infinitely thin membrane: this residual electroosmotic resistance (inverse of conductance) is analogous to the concept of "access resistance of a pore" in the corresponding electrical problem. The dependence of the electroosmotic conductance on pore radius, Debye length and membrane thickness is investigated. Reference: JFM (2014) 749, 167; Langmuir (in press) 1 Supported by the NIH under grant 4R01HG004842. SG acknowledges a visiting professorship at Cambridge University funded by the Leverhulme Trust, UK. JDS thanks DAMTP (
Bulletin of the American Physical Society, Nov 18, 2007
Submitted for the DFD07 Meeting of The American Physical Society DNA translocation through nanopo... more Submitted for the DFD07 Meeting of The American Physical Society DNA translocation through nanopores: effect of salt concentration SANDIP GHOSAL, Northwestern University-Recent experiments on the detection of single molecules of linear polyelectrolytes through nanopores could lead to an ultrafast and inexpensive method of rapidly sequencing linear polymer chains such as DNA and RNA. In earlier work (see Ghosal, S. in APS DFD06 abstracts) a hydrodynamic model for determining the electrophoretic speed of a polyelectrolyte through an axially symmetric slowly varying nanopore was presented in the limit of vanishingly small Debye length. Here the case of a finite Debye layer thickness is considered within the Debye-Hückel and Stokes flow approximations while restricting the pore geometry to that of a cylinder of length much larger than the diameter. Further, the possibility of a uniform surface charge on the walls of the nanopore is taken into account. The model admits an exact analytical solution from which translocation times are calculated and found to be consistent with recent measurements in solidstate nanopores. It is suggested, based on the solution to the model problem, that the translocation speed can be greatly reduced if the ζ-potential of the nanopore walls could be fine tuned to match closely the ζ-potential of the polyelectrolyte. Physically this amounts to balancing the net electrical force on the polyelectrolyte inside the pore by the viscous drag of the electroosmotic flow inside the nanopore. Reduction of the translocation speed by several orders of magnitude is essential for achieving single base resolution.
Bulletin of the American Physical Society, Nov 22, 2005
Submitted for the DFD05 Meeting of The American Physical Society Electroosmotic flow in rectangul... more Submitted for the DFD05 Meeting of The American Physical Society Electroosmotic flow in rectangular microchannels: numerical simulation and asymptotic theory 1 SUBHRA DATTA, SANDIP GHOSAL, NEE-LESH PATANKAR, Northwestern University-The problem of fluid flow in a microfluidic channel of rectangular cross-section is solved numerically when the zeta potential is not uniform. Variations in the axial direction as well as along the perimeter of the channel cross-section is considered. Excellent agreement is found with a previously published (Ghosal, 2002 JFM vol.459 pg. 103) asymptotic theory based on the lubrication approximation, even when the length scale of axial variations is of the same order as the characteristic channel width.
Bulletin of the American Physical Society, Mar 6, 2007
Submitted for the MAR07 Meeting of The American Physical Society Electrophoretic speed of a polye... more Submitted for the MAR07 Meeting of The American Physical Society Electrophoretic speed of a polyelectrolyte in a nanopore 1 SANDIP GHOSAL, Northwestern University-A hydrodynamic model for determining the electrophoretic speed of a polyelectrolyte through a nanopore is presented. It is assumed that the speed is determined by a balance of electrical and viscous forces arising from within the pore in the presence of co and counter ions. Further, classical continuum electrostatics and hydrodynamics as well as the mean field description of Poisson-Boltzmann is assumed to be applicable after accounting for Manning condensation on the polyelectrolyte. An explicit formula for the translocation speed as a function of the pore geometry and other physical parameters is obtained and is shown to be consistent with recent experimental measurements on DNA translocation through nanopores in silicon membranes.
Diffusion of colored dye on water saturated paper substrates has been traditionally exploited wit... more Diffusion of colored dye on water saturated paper substrates has been traditionally exploited with great skill by renowned water color artists. The same physics finds more recent practical applications in paper based diagnostic devices deploying chemicals that react with a bodily fluid yielding colorimetric signals for disease detection. During spontaneous imbibition through the tortuous pathways of a porous electrolyte saturated paper matrix, a dye molecule undergoes diffusion in a complex network of pores. The advancing front forms a strongly correlated interface that propagates diffusively but with an enhanced effective diffusivity. We measure this effective diffusivity and show that it is several orders of magnitude greater than the free solution diffusivity and has a significant dependence on the solution pH and salt concentration in the background electrolyte. We attribute this to electrically mediated interfacial interactions between the ionic species in the liquid dye and spontaneous surface charges developed at porous interfaces, and introduce a simple theory to explain this phenomenon.
Single molecule experiments on bacteriophages show an exponential scaling for the dependence of m... more Single molecule experiments on bacteriophages show an exponential scaling for the dependence of mobility on the length of DNA within the capsid. It has been suggested that this could be due to the "capstan mechanism"-the exponential amplification of friction forces that result when a rope is wound around a cylinder as in a ship's capstan. Here we describe a desktop experiment that illustrates the effect. Though our model phage is a million times larger, it exhibits the same scaling observed in single molecule experiments.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2017
The screened Coulomb interaction between a pair of infinite parallel planes with spatially varyin... more The screened Coulomb interaction between a pair of infinite parallel planes with spatially varying surface charge is considered in the limit of small electrical potentials for arbitrary Debye lengths. A simple expression for the disjoining pressure is derived in terms of a two-dimensional integral in Fourier space. The integral is evaluated for periodic and random charge distributions and the disjoining pressure is expressed as a sum over Fourier–Bloch reciprocal lattice vectors or in terms of an integral involving the autocorrelation function, respectively. The force between planes with a finite area of uniform charge, a model for the DLVO interaction between finite surfaces, is also calculated. It is shown that the overspill of the charge cloud beyond the region immediately between the charged areas results in a reduction of the disjoining pressure, as reported by us recently in the long Debye length limit for planes of finite width.
Langmuir : the ACS journal of surfaces and colloids, Jan 8, 2016
Screened Coulomb interactions between uniformly charged flat plates are considered at very small ... more Screened Coulomb interactions between uniformly charged flat plates are considered at very small plate separations for which the Debye layers are strongly overlapped, in the limit of small electrical potentials. If the plates are of infinite length, the disjoining pressure between the plates decays as an inverse power of the plate separation. If the plates are of finite length, we show that screening Debye layer charges close to the edge of the plates are no longer constrained to stay between the plates, but instead spill out into the surrounding electrolyte. The resulting change in the disjoining pressure is calculated analytically: the force between the plates is reduced by this edge correction when the charge density is uniform over the surface of the plates, and is increased when the surface is at constant potential. A similar change in disjoining pressure due to loss of lateral confinement of the Debye layer charges should occur whenever the sizes of the interacting charged obj...
and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a... more and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a solid-state membrane under an applied voltage. The ion distribution near the surface of the membrane arises due to the combined effect of the intrinsic surface charge as well as concentration polarization due to the applied field. It gives rise to an electric pressure that drives hydrodynamic flow in the vicinity of the pore. There is a net hydrodynamic flow through the nanopore due to the asymmetry in the Debye layer induced by the membrane surface charge. The qualitative behavior is similar to that observed in a previous study using molecular dynamic simulations. The flow strength is a strongly nonlinear function of the applied field. Combination of electrophoretic and hydrodynamic effects can lead to ion selectivity in terms of valences and this could have some practical applications in separations.
flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density ... more flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density is considered. In the limit where the surface charge is small and the applied electric field weak, the reciprocal theorem is used to derive an expression for the electroosmotic flux through the pore up to quadratures over the fluid volume. Thus, an "electroosmotic conductance" (the fluid flux per unit applied voltage) may be defined in analogy to the corresponding electrical conductance of a hole in an insulating membrane immersed in a uniform conductor. In the limit when the membrane is thick compared to the pore diameter, the usual result for the electroosmotic conductance through long cylindrical channels (which varies inversely as the membrane thickness) is recovered. The electroosmotic conductance is shown to approach a finite value for an infinitely thin membrane: this residual electroosmotic resistance (inverse of conductance) is analogous to the concept of "access resistance of a pore" in the corresponding electrical problem. The dependence of the electroosmotic conductance on pore radius, Debye length and membrane thickness is investigated. Reference: JFM (2014) 749, 167; Langmuir (in press) 1 Supported by the NIH under grant 4R01HG004842. SG acknowledges a visiting professorship at Cambridge University funded by the Leverhulme Trust, UK. JDS thanks DAMTP (
Submitted for the DFD14 Meeting of The American Physical Society Electroosmotic flow through a cy... more Submitted for the DFD14 Meeting of The American Physical Society Electroosmotic flow through a cylindrical nanopore in a charged membrane of finite thickness 1 MAO MAO, SANDIP GHOSAL, Northwestern University, JOHN D. SHERWOOD, University of Cambridge-We present numerical solutions to the coupled Nernst-Planck-Poisson-Stokes equation for electroosmotic flow through a cylindrical nanopore. The pore traverses a dielectric membrane with uniform surface charge. A multi-physics solver that incorporates electrostatics, ionic transport and electroosmotic flow is developed using the OpenFOAM CFD library. In the limit of small surface charge and weak applied electric field, the numerical results of fluid flux agree with theory when the thickness of the pore h is either very small or very large compared to the pore radius a. For intermediate h/a, our simulation agrees with the composite model of electroosmotic conductance [Sherwood et al. Langmuir (in press)]. When the finite permittivity of the dieletric membrane is taken into account, pairs of toroidal counter rotating eddies appear at the corner of the nanopore that expand to fill the entire pore as the pore radius is decreased. We discuss how the topology of the eddies/stagnation points varies as the aspect ratio of the pore increases. 1 Supported by the NIH under grant 4R01HG004842.
, except for brief excerpts in connection with reviews or scholarly analysis. Use in connection w... more , except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.
Bacteriophages infect cells by attaching to the outer membrane and injecting their DNA into the c... more Bacteriophages infect cells by attaching to the outer membrane and injecting their DNA into the cell. The phage DNA is then transcribed by the cell's transcription machinery. A number of physical mechanisms by which DNA can be translocated from the phage capsid into the cell have been identified. A fast ejection driven by the elastic and electrostatic potential energy of the compacted DNA within the viral capsid appears to be used by most phages, at least to initiate infection. In recent in vitro experiments, the speed of DNA translocation from a λ phage capsid has been measured as a function of ejected length over the entire duration of the event. Here a mechanical model is proposed that is able to explain the observed dependence of exit velocity on ejected length, and that is also consistent with the accepted picture of the geometric arrangement of DNA within the viral capsid.
Gelcasting is a promising new technology for manufacturing advanced structural ceramic components... more Gelcasting is a promising new technology for manufacturing advanced structural ceramic components. The process involves drying of the 'green' gelcast part before densification. The physical mechanisms controlling this relatively long drying process are not well understood. In this study, several controlled experiments were performed to elucidate the key mechanisms. A one-dimensional drying model was formulated based on evaporation and gaseous diffusion through the part. Experimental data were used to obtain correlations for model parameters. This model predicts the instantaneous moisture content of a gelcast sample with an accuracy of better than 10% when dryer humidity and temperature, and sample thickness are specified.
Flame holes and flame disks in a laminar axisymmetric counterflow configuration are numerically i... more Flame holes and flame disks in a laminar axisymmetric counterflow configuration are numerically investigated for unity Lewis number, with the strain rate as the control parameter. The temporal evolution of the topological structure of flame holes and flame disks is described in detail for different representative strain rates. It is found that corresponding to each given strain rate, there exists a critical hole (disk) radius r c that separates the shrinking and expanding hole (disk) regimes. The value of r c decreases monotonically with the increase (decrease) of strain rate and reaches a finite minimum at the extinction (ignition) limit of the strain rate, which indicates that one cannot ignite a mixing layer by an infinitesimal energy source, nor can one quench a diffusion flame by making an infinitesimal extinction hole on it. An examination of the phase diagrams of flame holes (disks) justifies the existence of a unique edge-flame velocity v f as a smooth continuous function of the hole (disk) radius r f in the entire range 0 < r f < ∞, with the strain rate (or equivalently, Damköhler number) as a parameter. For the flame hole case, it is found that in the final stage of collapse of a hole, the edge-flame velocity is essentially proportional to the inverse of the hole radius, except when the strain rate is very close to the extinction limit. Flame interactions induced by overlapping of pre-heat zones are mainly responsible for the acceleration of the edge-flame velocity when the hole radius approaches zero, and it is further enhanced by the focusing effects of hole curvature in the plane of the stoichiometric surface. For the flame disk, the increasing heat loss rate plays a major role on the acceleration of the shrinking speed when the disk radius approaches zero.
Bulletin of the American Physical Society, Nov 19, 2006
The transport of a species, in a microfluidic capillary of arbitrary but axially-invariant cross-... more The transport of a species, in a microfluidic capillary of arbitrary but axially-invariant cross-sectional shape in the presence of an adsorption-desorption reaction on the capillary wall is studied, motivated by applications to capillary electrophoresis and open-tubular capillary electrochromatography. An asymptotic approach based on the long time limit is adopted. Numerical simulations are conducted for (a) an axially homogeneous flow with arbitrary crosssectional variation of the axial velocity and (b) an electroosmotic flow rendered temporally and axially variable by the adsorbed species concentration on the wall. The predictions from asymptotic theory are tested against numerical results obtained from the full three-dimensional solution of the governing equations.
Proceedings of The Royal Society A: Mathematical, Physical and Engineering Sciences, Mar 1, 2017
The screened Coulomb interaction between a pair of infinite parallel planes with spatially varyin... more The screened Coulomb interaction between a pair of infinite parallel planes with spatially varying surface charge is considered in the limit of small electrical potentials for arbitrary Debye lengths. A simple expression for the disjoining pressure is derived in terms of a two-dimensional integral in Fourier space. The integral is evaluated for periodic and random charge distributions and the disjoining pressure is expressed as a sum over Fourier-Bloch reciprocal lattice vectors or in terms of an integral involving the autocorrelation function, respectively. The force between planes with a finite area of uniform charge, a model for the DLVO interaction between finite surfaces, is also calculated. It is shown that the overspill of the charge cloud beyond the region immediately between the charged areas results in a reduction of the disjoining pressure, as reported by us recently in the long Debye length limit for planes of finite width.
Single molecule experiments on bacteriophages show an exponential scaling for the dependence of m... more Single molecule experiments on bacteriophages show an exponential scaling for the dependence of mobility on the length of DNA within the capsid. It has been suggested that this could be due to the "capstan mechanism"-the exponential amplification of friction forces that result when a rope is wound around a cylinder as in a ship's capstan. Here we describe a desktop experiment that illustrates the effect. Though our model phage is a million times larger, it exhibits the same scaling observed in single molecule experiments.
Bulletin of the American Physical Society, Nov 25, 2013
and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a... more and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a solid-state membrane under an applied voltage. The ion distribution near the surface of the membrane arises due to the combined effect of the intrinsic surface charge as well as concentration polarization due to the applied field. It gives rise to an electric pressure that drives hydrodynamic flow in the vicinity of the pore. There is a net hydrodynamic flow through the nanopore due to the asymmetry in the Debye layer induced by the membrane surface charge. The qualitative behavior is similar to that observed in a previous study using molecular dynamic simulations. The flow strength is a strongly nonlinear function of the applied field. Combination of electrophoretic and hydrodynamic effects can lead to ion selectivity in terms of valences and this could have some practical applications in separations.
Bulletin of the American Physical Society, Nov 24, 2014
flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density ... more flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density is considered. In the limit where the surface charge is small and the applied electric field weak, the reciprocal theorem is used to derive an expression for the electroosmotic flux through the pore up to quadratures over the fluid volume. Thus, an "electroosmotic conductance" (the fluid flux per unit applied voltage) may be defined in analogy to the corresponding electrical conductance of a hole in an insulating membrane immersed in a uniform conductor. In the limit when the membrane is thick compared to the pore diameter, the usual result for the electroosmotic conductance through long cylindrical channels (which varies inversely as the membrane thickness) is recovered. The electroosmotic conductance is shown to approach a finite value for an infinitely thin membrane: this residual electroosmotic resistance (inverse of conductance) is analogous to the concept of "access resistance of a pore" in the corresponding electrical problem. The dependence of the electroosmotic conductance on pore radius, Debye length and membrane thickness is investigated. Reference: JFM (2014) 749, 167; Langmuir (in press) 1 Supported by the NIH under grant 4R01HG004842. SG acknowledges a visiting professorship at Cambridge University funded by the Leverhulme Trust, UK. JDS thanks DAMTP (
Bulletin of the American Physical Society, Nov 18, 2007
Submitted for the DFD07 Meeting of The American Physical Society DNA translocation through nanopo... more Submitted for the DFD07 Meeting of The American Physical Society DNA translocation through nanopores: effect of salt concentration SANDIP GHOSAL, Northwestern University-Recent experiments on the detection of single molecules of linear polyelectrolytes through nanopores could lead to an ultrafast and inexpensive method of rapidly sequencing linear polymer chains such as DNA and RNA. In earlier work (see Ghosal, S. in APS DFD06 abstracts) a hydrodynamic model for determining the electrophoretic speed of a polyelectrolyte through an axially symmetric slowly varying nanopore was presented in the limit of vanishingly small Debye length. Here the case of a finite Debye layer thickness is considered within the Debye-Hückel and Stokes flow approximations while restricting the pore geometry to that of a cylinder of length much larger than the diameter. Further, the possibility of a uniform surface charge on the walls of the nanopore is taken into account. The model admits an exact analytical solution from which translocation times are calculated and found to be consistent with recent measurements in solidstate nanopores. It is suggested, based on the solution to the model problem, that the translocation speed can be greatly reduced if the ζ-potential of the nanopore walls could be fine tuned to match closely the ζ-potential of the polyelectrolyte. Physically this amounts to balancing the net electrical force on the polyelectrolyte inside the pore by the viscous drag of the electroosmotic flow inside the nanopore. Reduction of the translocation speed by several orders of magnitude is essential for achieving single base resolution.
Bulletin of the American Physical Society, Nov 22, 2005
Submitted for the DFD05 Meeting of The American Physical Society Electroosmotic flow in rectangul... more Submitted for the DFD05 Meeting of The American Physical Society Electroosmotic flow in rectangular microchannels: numerical simulation and asymptotic theory 1 SUBHRA DATTA, SANDIP GHOSAL, NEE-LESH PATANKAR, Northwestern University-The problem of fluid flow in a microfluidic channel of rectangular cross-section is solved numerically when the zeta potential is not uniform. Variations in the axial direction as well as along the perimeter of the channel cross-section is considered. Excellent agreement is found with a previously published (Ghosal, 2002 JFM vol.459 pg. 103) asymptotic theory based on the lubrication approximation, even when the length scale of axial variations is of the same order as the characteristic channel width.
Bulletin of the American Physical Society, Mar 6, 2007
Submitted for the MAR07 Meeting of The American Physical Society Electrophoretic speed of a polye... more Submitted for the MAR07 Meeting of The American Physical Society Electrophoretic speed of a polyelectrolyte in a nanopore 1 SANDIP GHOSAL, Northwestern University-A hydrodynamic model for determining the electrophoretic speed of a polyelectrolyte through a nanopore is presented. It is assumed that the speed is determined by a balance of electrical and viscous forces arising from within the pore in the presence of co and counter ions. Further, classical continuum electrostatics and hydrodynamics as well as the mean field description of Poisson-Boltzmann is assumed to be applicable after accounting for Manning condensation on the polyelectrolyte. An explicit formula for the translocation speed as a function of the pore geometry and other physical parameters is obtained and is shown to be consistent with recent experimental measurements on DNA translocation through nanopores in silicon membranes.
Diffusion of colored dye on water saturated paper substrates has been traditionally exploited wit... more Diffusion of colored dye on water saturated paper substrates has been traditionally exploited with great skill by renowned water color artists. The same physics finds more recent practical applications in paper based diagnostic devices deploying chemicals that react with a bodily fluid yielding colorimetric signals for disease detection. During spontaneous imbibition through the tortuous pathways of a porous electrolyte saturated paper matrix, a dye molecule undergoes diffusion in a complex network of pores. The advancing front forms a strongly correlated interface that propagates diffusively but with an enhanced effective diffusivity. We measure this effective diffusivity and show that it is several orders of magnitude greater than the free solution diffusivity and has a significant dependence on the solution pH and salt concentration in the background electrolyte. We attribute this to electrically mediated interfacial interactions between the ionic species in the liquid dye and spontaneous surface charges developed at porous interfaces, and introduce a simple theory to explain this phenomenon.
Single molecule experiments on bacteriophages show an exponential scaling for the dependence of m... more Single molecule experiments on bacteriophages show an exponential scaling for the dependence of mobility on the length of DNA within the capsid. It has been suggested that this could be due to the "capstan mechanism"-the exponential amplification of friction forces that result when a rope is wound around a cylinder as in a ship's capstan. Here we describe a desktop experiment that illustrates the effect. Though our model phage is a million times larger, it exhibits the same scaling observed in single molecule experiments.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2017
The screened Coulomb interaction between a pair of infinite parallel planes with spatially varyin... more The screened Coulomb interaction between a pair of infinite parallel planes with spatially varying surface charge is considered in the limit of small electrical potentials for arbitrary Debye lengths. A simple expression for the disjoining pressure is derived in terms of a two-dimensional integral in Fourier space. The integral is evaluated for periodic and random charge distributions and the disjoining pressure is expressed as a sum over Fourier–Bloch reciprocal lattice vectors or in terms of an integral involving the autocorrelation function, respectively. The force between planes with a finite area of uniform charge, a model for the DLVO interaction between finite surfaces, is also calculated. It is shown that the overspill of the charge cloud beyond the region immediately between the charged areas results in a reduction of the disjoining pressure, as reported by us recently in the long Debye length limit for planes of finite width.
Langmuir : the ACS journal of surfaces and colloids, Jan 8, 2016
Screened Coulomb interactions between uniformly charged flat plates are considered at very small ... more Screened Coulomb interactions between uniformly charged flat plates are considered at very small plate separations for which the Debye layers are strongly overlapped, in the limit of small electrical potentials. If the plates are of infinite length, the disjoining pressure between the plates decays as an inverse power of the plate separation. If the plates are of finite length, we show that screening Debye layer charges close to the edge of the plates are no longer constrained to stay between the plates, but instead spill out into the surrounding electrolyte. The resulting change in the disjoining pressure is calculated analytically: the force between the plates is reduced by this edge correction when the charge density is uniform over the surface of the plates, and is increased when the surface is at constant potential. A similar change in disjoining pressure due to loss of lateral confinement of the Debye layer charges should occur whenever the sizes of the interacting charged obj...
and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a... more and analytical modeling is employed to study ion transport and fluid flow through a nanopore in a solid-state membrane under an applied voltage. The ion distribution near the surface of the membrane arises due to the combined effect of the intrinsic surface charge as well as concentration polarization due to the applied field. It gives rise to an electric pressure that drives hydrodynamic flow in the vicinity of the pore. There is a net hydrodynamic flow through the nanopore due to the asymmetry in the Debye layer induced by the membrane surface charge. The qualitative behavior is similar to that observed in a previous study using molecular dynamic simulations. The flow strength is a strongly nonlinear function of the applied field. Combination of electrophoretic and hydrodynamic effects can lead to ion selectivity in terms of valences and this could have some practical applications in separations.
flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density ... more flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density is considered. In the limit where the surface charge is small and the applied electric field weak, the reciprocal theorem is used to derive an expression for the electroosmotic flux through the pore up to quadratures over the fluid volume. Thus, an "electroosmotic conductance" (the fluid flux per unit applied voltage) may be defined in analogy to the corresponding electrical conductance of a hole in an insulating membrane immersed in a uniform conductor. In the limit when the membrane is thick compared to the pore diameter, the usual result for the electroosmotic conductance through long cylindrical channels (which varies inversely as the membrane thickness) is recovered. The electroosmotic conductance is shown to approach a finite value for an infinitely thin membrane: this residual electroosmotic resistance (inverse of conductance) is analogous to the concept of "access resistance of a pore" in the corresponding electrical problem. The dependence of the electroosmotic conductance on pore radius, Debye length and membrane thickness is investigated. Reference: JFM (2014) 749, 167; Langmuir (in press) 1 Supported by the NIH under grant 4R01HG004842. SG acknowledges a visiting professorship at Cambridge University funded by the Leverhulme Trust, UK. JDS thanks DAMTP (
Submitted for the DFD14 Meeting of The American Physical Society Electroosmotic flow through a cy... more Submitted for the DFD14 Meeting of The American Physical Society Electroosmotic flow through a cylindrical nanopore in a charged membrane of finite thickness 1 MAO MAO, SANDIP GHOSAL, Northwestern University, JOHN D. SHERWOOD, University of Cambridge-We present numerical solutions to the coupled Nernst-Planck-Poisson-Stokes equation for electroosmotic flow through a cylindrical nanopore. The pore traverses a dielectric membrane with uniform surface charge. A multi-physics solver that incorporates electrostatics, ionic transport and electroosmotic flow is developed using the OpenFOAM CFD library. In the limit of small surface charge and weak applied electric field, the numerical results of fluid flux agree with theory when the thickness of the pore h is either very small or very large compared to the pore radius a. For intermediate h/a, our simulation agrees with the composite model of electroosmotic conductance [Sherwood et al. Langmuir (in press)]. When the finite permittivity of the dieletric membrane is taken into account, pairs of toroidal counter rotating eddies appear at the corner of the nanopore that expand to fill the entire pore as the pore radius is decreased. We discuss how the topology of the eddies/stagnation points varies as the aspect ratio of the pore increases. 1 Supported by the NIH under grant 4R01HG004842.
, except for brief excerpts in connection with reviews or scholarly analysis. Use in connection w... more , except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.
Bacteriophages infect cells by attaching to the outer membrane and injecting their DNA into the c... more Bacteriophages infect cells by attaching to the outer membrane and injecting their DNA into the cell. The phage DNA is then transcribed by the cell's transcription machinery. A number of physical mechanisms by which DNA can be translocated from the phage capsid into the cell have been identified. A fast ejection driven by the elastic and electrostatic potential energy of the compacted DNA within the viral capsid appears to be used by most phages, at least to initiate infection. In recent in vitro experiments, the speed of DNA translocation from a λ phage capsid has been measured as a function of ejected length over the entire duration of the event. Here a mechanical model is proposed that is able to explain the observed dependence of exit velocity on ejected length, and that is also consistent with the accepted picture of the geometric arrangement of DNA within the viral capsid.
Gelcasting is a promising new technology for manufacturing advanced structural ceramic components... more Gelcasting is a promising new technology for manufacturing advanced structural ceramic components. The process involves drying of the 'green' gelcast part before densification. The physical mechanisms controlling this relatively long drying process are not well understood. In this study, several controlled experiments were performed to elucidate the key mechanisms. A one-dimensional drying model was formulated based on evaporation and gaseous diffusion through the part. Experimental data were used to obtain correlations for model parameters. This model predicts the instantaneous moisture content of a gelcast sample with an accuracy of better than 10% when dryer humidity and temperature, and sample thickness are specified.
Flame holes and flame disks in a laminar axisymmetric counterflow configuration are numerically i... more Flame holes and flame disks in a laminar axisymmetric counterflow configuration are numerically investigated for unity Lewis number, with the strain rate as the control parameter. The temporal evolution of the topological structure of flame holes and flame disks is described in detail for different representative strain rates. It is found that corresponding to each given strain rate, there exists a critical hole (disk) radius r c that separates the shrinking and expanding hole (disk) regimes. The value of r c decreases monotonically with the increase (decrease) of strain rate and reaches a finite minimum at the extinction (ignition) limit of the strain rate, which indicates that one cannot ignite a mixing layer by an infinitesimal energy source, nor can one quench a diffusion flame by making an infinitesimal extinction hole on it. An examination of the phase diagrams of flame holes (disks) justifies the existence of a unique edge-flame velocity v f as a smooth continuous function of the hole (disk) radius r f in the entire range 0 < r f < ∞, with the strain rate (or equivalently, Damköhler number) as a parameter. For the flame hole case, it is found that in the final stage of collapse of a hole, the edge-flame velocity is essentially proportional to the inverse of the hole radius, except when the strain rate is very close to the extinction limit. Flame interactions induced by overlapping of pre-heat zones are mainly responsible for the acceleration of the edge-flame velocity when the hole radius approaches zero, and it is further enhanced by the focusing effects of hole curvature in the plane of the stoichiometric surface. For the flame disk, the increasing heat loss rate plays a major role on the acceleration of the shrinking speed when the disk radius approaches zero.
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Papers by Sandip Ghosal