The electrophoretic motion of cylindrical macroions inside a nanochannel using molecular dynamics simulation (original) (raw)
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Springer, 2014
The electro-osmotic flow of an aqueous solution of NaCl between two parallel silicon walls is studied through a molecular dynamics simulation. The objective here is to examine the dependency of the electro-osmotic flow on the surface charge density by considering the changes made in the structural properties of the electric double layer (EDL). The ion concentration, velocity profiles, and electric charge density of the electrolyte solution are investigated. Due to the partially charged atoms of the water molecules, water concentration is of a layered type near the wall. The obtained profiles revealed that an increase in the surface charge density, at low surface charges where the governing electrostatic coupling regime is Debye–Hu¨ckel, increases both the electro-osmotic velocity and the EDL thickness; whereas, a decreasing trend is observed in these two parameters in the intermediate regime. For high values of surface charge density, due to the charge inversion phenomenon, the reversed electroosmotic flow will be generated in the channel. Results indicate that the absolute value of the reversed electroosmotic velocity rises with an increase in the surface charge density.
ScienceDirect, 2015
Electro-osmotic flow of an aqueous solution of NaCl has been studied using the molecular dynamics simulation. The main objective of this work is to investigate the effects of the electric field and temperature on the flow properties considering the role of the stern layer. By increasing any of the mentioned parameters, the electro-osmotic velocity grows. It is found that the electro-osmotic velocity is a fourth order function of the electric field, while it changes linearly with temperature. Similar trends of change are found for the EDL thickness. By an increase in the studied parameters, a reduction in the stern layer capacity is observed. In this situation, more moving ions are located in the diffuse layer, which are dragging other particles. This is one of the causes that increase the electro-osmotic velocity, a matter which was not predicted by previous researches. A consequence of the stern layer capacity reduction is that in the systems under the influence of higher temperatures or stronger electric fields, charge inversion phenomenon occurs at higher wall charges.
Polyelectrolyte Electrophoresis in Nanochannels: A Dissipative Particle Dynamics Simulation
The Journal of Physical Chemistry B, 2010
We present mesoscopic DPD-simulations of polyelectrolyte electrophoresis in confined nanogeometries, for varying salt concentration and surface slip conditions. Special attention is given to the influence of electroosmotic flow (EOF) on the migration of the polyelectrolyte. The effective polyelectrolyte mobility is found to depend strongly on the boundary properties, i.e., the slip length and the width of the electric double layer. Analytic expressions for the electroosmotic mobility and the total mobility are derived which are in good agreement with the numerical results. The relevant quantity characterizing the effect of slippage is found to be the dimensionless quantity κ δB, where δB is the slip length, and κ −1 an effective electrostatic screening length at the channel boundaries.
Microgravity Science and Technology, 2010
The electrophoretic motion of a charged spherical nanoparticle along the axis of a nanopore connecting two fluid reservoirs, subjected to an axial electric field and electrolyte concentration gradient, has been investigated using a continuum model. The model consists of the Poisson and Nernst-Planck equations for the electric potential and ionic concentrations and the Stokes equations for the hydrodynamic field with zero gravity. In addition to the electrophoresis generated by the externally imposed electric field, the particle also experiences diffusiophoresis arising from the externally imposed concentration gradient. The effects of the diffusiophoresis on the axial electrophoretic motion are examined with changes in the ratio of the particle size to the thickness of the electric double layer (EDL), and the imposed concentration gradient. Since the EDL thickness, the particle size, and the nanopore size are of the same order of magnitude, the diffusiophoresis is dominated by the induced electrophoresis driven by the generated electric field arising from the doublelayer polarization (DLP). For a relatively small κa p , the ratio of the particle size to the EDL thickness, the diffusiophoresis is dominated by the induced elec-trophoresis from the type II DLP, which propels the particle toward regions with lower salt concentration. Depending on the magnitude and direction of the externally imposed concentration gradient, the electrophoretic motion can be accelerated, decelerated, and even reversed by the diffusiophoresis.
Mesoscopic simulations of electroosmotic flow and electrophoresis in nanochannels
Computer Physics Communications, 2011
We review recent dissipative particle dynamics (DPD) simulations of electrolyte flow in nanochannels. A method is presented by which the slip length δ B at the channel boundaries can be tuned systematically from negative to infinity by introducing suitably adjusted wall-fluid friction forces. Using this method, we study electroosmotic flow (EOF) in nanochannels for varying surface slip conditions and fluids of different ionic strength. Analytic expressions for the flow profiles are derived from the Stokes equation, which are in good agreement with the numerical results. Finally, we investigate the influence of EOF on the effective mobility of polyelectrolytes in nanochannels. The relevant quantity characterizing the effect of slippage is found to be the dimensionless quantity κδ B , where 1/κ is an effective electrostatic screening length at the channel boundaries.
Effects of surface charge density and distribution on the nanochannel electro-osmotic flow
International Journal of Theoretical and Applied Multiscale Mechanics, 2011
Surface charge density and distribution dependence of a nanochannel electro-osmotic flow was examined using a molecular dynamics (MD) model. Systems consisting of Na + and Cl − ions in water confined between crystalline walls with varying negative charge on inner surfaces in an external electric field were investigated. At low surface charge densities, water flows as expected by common interpretations of electro-osmosis. At intermediate surface charge density, the flow exhibits a maximum. Strongly charged surfaces cause adsorption of counterions, immobilization of the near-wall fluid layers, and subsequent flow reversal. An effect of increase in the viscosity of water near the strongly charged surface was demonstrated. When the discrete −1e charge was distributed on a subgrid of surface atoms, the flow deteriorated and reversed at much lower surface charge densities than when all the surface atoms carried equal partial charge. . (2011) 'Effects of surface charge density and distribution on the nanochannel electro-osmotic flow', Int.
Electrodiffusiophoretic Motion of a Charged Spherical Particle in a Nanopore
The electrodiffusiophoretic motion of a charged spherical nanoparticle in a nanopore subjected to an axial electric field and electrolyte concentration gradient has been investigated using a continuum model, composed of the Poisson-Nernst-Planck equations for the ionic mass transport and the Navier-Stokes equations for the flow field. The charged particle experiences electrophoresis in response to the imposed electric field and diffusiophoresis caused solely by the imposed concentration gradient. The diffusiophoretic motion is induced by two different mechanisms, an electrophoresis driven by the generated electric field arising from the difference of ionic diffusivities and the double layer polarization and a chemiphoresis due to the induced osmotic pressure gradient around the charged nanoparticle. The electrodiffusiophoretic motion along the axis of a nanopore is investigated as a function of the ratio of the particle size to the thickness of the electrical double layer, the imposed concentration gradient, the ratio of the surface charge density of the nanopore to that of the particle, and the type of electrolyte. Depending on the magnitude and direction of the imposed concentration gradient, one can accelerate, decelerate, and even reverse the particle's electrophoretic motion in a nanopore by the superimposed diffusiophoresis. The induced electroosmotic flow in the vicinity of the charged nanopore wall driven by both the imposed and the generated electric fields also significantly affects the electrodiffusiophoretic motion.
Electrokinetic motion of a rectangular nanoparticle in a nanochannel
Journal of Nanoparticle Research, 2012
This article presents a theoretical study of electrokinetic motion of a negatively charged cubic nanoparticle in a three-dimensional nanochannel with a circular cross-section. Effects of the electrophoretic and the hydrodynamic forces on the nanoparticle motion are examined. Because of the large applied electric field over the nanochannel, the impact of the Brownian force is negligible in comparison with the electrophoretic and the hydrodynamic forces. The conventional theories of electrokinetics such as the Poisson-Boltzmann equation and the Helmholtz-Smoluchowski slip velocity approach are no longer applicable in the small nanochannels. In this study, and at each time step, first, a set of highly coupled partial differential equations including the Poisson-Nernst-Plank equation, the Navier-Stokes equations, and the continuity equation was solved to find the electric potential, ionic concentration field, and the flow field around the nanoparticle. Then, the electrophoretic and hydrodynamic forces acting on the negatively charged nanoparticle were determined. Following that, the Newton second law was utilized to find the velocity of the nanoparticle. Using this model, effects of surface electric charge of the nanochannel, bulk ionic concentration, the size of the nanoparticle, and the radius of the nanochannel on the nanoparticle motion were investigated. Increasing the bulk ionic concentration or the surface charge of the nanochannel will increase the electroosmotic flow, and hence affect the particle's motion. It was also shown that, unlike microchannels with thin EDL, the change in nanochannel size will change the EDL field and the ionic concentration field in the nanochannel, affecting the particle's motion. If the nanochannel size is fixed, a larger particle will move faster than a smaller particle under the same conditions.
Electrophoretic mobility of a charged colloidal particle: a computer simulation study
Journal of Physics: Condensed Matter, 2004
We study the mobility of a charged colloidal particle in a constant homogeneous electric field by means of computer simulations. The simulation method combines a lattice Boltzmann scheme for the fluid with standard Langevin dynamics for the colloidal particle, which is built up from a net of bonded particles forming the surface of the colloid. The coupling between the two subsystems is introduced via friction forces. In addition, explicit counterions, also coupled to the fluid, are present. We observe a non-monotonic dependence of the electrophoretic mobility on the bare colloidal charge. At low surface charge density we observe a linear increase of the mobility with bare charge, whereas at higher charges, where more than half of the ions are co-moving with the colloid, the mobility decreases with increasing bare charge.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2013
Optical Tweezers are employed to study the electrophoretic and the electroosmotic motion of a single colloid immersed in electrolyte solutions of ion concentrations between 10 −5 and 1 mol/l and of different valencies (KCl, CaCl 2 , LaCl 3 ). The measured particle mobility in monovalent salt is found to be in agreement with computations combining primitive model molecular dynamics simulations of the ionic double layer with the standard electrokinetic model. Mobility reversal of a single colloid-for the first time-is observed in the presence of trivalent ions (LaCl 3 ) at ionic strengths larger than 10 −2 mol/l. In this case, our numerical model is in a quantitative agreement with the experiment only when ion specific attractive forces are added to the primitive model, demonstrating that at low colloidal charge densities, ion correlation effects alone do not suffice to produce mobility reversal.