DPD Simulation of Electroosmotic Flow in Nanochannels and the Evaluation of Effective Parameters (original) (raw)
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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.
Advances in Microfluidics, 2012
In the past decades, several mesoscale simulation techniques have emerged as tools to study hydrodynamic flow phenomena on scales in the range of nano-to micrometers. Examples are Dissipative Particle Dynamics (DPD), Multiparticle Collision Dynamics (MPCD), or Lattice Boltzmann (LB) methods. These methods allow one to access time and length scales which are not yet within reach of atomistic Molecular Dynamics (MD) simulations, often at relatively moderate computational expense. They can be coupled with particle-based (e.g., molecular dynamics) simulation methods for thermally fluctuating nanoscale objects, such as colloids or large molecules. This makes them particularly attractive for the application in microfluidic or nanofluidic research.
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Fluid-ion transport through a nanochannel is studied to understand the role and impact of different physical phenomena and medium properties on the flow. Mathematically, the system is described through coupled fourth order Poisson–Nernst–Planck–Bikerman and Navier–Stokes equations. The fourth order-Poisson–Nernst–Planck–Bikerman model accounts for ionic and nonionic interactions between particles, the effect of finite size of the particles, polarization of the medium, solvation of the ions, etc. Navier–Stokes equations are modified accordingly to include both electroviscous and viscoelectric effects and the velocity slip. The governing equations are discretized using the lattice Boltzmann method. The mathematical model is validated by comparing the analytical and experimental ion activity while the numerical model is validated by comparing the analytical and numerical velocity profiles for electro-osmotic flow through a microchannel. For a pressure driven flow, the electroviscous an...
Transient Analysis of Electroosmotic Flow in Nano-diameter Channels
2002
Transient analysis of electroosmotic flow in a nanodiameter channel is presented. The time for flow to reach steady state in a nano-diameter channel for various Debye lengths is investigated by solving the Navier-Stokes equations in the presence of electrical forces. The results indicate that the time for flow to reach steady state depends on the interaction of Debye layers. Preliminary results on molecular dynamics simulation of electroosmotic flow are also presented. A comparison between continuum theory results and molecular dynamics simulation results indicates that for a 5 nm channel, continuum theory based on the Poisson-Boltzmann equation can overestimate the ion density near the channel wall, but agrees fairly well in the rest of the channel region. The velocity profile obtained from the continuum model agrees qualitatively with the molecular dynamics simulation result.
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Molecular dynamics (MD) simulation is a powerful tool to investigate the nanoscale fluid flow. In this article, we review the methods and the applications of MD simulation in liquid flows in nanochannels. For pressuredriven flows, we focus on the fundamental research and the rationality of the model hypotheses. For electrokineticdriven flows and the thermal-driven flows, we concentrate on the principle of generating liquid motion. The slip boundary condition is one of the marked differences between the macro-and micro-scale flows and the nanoscale flows. In this article, we review the parameters controlling the degree of boundary slip and the new findings. MD simulation is based on the Newton's second law to simulate the particles' interactions and consists of several important processing methods, such as the thermal wall model, the cut-off radius, and the initial condition. Therefore, we also reviewed the recent improvement in these key methods to make the MD simulation more rational and efficient. Finally, we summarized the important discoveries in this research field and proposed some worthwhile future research directions.
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.
Numerical Modelling of Electrokinetic Flow in Microchannels: Streaming Potential and Electroosmosis
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Investigating the flow-behavior in microfluidic systems has become of interest due to the need for precise control of the mass and momentum transport in microfluidic devices. In multiphase flows, precise control of the flow behavior is much more challenging as it depends on multiple parameters. The following thesis focuses on two aspects of microfluidics discussed in two chapters: the flow reversal phenomenon in streaming potential flows and the magnetic fields generated by electroosmotic and streaming potential flows. In the first chapter, the proposed microfluidic system consists of an aqueous solution between a moving plate and a stationary wall, where the moving plate represents a charged oil-water interface. A numerical model was developed to predict the streaming potential flow created due to the shear-driven motion of the charged upper wall along with its associated electric double layer (EDL) effect. Additionally, analytical expressions were derived by solving the nonlinear Poisson-Boltzmann equation along with the simplified Navier-Stokes equation in order to describe the effect of the EDL on the sheardriven flow of the aqueous electrolyte solution. Results show that the interfacial charge of the moving interface greatly impacts the velocity profile of the flow and can reverse its overall direction. The numerical results were validated by the analytical expressions, where both models predicted that flow can reverse its overall direction when the surface potential of the oil-water interface exceeds 120mV. For the second chapter, models were constructed for the transient electrokinetics, for both the electroosmotic flow and for the shear driven streaming potential flow, in a charged nanocapillary channel. Additionally, the transient effects of ionic currents and the magnetic field generated both inside and outside the microchannel were evaluated, and the results compared with known iii analytical solutions for verification purposes. In order to correctly simulate the above models, the following partial differential equations are solved together for the electrolyte continuum to capture the physics of the problem: a) the Navier-Stokes equation for the fluid flow b) Poisson-Nernst-Planck equations for the electric potential distribution and ion transport and c) Ampere-Maxwell's law for the associated magnetic field. The obtained results showed that the magnetic field detected outside of the nanochannels can be used as a secondary electromagnetic signal for biomolecules as a part of a sequencing technique.