Multiscale modeling of electroosmotic flow: Effects of discrete ion, enhanced viscosity, and surface friction

Abstract

We propose an isothermal, one-dimensional, electroosmotic flow model for slit-shaped nanochannels. Nanoscale confinement effects are embedded into the transport model by incorporating the spatially varying solvent and ion concentration profiles that correspond to the electrochemical potential of mean force. The local viscosity is dependent on the solvent local density and is modeled using the local average density method. Excess contributions to the local viscosity are included using the Onsager-Fuoss expression that is dependent on the local ionic strength. A Dirichlet-type boundary condition is provided in the form of the slip velocity that is dependent on the macroscopic interfacial friction. This solvent-surface specific interfacial friction is estimated using a dynamical generalized Langevin equation based framework. The electroosmotic flow of Na + and Cl as single counterions and NaCl salt solvated in Extended Simple Point Charge (SPC/E) water confined between graphene and silicon slit-shaped nanochannels are considered as examples. The proposed model yields a good quantitative agreement with the solvent velocity profiles obtained from the non-equilibrium molecular dynamics simulations. Published by AIP Publishing. [http://dx.

Molecular and Continuum Perspectives on Intermediate and Flow Reversal Regimes in Electroosmotic Transport

“…Modeling of electrokinetic effects has been relied on meanfield approximation, which can only capture the exponential nature (i.e., monotonic decrease) of the charge distribution and does not take into account discrete nature of ions and variations in dielectric permittivity. 18 Thus, classical PB theory cannot predict the anomalous nature of overscreening and/or crowding accurately. Nonetheless, these effects may be integrated into a model by relaxing the mean-field approximation with a local density approximation, so that a better estimation of ion distributions and charge densities can be made.…”

What Controls Thermo-osmosis? Molecular Simulations Show the Critical Role of Interfacial Hydrodynamics

“…Thermophoretic phenomena, referring to the influence of temperature gradients on the flux of colloidal particles, were firstly studied for numerous applications such as optothermal DNA trapping or diseaserelated protein aggregates identification [1][2][3][4][5][6][7][8], and this interest for thermophoresis fostered work on its theoretical description [9][10][11][12][13]. On the other hand, at variance with what has been done for electro-osmosis [14][15][16][17][18][19][20][21][22] and diffusio-osmosis [23][24][25][26], very limited theoretical work has been done so far on thermo-osmosis at solid-liquid interfaces.…”

A multiscale transport model for non-classical nanochannel electroosmosis

“…The variation of ε r across the confinement is modeled using the polarization model discussed in detail in our previous work. 19 The variation of ionic conductivity in the confinement is modeled using a modified Nernst-Einstein model as…”

“…17 Near charged surfaces, quantities such as the solvent interfacial friction and slip length are affected due to combined electro-hydrodynamic effects. 18,19 In our previous work, 19 we presented an EOF model that sought to mitigate the aforementioned shortcomings of the classical model. The pertinent momentum equation for the solvent included the spatial variation of ion concentration and solvent viscosity.…”

“…20 The no-slip condition was replaced by a slip velocity that is dependent on the system specific solvent interfacial friction, which in turn can be readily obtained using a Generalized Langevin Equation (GLE) based interfacial particle dynamics framework developed previously in our laboratory. 19,21,22 However, the accuracy of the model, as we will demonstrate later in the text, was limited to low to moderate wall-charge density cases as it did not account for viscous enhancement due to ion-solvent interactions.…”