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Struttura della materia
Multiscale hydrodynamics and electrokinetics in clay and silica nanopores
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Aula Rasetti (Dip. di Fisica - Edificio G. Marconi)
Aula Rasetti
Dip. di Fisica - Edificio G. Marconi
Description
Hydrodynamic and electrokinetic flows in porous media are usually described using the Navier-Stokes and Nernst-Planck equations. The validity of such continuous descriptions needs to be questioned in nanopores. Indeed, the molecular nature of the solvent and ions plays an important role in the vicinity of a surface, resulting among others in the layering of the solvent and oscillatory ionic profiles. We use molecular simulations to investigate hydrodynamic and electroosmotic flows in clay nanopores, with pore sizes ranging from 3 to 10 nm, and address the following questions: - What is the smallest pore size for which the predictions of the Navier-Stokes and Nernst-Planck equations may apply? - When this is the case, how far from the surface are the solvent and ionic profiles well described by the continuous result (e.g. Poiseuille flow)?
Is the use of the bulk density and viscosity of the fluid appropriate to determine the properties of the confined fluid? - Can the continuous description be improved using locally varying properties (density, viscosity) determined from molecular simulations? - Can the continuous description be improved by using boundary conditions (slip length) determined from molecular simulations ? More precisely, we first use Grand-Canonical Monte-Carlo simulations to determine the density of the confined fluid. We then use equilibrium Molecular Dynamics to study the structure of the interface on the molecular scale and the diffusion of all species as a function of the distance to the surface. We finally use Non-Equilibrium Molecular Dynamics to investigate the transport properties, by applying a constant force to the fluid atoms (and compare the fluid flow profile to the Poiseuille flow) or a constant electric field to the ions (and compare to the prediction of Poisson-Nernst-Planck + Stokes equations). In the case of electroosmotic flows, we also study the effect of added salt. Comparison with Smoluchowsky’selectrosmotic model, exhibits slipping of the solvent and solute in the vicinity of the surface, related to it’s hydorphobic/hydrophilic character
Is the use of the bulk density and viscosity of the fluid appropriate to determine the properties of the confined fluid? - Can the continuous description be improved using locally varying properties (density, viscosity) determined from molecular simulations? - Can the continuous description be improved by using boundary conditions (slip length) determined from molecular simulations ? More precisely, we first use Grand-Canonical Monte-Carlo simulations to determine the density of the confined fluid. We then use equilibrium Molecular Dynamics to study the structure of the interface on the molecular scale and the diffusion of all species as a function of the distance to the surface. We finally use Non-Equilibrium Molecular Dynamics to investigate the transport properties, by applying a constant force to the fluid atoms (and compare the fluid flow profile to the Poiseuille flow) or a constant electric field to the ions (and compare to the prediction of Poisson-Nernst-Planck + Stokes equations). In the case of electroosmotic flows, we also study the effect of added salt. Comparison with Smoluchowsky’selectrosmotic model, exhibits slipping of the solvent and solute in the vicinity of the surface, related to it’s hydorphobic/hydrophilic character
