arXiv subjects

Naren Bag

Publications and source records attributed to Naren Bag.

3 recordsLinked to original sources

arXiv2021

Surface and zeta potentials of charged permeable nanocoatings

An electrokinetic (zeta) potential of charged permeable porous films on solid supports generally exceeds their surface potential, which often builds up to a quite high value itself. Recent work provided a quantitative understanding of zeta potentials of thick, compared to the extension of an inner electrostatic diffuse layer, porous films. Here, we consider porous coatings of a thickness comparable or smaller than that of the inner diffuse layer. Our theory, which is valid even when electrostatic potentials become quite high and accounts for a finite hydrodynamic permeability of the porous materials, provides a framework for interpreting the difference between values of surface and zeta potentials in various situations. Analytic approximations for the zeta potential in the experimentally relevant limits provide a simple explanation of transitions between different regimes of electro-osmotic flows, and also suggest strategies for its tuning in microfluidic applications.

physics.chem-ph↗

arXiv2020

Electro-osmotic properties of porous permeable films

Permeable porous coatings on a flat solid support significantly impact its electrostatic and electrokinetic properties. Existing work has focused on simplified cases, such as weakly charged and/or thick porous films, with limited theoretical guidance. Here, we consider the general case of coatings of any given volume charge density and obtain analytic formulas for electrostatic potential profiles, valid for any film thickness and salt concentration. They allow us to calculate analytically the difference between potentials at solid support and at interface with an outer electrolyte, that is the key parameter ascertaining the functionality of permeable coatings. Our analysis provides a framework for interpreting and predicting specific for porous films super-properties, from an enhanced ion absorption to a giant amplification of electro-osmotic flows. The results are relevant for hydrogel and zeolite coatings, porous carbon and ion-exchange resins, polyelectrolyte brushes, and more.

cond-mat.soft↗

arXiv2020

Achieving large zeta-potentials with charged porous surfaces

We discuss an electro-osmotic flow near charged porous coatings of a finite hydrodynamic permeability, impregnated with an outer electrolyte solution. It is shown that their electrokinetic (zeta) potential is generally augmented compared to the surface electrostatic potential, thanks to a large liquid slip at their surface emerging due to an electro-osmotic flow in the enriched by counter-ions porous films. The inner flow shows a very rich behavior controlled by the volume charge density of the coating, its Brinkman length, and concentration of added salt. Interestingly, even for relatively small Brinkman length the zeta-potential can, in some cases, become huge, providing a very fast outer flow in the bulk electrolyte. When the Brinkman length is large enough, the zeta-potential could be extremely high even at practically vanishing surface potential. To describe the slip velocity in a simple manner, we introduce a concept of an electro-osmotic slip length and demonstrate that the latter is always defined by the hydrodynamic permeability of the porous film, and also, depending on the regime, either by its volume charge density or the salt concentration. These results provide a framework for the rational design of porous coatings to enhance electrokineic phenomena, and for tuning their properties by adjusting bulk electrolyte concentrations, with direct applications in microfluidics.

physics.flu-dyn↗