Viscous interfacial layer formation causes electroosmotic mobility reversal in monovalent electrolytes

We study the ion density, shear viscosity and electroosmotic mobility of an aqueous monovalent electrolyte at a charged solid surface using molecular dynamics simulations. Upon increasing the surface charge density, ions are displaced first from the diffuse layer to the outer Helmholtz layer, increasing its viscosity, and subsequently to the hydrodynamically stagnant inner Helmholtz layer. The ion redistribution causes both charge inversion and reversal of the electroosmotic mobility. Because of the surface-charge dependent interfacial hydrodynamic properties, however, the charge density of mobility reversal differs from the charge density of charge inversion, depending on the salt concentration and the chemical details of the ions and the surface. Mobility reversal cannot be described by an effective slip boundary condition alone – the spatial dependence of the viscosity is essential

Numerical Simulation of Jet Mode in Electrospraying of Newtonian and Viscoelastic Fluids

The purpose of this article is to explore the role of viscoelastic properties of polymeric solutions on the jet mode in the electrospray process. In this research, several numerical simulations were performed to model the behavior of electrified Newtonian and viscoelastic jets. First, used for validating viscoelastic constitutive equations and their implementation, the benchmark problem of the sedimenting sphere is stabilized beyond the previously suggested threshold. Then, an electrified DI water jet was simulated, and the obtained jet profile was compared with the experimental data from previous publications. Finally, the proposed algorithm was used to simulate viscoelastic electrified jets, where the effect of the Weissenberg number (Wi) on the jet profile was examined. In agreement with the previously obtained experimental results, by increasing the solution concentration, the asymptotic profile of the jet is reached at a smaller length from the nozzle, while the final thickness of the jet is slightly reduced.Comment: 17 pages, 12 figures, The following article is going to be submitted to the international journal of multiphase flow. Once accepted, it will be replaced accordingl

Numerical and experimental study on the steady cone-jet mode of electro-centrifugal spinning

This study focuses on a numerical investigation of an initial stable jet through the air-sealed electro-centrifugal spinning process, which is known as a viable method for the mass production of nanofibers. A liquid jet undergoing electric and centrifugal forces, as well as other forces, first travels in a stable trajectory and then goes through an unstable curled path to the collector. In numerical modeling, hydrodynamic equations have been solved using the perturbation method—and the boundary integral method has been implemented to efficiently solve the electric potential equation. Hydrodynamic equations have been coupled with the electric field using stress boundary conditions at the fluid-fluid interface. Perturbation equations were discretized by a second order finite difference method, and the Newton method was implemented to solve the discretized non-linear system. Also, the boundary element method was utilized to solve electrostatic equations. In the theoretical study, the fluid was described as a leaky dielectric with charges only on the surface of the jet traveling in dielectric air. The effect of the electric field induced around the nozzle tip on the jet instability and trajectory deviation was also experimentally studied through plate-plate geometry as well as point-plate geometry. It was numerically found that the centrifugal force prevails on electric force by increasing the rotational speed. Therefore, the alteration of the applied voltage does not significantly affect the jet thinning profile or the jet trajectory

Numerical and experimental investigation on static electric charge model at stable cone-jet region

In a typical electro-spinning process, the steady stretching process of the jet beyond the Taylor cone has a significant effect on the dimensions of resulting nanofibers. Also, it sets up the conditions for the onset of the bending instability. The focus of this work is the modeling and simulation of the initial stable jet phase seen during the electro-spinning process. The perturbation method was applied to solve hydrodynamic equations, and the electrostatic equation was solved by a boundary integral method. These equations were coupled with the stress boundary conditions derived appropriate at the fluid-fluid interface. Perturbation equations were discretized by the second-order finite difference method, and the Newton method was implemented to solve the discretized nonlinear system. Also, the boundary element method was utilized to solve the electrostatic equation. In the theoretical study, the fluid is described as a leaky dielectric with charges only on the jet surface in dielectric air. In this study, electric charges were modeled as static. Comparison of numerical and experimental results shows that at low flow rates and high electric field, good agreement was achieved because of the superior importance of the charge transport by conduction rather than convection and charge concentration. In addition, the effect of unevenness of the electric field around the nozzle tip was experimentally studied through plate-plate geometry as well as point-plate geometry

Numerical Simulation of Steady Supercavitating Flows

In this research, the Supercavitation phenomenon in compressible liquid flows is simulated. The one-fluid method based on a new exact two-phase Riemann solver is used for modeling. The cavitation is considered as an isothermal process and a consistent equation of state with the physical behavior of the water is used. High speed flow of water over a cylinder and a projectile are simulated and the results are compared with the previous numerical and experimental results. The cavitation bubble profile in both cases agrees well with the previous experimental results reported in the literature. As the result shows, coupling the two-phase Riemann solver with the considered EOS prepares a robust method for simulating the compressible fluid flow with cavitation which can undertake the whole physical behavior of water in a supercavitation process. Furthermore, the influence of the cavitator head and the flow speed on the supercavitation bubble is explored. The results show that cavitators with sharper head results in a smaller supercavitating bubble. Increasing the flow speed beyond a specific limit does not have any significant effect on the cavitation bubble and slightly increases the bubble size

Evaluating the capability of the flux-limiter schemes in capturing strong shocks and discontinuities

Abstract. A numerical study is conducted to investigate the capability of the flux-limiter TVD schemes in capturing sharp discontinuities like shock waves. For this purpose, four classical test problems are considered such as slowly moving shock, gas Riemann problem with high density and pressure ratios, shock wave interaction with a density disturbance and shock-acoustic interaction. The governing equations consist of one-dimensional and quasi-one-dimensional Euler equations solved using an in-house numerical code. In order to validate the solution, the obtained results are compared with other results found in the literature

Evaluating the Capability of the Flux-Limiter Schemes in Capturing Strong Shocks and Discontinuities

A numerical study is conducted to investigate the capability of the flux-limiter TVD schemes in capturing sharp discontinuities like shock waves. For this purpose, four classical test problems are considered such as slowly moving shock, gas Riemann problem with high density and pressure ratios, shock wave interaction with a density disturbance and shock-acoustic interaction. The governing equations consist of one-dimensional and quasi-one-dimensional Euler equations solved using an in-house numerical code. In order to validate the solution, the obtained results are compared with other results found in the literature