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Collisions of charged droplets in the shear flow of an electrolyte were simulated to investigate the effects of surface charges and double layers on the critical conditions for coalescence. A lattice Boltzmann method phase field flow solver was coupled with an iterative finite-difference solver for the linearized Poisson–Boltzmann equation describing electric double layers. The simulations resolve both the phase field diffuse interface and electric double layer. The critical capillary numbers for coalescence were determined under varying strengths of the electric interactions. Critical capillary numbers between 0.03 and 0.2 were found for droplet radii spanning 25–50 lattice nodes (12.5–25 times the characteristic interface thickness), with lower critical values for larger droplets. The droplet interfaces had a constant potential. Once electric repulsion becomes comparable to the viscous shear force on the drops, the critical capillary numbers decrease, and the decrease is smaller for longer Debye lengths. Though the ratio of droplet size and Debye length that can be achieved in the simulations is constrained by high computational demands, the simulations provide insight into the effects of surface charge on the interactions between interfaces in multiphase flows. [ABSTRACT FROM AUTHOR]

In this paper, we provide a theory to pinpoint the role of electric double layer (EDL) interactions in governing the contact angle of an electrolyte drop on a charged solid in air or a bubble on a charged surface within an electrolyte solution. The EDL interactions are analytically solved by representing the three phase contact line as a wedge edge, with the wedge being formed by the solid-liquid and the air-liquid interfaces, and calculating the corresponding Maxwell stresses. We demonstrate that the EDL effects induce an "electrowetting-like" behavior, resulting in a lowering of the contact angle. As a specific example, we use this model to analyze the effect of added salt on preformed surface nanobubbles, and find, in contrast to what has been reported earlier, that even for most moderate conditions, added salt may have remarkable effect in altering the contact angle in preformed surface nanobubbles. [ABSTRACT FROM AUTHOR]

In this paper, we develop a theory to delineate the consequences of finite solvent polarization in electric double layer interaction or the osmotic pressure between two similar or oppositely charged surfaces. We use previously published Langevin-Bikerman equations to calculate this electric double layer interaction force or the osmotic pressure between the charged surfaces. The osmotic pressure between oppositely charged surfaces is found to be much larger than that between similarly charged surfaces, and for either case, the influence of solvent polarization ensures a larger pressure than that predicted by the Poisson-Boltzmann (PB) model. We derive distinct scaling relationships to explain the increase of the pressure as a function of the separation between the surfaces, the solvent polarizability, and the number density of water molecules. Most importantly, we demonstrate that our theory can successfully reproduce the experimental results of interaction force between similar and oppositely charged surfaces, by accounting for the large under-prediction made by the corresponding PB model. [ABSTRACT FROM AUTHOR]

Highlights: [•] Streaming potential and electroviscous effects are calculated for a nanocapillary. [•] Results are obtained for thick overlapped Electric Double Layers (EDLs). [•] Streaming potential exhibits distinct scaling variations with the EDL thickness. [•] Electroviscous effect obeys streaming potential variation at small EDL values. [•] Electroviscous effect is independent of streaming potential for overlapped EDL. [Copyright &y& Elsevier]

Abstract: In this paper, we provide for the first time an analytical framework for calculating the zeta (ζ) potential of asphaltene in aqueous solution in presence and in absence of an electrolyte salt. The analytical model is based on the assumption that the ionizable (in aqueous solution) carboxylic and hydroxyl groups present on the asphaltene molecule lead to its charging. The model predicts the ζ potential as a function of parameters such as buffer pH and electrolyte salt concentration. For the case with the added salt, ζ potential decreases with pH monotonically, and we obtain quantitative match with the experimental results. For the case without the added salt, the variation with pH is non-monotonic, represented by presence of two distinct extrema, and can be attributed to the role of the corresponding Electric Double Layer (EDL) thickness, which (in absence of any added salt) is a sole function of the pH. [Copyright &y& Elsevier]

In this paper we delineate the consequences of field-dependent solvent polarization in the electric double layer (EDL) electrostatic potential distribution, and the effective EDL thickness in narrow nanofluidic confinements with thick (or overlapping) EDLs. The EDL, formed at the interface between a charged substrate and an electrolyte solution, induces a large electric field spanning across few nanometer distances from the interface. As a result, a polar solvent like water gets polarized, making its relative permittivity a function of the EDL electric field. This affects the overall EDL electrostatic potential distribution and most importantly, leads to a significant reduction of the effective EDL thickness, with the extent of the reduction being dictated by the value of field independent EDL thickness, strength of the solvent polarization, and the substrate-liquid interfacial electrostatic potential. Such a finding will necessitate redefining the classical EDL thickness, which will be of overwhelming significance in nanofluidic transport. [ABSTRACT FROM AUTHOR]

We provide an analytical model to describe the filling dynamics of horizontal cylindrical capillaries having charged walls. The presence of surface charge leads to two distinct effects: It leads to a retarding electrical force on the liquid column and also causes a reduced viscous drag force because of decreased velocity gradients at the wall. Both these effects essentially stem from the spontaneous formation of an electric double layer (EDL) and the resulting streaming potential caused by the net capillary-flow-driven advection of ionic species within the EDL. Our results demonstrate that filling of charged capillaries also exhibits the well-known linear and Washburn regimes witnessed for uncharged capillaries, although the filling rate is always lower than that of the uncharged capillary. We attribute this to a competitive success of the lowering of the driving forces (because of electroviscous effects), in comparison to the effect of weaker drag forces. We further reveal that the time at which the transition between the linear and the Washburn regime occurs may become significantly altered with the introduction of surface charges, thereby altering the resultant capillary dynamics in a rather intricate manner. [ABSTRACT FROM AUTHOR]