Influence of surfactants on the electrohydrodynamic stretching of water drops in oil

Ervik, Åsmund; Penne, Torstein Eidsnes; Hellesø, Svein Magne; Munkejord, Svend Tollak; Müller, Bernhard

doi:10.1016/j.ijmultiphaseflow.2017.08.011

Cited by 17 publications

(13 citation statements)

References 39 publications

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“…To extend this to two-phase flow, several methods are available. In previous work we have employed the combination of the level-set and ghost-fluid methods, which gives results that agree well with theory and experiments [84,85,86,87,88,89], and which can handle topological changes in the interface, e.g. during drop coalescence.…”

Section: Macroscale: Numerical Methodssupporting

confidence: 53%

“…during drop coalescence. This method can handle a varying interfacial tension, and has been coupled with the Langmuir equation of state to simulate the effects of insoluble surfactants [86,89].…”

Section: Macroscale: Numerical Methodsmentioning

confidence: 99%

“…The handling of the viscosity and density jumps must also be verified to be correctly coupled with the immersed boundary. The technique chosen for this was to compare the proposed method, Section 3, with a reference method, the level-set method with the ghost fluid method, which has previously been verified and validated [84,85,86,87,88,89]. To have some measure of the drop dimensions during oscillations, the horizontal and vertical axis lengths are used.…”

Section: Comparison With Reference Methodsmentioning

confidence: 99%

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A multiscale method for simulating fluid interfaces covered with large molecules such as asphaltenes

Ervik

Lysgaard

Herdes

et al. 2016

Journal of Computational Physics

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The interface between two liquids is fully described by the interfacial tension only for very pure liquids. In most cases the system also contains surfactant molecules which modify the interfacial tension according to their concentration at the interface. This has been widely studied over the years, and interesting phenomena arise, e.g. the Marangoni effect. An even more complicated situation arises for complex fluids like crude oil, where large molecules such as asphaltenes migrate to the interface and give rise to further phenomena not seen in surfactant-contaminated systems. An example of this is the "crumpling drop" experiments, where the interface of a drop being deflated becomes non-smooth at some point. In this paper we report on the development of a multiscale method for simulating such complex liquid-liquid systems. We consider simulations where water drops covered with asphaltenes are deflated, and reproduce the crumpling observed in experiments. The method on the nanoscale is based on using coarse-grained molecular dynamics simulations of the interface, with an accurate model for the asphaltene molecules. This enables the calculation of interfacial properties. These properties are then used in the macroscale simulation, which is performed with a two-phase incompressible flow solver using a novel hybrid level-set/ghost-fluid/immersed-boundary method for taking the complex interface behaviour into account. We validate both the nano-and macroscale methods. Results are presented from nano-and macroscale simulations which showcase some of the interesting behaviour caused by asphaltenes affecting the interface. The molecular simulations presented here are the first in the literature to obtain the correct interfacial orientation of asphaltenes. Results from the macroscale simulations present a new physical explanation of the crumpled drop phenomenon, while highlighting shortcomings in previous hypotheses.

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Section: Macroscale: Numerical Methodssupporting

confidence: 53%

Section: Macroscale: Numerical Methodsmentioning

confidence: 99%

Section: Comparison With Reference Methodsmentioning

confidence: 99%

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A multiscale method for simulating fluid interfaces covered with large molecules such as asphaltenes

Ervik

Lysgaard

Herdes

et al. 2016

Journal of Computational Physics

Self Cite

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“…When the R → ∞ limit is reached the drop behaves as a perfectly conducting one. Such a condition closely resembles various experimental situations as in Mhatre & Thaokar (2013) and Ervik et al (2018) where aqueous drops are considered in an oil medium. In such conditions the electric field acts perpendicularly to the drop surface and the tangential electric field components becomes identically zero, having no effect on O(Re E ) velocity perturbation.…”

Section: Effects Of Surfactant On a Deformable Dropmentioning

confidence: 58%

Sedimentation of a surfactant-laden drop under the influence of an electric field

et al. 2018

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The sedimentation of a surfactant-laden deformable viscous drop acted upon by an electric field is considered theoretically. The convection of surfactants in conjunction with the the combined effect electrohydrodynamic flow and sedimentation leads to a locally varying surface tension, which subsequently alters the drop dynamics via the interplay of Marangoni, Maxwell and hydrodynamic stresses. Assuming small capillary number and small electric Reynolds number, we employ a regular perturbation technique to solve the coupled system of governing equations. It is shown that when a leaky dielectric drop is sedimenting in another leaky dielectric fluid, Marangoni stress can oppose the electrohydrodynamic motion severely, thereby causing corresponding changes in internal flow pattern. Such effects further result in retardation of drop settling velocity, which would have otherwise increased due to the influence of charge convection. For highly mobile surfactants (high Péclet number limit), the drop surface becomes immobilized and the charge convection effect gets completely eliminated. For non-spherical drop shapes, the effect of Marangoni stress is overcome by the 'tip stretching' effect on the flow field. As a result, the drop deformation gets intensified with increment in sensitivity of surface tension to the local surfactant concentration. Consequently, for oblate type of deformation the elevated drag force causes further reduction in velocity. Owing to similar reasons, prolate drops experience lesser drag and settles faster than the surfactant-free case. In addition to this, with increased sensitivity of interfacial tension on the surfactant concentration, the asymmetric deformation about the equator gets suppressed. These findings may turn out to be of fundamental significance towards designing electrohydrodynamically actuated droplet based microfluidic systems that are intrinsically tunable by varying the surfactant concentration. *

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“…The effect of surfactants on the electrically actuated drops, is a less studied domain in the literature (Ha & Yang, 1995;Teigen & Munkejord, 2010;Nganguia et al, 2013;Poddar et al, 2018;Ervik et al, 2018). The electrorheology of concentrated emulsions in the presence of nonionic surfactants has only been experimentally investigated by Ha et al (1999).…”

Section: Introductionmentioning

confidence: 99%

Electrorheology of a dilute emulsion of surfactant-covered drops

et al. 2019

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The effects of surfactant coating on a deformable viscous drop under the combined action of a shear flow and a uniform electric field, are investigated by solving the coupled equations of electrostatics, fluid flow and surfactant transport. Employing a comprehensive three-dimensional solution technique, the non-Newtonian shearing response of the bulk emulsion is analyzed in the dilute suspension regime. The present results reveal that the surfactant non-uniformity creates significant alterations in the flow disturbance around the drop, thereby influencing the viscous dissipation from the flowing emulsion. This, in effect, triggers changes in the bulk shear viscosity. It is striking to observe that the balance between electrical and hydrodynamic stresses is affected in such a way that surface tension gradient on the drop surface vanishes for some specific shear rates and the corresponding effective change in the bulk viscosity becomes negligible too. This critical condition hugely depends on the electrical permittivity and conductivity ratios of the two fluids and orientation of the applied electric field. Also the physical mechanisms of charge convection of surface deformation play their role in determining this critical shear rate. The charge convection instigated shear thinning or shear thickening behavior of the emulsion gets reversed due to a coupled interaction of the charge convection and Marangoni stress. In addition, the electrically created anisotropic normal stresses in the bulk rheology, get reduced due to the presence of surfactants, especially when the drop viscosity is much lesser than the continuous fluid. A thorough description of the drop-level flow physics and its connection to the bulk rheology of a dilute emulsion, may provide a fundamental understanding of a more complex emulsion system. *

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Influence of surfactants on the electrohydrodynamic stretching of water drops in oil

Cited by 17 publications

References 39 publications

A multiscale method for simulating fluid interfaces covered with large molecules such as asphaltenes

A multiscale method for simulating fluid interfaces covered with large molecules such as asphaltenes

Sedimentation of a surfactant-laden drop under the influence of an electric field

Electrorheology of a dilute emulsion of surfactant-covered drops

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