Electrorheology of a dilute emulsion of surfactant-covered drops

Abstract

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. *

Electro-Hydrodynamics of Emulsion Droplets: Physical Insights to Applications

“…The high voltage is supplied across the electrodes and dynamics are captured by using cameras. In these studies, some authors have also introduced surfactants at the liquid-liquid interface [ 27 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 ]. A summary of the most prominent works related to droplet EHD are summarized in Table 1 .…”

“…The influence of interfacial properties on the droplet deformation and breakup under an electric field has also been a topic of profound interest for decades [ 27 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 ]. The surface-active molecules prompt to change in the interracial and rheological properties, leading to interfacial perturbations and breakups [ 23 , 71 , 136 , 137 , 138 ].…”

Effects of Surfactant Solubility on the Hydrodynamics of a Viscous Drop in a DC Electric Field

“…Electric field Surfactants Method References Stokes dc, uniform insoluble analytical (SD) [42] N.-S. dc, uniform insoluble numerical (LS-RegM) [43] Stokes dc, uniform insoluble (semi-) analytical (LD) [44,45] Stokes dc, uniform insoluble numerical (BIM) [46,47] Stokes dc, uniform insoluble analytical & numerical [48][49][50] Stokes dc, nonuniform insoluble analytical & numerical [51] Stokes dc, uniform soluble numerical (IIM) Present Work electrodeformation of a viscous drop [42,46,[52][53][54]. By reducing the surface tension and inducing a significant Marangoni stress due to the surfactant transport on the interface, surfactant could lead to drastically different EHD of a surfactant-laden viscous drop.…”

Steady axial electric field may lead to controllable cross-stream migration of droplets in confined oscillatory microflows

“…Motivated by the classical work of Taylor (1966), several studies have been reported on electrically modulated morpho-dynamics of droplets in an external flow (Hase, Watanabe & Yoshikawa 2006; Ristenpart et al 2009; Mhatre & Thaokar 2013; Zhang et al 2016; Brosseau & Vlahovska 2017; Poddar et al 2018, 2019 b ; Behera et al 2019; Santra et al 2018 b , 2019 a ; Santra, Das & Chakraborty 2020). Mandal, Bandopadhyay & Chakraborty (2016) studied the cross-stream migration of the droplet in unbounded plane Poiseuille flow under uniform electric field.…”