1,720,992 research outputs found
Deformation and breakup of Newtonian and non-Newtonian conducting drops in an electric field
No full textIn this article, we considered experimentally the deformation and breakup of Newtonian and non-Newtonian conducting drops in surrounding fluid subjected to a uniform electric field. First, we examined three distinctive cases of Newtonian-fluid pairs with different relative conductivities, namely highly conducting drops, conducting drops and slightly conducting drops. The results on the Newtonian fluids demonstrated that when the conductivity of the drop is very large relative to that of the surrounding fluid, the deformation response of such highly conducting drops is described well by the electrohydrostatic theory, especially with regard to the prediction of the critical point. Specifically, when the ratio of drop to continuous-phase resistivity, R, was less than 10(-5), the electrohydrostatic theory was quite satisfactory. Then, the non-Newtonian effect on the drop deformation and breakup was studied for highly conducting drops which satisfied the condition R < 0(10(-5)). The highly conducting drop became stable in a weak or moderate field strength when either the drop or the continuous phase was non-Newtonian. On the other hand, when both the phases were non-Newtonian, more complicated responses were observed depending on the ratio of zero-shear-rate viscosities. Although the effects of the rheological properties are minimal on all features away from the critical conditions for breakup or prior to the instability, the non-Newtonian properties have a significant influence during drop burst, which is accompanied by large velocities and velocity gradients. In particular, when the ratio of the zero-shear-rate viscosity of the drop to that of the ambient fluid was much larger than unity, non-Newtonian properties of the drop phase enhanced the drop stability. Conversely, the elasticity of the continuous phase deteriorated the drop stability. Meanwhile if the zero-shear-rate viscosity ratio was much smaller than unity, the elasticity of the continuous phase produced a stabilizing effect. The effects of resistivity and viscosity ratios on the breakup modes were also investigated. When at least one of the two contiguous phases possessed considerable non-Newtonian properties, tip streaming appeared.This work has been supported partly by the Ministry of Science and Technology
and by a contracted research project with Hyundai Heavy Industries Co., Ltd. The
authors appreciate their support
Electrohydrodynamics and electrorotation of a drop with fluid less conductive than that of the ambient fluid
No full textIn this article, we investigated the electrohydrodynamic responses of a deformable fluid drop in another immiscible fluid under the action of a dc electric field. Both the ambient and drop fluids considered here were incompressible Newtonian and all of the drop phases were less conductive than their ambient fluids. Under these circumstances, the drops experienced the so-called electrorotation owing to the reverse dipole generated by the external electric field when the electric field strength exceeded a certain threshold value. The experimental observation showed that the threshold electric field strength was dependent on the drop size as well as the viscosity ratio. Also noted was the effect of the electrorotation on both the deformation behavior and the mode of the drop breakup. Specifically, we determined the critical electric capillary number beyond which the steady-state drop shape did not exist and the drop eventually broke up. Finally, the validity of Taylor's leaky dielectric theory was discussed in the presence of the electrorotation of the drop. (C) 2000 American Institute of Physics. [S1070-6631(00)00704-2]
Electrohydrodynamic effects on the deformation and orientation of a liquid capsule in a linear flow
No full textThe role of a uniform electric field on the deformation and orientation of a liquid capsule with a viscoelastic membrane is considered analytically in the small deformation limit. The capsule is freely suspended either in a quiescent fluid or in a shear flow. The viscoelasticity of the membrane is taken into account by the Kelvin-Voigt model and the electrohydrodynamic flow is analyzed on the basis of the leaky dielectric model. In this article, we consider three different prototype models of capsules; viz., a neo-Hookean (incompressible isotropic) membrane, a red blood cell-type (area-preserving) membrane, and an interfacial-tension droplet. The deformed capsule shape from its initial sphericity and its orientaion are determined from the linearized governing equations and boundary conditions in the limit of small deformations. The asymptotic theory shows that the degree of capsule deformation induced by a uniform electric field alone is independent of the surface viscosity of the capsule as well as the viscosity ratio between the two fluids inside and outside the capsule. Meanwhile, in the presence of an imposed shear flow, the degree of deformation depends on the surface viscosity with preserving still the independence of the viscosity ratio. For an illustrative purpose, experimental results for the role of a uniform electric field on the orientation of an interfacial-tension droplet in a shear flow are discussed briefly. (C) 2000 American Institute of Physics. [S1070-6631(00)00807-2].This work was supported partly by a grant from the Ministry
of Science and Technology and by a contracted research
project with Hyundai Heavy Industries Co., Ltd. The authors
appreciate their support. One of the referees pointed to one
very important issue regarding the boundary conditions,
which was essential to correct the asymptotic analysis
Effect of nonionic surfactant on the deformation and breakup of a drop in an electric field
No full textWe have examined deformation and breakup of fluid drops suspended in another immiscible fluid under the action of an electric field. The contiguous fluids are incompressible Newtonian and the fluid-fluid interface is populated by nonionic surfactant molecules. The presence of the nonionic surfactant affects both the degree of deformation and the modes of breakup through the so-called Marangoni flow resulting from its nonuniform distribution on the interface. The drop is deformed into either a prolate or an oblate spheroid depending upon the electrical properties of the fluids and sustains a steady-state shape until the electrical Weber number is above a certain critical value. Two distinctively different modes of the drop breakup are observed depending on the surfactant concentration. When the interface is clean or contaminated by a very small amount of surfactant molecules, the drop bursts into several small droplets after forming bulbous ends. There exists a certain range of the surfactant concentration in which tip-steaming is a prevalent drop breakup mode. If the surfactant concentration exceeds this range, the breakup mode goes back to the fragmentation with bulbous end formation. This shows that, although not pronounced in the small deformation limit, nonuniformity in the surfactant distribution is a decisive factor for the breakup mechanism of a prolate spheroid. The results also show that when the drop deforms into an oblate spheroid, the effect of nonuniform distribution of surfactant can be significant. (C) 1998 Academic Press
Rheological responses of oil-in-oil emulsions in an electric field
No full textIn the present article, the rheological responses of oil-in-oil emulsions in a de electric field were investigated experimentally. Specifically, the dispersed phase of the emulsions considered in this work was less conducting than the continuous phase. Depending on the relative strength between the shear flow and electric fields, three distinctive responses of the emulsions were observed in steady and dynamic oscillatory shear tests. First, the apparent viscosity enhancement (positive electrorheological effect) was produced when the electric field was predominant. Second, when the shear flow was strong and dominant, the electric field played a trivial role on the rheological behavior of emulsions. Finally, the viscosity reduction (negative electrorheological effect) was generated when the shear flow and electric fields were competitive. The viscosity enhancement was induced by the formation of chain-like microstructures of the dispersed droplets as in a typical electric-field responsive particle suspension. Meanwhile, the viscosity reduction was closely associated with the electric-field-induced rotation of the dispersed droplets, which was confirmed by the electrohydrodynamics of a single conducting drop in a more conducting ambient fluid. Finally, the rheological responses of the model emulsions to a dynamic small-amplitude oscillatory shearing were considered in conjunction with the morphology evolution under the action of the electric and flow fields. The results showed that the interfacial contribution to the rheological response appeared quite differently depending on the conductivity ratio of the two contiguous fluids. (C) 2000 The Society of Rheology. [S0148 -6055(00)00502-2]
Fluid dynamics of a double emulsion droplet in an electric field
No full textOne of general free boundary problems concerning the electrohydrodynamic effects on a concentric double emulsion drop is studied theoretically for the three constituent phases of leaky dielectric fluids. In order to proceed the problem analytically, the domain perturbation procedure is utilized in the small deformation limit. The patterns of electric-field-driven flow are successfully characterized by examining the distribution of induced surface charges at the inner and outer drop interfaces. The second recirculating flow is generated in the annular phase when the inner and outer interfaces are charged with the same sense. The deformation type of inner and outer interfaces can be roughly interpreted by the flow patterns, although the exact description on the deformation requires consideration of the combined contributions from both electric and flow fields. In addition, the presence of double emulsion droplets alters the stress field of the continuous phase. The electric-field-induced ''particle stress'' not only changes the effective viscosity of dispersion of the double emulsion droplets but yields the normal stress difference, which is typical of a viscoelastic fluid. Finally, the heat transfer rate enhanced by the electric-field-driven flow is also considered. (C) 1999 American Institute of Physics. [S1070-6631(99)01605-0]
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