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Electrohydrodynamics in oil-in-oil emulsion

The fundamental phenomena observed in leaky dielectrics are related to the accumulation of free charge at the oil-oil interface, which adds a tangential component to electric stresses. These tangential stresses compete with normal stresses that arise in any dielectric. The angular variation of the normal stresses on the drop gives rise to shape deformations of droplets while the magnitude of the total electric stresses determine the conditions for the break-up of drops. In the presence of an oscillating electric field of frequency, a spherical oil drop immersed in a surrounding leaky-dielectric oil medium experiences normal and tangential electric stresses that have a steady and a time-dependent part. The total stress is a function of frequency and exhibits a transition from hydrodynamics-dominated to dipolar-dominated, where in the latter regime, the tangential electric stress is zero. The time-dependent normal stresses also induce pulsating droplet shapes. The periodic volume displacement of these pulsating drops are an additional source of hydrodynamic disturbances. With increasing frequency, the spatial extent of the droplet oscillation decreases, reducing the strength of hydrodynamics.

Our experimental system is a two-component mixture of oil drops of one component inside a surrounding medium of a second oil which belongs to the class of ‘‘leaky dielectrics’’, where an electric field is a good control parameter for tuning the strength of hydrodynamic fields.

An overview of the self-organized structures we observe as a function of frequency is shown in figure.

self-organized structures

Atul Varshney, Shankar Ghosh, S. Bhattacharya, and Anand Yethiraj "Self organization of exotic oil-in-oil phases driven by tunable electrohydrodynamics" Scientific Reports (Nature) 2, 738 (2012) [pdf] [supplementary information including movies].

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Turbulent breakup of a drop in a dc field

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Transition from strong to weak hydrodynamics

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Spherical to oblate pulsation of oil droplets

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Polygonal facets on pulsating droplets

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Droplet coalescence

 

 

 

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