Electric field centering of double-emulsion droplets suspended in a density gradient

Abstract

Highly concentric double-emulsion droplets can be formed in a suspending fluid by application of a uniform, AC electric field. A dipole/dipole interaction acting on the inner droplet achieves this centering effect. The dielectric constant of the outer liquid shell must be higher than the dielectric constant of the suspending liquid; the electric field frequency must overcome electrostatic shielding due to the electrical conductivity of the liquid forming the outer shell; and the liquid densities must be matched within ∼0.1%. Early experiments using a thin glass plate to hold the droplet between the electrodes demonstrated centering in ∼60 seconds with an electric field of magnitude >10⁴ V_RMS m⁻¹, but the plate caused a systematic ∼10% vertical offset between the centers of the inner and outer surfaces. Suspending the double-emulsion droplet in a density gradient reduces the centering error to less than ±5% and eliminates high-spatial-frequency distortion. Furthermore, the electric field and the density gradient effects upon droplet distortion tend to counteract each other.