Electrically stressed water drops in oil

The deformation of water droplets in an electric field has been studied in hydrocarbon liquids. A water drop will elongate in an electric field due to the electrostatic pressure, and becomes unstable when a critical field limit is reached. By applying different voltage waveforms it is possible to measure both transient, time varying and static effects. An experimental method has been developed to study the rheological properties of the water droplet. The experimental results in surfactant-free model oil are in good agreement with classic Taylor theory. In diluted crude oil a correlation was found between the drop behavior in an electric field and the interface elasticity modulus, measured by dilational interfacial rheology. The static drop deformation was reduced with increasing elastic modulus due to adsorption of polar surfactants to the water/oil interface. Drop surface oscillations will be damped by the viscosity of the bulk liquid. The dynamic drop behavior was studied in viscosity standards prepared from different concentrations of polystyrene in toluene. The drop oscillations were modeled as damped oscillator, and the influence of viscosity on the eigen-frequency and damping coefficient was studied for different drop sizes. The oscillations can also explain the premature break-up of water droplets sometimes observed in the experiments. (C) 2010 Elsevier B.V. All rights reserved.