Drop-on-demand technique was developed, based on multi-stage high voltage electric pulse applied on conductive liquid meniscus hanging from a needle [1]. It is applied to the injection of a water droplet in oil, and allows injecting droplets from the same needle in a wide range of diameter, smaller than the needle one. The paper presents experiments of droplet injection in two types of mineral oil: Marcol 52, Primol 352, for varying interface tension and viscosity parameters. High speed video and numerical simulations helped understanding the main physical effects occurring during water droplet formation.

The basic process of coalescence of droplets in a flowing water-in-oil emulsion under the action of an electric field is considered. The coalescence probability depends on the ratio of time of close proximity of droplets and time of decrease of spacing down to drops contact. For two free drops aligned with the field, the dynamical problem consists in the deformation of the drops, their motion and the thinning of the oil film between the drops. For very small droplets, assuming a negligible interface deformation, a very small initial spacing and a high value of viscosity ratio leads to an order of magnitude estimate of the time required for the drops to achieve contact. Numerical simulations confirm that this time is roughly inversely proportional to the maximum initial electrostatic pressure p e0 at the facing interfaces and point up an influence of the electric Bond number defined as the ratio of electric and capillary forces.

Present research aims at determining the conditions leading to electrocoalescence of water drops in oil flows and at characterizing the interplay between fluid dynamics and electric field in the mechanism. We describe here the build-up of an experimental set-up designed to investigate the critical coalescence conditions in the case of two free water droplets in an oil shear flow under the action of an applied electric field. Drop pairs will be injected in a Poiseuille flow to study, by optical means, their relative motion, deformation and possible coalescence. Important part of this preliminary work concerns the injection of controlled charge free drop pairs using a newly developed EHD actuation technique. Second paper [1] deals with theoretical and numerical analyses in the same configuration.

— As a part of a study of electrocoalescence of water droplets in oil, the electrically induced deformations of water-oil interfaces are studied. Cases of large deformations of the interfaces involve the strong coupling of hydrodynamics and electrostatics, in which cases numerical simulation is needed. The paper presents different numerical simulations performed using the commercial software COMSOL MULTIPHYSICS™ in cases of water-air or water-oil configurations with axial symmetry. Electrohydrodynamics problems are solved using ALE approach in deforming meshes. Comparisons are presented with the results of analytical developments, asymptotic approaches or experiments.

The problem of instability of a horizontal interface between water and an insulating fluid, electrically influenced by a metallic sphere located just above it, is considered here with its relevance to the basic electrocoalescence phenomenon of close water droplets suspended in an insulating medium. Results are presented of preliminary experiments performed using visualisation and shadowscopy techniques. The evolution of the interface shape is characterised for various applied step voltages and for different values of the initial spacing s 0 between sphere and interface. An order of magnitude analysis leads to good estimates of the characteristic parameters and time scales.

As a part of a study of electrocoalescence of water droplets in oil, the controlled generation of small drops (diameter ~ 100 mum) is considered. The technique used consists in applying a voltage pulse promoting the deformation of a meniscus at the end of a capillary tube through the action of electric forces. For pulses of short enough duration, the transient deformation can lead to the ejection of a small drop electrically neutral. The experimental results of water drops extraction in oil are presented. Using capillary tubes of outer tip diameter varying from 0.5 mm to 1 mm, it is possible to obtain in a reproducible way drops of diameter ranging from 50 mum to 200 mum. For a given meniscus shape, the diameter of the extracted droplet depends on the voltage amplitude V and on the pulse duration Deltat. Order of magnitude considerations on the meniscus deformation process suggest that the main parameter which determines the size of the generated droplet is the product V2 Deltat. The experimental results support this guess for pulse durations low enough so that there is no electrical field (and, therefore, no surface charge) during the last stage of meniscus elongation and break-up. The possible use of this technique of drop-on-demand generation is discussed, taking into account the transient oil flow around the meniscus which most often brings the droplet off the system axis.