The movement of drops in electric fields plays a role in processes as diverse as storm cloud formation, ink-jet printing and lab-on-a-chip manipulations. An important factor in practical applications is the tendency for adjacent drops to coalesce, usually assumed to be favoured if drops are oppositely charged and attracted to each other. Now Ristenpart et al. show that when oppositely charged drops move towards each other in an electric field whose strength exceeds a critical value, the drops simply 'bounce' off one another. This observation calls for a re-evaluation of our understanding of all processes involving electrically induced drop motion. Adjacent drops of fluid coalesce, and oppositely charged drops have long been assumed to experience an attractive force that favours their coalescence. However, here it is observed that oppositely charged drops moving towards each other in a strong electric field do not coalesce when the field strength exceeds a certain value but rather 'bounce' off one another. This observation calls for a re-evaluation of our understanding of processes such as storm cloud formation and ink-jet printing, which involve electrically induced droplet motion. Electric fields induce motion in many fluid systems, including polymer melts1, surfactant micelles2 and colloidal suspensions3. Likewise, electric fields can be used to move liquid drops4. Electrically induced droplet motion manifests itself in processes as diverse as storm cloud formation5, commercial ink-jet printing6, petroleum and vegetable oil dehydration7, electrospray ionization for use in mass spectrometry8, electrowetting9 and lab-on-a-chip manipulations10. An important issue in practical applications is the tendency for adjacent drops to coalesce, and oppositely charged drops have long been assumed to experience an attractive force that favours their coalescence11,12,13. Here we report the existence of a critical field strength above which oppositely charged drops do not coalesce. We observe that appropriately positioned and oppositely charged drops migrate towards one another in an applied electric field; but whereas the drops coalesce as expected at low field strengths, they are repelled from one another after contact at higher field strengths. Qualitatively, the drops appear to ‘bounce’ off one another. We directly image the transient formation of a meniscus bridge between the bouncing drops, and propose that this temporary bridge is unstable with respect to capillary pressure when it forms in an electric field exceeding a critical strength. The observation of oppositely charged drops bouncing rather than coalescing in strong electric fields should affect our understanding of any process involving charged liquid drops, including de-emulsification, electrospray ionization and atmospheric conduction.

http://absimage.aps.org/image/DFD09/MWS_DFD09-2009-001564.pdf

Non-coalescence of oppositely charged drops

Durable superhydrophobic and superoleophilic electrospun nanofibrous membrane for oil-water emulsion separation

The coalescence of an aqueous droplet at an oil–water interface under an electric field has been investigated, with a view to quantify conditions that give rise to secondary droplet formation. Two patterns of drop-interface coalescence may occur: complete coalescence and partial coalescence. The former is obviously the desirable pattern for industrial coalescers. However in practice, the process of coalescence could actually produce smaller droplets, which become more difficult to remove, and hence undesirable. This is caused by either necking, due to extensive elongation of the droplet, or reaction to a fast and energetic coalescence and is referred to as partial coalescence. The volume of the droplets formed in this way has been analyzed as a function of the initial droplet size, electric field strength and the distance between the droplet and the interface. The expansion speed of the neck connecting the droplet and interface at the beginning of the pumping process has also been quantified. These results are useful in optimizing the electro-coalescence process.

Electro-coalescence of an aqueous droplet at an oil–water interface

Electrostatic destabilization of water-in-crude oil emulsions: Application to a real case and evaluation of the Aibel VIEC technology

The approaching movement and consequent coalescence of binary water droplets falling in stagnant oil and exposed to an external electric field are investigated using a high speed camera. Different situation of the droplets and electric field intensities are applied in the experiments. The qualitative results of the experimental observations are exhibited through the scaled images of the binary droplets snapshots in milliseconds. Furthermore, different approaching trends of the droplets are presented as quantitative plots and discussed based on the theoretical electrostatic and hydrodynamic models. The effect of the applied voltage amplitude, initial distance of the drop pair, and skew angle of the electric field are investigated. The experimental results prove the electrostatic theories; as acceleration in electrocoalescence demonstrated using a stronger electric field as well as closer distance between the droplets. It was also revealed that the skew angle of the electric field decelerates the electrocoalescence until alignment of the droplets.

Electrocoalescence of binary water droplets falling in oil: Experimental study

Electrocoalescence for oil-water separation : Fundamental aspects

Results are presented concerning the influence of an applied electric field on the coalescence of a water droplet with a much bigger water drop, both drops being immersed in a crude oil. This original study of electrocoalescence in crude oil was performed through high-speed optical observations, using a near-infrared camera, of a droplet falling onto a bottom drop, a bipolar square voltage being applied. For low electrical field strength, the electrostatic pressure at the water/oil interface of the merging droplet partly counteracts the capillary forces, which slows down the drop coalescence process. Above threshold field strength, the electric forces drastically affect the dynamics of drop deformation and merging. At a working temperature T = 60 °C, partial coalescence was observed, leaving a daughter drop of size increasing with the applied field. At T = 40 °C, there was an abrupt transition from coalescence to non-coalescence, the top droplet inducing an upward directed nearly conical deformation of the bottom drop. This is attributed to charge exchange between the colliding water drops at each polarity reversal of the applied voltage, thus generating a bouncing-like behaviour at a frequency twice the voltage frequency. The charge exchange occurs through a very thin filament interconnecting the drops during a short time and presumably generated by interface instability. Under strong enough applied field, there was also formation of a very fine mist around the zone of drop quasi contact; this mist is ascribed to the break-up of the transitory very thin ligament bridging the drops. Some explanations and considerations are proposed for the various observations; in particular, it is proposed that the contrast between the transitions to partial coalescence (at 60 °C) and to non-coalescence (at 40 °C) arises from geometrical differences in the temporary small bridge interconnecting the drops.

Experimental study of electrocoalescence of water drops in crude oil using near-infrared camera

This paper deals with the extraction from a meniscus of water droplets of small size into an immiscible insulating liquid by the use of a multistage voltage pulse. A previous study on a particular oil established that this EHD Drop-on-demand technique allows producing droplets of size smaller or much smaller than the needle diameter. The case of a dielectric liquid characterized by a rather high value of the water/oil interfacial tension is considered here. It is shown that by adequately modifying the shape and level of the multistage voltage pulse, droplets are produced with a diameter ranging over a large interval. It was expected that the short enough pulse duration would give electrically neutral droplets; in practice it is found that the droplets carry minute but nonzero charges. An explanation is given for the charging mechanism and other changes of the pulse shape allow to further minimize the droplets charge. Finally having in mind the possibility to investigate experimentally the electrocoalescence of two close drops, results are presented concerning the injection of two successive droplets.

Pierre Atten

Papers

Electrocoalescence for oil-water separation : Fundamental aspects

Experimental study of electrocoalescence of water drops in crude oil using near-infrared camera

Improvements of on-demand electrohydrodynamic injection of droplet and droplet pair in a dielectric liquid