Thin film breakup by electric fields: development of microfluidic applications in deemulsification

Stability of emulsions, deemulsification of microemulsions by coalescing the dispersed phase using chemical additives or external forces, and stability of thin liquid films are of paramount importance in a wide array of industrial applications. Fundamental to all these processes is a clear understanding of the role of different physical and chemical factors on the stability of a thin film formed by one fluid phase in another. Several experimental techniques are currently available to explore the thermodynamic properties and stability of thin films, such as, micropipettes and thin film cells employing a porous plate, all of which provide a platform for following a thin film in a macroscopic sense. Setting up these experimental techniques to quantitatively and reproducibly measure thin film behaviour requires considerable time and dexterity. A novel microfluidic chip has been developed to study the break up of oily thin films in aqueous media under the influence of an externally applied electrical field. The basic concept involves two crossing micro channels etched into the surface of a glass substrate. Carefully designing the intersection of the two perpendicular channels can lead to formation of a film at the intersection. One channel would be filled with oil and the second channel, divided to half by oil channel, would deliver two water droplets toward the oil channel from both sides. Once the thin liquid film is created, an electrical potential difference can be applied across the channel. The electrical stresses developed at the interfaces of the two fluids forming the film will lead to a collapse of the film beyond a certain threshold voltage. The break up of the film was performed using a ramped DC potential, and a series of capacitance measurements determined the variation of the capacitance of the film with regards to the film thickness. This new microfluidics chip shows promise as a low cost, reproducible, microscale measurement tool to study the thermodynamic behaviour and stability of thin liquid films.